mz_storage/source/

kafka.rs

// Copyright Materialize, Inc. and contributors. All rights reserved.
//
// Use of this software is governed by the Business Source License
// included in the LICENSE file.
//
// As of the Change Date specified in that file, in accordance with
// the Business Source License, use of this software will be governed
// by the Apache License, Version 2.0.

use std::collections::BTreeMap;
use std::collections::btree_map::Entry;
use std::str::{self};
use std::sync::Arc;
use std::thread;
use std::time::Duration;

use anyhow::anyhow;
use chrono::{DateTime, NaiveDateTime};
use differential_dataflow::{AsCollection, Hashable};
use futures::StreamExt;
use itertools::Itertools;
use maplit::btreemap;
use mz_kafka_util::client::{
    GetPartitionsError, MzClientContext, PartitionId, TunnelingClientContext, get_partitions,
};
use mz_ore::assert_none;
use mz_ore::cast::CastFrom;
use mz_ore::error::ErrorExt;
use mz_ore::future::InTask;
use mz_ore::iter::IteratorExt;
use mz_repr::adt::timestamp::CheckedTimestamp;
use mz_repr::{Datum, Diff, GlobalId, Row, adt::jsonb::Jsonb};
use mz_ssh_util::tunnel::SshTunnelStatus;
use mz_storage_types::errors::{
    ContextCreationError, DataflowError, SourceError, SourceErrorDetails,
};
use mz_storage_types::sources::kafka::{
    KafkaMetadataKind, KafkaSourceConnection, KafkaTimestamp, RangeBound,
};
use mz_storage_types::sources::{MzOffset, SourceExport, SourceExportDetails, SourceTimestamp};
use mz_timely_util::antichain::AntichainExt;
use mz_timely_util::builder_async::{
    Event, OperatorBuilder as AsyncOperatorBuilder, PressOnDropButton,
};
use mz_timely_util::containers::stack::AccountedStackBuilder;
use mz_timely_util::order::Partitioned;
use rdkafka::consumer::base_consumer::PartitionQueue;
use rdkafka::consumer::{BaseConsumer, Consumer, ConsumerContext};
use rdkafka::error::KafkaError;
use rdkafka::message::{BorrowedMessage, Headers};
use rdkafka::statistics::Statistics;
use rdkafka::topic_partition_list::Offset;
use rdkafka::{ClientContext, Message, TopicPartitionList};
use serde::{Deserialize, Serialize};
use timely::PartialOrder;
use timely::container::CapacityContainerBuilder;
use timely::dataflow::channels::pact::Pipeline;
use timely::dataflow::operators::Capability;
use timely::dataflow::operators::core::Partition;
use timely::dataflow::operators::vec::Broadcast;
use timely::dataflow::{Scope, StreamVec};
use timely::progress::Antichain;
use timely::progress::Timestamp;
use tokio::sync::{Notify, mpsc};
use tracing::{error, info, trace};

use crate::healthcheck::{HealthStatusMessage, HealthStatusUpdate, StatusNamespace};
use crate::metrics::source::kafka::KafkaSourceMetrics;
use crate::source::types::{Probe, SignaledFuture, SourceRender, StackedCollection};
use crate::source::{RawSourceCreationConfig, SourceMessage, probe};
use crate::statistics::SourceStatistics;

#[derive(
    Clone,
    Debug,
    Default,
    PartialEq,
    Eq,
    PartialOrd,
    Ord,
    Serialize,
    Deserialize
)]
struct HealthStatus {
    kafka: Option<HealthStatusUpdate>,
    ssh: Option<HealthStatusUpdate>,
}

impl HealthStatus {
    fn kafka(update: HealthStatusUpdate) -> Self {
        Self {
            kafka: Some(update),
            ssh: None,
        }
    }

    fn ssh(update: HealthStatusUpdate) -> Self {
        Self {
            kafka: None,
            ssh: Some(update),
        }
    }
}

/// Contains all information necessary to ingest data from Kafka
pub struct KafkaSourceReader {
    /// Name of the topic on which this source is backed on
    topic_name: String,
    /// Name of the source (will have format kafka-source-id)
    source_name: String,
    /// Source global ID
    id: GlobalId,
    /// Kafka consumer for this source
    consumer: Arc<BaseConsumer<TunnelingClientContext<GlueConsumerContext>>>,
    /// List of consumers. A consumer should be assigned per partition to guarantee fairness
    partition_consumers: Vec<PartitionConsumer>,
    /// Worker ID
    worker_id: usize,
    /// Total count of workers
    worker_count: usize,
    /// The most recently read offset for each partition known to this source
    /// reader by output-index. An offset of -1 indicates that no prior message
    /// has been read for the given partition.
    last_offsets: BTreeMap<usize, BTreeMap<PartitionId, i64>>,
    /// The offset to start reading from for each partition.
    start_offsets: BTreeMap<PartitionId, i64>,
    /// Channel to receive Kafka statistics JSON blobs from the stats callback.
    stats_rx: crossbeam_channel::Receiver<Jsonb>,
    /// A handle to the partition specific metrics
    partition_metrics: KafkaSourceMetrics,
    /// Per partition capabilities used to produce messages
    partition_capabilities: BTreeMap<PartitionId, PartitionCapability>,
}

struct PartitionCapability {
    /// The capability of the data produced
    data: Capability<KafkaTimestamp>,
}

/// The high watermark offsets of a Kafka partition.
///
/// This is the offset of the latest message in the topic/partition available for consumption + 1.
type HighWatermark = u64;

/// Processes `resume_uppers` stream updates, committing them upstream and
/// storing them in the `progress_statistics` to be emitted later.
pub struct KafkaResumeUpperProcessor {
    config: RawSourceCreationConfig,
    topic_name: String,
    consumer: Arc<BaseConsumer<TunnelingClientContext<GlueConsumerContext>>>,
    statistics: Vec<SourceStatistics>,
}

/// Computes whether this worker is responsible for consuming a partition. It assigns partitions to
/// workers in a round-robin fashion, starting at an arbitrary worker based on the hash of the
/// source id.
fn responsible_for_pid(config: &RawSourceCreationConfig, pid: i32) -> bool {
    let pid = usize::try_from(pid).expect("positive pid");
    ((config.responsible_worker(config.id) + pid) % config.worker_count) == config.worker_id
}

struct SourceOutputInfo {
    id: GlobalId,
    output_index: usize,
    resume_upper: Antichain<KafkaTimestamp>,
    metadata_columns: Vec<KafkaMetadataKind>,
}

impl SourceRender for KafkaSourceConnection {
    // TODO(petrosagg): The type used for the partition (RangeBound<PartitionId>) doesn't need to
    // be so complicated and we could instead use `Partitioned<PartitionId, Option<u64>>` where all
    // ranges are inclusive and a time of `None` signifies that a particular partition is not
    // present. This requires an shard migration of the remap shard.
    type Time = KafkaTimestamp;

    const STATUS_NAMESPACE: StatusNamespace = StatusNamespace::Kafka;

    fn render<G: Scope<Timestamp = KafkaTimestamp>>(
        self,
        scope: &mut G,
        config: &RawSourceCreationConfig,
        resume_uppers: impl futures::Stream<Item = Antichain<KafkaTimestamp>> + 'static,
        start_signal: impl std::future::Future<Output = ()> + 'static,
    ) -> (
        BTreeMap<GlobalId, StackedCollection<G, Result<SourceMessage, DataflowError>>>,
        StreamVec<G, HealthStatusMessage>,
        StreamVec<G, Probe<KafkaTimestamp>>,
        Vec<PressOnDropButton>,
    ) {
        let (metadata, probes, metadata_token) =
            render_metadata_fetcher(scope, self.clone(), config.clone());
        let (data, health, reader_token) = render_reader(
            scope,
            self,
            config.clone(),
            resume_uppers,
            metadata,
            start_signal,
        );

        let partition_count = u64::cast_from(config.source_exports.len());
        let data_streams: Vec<_> = data.inner.partition::<CapacityContainerBuilder<_>, _, _>(
            partition_count,
            |((output, data), time, diff)| {
                let output = u64::cast_from(*output);
                (output, (data.clone(), time.clone(), diff.clone()))
            },
        );
        let mut data_collections = BTreeMap::new();
        for (id, data_stream) in config.source_exports.keys().zip_eq(data_streams) {
            data_collections.insert(*id, data_stream.as_collection());
        }

