mz_storage/source/postgres/

replication.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.

//! Renders the logical replication side of the [`PostgresSourceConnection`] ingestion dataflow.
//!
//! ```text
//!              o
//!              │rewind
//!              │requests
//!          ╭───┴────╮
//!          │exchange│ (collect all requests to one worker)
//!          ╰───┬────╯
//!           ┏━━v━━━━━━━━━━┓
//!           ┃ replication ┃ (single worker)
//!           ┃   reader    ┃
//!           ┗━┯━━━━━━━━┯━━┛
//!             │raw     │
//!             │data    │
//!        ╭────┴─────╮  │
//!        │distribute│  │ (distribute to all workers)
//!        ╰────┬─────╯  │
//! ┏━━━━━━━━━━━┷━┓      │
//! ┃ replication ┃      │ (parallel decode)
//! ┃   decoder   ┃      │
//! ┗━━━━━┯━━━━━━━┛      │
//!       │ replication  │ progress
//!       │ updates      │ output
//!       v              v
//! ```
//!
//! # Progress tracking
//!
//! In order to avoid causing excessive resource usage in the upstream server it's important to
//! track the LSN that we have successfully committed to persist and communicate that back to
//! PostgreSQL. Under normal operation this gauge of progress is provided by the presence of
//! transactions themselves. Since at a given LSN offset there can be only a single message, when a
//! transaction is received and processed we can infer that we have seen all the messages that are
//! not beyond `commit_lsn + 1`.
//!
//! Things are a bit more complicated in the absence of transactions though because even though we
//! don't receive any the server might very well be generating WAL records. This can happen if
//! there is a separate logical database performing writes (which is the case for RDS databases),
//! or, in servers running PostgreSQL version 15 or greater, the logical replication process
//! includes an optimization that omits empty transactions, which can happen if you're only
//! replicating a subset of the tables and there writes going to the other ones.
//!
//! If we fail to detect this situation and don't send LSN feedback in a timely manner the server
//! will be forced to keep around WAL data that can eventually lead to disk space exhaustion.
//!
//! In the absence of transactions the only available piece of information in the replication
//! stream are keepalive messages. Keepalive messages are documented[1] to contain the current end
//! of WAL on the server. That is a useless number when it comes to progress tracking because there
//! might be pending messages at LSNs between the last received commit_lsn and the current end of
//! WAL.
//!
//! Fortunately for us, the documentation for PrimaryKeepalive messages is wrong and it actually
//! contains the last *sent* LSN[2]. Here sent doesn't necessarily mean sent over the wire, but
//! sent to the upstream process that is handling producing the logical stream. Therefore, if we
//! receive a keepalive with a particular LSN we can be certain that there are no other replication
//! messages at previous LSNs, because they would have been already generated and received. We
//! therefore connect the keepalive messages directly to our capability.
//!
//! [1]: https://www.postgresql.org/docs/15/protocol-replication.html#PROTOCOL-REPLICATION-START-REPLICATION
//! [2]: https://www.postgresql.org/message-id/CAFPTHDZS9O9WG02EfayBd6oONzK%2BqfUxS6AbVLJ7W%2BKECza2gg%40mail.gmail.com

use std::collections::BTreeMap;
use std::convert::Infallible;
use std::pin::pin;
use std::rc::Rc;
use std::str::FromStr;
use std::sync::Arc;
use std::sync::LazyLock;
use std::time::Instant;
use std::time::{Duration, SystemTime, UNIX_EPOCH};

use differential_dataflow::AsCollection;
use futures::{FutureExt, Stream as AsyncStream, StreamExt, TryStreamExt};
use mz_ore::cast::CastFrom;
use mz_ore::future::InTask;
use mz_postgres_util::PostgresError;
use mz_postgres_util::{Client, simple_query_opt};
use mz_repr::{Datum, DatumVec, Diff, Row};
use mz_sql_parser::ast::{Ident, display::AstDisplay};
use mz_storage_types::dyncfgs::{PG_OFFSET_KNOWN_INTERVAL, PG_SCHEMA_VALIDATION_INTERVAL};
use mz_storage_types::errors::DataflowError;
use mz_storage_types::sources::{MzOffset, PostgresSourceConnection};
use mz_timely_util::builder_async::{
    AsyncOutputHandle, Event as AsyncEvent, OperatorBuilder as AsyncOperatorBuilder,
    PressOnDropButton,
};
use postgres_replication::LogicalReplicationStream;
use postgres_replication::protocol::{LogicalReplicationMessage, ReplicationMessage, TupleData};
use serde::{Deserialize, Serialize};
use timely::container::CapacityContainerBuilder;
use timely::dataflow::channels::pact::{Exchange, Pipeline};
use timely::dataflow::channels::pushers::Tee;
use timely::dataflow::operators::Capability;
use timely::dataflow::operators::Concat;
use timely::dataflow::operators::Operator;
use timely::dataflow::operators::core::Map;
use timely::dataflow::{Scope, Stream};
use timely::progress::Antichain;
use tokio::sync::{mpsc, watch};
use tokio_postgres::error::SqlState;
use tokio_postgres::types::PgLsn;
use tracing::{error, trace};

use crate::metrics::source::postgres::PgSourceMetrics;
use crate::source::RawSourceCreationConfig;
use crate::source::postgres::verify_schema;
use crate::source::postgres::{DefiniteError, ReplicationError, SourceOutputInfo, TransientError};
use crate::source::probe;
use crate::source::types::{Probe, SignaledFuture, SourceMessage, StackedCollection};

