timely/dataflow/operators/
capability.rs

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1//! Capabilities to send data from operators
2//!
3//! Timely dataflow operators are only able to send data if they possess a "capability",
4//! a system-created object which warns the runtime that the operator may still produce
5//! output records.
6//!
7//! The timely dataflow runtime creates a capability and provides it to an operator whenever
8//! the operator receives input data. The capabilities allow the operator to respond to the
9//! received data, immediately or in the future, for as long as the capability is held.
10//!
11//! Timely dataflow's progress tracking infrastructure communicates the number of outstanding
12//! capabilities across all workers.
13//! Each operator may hold on to its capabilities, and may clone, advance, and drop them.
14//! Each of these actions informs the timely dataflow runtime of changes to the number of outstanding
15//! capabilities, so that the runtime can notice when the count for some capability reaches zero.
16//! While an operator can hold capabilities indefinitely, and create as many new copies of them
17//! as it would like, the progress tracking infrastructure will not move forward until the
18//! operators eventually release their capabilities.
19//!
20//! Note that these capabilities currently lack the property of "transferability":
21//! An operator should not hand its capabilities to some other operator. In the future, we should
22//! probably bind capabilities more strongly to a specific operator and output.
23
24use std::{borrow, error::Error, fmt::Display, ops::Deref};
25use std::rc::Rc;
26use std::cell::RefCell;
27use std::fmt::{self, Debug};
28
29use crate::order::PartialOrder;
30use crate::progress::Timestamp;
31use crate::progress::ChangeBatch;
32use crate::progress::operate::PortConnectivity;
33use crate::scheduling::Activations;
34use crate::dataflow::channels::pullers::counter::ConsumedGuard;
35
36/// An internal trait expressing the capability to send messages with a given timestamp.
37pub trait CapabilityTrait<T: Timestamp> {
38    /// The timestamp associated with the capability.
39    fn time(&self) -> &T;
40    fn valid_for_output(&self, query_buffer: &Rc<RefCell<ChangeBatch<T>>>) -> bool;
41}
42
43impl<T: Timestamp, C: CapabilityTrait<T>> CapabilityTrait<T> for &C {
44    fn time(&self) -> &T { (**self).time() }
45    fn valid_for_output(&self, query_buffer: &Rc<RefCell<ChangeBatch<T>>>) -> bool {
46        (**self).valid_for_output(query_buffer)
47    }
48}
49impl<T: Timestamp, C: CapabilityTrait<T>> CapabilityTrait<T> for &mut C {
50    fn time(&self) -> &T { (**self).time() }
51    fn valid_for_output(&self, query_buffer: &Rc<RefCell<ChangeBatch<T>>>) -> bool {
52        (**self).valid_for_output(query_buffer)
53    }
54}
55
56/// The capability to send data with a certain timestamp on a dataflow edge.
57///
58/// Capabilities are used by timely dataflow's progress tracking machinery to restrict and track
59/// when user code retains the ability to send messages on dataflow edges. All capabilities are
60/// constructed by the system, and should eventually be dropped by the user. Failure to drop
61/// a capability (for whatever reason) will cause timely dataflow's progress tracking to stall.
62pub struct Capability<T: Timestamp> {
63    time: T,
64    internal: Rc<RefCell<ChangeBatch<T>>>,
65}
66
67impl<T: Timestamp> CapabilityTrait<T> for Capability<T> {
68    fn time(&self) -> &T { &self.time }
69    fn valid_for_output(&self, query_buffer: &Rc<RefCell<ChangeBatch<T>>>) -> bool {
70        Rc::ptr_eq(&self.internal, query_buffer)
71    }
72}
73
74impl<T: Timestamp> Capability<T> {
75    /// Creates a new capability at `time` while incrementing (and keeping a reference to) the provided
76    /// [`ChangeBatch`].
77    pub(crate) fn new(time: T, internal: Rc<RefCell<ChangeBatch<T>>>) -> Self {
78        internal.borrow_mut().update(time.clone(), 1);
79
80        Self {
81            time,
82            internal,
83        }
84    }
85
86    /// The timestamp associated with this capability.