        (
            data_collections,
            health,
            probes,
            vec![metadata_token, reader_token],
        )
    }
}

/// Render the reader of a Kafka source.
///
/// The reader is responsible for polling the Kafka topic partitions for new messages, and
/// transforming them into a `SourceMessage` collection.
fn render_reader<G: Scope<Timestamp = KafkaTimestamp>>(
    scope: &G,
    connection: KafkaSourceConnection,
    config: RawSourceCreationConfig,
    resume_uppers: impl futures::Stream<Item = Antichain<KafkaTimestamp>> + 'static,
    metadata_stream: StreamVec<G, (mz_repr::Timestamp, MetadataUpdate)>,
    start_signal: impl std::future::Future<Output = ()> + 'static,
) -> (
    StackedCollection<G, (usize, Result<SourceMessage, DataflowError>)>,
    StreamVec<G, HealthStatusMessage>,
    PressOnDropButton,
) {
    let name = format!("KafkaReader({})", config.id);
    let mut builder = AsyncOperatorBuilder::new(name, scope.clone());

    let (data_output, stream) = builder.new_output::<AccountedStackBuilder<_>>();
    let (health_output, health_stream) = builder.new_output::<CapacityContainerBuilder<Vec<_>>>();

    let mut metadata_input = builder.new_disconnected_input(metadata_stream.broadcast(), Pipeline);

    let mut outputs = vec![];

    // Contains the `SourceStatistics` entries for exports that require a snapshot.
    let mut all_export_stats = vec![];
    let mut snapshot_export_stats = vec![];
    for (idx, (id, export)) in config.source_exports.iter().enumerate() {
        let SourceExport {
            details,
            storage_metadata: _,
            data_config: _,
        } = export;
        let resume_upper = Antichain::from_iter(
            config
                .source_resume_uppers
                .get(id)
                .expect("all source exports must be present in source resume uppers")
                .iter()
                .map(Partitioned::<RangeBound<PartitionId>, MzOffset>::decode_row),
        );

        let metadata_columns = match details {
            SourceExportDetails::Kafka(details) => details
                .metadata_columns
                .iter()
                .map(|(_name, kind)| kind.clone())
                .collect::<Vec<_>>(),
            _ => panic!("unexpected source export details: {:?}", details),
        };

        let statistics = config
            .statistics
            .get(id)
            .expect("statistics have been initialized")
            .clone();
        // export requires snapshot
        if resume_upper.as_ref() == &[Partitioned::minimum()] {
            snapshot_export_stats.push(statistics.clone());
        }
        all_export_stats.push(statistics);

        let output = SourceOutputInfo {
            id: *id,
            resume_upper,
            output_index: idx,
            metadata_columns,
        };
        outputs.push(output);
    }

    let busy_signal = Arc::clone(&config.busy_signal);
    let button = builder.build(move |caps| {
        SignaledFuture::new(busy_signal, async move {
            let [mut data_cap, health_cap] = caps.try_into().unwrap();

            let client_id = connection.client_id(
                config.config.config_set(),
                &config.config.connection_context,
                config.id,
            );
            let group_id = connection.group_id(&config.config.connection_context, config.id);
            let KafkaSourceConnection {
                connection,
                topic,
                topic_metadata_refresh_interval,
                start_offsets,
                metadata_columns: _,
                // Exhaustive match protects against forgetting to apply an
                // option. Ignored fields are justified below.
                connection_id: _,   // not needed here
                group_id_prefix: _, // used above via `connection.group_id`
            } = connection;

            // Start offsets is a map from partition to the next offset to read from.
            let mut start_offsets: BTreeMap<_, i64> = start_offsets
                .clone()
                .into_iter()
                .filter(|(pid, _offset)| responsible_for_pid(&config, *pid))
                .map(|(k, v)| (k, v))
                .collect();

            let mut partition_capabilities = BTreeMap::new();
            let mut max_pid = None;
            let resume_upper = Antichain::from_iter(
                outputs
                    .iter()
                    .map(|output| output.resume_upper.clone())
                    .flatten(),
            );

            for ts in resume_upper.elements() {
                if let Some(pid) = ts.interval().singleton() {
                    let pid = pid.unwrap_exact();
                    max_pid = std::cmp::max(max_pid, Some(*pid));
                    if responsible_for_pid(&config, *pid) {
                        let restored_offset = i64::try_from(ts.timestamp().offset)
                            .expect("restored kafka offsets must fit into i64");
                        if let Some(start_offset) = start_offsets.get_mut(pid) {
                            *start_offset = std::cmp::max(restored_offset, *start_offset);
                        } else {
                            start_offsets.insert(*pid, restored_offset);
                        }

                        let part_ts = Partitioned::new_singleton(
                            RangeBound::exact(*pid),
                            ts.timestamp().clone(),
                        );
                        let part_cap = PartitionCapability {
                            data: data_cap.delayed(&part_ts),
                        };
                        partition_capabilities.insert(*pid, part_cap);
                    }
                }
            }
            let lower = max_pid
                .map(RangeBound::after)
                .unwrap_or(RangeBound::NegInfinity);
            let future_ts =
                Partitioned::new_range(lower, RangeBound::PosInfinity, MzOffset::from(0));
            data_cap.downgrade(&future_ts);

            info!(
                source_id = config.id.to_string(),
                worker_id = config.worker_id,
                num_workers = config.worker_count,
                "instantiating Kafka source reader at offsets {start_offsets:?}"
            );

            let (stats_tx, stats_rx) = crossbeam_channel::unbounded();
            let notificator = Arc::new(Notify::new());

            let consumer: Result<BaseConsumer<_>, _> = connection
                .create_with_context(
                    &config.config,
                    GlueConsumerContext {
                        notificator: Arc::clone(&notificator),
                        stats_tx,
                        inner: MzClientContext::default(),
                    },
                    &btreemap! {
                        // Disable Kafka auto commit. We manually commit offsets
                        // to Kafka once we have reclocked those offsets, so
                        // that users can use standard Kafka tools for progress
                        // tracking.
                        "enable.auto.commit" => "false".into(),
                        // Always begin ingest at 0 when restarted, even if Kafka
                        // contains committed consumer read offsets
                        "auto.offset.reset" => "earliest".into(),
                        // Use the user-configured topic metadata refresh
                        // interval.
                        "topic.metadata.refresh.interval.ms" =>
                            topic_metadata_refresh_interval
                            .as_millis()
                            .to_string(),
                        // TODO: document the rationale for this.
                        "fetch.message.max.bytes" => "134217728".into(),
                        // Consumer group ID, which may have been overridden by
                        // the user. librdkafka requires this, and we use offset
                        // committing to provide a way for users to monitor
                        // ingest progress, though we do not rely on the
                        // committed offsets for any functionality.
                        "group.id" => group_id.clone(),
                        // Allow Kafka monitoring tools to identify this
                        // consumer.
                        "client.id" => client_id.clone(),
                    },
                    InTask::Yes,
                )
                .await;