/// Postgres epoch is 2000-01-01T00:00:00Z
static PG_EPOCH: LazyLock<SystemTime> =
    LazyLock::new(|| UNIX_EPOCH + Duration::from_secs(946_684_800));

// A request to rewind a snapshot taken at `snapshot_lsn` to the initial LSN of the replication
// slot. This is accomplished by emitting `(data, 0, -diff)` for all updates `(data, lsn, diff)`
// whose `lsn <= snapshot_lsn`. By convention the snapshot is always emitted at LSN 0.
#[derive(Debug, Clone, Serialize, Deserialize)]
pub(crate) struct RewindRequest {
    /// The output index that should be rewound.
    pub(crate) output_index: usize,
    /// The LSN that the snapshot was taken at.
    pub(crate) snapshot_lsn: MzOffset,
}

/// Renders the replication dataflow. See the module documentation for more information.
pub(crate) fn render<G: Scope<Timestamp = MzOffset>>(
    scope: G,
    config: RawSourceCreationConfig,
    connection: PostgresSourceConnection,
    table_info: BTreeMap<u32, BTreeMap<usize, SourceOutputInfo>>,
    rewind_stream: &Stream<G, RewindRequest>,
    slot_ready_stream: &Stream<G, Infallible>,
    committed_uppers: impl futures::Stream<Item = Antichain<MzOffset>> + 'static,
    metrics: PgSourceMetrics,
) -> (
    StackedCollection<G, (usize, Result<SourceMessage, DataflowError>)>,
    Stream<G, Infallible>,
    Option<Stream<G, Probe<MzOffset>>>,
    Stream<G, ReplicationError>,
    PressOnDropButton,
) {
    let op_name = format!("ReplicationReader({})", config.id);
    let mut builder = AsyncOperatorBuilder::new(op_name, scope.clone());

    let slot_reader = u64::cast_from(config.responsible_worker("slot"));
    let (data_output, data_stream) = builder.new_output();
    let (_upper_output, upper_stream) = builder.new_output::<CapacityContainerBuilder<_>>();
    let (definite_error_handle, definite_errors) = builder.new_output();
    let (probe_output, probe_stream) = builder.new_output::<CapacityContainerBuilder<_>>();

    let mut rewind_input =
        builder.new_disconnected_input(rewind_stream, Exchange::new(move |_| slot_reader));
    let mut slot_ready_input = builder.new_disconnected_input(slot_ready_stream, Pipeline);
    let output_uppers = table_info
        .iter()
        .flat_map(|(_, outputs)| outputs.values().map(|o| o.resume_upper.clone()))
        .collect::<Vec<_>>();
    metrics.tables.set(u64::cast_from(output_uppers.len()));

    let reader_table_info = table_info.clone();
    let (button, transient_errors) = builder.build_fallible(move |caps| {
        let mut table_info = reader_table_info;
        let busy_signal = Arc::clone(&config.busy_signal);
        Box::pin(SignaledFuture::new(busy_signal, async move {
            let (id, worker_id) = (config.id, config.worker_id);
            let [
                data_cap_set,
                upper_cap_set,
                definite_error_cap_set,
                probe_cap,
            ]: &mut [_; 4] = caps.try_into().unwrap();

            if !config.responsible_for("slot") {
                // Emit 0, to mark this worker as having started up correctly.
                for stat in config.statistics.values() {
                    stat.set_offset_known(0);
                    stat.set_offset_committed(0);
                }
                return Ok(());
            }

            // Determine the slot lsn.
            let connection_config = connection
                .connection
                .config(
                    &config.config.connection_context.secrets_reader,
                    &config.config,
                    InTask::Yes,
                )
                .await?;

            let slot = &connection.publication_details.slot;
            let replication_client = connection_config
                .connect_replication(&config.config.connection_context.ssh_tunnel_manager)
                .await?;

            let metadata_client = connection_config
                .connect(
                    "replication metadata",
                    &config.config.connection_context.ssh_tunnel_manager,
                )
                .await?;
            let metadata_client = Arc::new(metadata_client);

            while let Some(_) = slot_ready_input.next().await {
                // Wait for the slot to be created
            }

            // The slot is always created by the snapshot operator. If the slot doesn't exist,
            // when this check runs, this operator will return an error.
            let slot_metadata = super::fetch_slot_metadata(
                &*metadata_client,
                slot,
                mz_storage_types::dyncfgs::PG_FETCH_SLOT_RESUME_LSN_INTERVAL
                    .get(config.config.config_set()),
            )
            .await?;

            // We're the only application that should be using this replication
            // slot. The only way that there can be another connection using
            // this slot under normal operation is if there's a stale TCP
            // connection from a prior incarnation of the source holding on to
            // the slot. We don't want to wait for the WAL sender timeout and/or
            // TCP keepalives to time out that connection, because these values
            // are generally under the control of the DBA and may not time out
            // the connection for multiple minutes, or at all. Instead we just
            // force kill the connection that's using the slot.
            //
            // Note that there's a small risk that *we're* the zombie cluster
            // that should not be using the replication slot. Kubernetes cannot
            // 100% guarantee that only one cluster is alive at a time. However,
            // this situation should not last long, and the worst that can
            // happen is a bit of transient thrashing over ownership of the
            // replication slot.
            if let Some(active_pid) = slot_metadata.active_pid {
                tracing::warn!(
                    %id, %active_pid,
                    "replication slot already in use; will attempt to kill existing connection",
                );

                match metadata_client
                    .execute("SELECT pg_terminate_backend($1)", &[&active_pid])
                    .await
                {
                    Ok(_) => {
                        tracing::info!(
                            "successfully killed existing connection; \
                            starting replication is likely to succeed"
                        );
                        // Note that `pg_terminate_backend` does not wait for
                        // the termination of the targeted connection to
                        // complete. We may try to start replication before the
                        // targeted connection has cleaned up its state. That's
                        // okay. If that happens we'll just try again from the
                        // top via the suspend-and-restart flow.
                    }
                    Err(e) => {
                        tracing::warn!(
                            %e,
                            "failed to kill existing replication connection; \
                            replication will likely fail to start"
                        );
                        // Continue on anyway, just in case the replication slot
                        // is actually available. Maybe PostgreSQL has some
                        // staleness when it reports `active_pid`, for example.
                    }
                }
            }