87    pub fn time(&self) -> &T {
88        &self.time
89    }
90
91    /// Makes a new capability for a timestamp `new_time` greater or equal to the timestamp of
92    /// the source capability (`self`).
93    ///
94    /// This method panics if `self.time` is not less or equal to `new_time`.
95    pub fn delayed(&self, new_time: &T) -> Capability<T> {
96        /// Makes the panic branch cold & outlined to decrease code bloat & give
97        /// the inner function the best chance possible of being inlined with
98        /// minimal code bloat
99        #[cold]
100        #[inline(never)]
101        fn delayed_panic(capability: &dyn Debug, invalid_time: &dyn Debug) -> ! {
102            // Formatting & panic machinery is relatively expensive in terms of code bloat, so
103            // we outline it
104            panic!(
105                "Attempted to delay {:?} to {:?}, which is not beyond the capability's time.",
106                capability,
107                invalid_time,
108            )
109        }
110
111        self.try_delayed(new_time)
112            .unwrap_or_else(|| delayed_panic(self, new_time))
113    }
114
115    /// Attempts to make a new capability for a timestamp `new_time` that is
116    /// greater or equal to the timestamp of the source capability (`self`).
117    ///
118    /// Returns [`None`] `self.time` is not less or equal to `new_time`.
119    pub fn try_delayed(&self, new_time: &T) -> Option<Capability<T>> {
120        if self.time.less_equal(new_time) {
121            Some(Self::new(new_time.clone(), Rc::clone(&self.internal)))
122        } else {
123            None
124        }
125    }
126
127    /// Downgrades the capability to one corresponding to `new_time`.
128    ///
129    /// This method panics if `self.time` is not less or equal to `new_time`.
130    pub fn downgrade(&mut self, new_time: &T) {
131        /// Makes the panic branch cold & outlined to decrease code bloat & give
132        /// the inner function the best chance possible of being inlined with
133        /// minimal code bloat
134        #[cold]
135        #[inline(never)]
136        fn downgrade_panic(capability: &dyn Debug, invalid_time: &dyn Debug) -> ! {
137            // Formatting & panic machinery is relatively expensive in terms of code bloat, so
138            // we outline it
139            panic!(
140                "Attempted to downgrade {:?} to {:?}, which is not beyond the capability's time.",
141                capability,
142                invalid_time,
143            )
144        }
145
146        self.try_downgrade(new_time)
147            .unwrap_or_else(|_| downgrade_panic(self, new_time))
148    }
149
150    /// Attempts to downgrade the capability to one corresponding to `new_time`.
151    ///
152    /// Returns a [DowngradeError] if `self.time` is not less or equal to `new_time`.
153    pub fn try_downgrade(&mut self, new_time: &T) -> Result<(), DowngradeError> {
154        if let Some(new_capability) = self.try_delayed(new_time) {
155            *self = new_capability;
156            Ok(())
157        } else {
158            Err(DowngradeError(()))
159        }
160    }
161}
162
163// Necessary for correctness. When a capability is dropped, the "internal" `ChangeBatch` needs to be
164// updated accordingly to inform the rest of the system that the operator has released its permit
165// to send data and request notification at the associated timestamp.
166impl<T: Timestamp> Drop for Capability<T> {
167    fn drop(&mut self) {
168        self.internal.borrow_mut().update(self.time.clone(), -1);
169    }
170}
171
172impl<T: Timestamp> Clone for Capability<T> {
173    fn clone(&self) -> Capability<T> {
174        Self::new(self.time.clone(), Rc::clone(&self.internal))
175    }
176}
177
178impl<T: Timestamp> Deref for Capability<T> {
179    type Target = T;
180
181    fn deref(&self) -> &T {
182        &self.time
183    }
184}
185
186impl<T: Timestamp> Debug for Capability<T> {
187    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
188        f.debug_struct("Capability")
189            .field("time", &self.time)
190            .field("internal", &"...")