            let consumer = match consumer {
                Ok(consumer) => Arc::new(consumer),
                Err(e) => {
                    let update = HealthStatusUpdate::halting(
                        format!(
                            "failed creating kafka reader consumer: {}",
                            e.display_with_causes()
                        ),
                        None,
                    );
                    health_output.give(
                        &health_cap,
                        HealthStatusMessage {
                            id: None,
                            namespace: if matches!(e, ContextCreationError::Ssh(_)) {
                                StatusNamespace::Ssh
                            } else {
                                StatusNamespace::Kafka
                            },
                            update: update.clone(),
                        },
                    );
                    for (output, update) in outputs.iter().repeat_clone(update) {
                        health_output.give(
                            &health_cap,
                            HealthStatusMessage {
                                id: Some(output.id),
                                namespace: if matches!(e, ContextCreationError::Ssh(_)) {
                                    StatusNamespace::Ssh
                                } else {
                                    StatusNamespace::Kafka
                                },
                                update,
                            },
                        );
                    }
                    // IMPORTANT: wedge forever until the `SuspendAndRestart` is processed.
                    // Returning would incorrectly present to the remap operator as progress to the
                    // empty frontier which would be incorrectly recorded to the remap shard.
                    std::future::pending::<()>().await;
                    unreachable!("pending future never returns");
                }
            };

            // Note that we wait for this AFTER we downgrade to the source `resume_upper`. This
            // allows downstream operators (namely, the `reclock_operator`) to downgrade to the
            // `resume_upper`, which is necessary for this basic form of backpressure to work.
            start_signal.await;
            info!(
                source_id = config.id.to_string(),
                worker_id = config.worker_id,
                num_workers = config.worker_count,
                "kafka worker noticed rehydration is finished, starting partition queues..."
            );

            let partition_ids = start_offsets.keys().copied().collect();
            let offset_commit_metrics = config.metrics.get_offset_commit_metrics(config.id);

            let mut reader = KafkaSourceReader {
                topic_name: topic.clone(),
                source_name: config.name.clone(),
                id: config.id,
                partition_consumers: Vec::new(),
                consumer: Arc::clone(&consumer),
                worker_id: config.worker_id,
                worker_count: config.worker_count,
                last_offsets: outputs
                    .iter()
                    .map(|output| (output.output_index, BTreeMap::new()))
                    .collect(),
                start_offsets,
                stats_rx,
                partition_metrics: config.metrics.get_kafka_source_metrics(
                    partition_ids,
                    topic.clone(),
                    config.id,
                ),
                partition_capabilities,
            };

            let offset_committer = KafkaResumeUpperProcessor {
                config: config.clone(),
                topic_name: topic.clone(),
                consumer,
                statistics: all_export_stats.clone(),
            };

            // Seed the progress metrics with `0` if we are snapshotting.
            if !snapshot_export_stats.is_empty() {
                if let Err(e) = offset_committer
                    .process_frontier(resume_upper.clone())
                    .await
                {
                    offset_commit_metrics.offset_commit_failures.inc();
                    tracing::warn!(
                        %e,
                        "timely-{worker_id} source({source_id}) failed to commit offsets: resume_upper={upper}",
                        worker_id = config.worker_id,
                        source_id = config.id,
                        upper = resume_upper.pretty()
                    );
                }
                // Reset snapshot statistics for any exports that are not involved
                // in this round of snapshotting. Those that are snapshotting this round will
                // see updates as the snapshot commences.
                for statistics in config.statistics.values() {
                    statistics.set_snapshot_records_known(0);
                    statistics.set_snapshot_records_staged(0);
                }
            }

            let resume_uppers_process_loop = async move {
                tokio::pin!(resume_uppers);
                while let Some(frontier) = resume_uppers.next().await {
                    if let Err(e) = offset_committer.process_frontier(frontier.clone()).await {
                        offset_commit_metrics.offset_commit_failures.inc();
                        tracing::warn!(
                            %e,
                            "timely-{worker_id} source({source_id}) failed to commit offsets: resume_upper={upper}",
                            worker_id = config.worker_id,
                            source_id = config.id,
                            upper = frontier.pretty()
                        );
                    }
                }
                // During dataflow shutdown this loop can end due to the general chaos caused by
                // dropping tokens as a means to shutdown. This call ensures this future never ends
                // and we instead rely on this operator being dropped altogether when *its* token
                // is dropped.
                std::future::pending::<()>().await;
            };
            tokio::pin!(resume_uppers_process_loop);

            let mut metadata_update: Option<MetadataUpdate> = None;
            let mut snapshot_total = None;

            let max_wait_time =
                mz_storage_types::dyncfgs::KAFKA_POLL_MAX_WAIT.get(config.config.config_set());
            loop {
                // Wait for data or metadata events while also making progress with offset
                // committing.
                tokio::select! {
                    // TODO(petrosagg): remove the timeout and rely purely on librdkafka waking us
                    // up
                    _ = tokio::time::timeout(max_wait_time, notificator.notified()) => {},

                    _ = metadata_input.ready() => {
                        // Collect all pending updates, then only keep the most recent one.
                        let mut updates = Vec::new();
                        while let Some(event) = metadata_input.next_sync() {
                            if let Event::Data(_, mut data) = event {
                                updates.append(&mut data);
                            }
                        }
                        metadata_update = updates
                            .into_iter()
                            .max_by_key(|(ts, _)| *ts)
                            .map(|(_, update)| update);
                    }

                    // This future is not cancel safe but we are only passing a reference to it in
                    // the select! loop so the future stays on the stack and never gets cancelled
                    // until the end of the function.
                    _ = resume_uppers_process_loop.as_mut() => {},
                }

                match metadata_update.take() {
                    Some(MetadataUpdate::Partitions(partitions)) => {
                        let max_pid = partitions.keys().last().cloned();
                        let lower = max_pid
                            .map(RangeBound::after)
                            .unwrap_or(RangeBound::NegInfinity);
                        let future_ts = Partitioned::new_range(
                            lower,
                            RangeBound::PosInfinity,
                            MzOffset::from(0),
                        );

                        let mut offset_known = 0;
                        for (&pid, &high_watermark) in &partitions {
                            if responsible_for_pid(&config, pid) {
                                offset_known += high_watermark;
                                reader.ensure_partition(pid);
                                if let Entry::Vacant(entry) =
                                    reader.partition_capabilities.entry(pid)
                                {
                                    let start_offset = match reader.start_offsets.get(&pid) {
                                        Some(&offset) => offset.try_into().unwrap(),
                                        None => 0u64,
                                    };
                                    let part_since_ts = Partitioned::new_singleton(
                                        RangeBound::exact(pid),
                                        MzOffset::from(start_offset),
                                    );

                                    entry.insert(PartitionCapability {
                                        data: data_cap.delayed(&part_since_ts),
                                    });
                                }
                            }
                        }