            // The overall resumption point for this source is the minimum of the resumption points
            // contributed by each of the outputs.
            let resume_lsn = output_uppers
                .iter()
                .flat_map(|f| f.elements())
                .map(|&lsn| {
                    // An output is either an output that has never had data committed to it or one
                    // that has and needs to resume. We differentiate between the two by checking
                    // whether an output wishes to "resume" from the minimum timestamp. In that case
                    // its contribution to the overal resumption point is the earliest point available
                    // in the slot. This information would normally be something that the storage
                    // controller figures out in the form of an as-of frontier, but at the moment the
                    // storage controller does not have visibility into what the replication slot is
                    // doing.
                    if lsn == MzOffset::from(0) {
                        slot_metadata.confirmed_flush_lsn
                    } else {
                        lsn
                    }
                })
                .min();
            let Some(resume_lsn) = resume_lsn else {
                std::future::pending::<()>().await;
                return Ok(());
            };
            upper_cap_set.downgrade([&resume_lsn]);
            trace!(%id, "timely-{worker_id} replication reader started lsn={resume_lsn}");

            // Emitting an initial probe before we start waiting for rewinds ensures that we will
            // have a timestamp binding in the remap collection while the snapshot is processed.
            // This is important because otherwise the snapshot updates would need to be buffered
            // in the reclock operator, instead of being spilled to S3 in the persist sink.
            //
            // Note that we need to fetch the probe LSN _after_ having created the replication
            // slot, to make sure the fetched LSN will be included in the replication stream.
            let probe_ts = (config.now_fn)().into();
            let max_lsn = super::fetch_max_lsn(&*metadata_client).await?;
            let probe = Probe {
                probe_ts,
                upstream_frontier: Antichain::from_elem(max_lsn),
            };
            probe_output.give(&probe_cap[0], probe);

            let mut rewinds = BTreeMap::new();
            while let Some(event) = rewind_input.next().await {
                if let AsyncEvent::Data(_, data) = event {
                    for req in data {
                        if resume_lsn > req.snapshot_lsn + 1 {
                            let err = DefiniteError::SlotCompactedPastResumePoint(
                                req.snapshot_lsn + 1,
                                resume_lsn,
                            );
                            // If the replication stream cannot be obtained from the resume point there is nothing
                            // else to do. These errors are not retractable.
                            for (oid, outputs) in table_info.iter() {
                                for output_index in outputs.keys() {
                                    // We pick `u64::MAX` as the LSN which will (in practice) never conflict
                                    // any previously revealed portions of the TVC.
                                    let update = (
                                        (
                                            *oid,
                                            *output_index,
                                            Err(DataflowError::from(err.clone())),
                                        ),
                                        MzOffset::from(u64::MAX),
                                        Diff::ONE,
                                    );
                                    data_output.give_fueled(&data_cap_set[0], update).await;
                                }
                            }
                            definite_error_handle.give(
                                &definite_error_cap_set[0],
                                ReplicationError::Definite(Rc::new(err)),
                            );
                            return Ok(());
                        }
                        rewinds.insert(req.output_index, req);
                    }
                }
            }
            trace!(%id, "timely-{worker_id} pending rewinds {rewinds:?}");

            let mut committed_uppers = pin!(committed_uppers);

            let stream_result = raw_stream(
                &config,
                replication_client,
                Arc::clone(&metadata_client),
                &connection.publication_details.slot,
                &connection.publication_details.timeline_id,
                &connection.publication,
                resume_lsn,
                committed_uppers.as_mut(),
                &probe_output,
                &probe_cap[0],
            )
            .await?;

            let stream = match stream_result {
                Ok(stream) => stream,
                Err(err) => {
                    // If the replication stream cannot be obtained in a definite way there is
                    // nothing else to do. These errors are not retractable.
                    for (oid, outputs) in table_info.iter() {
                        for output_index in outputs.keys() {
                            // We pick `u64::MAX` as the LSN which will (in practice) never conflict
                            // any previously revealed portions of the TVC.
                            let update = (
                                (*oid, *output_index, Err(DataflowError::from(err.clone()))),
                                MzOffset::from(u64::MAX),
                                Diff::ONE,
                            );
                            data_output.give_fueled(&data_cap_set[0], update).await;
                        }
                    }

                    definite_error_handle.give(
                        &definite_error_cap_set[0],
                        ReplicationError::Definite(Rc::new(err)),
                    );
                    return Ok(());
                }
            };
            let mut stream = pin!(stream.peekable());

            // Run the periodic schema validation on a separate task using a separate client,
            // to prevent it from blocking the replication reading progress.
            let ssh_tunnel_manager = &config.config.connection_context.ssh_tunnel_manager;
            let client = connection_config
                .connect("schema validation", ssh_tunnel_manager)
                .await?;
            let mut schema_errors = spawn_schema_validator(
                client,
                &config,
                connection.publication.clone(),
                table_info.clone(),
            );

