timely/dataflow/operators/capability.rs
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//! Capabilities to send data from operators
//!
//! Timely dataflow operators are only able to send data if they possess a "capability",
//! a system-created object which warns the runtime that the operator may still produce
//! output records.
//!
//! The timely dataflow runtime creates a capability and provides it to an operator whenever
//! the operator receives input data. The capabilities allow the operator to respond to the
//! received data, immediately or in the future, for as long as the capability is held.
//!
//! Timely dataflow's progress tracking infrastructure communicates the number of outstanding
//! capabilities across all workers.
//! Each operator may hold on to its capabilities, and may clone, advance, and drop them.
//! Each of these actions informs the timely dataflow runtime of changes to the number of outstanding
//! capabilities, so that the runtime can notice when the count for some capability reaches zero.
//! While an operator can hold capabilities indefinitely, and create as many new copies of them
//! as it would like, the progress tracking infrastructure will not move forward until the
//! operators eventually release their capabilities.
//!
//! Note that these capabilities currently lack the property of "transferability":
//! An operator should not hand its capabilities to some other operator. In the future, we should
//! probably bind capabilities more strongly to a specific operator and output.
use std::{borrow, error::Error, fmt::Display, ops::Deref};
use std::rc::Rc;
use std::cell::RefCell;
use std::fmt::{self, Debug};
use crate::order::PartialOrder;
use crate::progress::Antichain;
use crate::progress::Timestamp;
use crate::progress::ChangeBatch;
use crate::scheduling::Activations;
use crate::dataflow::channels::pullers::counter::ConsumedGuard;
/// An internal trait expressing the capability to send messages with a given timestamp.
pub trait CapabilityTrait<T: Timestamp> {
/// The timestamp associated with the capability.
fn time(&self) -> &T;
fn valid_for_output(&self, query_buffer: &Rc<RefCell<ChangeBatch<T>>>) -> bool;
}
impl<'a, T: Timestamp, C: CapabilityTrait<T>> CapabilityTrait<T> for &'a C {
fn time(&self) -> &T { (**self).time() }
fn valid_for_output(&self, query_buffer: &Rc<RefCell<ChangeBatch<T>>>) -> bool {
(**self).valid_for_output(query_buffer)
}
}
impl<'a, T: Timestamp, C: CapabilityTrait<T>> CapabilityTrait<T> for &'a mut C {
fn time(&self) -> &T { (**self).time() }
fn valid_for_output(&self, query_buffer: &Rc<RefCell<ChangeBatch<T>>>) -> bool {
(**self).valid_for_output(query_buffer)
}
}
/// The capability to send data with a certain timestamp on a dataflow edge.
///
/// Capabilities are used by timely dataflow's progress tracking machinery to restrict and track
/// when user code retains the ability to send messages on dataflow edges. All capabilities are
/// constructed by the system, and should eventually be dropped by the user. Failure to drop
/// a capability (for whatever reason) will cause timely dataflow's progress tracking to stall.
pub struct Capability<T: Timestamp> {
time: T,
internal: Rc<RefCell<ChangeBatch<T>>>,
}
impl<T: Timestamp> CapabilityTrait<T> for Capability<T> {
fn time(&self) -> &T { &self.time }
fn valid_for_output(&self, query_buffer: &Rc<RefCell<ChangeBatch<T>>>) -> bool {
Rc::ptr_eq(&self.internal, query_buffer)
}
}
impl<T: Timestamp> Capability<T> {
/// Creates a new capability at `time` while incrementing (and keeping a reference to) the provided
/// [`ChangeBatch`].
pub(crate) fn new(time: T, internal: Rc<RefCell<ChangeBatch<T>>>) -> Self {
internal.borrow_mut().update(time.clone(), 1);
Self {
time,
internal,
}
}
/// The timestamp associated with this capability.
pub fn time(&self) -> &T {
&self.time
}
/// Makes a new capability for a timestamp `new_time` greater or equal to the timestamp of
/// the source capability (`self`).