                        // If we are snapshotting, record our first set of partitions as the snapshot
                        // size.
                        if !snapshot_export_stats.is_empty() && snapshot_total.is_none() {
                            // Note that we want to represent the _number of offsets_, which
                            // means the watermark's frontier semantics is correct, without
                            // subtracting (Kafka offsets start at 0).
                            snapshot_total = Some(offset_known);
                        }

                        // Clear all the health namespaces we know about.
                        // Note that many kafka sources's don't have an ssh tunnel, but the
                        // `health_operator` handles this fine.
                        for output in &outputs {
                            for namespace in [StatusNamespace::Kafka, StatusNamespace::Ssh] {
                                health_output.give(
                                    &health_cap,
                                    HealthStatusMessage {
                                        id: Some(output.id),
                                        namespace,
                                        update: HealthStatusUpdate::running(),
                                    },
                                );
                            }
                        }
                        for namespace in [StatusNamespace::Kafka, StatusNamespace::Ssh] {
                            health_output.give(
                                &health_cap,
                                HealthStatusMessage {
                                    id: None,
                                    namespace,
                                    update: HealthStatusUpdate::running(),
                                },
                            );
                        }

                        for export_stat in all_export_stats.iter() {
                            export_stat.set_offset_known(offset_known);
                        }

                        data_cap.downgrade(&future_ts);
                    }
                    Some(MetadataUpdate::TransientError(status)) => {
                        if let Some(update) = status.kafka {
                            health_output.give(
                                &health_cap,
                                HealthStatusMessage {
                                    id: None,
                                    namespace: StatusNamespace::Kafka,
                                    update: update.clone(),
                                },
                            );
                            for (output, update) in outputs.iter().repeat_clone(update) {
                                health_output.give(
                                    &health_cap,
                                    HealthStatusMessage {
                                        id: Some(output.id),
                                        namespace: StatusNamespace::Kafka,
                                        update,
                                    },
                                );
                            }
                        }
                        if let Some(update) = status.ssh {
                            health_output.give(
                                &health_cap,
                                HealthStatusMessage {
                                    id: None,
                                    namespace: StatusNamespace::Ssh,
                                    update: update.clone(),
                                },
                            );
                            for (output, update) in outputs.iter().repeat_clone(update) {
                                health_output.give(
                                    &health_cap,
                                    HealthStatusMessage {
                                        id: Some(output.id),
                                        namespace: StatusNamespace::Ssh,
                                        update,
                                    },
                                );
                            }
                        }
                    }
                    Some(MetadataUpdate::DefiniteError(error)) => {
                        health_output.give(
                            &health_cap,
                            HealthStatusMessage {
                                id: None,
                                namespace: StatusNamespace::Kafka,
                                update: HealthStatusUpdate::stalled(
                                    error.to_string(),
                                    None,
                                ),
                            },
                        );
                        let error = Err(error.into());
                        let time = data_cap.time().clone();
                        for (output, error) in
                            outputs.iter().map(|o| o.output_index).repeat_clone(error)
                        {
                            data_output
                                .give_fueled(&data_cap, ((output, error), time, Diff::ONE))
                                .await;
                        }

                        return;
                    }
                    None => {}
                }

                // Poll the consumer once. We split the consumer's partitions out into separate
                // queues and poll those individually, but it's still necessary to drive logic that
                // consumes from rdkafka's internal event queue, such as statistics callbacks.
                //
                // Additionally, assigning topics and splitting them off into separate queues is
                // not atomic, so we expect to see at least some messages to show up when polling
                // the consumer directly.
                while let Some(result) = reader.consumer.poll(Duration::from_secs(0)) {
                    match result {
                        Err(e) => {
                            let error = format!(
                                "kafka error when polling consumer for source: {} topic: {} : {}",
                                reader.source_name, reader.topic_name, e
                            );
                            let status = HealthStatusUpdate::stalled(error, None);
                            health_output.give(
                                &health_cap,
                                HealthStatusMessage {
                                    id: None,
                                    namespace: StatusNamespace::Kafka,
                                    update: status.clone(),
                                },
                            );
                            for (output, status) in outputs.iter().repeat_clone(status) {
                                health_output.give(
                                    &health_cap,
                                    HealthStatusMessage {
                                        id: Some(output.id),
                                        namespace: StatusNamespace::Kafka,
                                        update: status,
                                    },
                                );
                            }
                        }
                        Ok(message) => {
                            let output_messages = outputs
                                .iter()
                                .map(|output| {
                                    let (message, ts) = construct_source_message(
                                        &message,
                                        &output.metadata_columns,
                                    );
                                    (output.output_index, message, ts)
                                })
                                // This vec allocation is required to allow obtaining a `&mut`
                                // on `reader` for the `reader.handle_message` call in the
                                // loop below since  `message` is borrowed from `reader`.
                                .collect::<Vec<_>>();
                            for (output_index, message, ts) in output_messages {
                                if let Some((msg, time, diff)) =
                                    reader.handle_message(message, ts, &output_index)
                                {
                                    let pid = time.interval().singleton().unwrap().unwrap_exact();
                                    let part_cap = &reader.partition_capabilities[pid].data;
                                    let msg = msg.map_err(|e| {
                                        DataflowError::SourceError(Box::new(SourceError {
                                            error: SourceErrorDetails::Other(e.to_string().into()),
                                        }))
                                    });
                                    data_output
                                        .give_fueled(part_cap, ((output_index, msg), time, diff))
                                        .await;
                                }
                            }
                        }
                    }
                }

                reader.update_stats();

                // Take the consumers temporarily to get around borrow checker errors
                let mut consumers = std::mem::take(&mut reader.partition_consumers);
                for consumer in consumers.iter_mut() {
                    let pid = consumer.pid();
                    // We want to make sure the rest of the actions in the outer loops get
                    // a chance to run. If rdkafka keeps pumping data at us we might find
                    // ourselves in a situation where we keep dumping data into the
                    // dataflow without signaling progress. For this reason we consume at most
                    // 10k messages from each partition and go around the loop.
                    let mut partition_exhausted = false;
                    for _ in 0..10_000 {
                        let Some(message) = consumer.get_next_message().transpose() else {
                            partition_exhausted = true;
                            break;
                        };

                        for output in outputs.iter() {
                            let message = match &message {
                                Ok((msg, pid)) => {
                                    let (msg, ts) =
                                        construct_source_message(msg, &output.metadata_columns);
                                    assert_eq!(*pid, ts.0);
                                    Ok(reader.handle_message(msg, ts, &output.output_index))
                                }
                                Err(err) => Err(err),
                            };
                            match message {
                                Ok(Some((msg, time, diff))) => {
                                    let pid = time.interval().singleton().unwrap().unwrap_exact();
                                    let part_cap = &reader.partition_capabilities[pid].data;
                                    let msg = msg.map_err(|e| {
                                        DataflowError::SourceError(Box::new(SourceError {
                                            error: SourceErrorDetails::Other(e.to_string().into()),
                                        }))
                                    });
                                    data_output
                                        .give_fueled(
                                            part_cap,
                                            ((output.output_index, msg), time, diff),
                                        )
                                        .await;
                                }
                                // The message was from an offset we've already seen.
                                Ok(None) => continue,
                                Err(err) => {
                                    let last_offset = reader
                                        .last_offsets
                                        .get(&output.output_index)
                                        .expect("output known to be installed")
                                        .get(&pid)
                                        .expect("partition known to be installed");