            // Instead of downgrading the capability for every transaction we process we only do it
            // if we're about to yield, which is checked at the bottom of the loop. This avoids
            // creating excessive progress tracking traffic when there are multiple small
            // transactions ready to go.
            let mut data_upper = resume_lsn;
            // A stash of reusable vectors to convert from bytes::Bytes based data, which is not
            // compatible with `columnation`, to Vec<u8> data that is.
            while let Some(event) = stream.as_mut().next().await {
                use LogicalReplicationMessage::*;
                use ReplicationMessage::*;
                match event {
                    Ok(XLogData(data)) => match data.data() {
                        Begin(begin) => {
                            let commit_lsn = MzOffset::from(begin.final_lsn());

                            let mut tx = pin!(extract_transaction(
                                stream.by_ref(),
                                &*metadata_client,
                                commit_lsn,
                                &mut table_info,
                                &metrics,
                                &connection.publication,
                            ));

                            trace!(
                                %id,
                                "timely-{worker_id} extracting transaction \
                                    at {commit_lsn}"
                            );
                            assert!(
                                data_upper <= commit_lsn,
                                "new_upper={data_upper} tx_lsn={commit_lsn}",
                            );
                            data_upper = commit_lsn + 1;
                            // We are about to ingest a transaction which has the possiblity to be
                            // very big and we certainly don't want to hold the data in memory. For
                            // this reason we eagerly downgrade the upper capability in order for
                            // the reclocking machinery to mint a binding that includes
                            // this transaction and therefore be able to pass the data of the
                            // transaction through as we stream it.
                            upper_cap_set.downgrade([&data_upper]);
                            while let Some((oid, output_index, event, diff)) = tx.try_next().await?
                            {
                                let event = event.map_err(Into::into);
                                let mut data = (oid, output_index, event);
                                if let Some(req) = rewinds.get(&output_index) {
                                    if commit_lsn <= req.snapshot_lsn {
                                        let update = (data, MzOffset::from(0), -diff);
                                        data_output.give_fueled(&data_cap_set[0], &update).await;
                                        data = update.0;
                                    }
                                }
                                let update = (data, commit_lsn, diff);
                                data_output.give_fueled(&data_cap_set[0], &update).await;
                            }
                        }
                        _ => return Err(TransientError::BareTransactionEvent),
                    },
                    Ok(PrimaryKeepAlive(keepalive)) => {
                        trace!( %id,
                            "timely-{worker_id} received keepalive lsn={}",
                            keepalive.wal_end()
                        );

                        // Take the opportunity to report any schema validation errors.
                        while let Ok(error) = schema_errors.try_recv() {
                            use SchemaValidationError::*;
                            match error {
                                Postgres(PostgresError::PublicationMissing(publication)) => {
                                    let err = DefiniteError::PublicationDropped(publication);
                                    for (oid, outputs) in table_info.iter() {
                                        for output_index in outputs.keys() {
                                            let update = (
                                                (
                                                    *oid,
                                                    *output_index,
                                                    Err(DataflowError::from(err.clone())),
                                                ),
                                                data_cap_set[0].time().clone(),
                                                Diff::ONE,
                                            );
                                            data_output.give_fueled(&data_cap_set[0], update).await;
                                        }
                                    }
                                    definite_error_handle.give(
                                        &definite_error_cap_set[0],
                                        ReplicationError::Definite(Rc::new(err)),
                                    );
                                    return Ok(());
                                }
                                Postgres(pg_error) => Err(TransientError::from(pg_error))?,
                                Schema {
                                    oid,
                                    output_index,
                                    error,
                                } => {
                                    let table = table_info.get_mut(&oid).unwrap();
                                    if table.remove(&output_index).is_none() {
                                        continue;
                                    }

                                    let update = (
                                        (oid, output_index, Err(error.into())),
                                        data_cap_set[0].time().clone(),
                                        Diff::ONE,
                                    );
                                    data_output.give_fueled(&data_cap_set[0], update).await;
                                }
                            }
                        }
                        data_upper = std::cmp::max(data_upper, keepalive.wal_end().into());
                    }
                    Ok(_) => return Err(TransientError::UnknownReplicationMessage),
                    Err(err) => return Err(err),
                }

                let will_yield = stream.as_mut().peek().now_or_never().is_none();
                if will_yield {
                    trace!(%id, "timely-{worker_id} yielding at lsn={data_upper}");
                    rewinds.retain(|_, req| data_upper <= req.snapshot_lsn);
                    // As long as there are pending rewinds we can't downgrade our data capability
                    // since we must be able to produce data at offset 0.
                    if rewinds.is_empty() {
                        data_cap_set.downgrade([&data_upper]);
                    }
                    upper_cap_set.downgrade([&data_upper]);
                }
            }
            // We never expect the replication stream to gracefully end
            Err(TransientError::ReplicationEOF)
        }))
    });