///
/// This method panics if `self.time` is not less or equal to `new_time`.
pub fn delayed(&self, new_time: &T) -> Capability<T> {
/// Makes the panic branch cold & outlined to decrease code bloat & give
/// the inner function the best chance possible of being inlined with
/// minimal code bloat
#[cold]
#[inline(never)]
fn delayed_panic(capability: &dyn Debug, invalid_time: &dyn Debug) -> ! {
// Formatting & panic machinery is relatively expensive in terms of code bloat, so
// we outline it
panic!(
"Attempted to delay {:?} to {:?}, which is not beyond the capability's time.",
capability,
invalid_time,
)
}
self.try_delayed(new_time)
.unwrap_or_else(|| delayed_panic(self, new_time))
}
/// Attempts to make a new capability for a timestamp `new_time` that is
/// greater or equal to the timestamp of the source capability (`self`).
///
/// Returns [`None`] `self.time` is not less or equal to `new_time`.
pub fn try_delayed(&self, new_time: &T) -> Option<Capability<T>> {
if self.time.less_equal(new_time) {
Some(Self::new(new_time.clone(), self.internal.clone()))
} else {
None
}
}
/// Downgrades the capability to one corresponding to `new_time`.
///
/// This method panics if `self.time` is not less or equal to `new_time`.
pub fn downgrade(&mut self, new_time: &T) {
/// Makes the panic branch cold & outlined to decrease code bloat & give
/// the inner function the best chance possible of being inlined with
/// minimal code bloat
#[cold]
#[inline(never)]
fn downgrade_panic(capability: &dyn Debug, invalid_time: &dyn Debug) -> ! {
// Formatting & panic machinery is relatively expensive in terms of code bloat, so
// we outline it
panic!(
"Attempted to downgrade {:?} to {:?}, which is not beyond the capability's time.",
capability,
invalid_time,
)
}
self.try_downgrade(new_time)
.unwrap_or_else(|_| downgrade_panic(self, new_time))
}
/// Attempts to downgrade the capability to one corresponding to `new_time`.
///
/// Returns a [DowngradeError] if `self.time` is not less or equal to `new_time`.
pub fn try_downgrade(&mut self, new_time: &T) -> Result<(), DowngradeError> {
if let Some(new_capability) = self.try_delayed(new_time) {
*self = new_capability;
Ok(())
} else {
Err(DowngradeError(()))
}
}
}
// Necessary for correctness. When a capability is dropped, the "internal" `ChangeBatch` needs to be
// updated accordingly to inform the rest of the system that the operator has released its permit
// to send data and request notification at the associated timestamp.
impl<T: Timestamp> Drop for Capability<T> {
fn drop(&mut self) {
self.internal.borrow_mut().update(self.time.clone(), -1);
}
}
impl<T: Timestamp> Clone for Capability<T> {
fn clone(&self) -> Capability<T> {
Self::new(self.time.clone(), self.internal.clone())
}
}
impl<T: Timestamp> Deref for Capability<T> {
type Target = T;
fn deref(&self) -> &T {
&self.time
}
}
impl<T: Timestamp> Debug for Capability<T> {
fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
f.debug_struct("Capability")
.field("time", &self.time)
.field("internal", &"...")