                                    let status = HealthStatusUpdate::stalled(
                                        format!(
                                            "error consuming from source: {} topic: {topic}:\
                                             partition: {pid} last processed offset:\
                                             {last_offset} : {err}",
                                            config.name
                                        ),
                                        None,
                                    );
                                    health_output.give(
                                        &health_cap,
                                        HealthStatusMessage {
                                            id: None,
                                            namespace: StatusNamespace::Kafka,
                                            update: status.clone(),
                                        },
                                    );
                                    health_output.give(
                                        &health_cap,
                                        HealthStatusMessage {
                                            id: Some(output.id),
                                            namespace: StatusNamespace::Kafka,
                                            update: status,
                                        },
                                    );
                                }
                            }
                        }
                    }
                    if !partition_exhausted {
                        notificator.notify_one();
                    }
                }
                // We can now put them back
                assert!(reader.partition_consumers.is_empty());
                reader.partition_consumers = consumers;

                let positions = reader.consumer.position().unwrap();
                let topic_positions = positions.elements_for_topic(&reader.topic_name);
                let mut snapshot_staged = 0;

                for position in topic_positions {
                    // The offset begins in the `Offset::Invalid` state in which case we simply
                    // skip this partition.
                    if let Offset::Offset(offset) = position.offset() {
                        let pid = position.partition();
                        let upper_offset = MzOffset::from(u64::try_from(offset).unwrap());
                        let upper =
                            Partitioned::new_singleton(RangeBound::exact(pid), upper_offset);

                        let part_cap = reader.partition_capabilities.get_mut(&pid).unwrap();
                        match part_cap.data.try_downgrade(&upper) {
                            Ok(()) => {
                                if !snapshot_export_stats.is_empty() {
                                    // The `.position()` of the consumer represents what offset we have
                                    // read up to.
                                    snapshot_staged += offset.try_into().unwrap_or(0u64);
                                    // This will always be `Some` at this point.
                                    if let Some(snapshot_total) = snapshot_total {
                                        // We will eventually read past the snapshot total, so we need
                                        // to bound it here.
                                        snapshot_staged =
                                            std::cmp::min(snapshot_staged, snapshot_total);
                                    }
                                }
                            }
                            Err(_) => {
                                // If we can't downgrade, it means we have already seen this offset.
                                // This is expected and we can safely ignore it.
                                info!(
                                    source_id = config.id.to_string(),
                                    worker_id = config.worker_id,
                                    num_workers = config.worker_count,
                                    "kafka source frontier downgrade skipped due to already \
                                     seen offset: {:?}",
                                    upper
                                );
                            }
                        };

                    }
                }

                if let (Some(snapshot_total), true) =
                    (snapshot_total, !snapshot_export_stats.is_empty())
                {
                    for export_stat in snapshot_export_stats.iter() {
                        export_stat.set_snapshot_records_known(snapshot_total);
                        export_stat.set_snapshot_records_staged(snapshot_staged);
                    }
                    if snapshot_total == snapshot_staged {
                        snapshot_export_stats.clear();
                    }
                }
            }
        })
    });

    (
        stream.as_collection(),
        health_stream,
        button.press_on_drop(),
    )
}

impl KafkaResumeUpperProcessor {
    async fn process_frontier(
        &self,
        frontier: Antichain<KafkaTimestamp>,
    ) -> Result<(), anyhow::Error> {
        use rdkafka::consumer::CommitMode;

        // Generate a list of partitions that this worker is responsible for
        let mut offsets = vec![];
        let mut offset_committed = 0;
        for ts in frontier.iter() {
            if let Some(pid) = ts.interval().singleton() {
                let pid = pid.unwrap_exact();
                if responsible_for_pid(&self.config, *pid) {
                    offsets.push((pid.clone(), *ts.timestamp()));

                    // Note that we do not subtract 1 from the frontier. Imagine
                    // that frontier is 2 for this pid. That means we have
                    // full processed offset 0 and offset 1, which means we have
                    // processed _2_ offsets.
                    offset_committed += ts.timestamp().offset;
                }
            }
        }

        for export_stat in self.statistics.iter() {
            export_stat.set_offset_committed(offset_committed);
        }

        if !offsets.is_empty() {
            let mut tpl = TopicPartitionList::new();
            for (pid, offset) in offsets {
                let offset_to_commit =
                    Offset::Offset(offset.offset.try_into().expect("offset to be vald i64"));
                tpl.add_partition_offset(&self.topic_name, pid, offset_to_commit)
                    .expect("offset known to be valid");
            }
            let consumer = Arc::clone(&self.consumer);
            mz_ore::task::spawn_blocking(
                || format!("source({}) kafka offset commit", self.config.id),
                move || consumer.commit(&tpl, CommitMode::Sync),
            )
            .await?;
        }
        Ok(())
    }
}

impl KafkaSourceReader {
    /// Ensures that a partition queue for `pid` exists.
    fn ensure_partition(&mut self, pid: PartitionId) {
        if self.last_offsets.is_empty() {
            tracing::info!(
                source_id = %self.id,
                worker_id = %self.worker_id,
                "kafka source does not have any outputs, not creating partition queue");

            return;
        }
        for last_offsets in self.last_offsets.values() {
            // early exit if we've already inserted this partition
            if last_offsets.contains_key(&pid) {
                return;
            }
        }

        let start_offset = self.start_offsets.get(&pid).copied().unwrap_or(0);
        self.create_partition_queue(pid, Offset::Offset(start_offset));

        for last_offsets in self.last_offsets.values_mut() {
            let prev = last_offsets.insert(pid, start_offset - 1);
            assert_none!(prev);
        }
    }

    /// Creates a new partition queue for `partition_id`.
    fn create_partition_queue(&mut self, partition_id: PartitionId, initial_offset: Offset) {
        info!(
            source_id = self.id.to_string(),
            worker_id = self.worker_id,
            num_workers = self.worker_count,
            "activating Kafka queue for topic {}, partition {}",
            self.topic_name,
            partition_id,
        );

        // Collect old partition assignments
        let tpl = self.consumer.assignment().unwrap();
        // Create list from assignments
        let mut partition_list = TopicPartitionList::new();
        for partition in tpl.elements_for_topic(&self.topic_name) {
            partition_list
                .add_partition_offset(partition.topic(), partition.partition(), partition.offset())
                .expect("offset known to be valid");
        }
        // Add new partition
        partition_list
            .add_partition_offset(&self.topic_name, partition_id, initial_offset)
            .expect("offset known to be valid");
        self.consumer
            .assign(&partition_list)
            .expect("assignment known to be valid");

        // Since librdkafka v1.6.0, we need to recreate all partition queues
        // after every call to `self.consumer.assign`.
        let context = Arc::clone(self.consumer.context());
        for pc in &mut self.partition_consumers {
            pc.partition_queue = self
                .consumer
                .split_partition_queue(&self.topic_name, pc.pid)
                .expect("partition known to be valid");
            pc.partition_queue.set_nonempty_callback({
                let context = Arc::clone(&context);
                move || context.inner().activate()
            });
        }

        let mut partition_queue = self
            .consumer
            .split_partition_queue(&self.topic_name, partition_id)
            .expect("partition known to be valid");
        partition_queue.set_nonempty_callback(move || context.inner().activate());
        self.partition_consumers
            .push(PartitionConsumer::new(partition_id, partition_queue));
        assert_eq!(
            self.consumer
                .assignment()
                .unwrap()
                .elements_for_topic(&self.topic_name)
                .len(),
            self.partition_consumers.len()
        );
    }