    // We now process the slot updates and apply the cast expressions
    let mut final_row = Row::default();
    let mut datum_vec = DatumVec::new();
    let mut next_worker = (0..u64::cast_from(scope.peers()))
        // Round robin on 1000-records basis to avoid creating tiny containers when there are a
        // small number of updates and a large number of workers.
        .flat_map(|w| std::iter::repeat_n(w, 1000))
        .cycle();
    let round_robin = Exchange::new(move |_| next_worker.next().unwrap());
    let replication_updates = data_stream
        .map::<Vec<_>, _, _>(Clone::clone)
        .unary(round_robin, "PgCastReplicationRows", |_, _| {
            move |input, output| {
                while let Some((time, data)) = input.next() {
                    let mut session = output.session(&time);
                    for ((oid, output_index, event), time, diff) in data.drain(..) {
                        let output = &table_info
                            .get(&oid)
                            .and_then(|outputs| outputs.get(&output_index))
                            .expect("table_info contains all outputs");
                        let event = event.and_then(|row| {
                            let datums = datum_vec.borrow_with(&row);
                            super::cast_row(&output.casts, &datums, &mut final_row)?;
                            Ok(SourceMessage {
                                key: Row::default(),
                                value: final_row.clone(),
                                metadata: Row::default(),
                            })
                        });

                        session.give(((output_index, event), time, diff));
                    }
                }
            }
        })
        .as_collection();

    let errors = definite_errors.concat(&transient_errors.map(ReplicationError::from));

    (
        replication_updates,
        upper_stream,
        Some(probe_stream),
        errors,
        button.press_on_drop(),
    )
}

/// Produces the logical replication stream while taking care of regularly sending standby
/// keepalive messages with the provided `uppers` stream.
///
/// The returned stream will contain all transactions that whose commit LSN is beyond `resume_lsn`.
async fn raw_stream<'a>(
    config: &'a RawSourceCreationConfig,
    replication_client: Client,
    metadata_client: Arc<Client>,
    slot: &'a str,
    timeline_id: &'a Option<u64>,
    publication: &'a str,
    resume_lsn: MzOffset,
    uppers: impl futures::Stream<Item = Antichain<MzOffset>> + 'a,
    probe_output: &'a AsyncOutputHandle<
        MzOffset,
        CapacityContainerBuilder<Vec<Probe<MzOffset>>>,
        Tee<MzOffset, Vec<Probe<MzOffset>>>,
    >,
    probe_cap: &'a Capability<MzOffset>,
) -> Result<
    Result<
        impl AsyncStream<Item = Result<ReplicationMessage<LogicalReplicationMessage>, TransientError>>
        + 'a,
        DefiniteError,
    >,
    TransientError,
> {
    if let Err(err) = ensure_publication_exists(&*metadata_client, publication).await? {
        // If the publication gets deleted there is nothing else to do. These errors
        // are not retractable.
        return Ok(Err(err));
    }

    // Skip the timeline ID check for sources without a known timeline ID
    // (sources created before the timeline ID was added to the source details)
    if let Some(expected_timeline_id) = timeline_id {
        if let Err(err) =
            ensure_replication_timeline_id(&replication_client, expected_timeline_id).await?
        {
            return Ok(Err(err));
        }
    }

    // How often a proactive standby status update message should be sent to the server.
    //
    // The upstream will periodically request status updates by setting the keepalive's reply field
    // value to 1. However, we cannot rely on these messages arriving on time. For example, when
    // the upstream is sending a big transaction its keepalive messages are queued and can be
    // delayed arbitrarily.
    //
    // See: <https://www.postgresql.org/message-id/CAMsr+YE2dSfHVr7iEv1GSPZihitWX-PMkD9QALEGcTYa+sdsgg@mail.gmail.com>
    //
    // For this reason we query the server's timeout value and proactively send a keepalive at
    // twice the frequency to have a healthy margin from the deadline.
    //
    // Note: We must use the metadata client here which is NOT in replication mode. Some Aurora
    // Postgres versions disallow SHOW commands from within replication connection.
    // See: https://github.com/readysettech/readyset/discussions/28#discussioncomment-4405671
    let row = simple_query_opt(&*metadata_client, "SHOW wal_sender_timeout;")
        .await?
        .unwrap();
    let wal_sender_timeout = match row.get("wal_sender_timeout") {
        // When this parameter is zero the timeout mechanism is disabled
        Some("0") => None,
        Some(value) => Some(
            mz_repr::adt::interval::Interval::from_str(value)
                .unwrap()
                .duration()
                .unwrap(),
        ),
        None => panic!("ubiquitous parameter missing"),
    };

    // This interval controls the cadence at which we send back status updates and, crucially,
    // request PrimaryKeepAlive messages. PrimaryKeepAlive messages drive the frontier forward in
    // the absence of data updates and we don't want a large `wal_sender_timeout` value to slow us
    // down. For this reason the feedback interval is set to one second, or less if the
    // wal_sender_timeout is less than 2 seconds.
    let feedback_interval = match wal_sender_timeout {
        Some(t) => std::cmp::min(Duration::from_secs(1), t.checked_div(2).unwrap()),
        None => Duration::from_secs(1),
    };

    let mut feedback_timer = tokio::time::interval(feedback_interval);
    // 'Delay' ensures we always tick at least 'feedback_interval'.
    feedback_timer.set_missed_tick_behavior(tokio::time::MissedTickBehavior::Delay);

    // Postgres will return all transactions that commit *at or after* after the provided LSN,
    // following the timely upper semantics.
    let lsn = PgLsn::from(resume_lsn.offset);
    let query = format!(
        r#"START_REPLICATION SLOT "{}" LOGICAL {} ("proto_version" '1', "publication_names" '{}')"#,
        Ident::new_unchecked(slot).to_ast_string_simple(),
        lsn,
        publication,
    );
    let copy_stream = match replication_client.copy_both_simple(&query).await {
        Ok(copy_stream) => copy_stream,
        Err(err) if err.code() == Some(&SqlState::OBJECT_NOT_IN_PREREQUISITE_STATE) => {
            return Ok(Err(DefiniteError::InvalidReplicationSlot));
        }
        Err(err) => return Err(err.into()),
    };