.finish()
}
}
impl<T: Timestamp> PartialEq for Capability<T> {
fn eq(&self, other: &Self) -> bool {
self.time() == other.time() && Rc::ptr_eq(&self.internal, &other.internal)
}
}
impl<T: Timestamp> Eq for Capability<T> { }
impl<T: Timestamp> PartialOrder for Capability<T> {
fn less_equal(&self, other: &Self) -> bool {
self.time().less_equal(other.time()) && Rc::ptr_eq(&self.internal, &other.internal)
}
}
impl<T: Timestamp> ::std::hash::Hash for Capability<T> {
fn hash<H: ::std::hash::Hasher>(&self, state: &mut H) {
self.time.hash(state);
}
}
/// An error produced when trying to downgrade a capability with a time
/// that's not less than or equal to the current capability
#[derive(Debug, Clone, Copy, PartialEq, Eq, PartialOrd, Ord, Hash)]
pub struct DowngradeError(());
impl Display for DowngradeError {
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
f.write_str("could not downgrade the given capability")
}
}
impl Error for DowngradeError {}
/// A shared list of shared output capability buffers.
type CapabilityUpdates<T> = Rc<RefCell<Vec<Rc<RefCell<ChangeBatch<T>>>>>>;
/// An capability of an input port. Holding onto this capability will implicitly holds onto a
/// capability for all the outputs ports this input is connected to, after the connection summaries
/// have been applied.
///
/// This input capability supplies a `retain_for_output(self)` method which consumes the input
/// capability and turns it into a [Capability] for a specific output port.
pub struct InputCapability<T: Timestamp> {
/// Output capability buffers, for use in minting capabilities.
internal: CapabilityUpdates<T>,
/// Timestamp summaries for each output.
summaries: Rc<RefCell<Vec<Antichain<T::Summary>>>>,
/// A drop guard that updates the consumed capability this InputCapability refers to on drop
consumed_guard: ConsumedGuard<T>,
}
impl<T: Timestamp> CapabilityTrait<T> for InputCapability<T> {
fn time(&self) -> &T { self.time() }
fn valid_for_output(&self, query_buffer: &Rc<RefCell<ChangeBatch<T>>>) -> bool {
let borrow = self.summaries.borrow();
self.internal.borrow().iter().enumerate().any(|(index, rc)| {
// To be valid, the output buffer must match and the timestamp summary needs to be the default.
Rc::ptr_eq(rc, query_buffer) && borrow[index].len() == 1 && borrow[index][0] == Default::default()
})
}
}
impl<T: Timestamp> InputCapability<T> {
/// Creates a new capability reference at `time` while incrementing (and keeping a reference to)
/// the provided [`ChangeBatch`].
pub(crate) fn new(internal: CapabilityUpdates<T>, summaries: Rc<RefCell<Vec<Antichain<T::Summary>>>>, guard: ConsumedGuard<T>) -> Self {
InputCapability {
internal,
summaries,
consumed_guard: guard,
}
}
/// The timestamp associated with this capability.
pub fn time(&self) -> &T {
self.consumed_guard.time()
}
/// Makes a new capability for a timestamp `new_time` greater or equal to the timestamp of
/// the source capability (`self`).
///
/// This method panics if `self.time` is not less or equal to `new_time`.
pub fn delayed(&self, new_time: &T) -> Capability<T> {
self.delayed_for_output(new_time, 0)
}
/// Delays capability for a specific output port.
pub fn delayed_for_output(&self, new_time: &T, output_port: usize) -> Capability<T> {
use crate::progress::timestamp::PathSummary;
if self.summaries.borrow()[output_port].iter().flat_map(|summary| summary.results_in(self.time())).any(|time| time.less_equal(new_time)) {
Capability::new(new_time.clone(), self.internal.borrow()[output_port].clone())
} else {
panic!("Attempted to delay to a time ({:?}) not greater or equal to the operators input-output summary ({:?}) applied to the capabilities time ({:?})", new_time, self.summaries.borrow()[output_port], self.time());
}
}
/// Transform to an owned capability.
///
/// This method produces an owned capability which must be dropped to release the
/// capability. Users should take care that these capabilities are only stored for
/// as long as they are required, as failing to drop them may result in livelock.
///
/// This method panics if the timestamp summary to output zero strictly advances the time.
pub fn retain(self) -> Capability<T> {
self.retain_for_output(0)
}
/// Transforms to an owned capability for a specific output port.