    /// Read any statistics JSON blobs generated via the rdkafka statistics callback.
    fn update_stats(&mut self) {
        while let Ok(stats) = self.stats_rx.try_recv() {
            match serde_json::from_str::<Statistics>(&stats.to_string()) {
                Ok(statistics) => {
                    let topic = statistics.topics.get(&self.topic_name);
                    match topic {
                        Some(topic) => {
                            for (id, partition) in &topic.partitions {
                                self.partition_metrics
                                    .set_offset_max(*id, partition.hi_offset);
                            }
                        }
                        None => error!("No stats found for topic: {}", &self.topic_name),
                    }
                }
                Err(e) => {
                    error!("failed decoding librdkafka statistics JSON: {}", e);
                }
            }
        }
    }

    /// Checks if the given message is viable for emission. This checks if the message offset is
    /// past the expected offset and returns None if it is not.
    fn handle_message(
        &mut self,
        message: Result<SourceMessage, KafkaHeaderParseError>,
        (partition, offset): (PartitionId, MzOffset),
        output_index: &usize,
    ) -> Option<(
        Result<SourceMessage, KafkaHeaderParseError>,
        KafkaTimestamp,
        Diff,
    )> {
        // Offsets are guaranteed to be 1) monotonically increasing *unless* there is
        // a network issue or a new partition added, at which point the consumer may
        // start processing the topic from the beginning, or we may see duplicate offsets
        // At all times, the guarantee : if we see offset x, we have seen all offsets [0,x-1]
        // that we are ever going to see holds.
        // Offsets are guaranteed to be contiguous when compaction is disabled. If compaction
        // is enabled, there may be gaps in the sequence.
        // If we see an "old" offset, we skip that message.

        // Given the explicit consumer to partition assignment, we should never receive a message
        // for a partition for which we have no metadata
        assert!(
            self.last_offsets
                .get(output_index)
                .unwrap()
                .contains_key(&partition)
        );

        let last_offset_ref = self
            .last_offsets
            .get_mut(output_index)
            .expect("output known to be installed")
            .get_mut(&partition)
            .expect("partition known to be installed");

        let last_offset = *last_offset_ref;
        let offset_as_i64: i64 = offset.offset.try_into().expect("offset to be < i64::MAX");
        if offset_as_i64 <= last_offset {
            info!(
                source_id = self.id.to_string(),
                worker_id = self.worker_id,
                num_workers = self.worker_count,
                "kafka message before expected offset: \
                 source {} (reading topic {}, partition {}, output {}) \
                 received offset {} expected offset {:?}",
                self.source_name,
                self.topic_name,
                partition,
                output_index,
                offset.offset,
                last_offset + 1,
            );
            // We explicitly should not consume the message as we have already processed it.
            None
        } else {
            *last_offset_ref = offset_as_i64;

            let ts = Partitioned::new_singleton(RangeBound::exact(partition), offset);
            Some((message, ts, Diff::ONE))
        }
    }
}

fn construct_source_message(
    msg: &BorrowedMessage<'_>,
    metadata_columns: &[KafkaMetadataKind],
) -> (
    Result<SourceMessage, KafkaHeaderParseError>,
    (PartitionId, MzOffset),
) {
    let pid = msg.partition();
    let Ok(offset) = u64::try_from(msg.offset()) else {
        panic!(
            "got negative offset ({}) from otherwise non-error'd kafka message",
            msg.offset()
        );
    };

    let mut metadata = Row::default();
    let mut packer = metadata.packer();
    for kind in metadata_columns {
        match kind {
            KafkaMetadataKind::Partition => packer.push(Datum::from(pid)),
            KafkaMetadataKind::Offset => packer.push(Datum::UInt64(offset)),
            KafkaMetadataKind::Timestamp => {
                let ts = msg
                    .timestamp()
                    .to_millis()
                    .expect("kafka sources always have upstream_time");

                let d: Datum = DateTime::from_timestamp_millis(ts)
                    .and_then(|dt| {
                        let ct: Option<CheckedTimestamp<NaiveDateTime>> =
                            dt.naive_utc().try_into().ok();
                        ct
                    })
                    .into();
                packer.push(d)
            }
            KafkaMetadataKind::Header { key, use_bytes } => {
                match msg.headers() {
                    Some(headers) => {
                        let d = headers
                            .iter()
                            .filter(|header| header.key == key)
                            .last()
                            .map(|header| match header.value {
                                Some(v) => {
                                    if *use_bytes {
                                        Ok(Datum::Bytes(v))
                                    } else {
                                        match str::from_utf8(v) {
                                            Ok(str) => Ok(Datum::String(str)),
                                            Err(_) => Err(KafkaHeaderParseError::Utf8Error {
                                                key: key.clone(),
                                                raw: v.to_vec(),
                                            }),
                                        }
                                    }
                                }
                                None => Ok(Datum::Null),
                            })
                            .unwrap_or_else(|| {
                                Err(KafkaHeaderParseError::KeyNotFound { key: key.clone() })
                            });
                        match d {
                            Ok(d) => packer.push(d),
                            //abort with a definite error when the header is not found or cannot be parsed correctly
                            Err(err) => return (Err(err), (pid, offset.into())),
                        }
                    }
                    None => packer.push(Datum::Null),
                }
            }
            KafkaMetadataKind::Headers => {
                packer.push_list_with(|r| {
                    if let Some(headers) = msg.headers() {
                        for header in headers.iter() {
                            match header.value {
                                Some(v) => r.push_list_with(|record_row| {
                                    record_row.push(Datum::String(header.key));
                                    record_row.push(Datum::Bytes(v));
                                }),
                                None => r.push_list_with(|record_row| {
                                    record_row.push(Datum::String(header.key));
                                    record_row.push(Datum::Null);
                                }),
                            }
                        }
                    }
                });
            }
        }
    }

    let key = match msg.key() {
        Some(bytes) => Row::pack([Datum::Bytes(bytes)]),
        None => Row::pack([Datum::Null]),
    };
    let value = match msg.payload() {
        Some(bytes) => Row::pack([Datum::Bytes(bytes)]),
        None => Row::pack([Datum::Null]),
    };
    (
        Ok(SourceMessage {
            key,
            value,
            metadata,
        }),
        (pid, offset.into()),
    )
}

/// Wrapper around a partition containing the underlying consumer
struct PartitionConsumer {
    /// the partition id with which this consumer is associated
    pid: PartitionId,
    /// The underlying Kafka partition queue
    partition_queue: PartitionQueue<TunnelingClientContext<GlueConsumerContext>>,
}

impl PartitionConsumer {
    /// Creates a new partition consumer from underlying Kafka consumer
    fn new(
        pid: PartitionId,
        partition_queue: PartitionQueue<TunnelingClientContext<GlueConsumerContext>>,
    ) -> Self {
        PartitionConsumer {
            pid,
            partition_queue,
        }
    }

    /// Returns the next message to process for this partition (if any).
    ///
    /// The outer `Result` represents irrecoverable failures, the inner one can and will
    /// be transformed into empty values.
    ///
    /// The inner `Option` represents if there is a message to process.
    fn get_next_message(&self) -> Result<Option<(BorrowedMessage<'_>, PartitionId)>, KafkaError> {
        match self.partition_queue.poll(Duration::from_millis(0)) {
            Some(Ok(msg)) => Ok(Some((msg, self.pid))),
            Some(Err(err)) => Err(err),
            _ => Ok(None),
        }
    }

    /// Return the partition id for this PartitionConsumer
    fn pid(&self) -> PartitionId {
        self.pid
    }
}