    // According to the documentation [1] we must check that the slot LSN matches our
    // expectations otherwise we risk getting silently fast-forwarded to a future LSN. In order
    // to avoid a TOCTOU issue we must do this check after starting the replication stream. We
    // cannot use the replication client to do that because it's already in CopyBoth mode.
    // [1] https://www.postgresql.org/docs/15/protocol-replication.html#PROTOCOL-REPLICATION-START-REPLICATION-SLOT-LOGICAL
    let slot_metadata = super::fetch_slot_metadata(
        &*metadata_client,
        slot,
        mz_storage_types::dyncfgs::PG_FETCH_SLOT_RESUME_LSN_INTERVAL
            .get(config.config.config_set()),
    )
    .await?;
    let min_resume_lsn = slot_metadata.confirmed_flush_lsn;
    tracing::info!(
        %config.id,
        "started replication using backend PID={:?}. wal_sender_timeout={:?}",
        slot_metadata.active_pid, wal_sender_timeout
    );

    let (probe_tx, mut probe_rx) = watch::channel(None);
    let config_set = Arc::clone(config.config.config_set());
    let now_fn = config.now_fn.clone();
    let max_lsn_task_handle =
        mz_ore::task::spawn(|| format!("pg_current_wal_lsn:{}", config.id), async move {
            let mut probe_ticker =
                probe::Ticker::new(|| PG_OFFSET_KNOWN_INTERVAL.get(&config_set), now_fn);

            while !probe_tx.is_closed() {
                let probe_ts = probe_ticker.tick().await;
                let probe_or_err = super::fetch_max_lsn(&*metadata_client)
                    .await
                    .map(|lsn| Probe {
                        probe_ts,
                        upstream_frontier: Antichain::from_elem(lsn),
                    });
                let _ = probe_tx.send(Some(probe_or_err));
            }
        })
        .abort_on_drop();

    let stream = async_stream::try_stream!({
        // Ensure we don't pre-drop the task
        let _max_lsn_task_handle = max_lsn_task_handle;

        // ensure we don't drop the replication client!
        let _replication_client = replication_client;

        let mut uppers = pin!(uppers);
        let mut last_committed_upper = resume_lsn;

        let mut stream = pin!(LogicalReplicationStream::new(copy_stream));

        if !(resume_lsn == MzOffset::from(0) || min_resume_lsn <= resume_lsn) {
            let err = TransientError::OvercompactedReplicationSlot {
                available_lsn: min_resume_lsn,
                requested_lsn: resume_lsn,
            };
            error!("timely-{} ({}) {err}", config.worker_id, config.id);
            Err(err)?;
        }

        loop {
            tokio::select! {
                Some(next_message) = stream.next() => match next_message {
                    Ok(ReplicationMessage::XLogData(data)) => {
                        yield ReplicationMessage::XLogData(data);
                        Ok(())
                    }
                    Ok(ReplicationMessage::PrimaryKeepAlive(keepalive)) => {
                        yield ReplicationMessage::PrimaryKeepAlive(keepalive);
                        Ok(())
                    }
                    Err(err) => Err(err.into()),
                    _ => Err(TransientError::UnknownReplicationMessage),
                },
                _ = feedback_timer.tick() => {
                    let ts: i64 = PG_EPOCH.elapsed().unwrap().as_micros().try_into().unwrap();
                    let lsn = PgLsn::from(last_committed_upper.offset);
                    trace!("timely-{} ({}) sending keepalive {lsn:?}", config.worker_id, config.id);
                    // Postgres only sends PrimaryKeepAlive messages when *it* wants a reply, which
                    // happens when out status update is late. Since we send them proactively this
                    // may never happen. It is therefore *crucial* that we set the last parameter
                    // (the reply flag) to 1 here. This will cause the upstream server to send us a
                    // PrimaryKeepAlive message promptly which will give us frontier advancement
                    // information in the absence of data updates.
                    let res = stream.as_mut().standby_status_update(lsn, lsn, lsn, ts, 1).await;
                    res.map_err(|e| e.into())
                },
                Some(upper) = uppers.next() => match upper.into_option() {
                    Some(lsn) => {
                        if last_committed_upper < lsn {
                            last_committed_upper = lsn;
                            for stat in config.statistics.values() {
                                stat.set_offset_committed(last_committed_upper.offset);
                            }
                        }
                        Ok(())
                    }
                    None => Ok(()),
                },
                Ok(()) = probe_rx.changed() => match &*probe_rx.borrow() {
                    Some(Ok(probe)) => {
                        if let Some(offset_known) = probe.upstream_frontier.as_option() {
                            for stat in config.statistics.values() {
                                stat.set_offset_known(offset_known.offset);
                            }
                        }
                        probe_output.give(probe_cap, probe);
                        Ok(())
                    },
                    Some(Err(err)) => Err(anyhow::anyhow!("{err}").into()),
                    None => Ok(()),
                },
                else => return
            }?;
        }
    });
    Ok(Ok(stream))
}