///
/// This method panics if the timestamp summary to `output_port` strictly advances the time.
pub fn retain_for_output(self, output_port: usize) -> Capability<T> {
use crate::progress::timestamp::PathSummary;
let self_time = self.time().clone();
if self.summaries.borrow()[output_port].iter().flat_map(|summary| summary.results_in(&self_time)).any(|time| time.less_equal(&self_time)) {
Capability::new(self_time, self.internal.borrow()[output_port].clone())
}
else {
panic!("Attempted to retain a time ({:?}) not greater or equal to the operators input-output summary ({:?}) applied to the capabilities time ({:?})", self_time, self.summaries.borrow()[output_port], self_time);
}
}
}
impl<T: Timestamp> Deref for InputCapability<T> {
type Target = T;
fn deref(&self) -> &T {
self.time()
}
}
impl<T: Timestamp> Debug for InputCapability<T> {
fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
f.debug_struct("InputCapability")
.field("time", self.time())
.field("internal", &"...")
.finish()
}
}
/// Capability that activates on drop.
#[derive(Clone, Debug)]
pub struct ActivateCapability<T: Timestamp> {
pub(crate) capability: Capability<T>,
pub(crate) address: Rc<[usize]>,
pub(crate) activations: Rc<RefCell<Activations>>,
}
impl<T: Timestamp> CapabilityTrait<T> for ActivateCapability<T> {
fn time(&self) -> &T { self.capability.time() }
fn valid_for_output(&self, query_buffer: &Rc<RefCell<ChangeBatch<T>>>) -> bool {
self.capability.valid_for_output(query_buffer)
}
}
impl<T: Timestamp> ActivateCapability<T> {
/// Creates a new activating capability.
pub fn new(capability: Capability<T>, address: Rc<[usize]>, activations: Rc<RefCell<Activations>>) -> Self {
Self {
capability,
address,
activations,
}
}
/// The timestamp associated with this capability.
pub fn time(&self) -> &T {
self.capability.time()
}
/// Creates a new delayed capability.
pub fn delayed(&self, time: &T) -> Self {
ActivateCapability {
capability: self.capability.delayed(time),
address: self.address.clone(),
activations: self.activations.clone(),
}
}
/// Downgrades this capability.
pub fn downgrade(&mut self, time: &T) {
self.capability.downgrade(time);
self.activations.borrow_mut().activate(&self.address);
}
}
impl<T: Timestamp> Drop for ActivateCapability<T> {
fn drop(&mut self) {
self.activations.borrow_mut().activate(&self.address);
}
}
/// A set of capabilities, for possibly incomparable times.
#[derive(Clone, Debug)]
pub struct CapabilitySet<T: Timestamp> {
elements: Vec<Capability<T>>,
}
impl<T: Timestamp> CapabilitySet<T> {
/// Allocates an empty capability set.
pub fn new() -> Self {
Self { elements: Vec::new() }
}
/// Allocates an empty capability set with space for `capacity` elements
pub fn with_capacity(capacity: usize) -> Self {
Self { elements: Vec::with_capacity(capacity) }
}
/// Allocates a capability set containing a single capability.
///
/// # Examples
/// ```
/// use std::collections::HashMap;
/// use timely::dataflow::{
/// operators::{ToStream, generic::Operator},
/// channels::pact::Pipeline,
/// };
/// use timely::dataflow::operators::CapabilitySet;
///
/// timely::example(|scope| {
/// vec![()].into_iter().to_stream(scope)
/// .unary_frontier(Pipeline, "example", |default_cap, _info| {
/// let mut cap = CapabilitySet::from_elem(default_cap);
/// let mut vector = Vec::new();
/// move |input, output| {
/// cap.downgrade(&input.frontier().frontier());
/// while let Some((time, data)) = input.next() {
/// data.swap(&mut vector);
/// }
/// let a_cap = cap.first();
/// if let Some(a_cap) = a_cap.as_ref() {
/// output.session(a_cap).give(());
/// }
/// }
/// })
/// .container::<Vec<_>>();
/// });
/// ```
pub fn from_elem(cap: Capability<T>) -> Self {
Self { elements: vec![cap] }
}
/// Inserts `capability` into the set, discarding redundant capabilities.