/// An implementation of [`ConsumerContext`] that forwards statistics to the
/// worker
struct GlueConsumerContext {
    notificator: Arc<Notify>,
    stats_tx: crossbeam_channel::Sender<Jsonb>,
    inner: MzClientContext,
}

impl ClientContext for GlueConsumerContext {
    fn stats_raw(&self, statistics: &[u8]) {
        match Jsonb::from_slice(statistics) {
            Ok(statistics) => {
                self.stats_tx
                    .send(statistics)
                    .expect("timely operator hung up while Kafka source active");
                self.activate();
            }
            Err(e) => error!("failed decoding librdkafka statistics JSON: {}", e),
        };
    }

    // The shape of the rdkafka *Context traits require us to forward to the `MzClientContext`
    // implementation.
    fn log(&self, level: rdkafka::config::RDKafkaLogLevel, fac: &str, log_message: &str) {
        self.inner.log(level, fac, log_message)
    }
    fn error(&self, error: rdkafka::error::KafkaError, reason: &str) {
        self.inner.error(error, reason)
    }
}

impl GlueConsumerContext {
    fn activate(&self) {
        self.notificator.notify_one();
    }
}

impl ConsumerContext for GlueConsumerContext {}

#[cfg(test)]
mod tests {
    use std::sync::Arc;
    use std::time::Duration;

    use mz_kafka_util::client::create_new_client_config_simple;
    use rdkafka::consumer::{BaseConsumer, Consumer};
    use rdkafka::{Message, Offset, TopicPartitionList};
    use uuid::Uuid;

    // Splitting off a partition queue with an `Offset` that is not `Offset::Beginning` seems to
    // lead to a race condition where sometimes we receive messages from polling the main consumer
    // instead of on the partition queue. This can be surfaced by running the test in a loop (in
    // the dataflow directory) using:
    //
    // cargo stress --lib --release source::kafka::tests::reproduce_kafka_queue_issue
    //
    // cargo-stress can be installed via `cargo install cargo-stress`
    //
    // You need to set up a topic "queue-test" with 1000 "hello" messages in it. Obviously, running
    // this test requires a running Kafka instance at localhost:9092.
    #[mz_ore::test]
    #[ignore]
    fn demonstrate_kafka_queue_race_condition() -> Result<(), anyhow::Error> {
        let topic_name = "queue-test";
        let pid = 0;

        let mut kafka_config = create_new_client_config_simple();
        kafka_config.set("bootstrap.servers", "localhost:9092".to_string());
        kafka_config.set("enable.auto.commit", "false");
        kafka_config.set("group.id", Uuid::new_v4().to_string());
        kafka_config.set("fetch.message.max.bytes", "100");
        let consumer: BaseConsumer<_> = kafka_config.create()?;

        let consumer = Arc::new(consumer);

        let mut partition_list = TopicPartitionList::new();
        // Using Offset:Beginning here will work fine, only Offset:Offset(0) leads to the race
        // condition.
        partition_list.add_partition_offset(topic_name, pid, Offset::Offset(0))?;

        consumer.assign(&partition_list)?;

        let partition_queue = consumer
            .split_partition_queue(topic_name, pid)
            .expect("missing partition queue");

        let expected_messages = 1_000;

        let mut common_queue_count = 0;
        let mut partition_queue_count = 0;

        loop {
            if let Some(msg) = consumer.poll(Duration::from_millis(0)) {
                match msg {
                    Ok(msg) => {
                        let _payload =
                            std::str::from_utf8(msg.payload().expect("missing payload"))?;
                        if partition_queue_count > 0 {
                            anyhow::bail!(
                                "Got message from common queue after we internally switched to partition queue."
                            );
                        }

                        common_queue_count += 1;
                    }
                    Err(err) => anyhow::bail!("{}", err),
                }
            }

            match partition_queue.poll(Duration::from_millis(0)) {
                Some(Ok(msg)) => {
                    let _payload = std::str::from_utf8(msg.payload().expect("missing payload"))?;
                    partition_queue_count += 1;
                }
                Some(Err(err)) => anyhow::bail!("{}", err),
                _ => (),
            }

            if (common_queue_count + partition_queue_count) == expected_messages {
                break;
            }
        }

        assert!(
            common_queue_count == 0,
            "Got {} out of {} messages from common queue. Partition queue: {}",
            common_queue_count,
            expected_messages,
            partition_queue_count
        );

        Ok(())
    }
}

/// Fetches the list of partitions and their corresponding high watermark.
fn fetch_partition_info<C: ConsumerContext>(
    consumer: &BaseConsumer<C>,
    topic: &str,
    fetch_timeout: Duration,
) -> Result<BTreeMap<PartitionId, HighWatermark>, GetPartitionsError> {
    let pids = get_partitions(consumer.client(), topic, fetch_timeout)?;

    let mut offset_requests = TopicPartitionList::with_capacity(pids.len());
    for pid in pids {
        offset_requests.add_partition_offset(topic, pid, Offset::End)?;
    }

    let offset_responses = consumer.offsets_for_times(offset_requests, fetch_timeout)?;

    let mut result = BTreeMap::new();
    for entry in offset_responses.elements() {
        let offset = match entry.offset() {
            Offset::Offset(offset) => offset,
            offset => Err(anyhow!("unexpected high watermark offset: {offset:?}"))?,
        };

        let pid = entry.partition();
        let watermark = offset.try_into().expect("invalid negative offset");
        result.insert(pid, watermark);
    }

    Ok(result)
}

/// An update produced by the metadata fetcher.
#[derive(Clone, Debug, PartialEq, Eq, PartialOrd, Ord, Serialize, Deserialize)]
enum MetadataUpdate {
    /// The current IDs and high watermarks of all topic partitions.
    Partitions(BTreeMap<PartitionId, HighWatermark>),
    /// A transient error.
    ///
    /// Transient errors stall the source until their cause has been resolved.
    TransientError(HealthStatus),
    /// A definite error.
    ///
    /// Definite errors cannot be recovered from. They poison the source until the end of time.
    DefiniteError(SourceError),
}

impl MetadataUpdate {
    /// Return the upstream frontier resulting from the metadata update, if any.
    fn upstream_frontier(&self) -> Option<Antichain<KafkaTimestamp>> {
        match self {
            Self::Partitions(partitions) => {
                let max_pid = partitions.keys().last().copied();
                let lower = max_pid
                    .map(RangeBound::after)
                    .unwrap_or(RangeBound::NegInfinity);
                let future_ts =
                    Partitioned::new_range(lower, RangeBound::PosInfinity, MzOffset::from(0));

                let mut frontier = Antichain::from_elem(future_ts);
                for (pid, high_watermark) in partitions {
                    frontier.insert(Partitioned::new_singleton(
                        RangeBound::exact(*pid),
                        MzOffset::from(*high_watermark),
                    ));
                }

                Some(frontier)
            }
            Self::DefiniteError(_) => Some(Antichain::new()),
            Self::TransientError(_) => None,
        }
    }
}

#[derive(Debug, thiserror::Error)]
pub enum KafkaHeaderParseError {
    #[error("A header with key '{key}' was not found in the message headers")]
    KeyNotFound { key: String },
    #[error(
        "Found ill-formed byte sequence in header '{key}' that cannot be decoded as valid utf-8 (original bytes: {raw:x?})"
    )]
    Utf8Error { key: String, raw: Vec<u8> },
}