/// Extracts a single transaction from the replication stream delimited by a BEGIN and COMMIT
/// message. The BEGIN message must have already been consumed from the stream before calling this
/// function.
fn extract_transaction<'a>(
    stream: impl AsyncStream<
        Item = Result<ReplicationMessage<LogicalReplicationMessage>, TransientError>,
    > + 'a,
    metadata_client: &'a Client,
    commit_lsn: MzOffset,
    table_info: &'a mut BTreeMap<u32, BTreeMap<usize, SourceOutputInfo>>,
    metrics: &'a PgSourceMetrics,
    publication: &'a str,
) -> impl AsyncStream<Item = Result<(u32, usize, Result<Row, DefiniteError>, Diff), TransientError>> + 'a
{
    use LogicalReplicationMessage::*;
    let mut row = Row::default();
    async_stream::try_stream!({
        let mut stream = pin!(stream);
        metrics.transactions.inc();
        metrics.lsn.set(commit_lsn.offset);
        while let Some(event) = stream.try_next().await? {
            // We can ignore keepalive messages while processing a transaction because the
            // commit_lsn will drive progress.
            let message = match event {
                ReplicationMessage::XLogData(data) => data.into_data(),
                ReplicationMessage::PrimaryKeepAlive(_) => {
                    metrics.ignored.inc();
                    continue;
                }
                _ => Err(TransientError::UnknownReplicationMessage)?,
            };
            metrics.total.inc();
            match message {
                Insert(body) if !table_info.contains_key(&body.rel_id()) => metrics.ignored.inc(),
                Update(body) if !table_info.contains_key(&body.rel_id()) => metrics.ignored.inc(),
                Delete(body) if !table_info.contains_key(&body.rel_id()) => metrics.ignored.inc(),
                Relation(body) if !table_info.contains_key(&body.rel_id()) => metrics.ignored.inc(),
                Insert(body) => {
                    metrics.inserts.inc();
                    let rel = body.rel_id();
                    for (output, info) in table_info.get(&rel).into_iter().flatten() {
                        let tuple_data = body.tuple().tuple_data();
                        let Some(ref projection) = info.projection else {
                            panic!("missing projection for {rel}");
                        };
                        let datums = projection.iter().map(|idx| &tuple_data[*idx]);
                        let row = unpack_tuple(datums, &mut row);
                        yield (rel, *output, row, Diff::ONE);
                    }
                }
                Update(body) => match body.old_tuple() {
                    Some(old_tuple) => {
                        metrics.updates.inc();
                        let new_tuple = body.new_tuple();
                        let rel = body.rel_id();
                        for (output, info) in table_info.get(&rel).into_iter().flatten() {
                            let Some(ref projection) = info.projection else {
                                panic!("missing projection for {rel}");
                            };
                            let old_tuple =
                                projection.iter().map(|idx| &old_tuple.tuple_data()[*idx]);
                            // If the new tuple contains unchanged toast values we reference the old ones
                            let new_tuple = std::iter::zip(
                                projection.iter().map(|idx| &new_tuple.tuple_data()[*idx]),
                                old_tuple.clone(),
                            )
                            .map(|(new, old)| match new {
                                TupleData::UnchangedToast => old,
                                _ => new,
                            });
                            let old_row = unpack_tuple(old_tuple, &mut row);
                            let new_row = unpack_tuple(new_tuple, &mut row);

                            yield (rel, *output, old_row, Diff::MINUS_ONE);
                            yield (rel, *output, new_row, Diff::ONE);
                        }
                    }
                    None => {
                        let rel = body.rel_id();
                        for (output, _) in table_info.get(&rel).into_iter().flatten() {
                            yield (
                                rel,
                                *output,
                                Err(DefiniteError::DefaultReplicaIdentity),
                                Diff::ONE,
                            );
                        }
                    }
                },
                Delete(body) => match body.old_tuple() {
                    Some(old_tuple) => {
                        metrics.deletes.inc();
                        let rel = body.rel_id();
                        for (output, info) in table_info.get(&rel).into_iter().flatten() {
                            let Some(ref projection) = info.projection else {
                                panic!("missing projection for {rel}");
                            };
                            let datums = projection.iter().map(|idx| &old_tuple.tuple_data()[*idx]);
                            let row = unpack_tuple(datums, &mut row);
                            yield (rel, *output, row, Diff::MINUS_ONE);
                        }
                    }
                    None => {
                        let rel = body.rel_id();
                        for (output, _) in table_info.get(&rel).into_iter().flatten() {
                            yield (
                                rel,
                                *output,
                                Err(DefiniteError::DefaultReplicaIdentity),
                                Diff::ONE,
                            );
                        }
                    }
                },
                Relation(body) => {
                    let rel_id = body.rel_id();
                    if let Some(outputs) = table_info.get_mut(&body.rel_id()) {
                        // Because the replication stream doesn't include columns' attnums, we need
                        // to check the current local schema against the current remote schema to
                        // ensure e.g. we haven't received a schema update with the same terminal
                        // column name which is actually a different column.
                        let upstream_info = mz_postgres_util::publication_info(
                            metadata_client,
                            publication,
                            Some(&[rel_id]),
                        )
                        .await?;

                        let mut schema_errors = vec![];