pub fn insert(&mut self, capability: Capability<T>) {
if !self.elements.iter().any(|c| c.less_equal(&capability)) {
self.elements.retain(|c| !capability.less_equal(c));
self.elements.push(capability);
}
}
/// Creates a new capability to send data at `time`.
///
/// This method panics if there does not exist a capability in `self.elements` less or equal to `time`.
pub fn delayed(&self, time: &T) -> Capability<T> {
/// Makes the panic branch cold & outlined to decrease code bloat & give
/// the inner function the best chance possible of being inlined with
/// minimal code bloat
#[cold]
#[inline(never)]
fn delayed_panic(invalid_time: &dyn Debug) -> ! {
// Formatting & panic machinery is relatively expensive in terms of code bloat, so
// we outline it
panic!(
"failed to create a delayed capability, the current set does not \
have an element less than or equal to {:?}",
invalid_time,
)
}
self.try_delayed(time)
.unwrap_or_else(|| delayed_panic(time))
}
/// Attempts to create a new capability to send data at `time`.
///
/// Returns [`None`] if there does not exist a capability in `self.elements` less or equal to `time`.
pub fn try_delayed(&self, time: &T) -> Option<Capability<T>> {
self.elements
.iter()
.find(|capability| capability.time().less_equal(time))
.and_then(|capability| capability.try_delayed(time))
}
/// Downgrades the set of capabilities to correspond with the times in `frontier`.
///
/// This method panics if any element of `frontier` is not greater or equal to some element of `self.elements`.
pub fn downgrade<B, F>(&mut self, frontier: F)
where
B: borrow::Borrow<T>,
F: IntoIterator<Item = B>,
{
/// Makes the panic branch cold & outlined to decrease code bloat & give
/// the inner function the best chance possible of being inlined with
/// minimal code bloat
#[cold]
#[inline(never)]
fn downgrade_panic() -> ! {
// Formatting & panic machinery is relatively expensive in terms of code bloat, so
// we outline it
panic!(
"Attempted to downgrade a CapabilitySet with a frontier containing an element \
that was not beyond an element within the set"
)
}
self.try_downgrade(frontier)
.unwrap_or_else(|_| downgrade_panic())
}
/// Attempts to downgrade the set of capabilities to correspond with the times in `frontier`.
///
/// Returns [`None`] if any element of `frontier` is not greater or equal to some element of `self.elements`.
///
/// **Warning**: If an error is returned the capability set may be in an inconsistent state and can easily
/// cause logic errors within the program if not properly handled.
///
pub fn try_downgrade<B, F>(&mut self, frontier: F) -> Result<(), DowngradeError>
where
B: borrow::Borrow<T>,
F: IntoIterator<Item = B>,
{
let count = self.elements.len();
for time in frontier.into_iter() {
let capability = self.try_delayed(time.borrow()).ok_or(DowngradeError(()))?;
self.elements.push(capability);
}
self.elements.drain(..count);
Ok(())
}
}
impl<T> From<Vec<Capability<T>>> for CapabilitySet<T>
where
T: Timestamp,
{
fn from(capabilities: Vec<Capability<T>>) -> Self {
let mut this = Self::with_capacity(capabilities.len());
for capability in capabilities {
this.insert(capability);
}
this
}
}
impl<T: Timestamp> Default for CapabilitySet<T> {
fn default() -> Self {
Self::new()
}
}
impl<T: Timestamp> Deref for CapabilitySet<T> {
type Target=[Capability<T>];
fn deref(&self) -> &[Capability<T>] {
&self.elements
}
}