/// Render the metadata fetcher of a Kafka source.
///
/// The metadata fetcher is a single-worker operator that is responsible for periodically fetching
/// the Kafka topic metadata (partition IDs and high watermarks) and making it available as a
/// Timely stream.
fn render_metadata_fetcher<G: Scope<Timestamp = KafkaTimestamp>>(
    scope: &G,
    connection: KafkaSourceConnection,
    config: RawSourceCreationConfig,
) -> (
    StreamVec<G, (mz_repr::Timestamp, MetadataUpdate)>,
    StreamVec<G, Probe<KafkaTimestamp>>,
    PressOnDropButton,
) {
    let active_worker_id = usize::cast_from(config.id.hashed());
    let is_active_worker = active_worker_id % scope.peers() == scope.index();

    let resume_upper = Antichain::from_iter(
        config
            .source_resume_uppers
            .values()
            .map(|uppers| uppers.iter().map(KafkaTimestamp::decode_row))
            .flatten(),
    );

    let name = format!("KafkaMetadataFetcher({})", config.id);
    let mut builder = AsyncOperatorBuilder::new(name, scope.clone());

    let (metadata_output, metadata_stream) =
        builder.new_output::<CapacityContainerBuilder<Vec<_>>>();
    let (probe_output, probe_stream) = builder.new_output::<CapacityContainerBuilder<Vec<_>>>();

    let button = builder.build(move |caps| async move {
        if !is_active_worker {
            return;
        }

        let [metadata_cap, probe_cap] = caps.try_into().unwrap();

        let client_id = connection.client_id(
            config.config.config_set(),
            &config.config.connection_context,
            config.id,
        );
        let KafkaSourceConnection {
            connection,
            topic,
            topic_metadata_refresh_interval,
            ..
        } = connection;

        let consumer: Result<BaseConsumer<_>, _> = connection
            .create_with_context(
                &config.config,
                MzClientContext::default(),
                &btreemap! {
                    // Use the user-configured topic metadata refresh
                    // interval.
                    "topic.metadata.refresh.interval.ms" =>
                        topic_metadata_refresh_interval
                        .as_millis()
                        .to_string(),
                    // Allow Kafka monitoring tools to identify this
                    // consumer.
                    "client.id" => format!("{client_id}-metadata"),
                },
                InTask::Yes,
            )
            .await;

        let consumer = match consumer {
            Ok(consumer) => consumer,
            Err(e) => {
                let msg = format!(
                    "failed creating kafka metadata consumer: {}",
                    e.display_with_causes()
                );
                let status_update = HealthStatusUpdate::halting(msg, None);
                let status = match e {
                    ContextCreationError::Ssh(_) => HealthStatus::ssh(status_update),
                    _ => HealthStatus::kafka(status_update),
                };
                let error = MetadataUpdate::TransientError(status);
                let timestamp = (config.now_fn)().into();
                metadata_output.give(&metadata_cap, (timestamp, error));

                // IMPORTANT: wedge forever until the `SuspendAndRestart` is processed.
                // Returning would incorrectly present to the remap operator as progress to the
                // empty frontier which would be incorrectly recorded to the remap shard.
                std::future::pending::<()>().await;
                unreachable!("pending future never returns");
            }
        };

        let (tx, mut rx) = mpsc::unbounded_channel();
        spawn_metadata_thread(config, consumer, topic, tx);

        let mut prev_upstream_frontier = resume_upper;

        while let Some((timestamp, mut update)) = rx.recv().await {
            if prev_upstream_frontier.is_empty() {
                return;
            }

            if let Some(upstream_frontier) = update.upstream_frontier() {
                // Topics are identified by name but it's possible that a user recreates a topic
                // with the same name. Ideally we'd want to catch all of these cases and
                // immediately error out the source, since the data is effectively gone.
                // Unfortunately this is not possible without something like KIP-516.
                //
                // The best we can do is check whether the upstream frontier regressed. This tells
                // us that the topic was recreated and now contains fewer offsets and/or fewer
                // partitions. Note that we are not able to detect topic recreation if neither of
                // the two are true.
                if !PartialOrder::less_equal(&prev_upstream_frontier, &upstream_frontier) {
                    let error = SourceError {
                        error: SourceErrorDetails::Other("topic was recreated".into()),
                    };
                    update = MetadataUpdate::DefiniteError(error);
                }
            }

            if let Some(upstream_frontier) = update.upstream_frontier() {
                prev_upstream_frontier = upstream_frontier.clone();

                let probe = Probe {
                    probe_ts: timestamp,
                    upstream_frontier,
                };
                probe_output.give(&probe_cap, probe);
            }

            metadata_output.give(&metadata_cap, (timestamp, update));
        }
    });

    (metadata_stream, probe_stream, button.press_on_drop())
}

fn spawn_metadata_thread<C: ConsumerContext>(
    config: RawSourceCreationConfig,
    consumer: BaseConsumer<TunnelingClientContext<C>>,
    topic: String,
    tx: mpsc::UnboundedSender<(mz_repr::Timestamp, MetadataUpdate)>,
) {
    // Linux thread names are limited to 15 characters. Use a truncated ID to fit the name.
    thread::Builder::new()
        .name(format!("kfk-mtdt-{}", config.id))
        .spawn(move || {
            trace!(
                source_id = config.id.to_string(),
                worker_id = config.worker_id,
                num_workers = config.worker_count,
                "kafka metadata thread: starting..."
            );

            let timestamp_interval = config.timestamp_interval;
            let mut ticker = probe::Ticker::new(move || timestamp_interval, config.now_fn);

            loop {
                let probe_ts = ticker.tick_blocking();
                let result = fetch_partition_info(
                    &consumer,
                    &topic,
                    config
                        .config
                        .parameters
                        .kafka_timeout_config
                        .fetch_metadata_timeout,
                );
                trace!(
                    source_id = config.id.to_string(),
                    worker_id = config.worker_id,
                    num_workers = config.worker_count,
                    "kafka metadata thread: metadata fetch result: {:?}",
                    result
                );
                let update = match result {
                    Ok(partitions) => {
                        trace!(
                            source_id = config.id.to_string(),
                            worker_id = config.worker_id,
                            num_workers = config.worker_count,
                            "kafka metadata thread: fetched partition metadata info",
                        );

                        MetadataUpdate::Partitions(partitions)
                    }
                    Err(GetPartitionsError::TopicDoesNotExist) => {
                        let error = SourceError {
                            error: SourceErrorDetails::Other("topic was deleted".into()),
                        };
                        MetadataUpdate::DefiniteError(error)
                    }
                    Err(e) => {
                        let kafka_status = Some(HealthStatusUpdate::stalled(
                            format!("{}", e.display_with_causes()),
                            None,
                        ));

                        let ssh_status = consumer.client().context().tunnel_status();
                        let ssh_status = match ssh_status {
                            SshTunnelStatus::Running => Some(HealthStatusUpdate::running()),
                            SshTunnelStatus::Errored(e) => {
                                Some(HealthStatusUpdate::stalled(e, None))
                            }
                        };

                        MetadataUpdate::TransientError(HealthStatus {
                            kafka: kafka_status,
                            ssh: ssh_status,
                        })
                    }
                };

                if tx.send((probe_ts, update)).is_err() {
                    break;
                }
            }

            info!(
                source_id = config.id.to_string(),
                worker_id = config.worker_id,
                num_workers = config.worker_count,
                "kafka metadata thread: receiver has gone away; shutting down."
            )
        })
        .unwrap();
}