                        outputs.retain(|output_index, info| {
                            match verify_schema(rel_id, info, &upstream_info) {
                                Ok(()) => true,
                                Err(err) => {
                                    schema_errors.push((
                                        rel_id,
                                        *output_index,
                                        Err(err),
                                        Diff::ONE,
                                    ));
                                    false
                                }
                            }
                        });
                        // Recalculate projection vector for the retained valid outputs. Here we
                        // must use the column names in the RelationBody message and not the
                        // upstream_info obtained above, since that one represents the current
                        // schema upstream which may be many versions head of the one we're about
                        // to receive after this Relation message.
                        let column_positions: BTreeMap<_, _> = body
                            .columns()
                            .iter()
                            .enumerate()
                            .map(|(idx, col)| (col.name().unwrap(), idx))
                            .collect();
                        for info in outputs.values_mut() {
                            let mut projection = vec![];
                            for col in info.desc.columns.iter() {
                                projection.push(column_positions[&*col.name]);
                            }
                            info.projection = Some(projection);
                        }
                        for schema_error in schema_errors {
                            yield schema_error;
                        }
                    }
                }
                Truncate(body) => {
                    for &rel_id in body.rel_ids() {
                        if let Some(outputs) = table_info.get_mut(&rel_id) {
                            for (output, _) in std::mem::take(outputs) {
                                yield (
                                    rel_id,
                                    output,
                                    Err(DefiniteError::TableTruncated),
                                    Diff::ONE,
                                );
                            }
                        }
                    }
                }
                Commit(body) => {
                    if commit_lsn != body.commit_lsn().into() {
                        Err(TransientError::InvalidTransaction)?
                    }
                    return;
                }
                // TODO: We should handle origin messages and emit an error as they indicate that
                // the upstream performed a point in time restore so all bets are off about the
                // continuity of the stream.
                Origin(_) | Type(_) => metrics.ignored.inc(),
                Begin(_) => Err(TransientError::NestedTransaction)?,
                // The enum is marked as non_exhaustive. Better to be conservative
                _ => Err(TransientError::UnknownLogicalReplicationMessage)?,
            }
        }
        Err(TransientError::ReplicationEOF)?;
    })
}

/// Unpacks an iterator of TupleData into a list of nullable bytes or an error if this can't be
/// done.
#[inline]
fn unpack_tuple<'a, I>(tuple_data: I, row: &mut Row) -> Result<Row, DefiniteError>
where
    I: IntoIterator<Item = &'a TupleData>,
    I::IntoIter: ExactSizeIterator,
{
    let iter = tuple_data.into_iter();
    let mut packer = row.packer();
    for data in iter {
        let datum = match data {
            TupleData::Text(bytes) => super::decode_utf8_text(bytes)?,
            TupleData::Null => Datum::Null,
            TupleData::UnchangedToast => return Err(DefiniteError::MissingToast),
            TupleData::Binary(_) => return Err(DefiniteError::UnexpectedBinaryData),
        };
        packer.push(datum);
    }
    Ok(row.clone())
}

/// Ensures the publication exists on the server. It returns an outer transient error in case of
/// connection issues and an inner definite error if the publication is dropped.
async fn ensure_publication_exists(
    client: &Client,
    publication: &str,
) -> Result<Result<(), DefiniteError>, TransientError> {
    // Figure out the last written LSN and then add one to convert it into an upper.
    let result = client
        .query_opt(
            "SELECT 1 FROM pg_publication WHERE pubname = $1;",
            &[&publication],
        )
        .await?;
    match result {
        Some(_) => Ok(Ok(())),
        None => Ok(Err(DefiniteError::PublicationDropped(
            publication.to_owned(),
        ))),
    }
}

/// Ensure the active replication timeline_id matches the one we expect such that we can safely
/// resume replication. It returns an outer transient error in case of
/// connection issues and an inner definite error if the timeline id does not match.
async fn ensure_replication_timeline_id(
    replication_client: &Client,
    expected_timeline_id: &u64,
) -> Result<Result<(), DefiniteError>, TransientError> {
    let timeline_id = mz_postgres_util::get_timeline_id(replication_client).await?;
    if timeline_id == *expected_timeline_id {
        Ok(Ok(()))
    } else {
        Ok(Err(DefiniteError::InvalidTimelineId {
            expected: *expected_timeline_id,
            actual: timeline_id,
        }))
    }
}

enum SchemaValidationError {
    Postgres(PostgresError),
    Schema {
        oid: u32,
        output_index: usize,
        error: DefiniteError,
    },
}

fn spawn_schema_validator(
    client: Client,
    config: &RawSourceCreationConfig,
    publication: String,
    table_info: BTreeMap<u32, BTreeMap<usize, SourceOutputInfo>>,
) -> mpsc::UnboundedReceiver<SchemaValidationError> {
    let (tx, rx) = mpsc::unbounded_channel();
    let source_id = config.id;
    let config_set = Arc::clone(config.config.config_set());

    mz_ore::task::spawn(|| format!("schema-validator:{}", source_id), async move {
        while !tx.is_closed() {
            trace!(%source_id, "validating schemas");

            let validation_start = Instant::now();

            let upstream_info = match mz_postgres_util::publication_info(
                &*client,
                &publication,
                Some(&table_info.keys().copied().collect::<Vec<_>>()),
            )
            .await
            {
                Ok(info) => info,
                Err(error) => {
                    let _ = tx.send(SchemaValidationError::Postgres(error));
                    continue;
                }
            };

            for (&oid, outputs) in table_info.iter() {
                for (&output_index, info) in outputs {
                    if let Err(error) = verify_schema(oid, info, &upstream_info) {
                        trace!(
                            %source_id,
                            "schema of output index {output_index} for oid {oid} invalid",
                        );
                        let _ = tx.send(SchemaValidationError::Schema {
                            oid,
                            output_index,
                            error,
                        });
                    } else {
                        trace!(
                            %source_id,
                            "schema of output index {output_index} for oid {oid} valid",
                        );
                    }
                }
            }

            let interval = PG_SCHEMA_VALIDATION_INTERVAL.get(&config_set);
            let elapsed = validation_start.elapsed();
            let wait = interval.saturating_sub(elapsed);
            tokio::time::sleep(wait).await;
        }
    });

    rx
}