191            .finish()
192    }
193}
194
195impl<T: Timestamp> PartialEq for Capability<T> {
196    fn eq(&self, other: &Self) -> bool {
197        self.time() == other.time() && Rc::ptr_eq(&self.internal, &other.internal)
198    }
199}
200impl<T: Timestamp> Eq for Capability<T> { }
201
202impl<T: Timestamp> PartialOrder for Capability<T> {
203    fn less_equal(&self, other: &Self) -> bool {
204        self.time().less_equal(other.time()) && Rc::ptr_eq(&self.internal, &other.internal)
205    }
206}
207
208impl<T: Timestamp> ::std::hash::Hash for Capability<T> {
209    fn hash<H: ::std::hash::Hasher>(&self, state: &mut H) {
210        self.time.hash(state);
211    }
212}
213
214/// An error produced when trying to downgrade a capability with a time
215/// that's not less than or equal to the current capability
216#[derive(Debug, Clone, Copy, PartialEq, Eq, PartialOrd, Ord, Hash)]
217pub struct DowngradeError(());
218
219impl Display for DowngradeError {
220    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
221        f.write_str("could not downgrade the given capability")
222    }
223}
224
225impl Error for DowngradeError {}
226
227/// A shared list of shared output capability buffers.
228type CapabilityUpdates<T> = Rc<RefCell<Vec<Rc<RefCell<ChangeBatch<T>>>>>>;
229
230/// An capability of an input port. 
231///
232/// Holding onto this capability will implicitly holds onto a capability for all the outputs 
233/// ports this input is connected to, after the connection summaries have been applied.
234///
235/// This input capability supplies a `retain_for_output(self)` method which consumes the input
236/// capability and turns it into a [Capability] for a specific output port.
237pub struct InputCapability<T: Timestamp> {
238    /// Output capability buffers, for use in minting capabilities.
239    internal: CapabilityUpdates<T>,
240    /// Timestamp summaries for each output.
241    summaries: Rc<RefCell<PortConnectivity<T::Summary>>>,
242    /// A drop guard that updates the consumed capability this InputCapability refers to on drop
243    consumed_guard: ConsumedGuard<T>,
244}
245
246impl<T: Timestamp> CapabilityTrait<T> for InputCapability<T> {
247    fn time(&self) -> &T { self.time() }
248    fn valid_for_output(&self, query_buffer: &Rc<RefCell<ChangeBatch<T>>>) -> bool {
249        let summaries_borrow = self.summaries.borrow();
250        let internal_borrow = self.internal.borrow();
251        // To be valid, the output buffer must match and the timestamp summary needs to be the default.
252        let result = summaries_borrow.iter_ports().any(|(port, path)| {
253            Rc::ptr_eq(&internal_borrow[port], query_buffer) && path.len() == 1 && path[0] == Default::default()
254        });
255        result
256    }
257}
258
259impl<T: Timestamp> InputCapability<T> {
260    /// Creates a new capability reference at `time` while incrementing (and keeping a reference to)
261    /// the provided [`ChangeBatch`].
262    pub(crate) fn new(internal: CapabilityUpdates<T>, summaries: Rc<RefCell<PortConnectivity<T::Summary>>>, guard: ConsumedGuard<T>) -> Self {
263        InputCapability {
264            internal,
265            summaries,
266            consumed_guard: guard,
267        }
268    }
269
270    /// The timestamp associated with this capability.
271    pub fn time(&self) -> &T {
272        self.consumed_guard.time()
273    }
274
275    /// Makes a new capability for a timestamp `new_time` greater or equal to the timestamp of
276    /// the source capability (`self`).
277    ///
278    /// This method panics if `self.time` is not less or equal to `new_time`.
279    pub fn delayed(&self, new_time: &T) -> Capability<T> {
280        self.delayed_for_output(new_time, 0)
281    }
282
283    /// Delays capability for a specific output port.
284    pub fn delayed_for_output(&self, new_time: &T, output_port: usize) -> Capability<T> {
285        use crate::progress::timestamp::PathSummary;
286        if let Some(path) = self.summaries.borrow().get(output_port) {
287            if path.iter().flat_map(|summary| summary.results_in(self.time())).any(|time| time.less_equal(new_time)) {
288                Capability::new(new_time.clone(), Rc::clone(&self.internal.borrow()[output_port]))
289            } else {
290                panic!("Attempted to delay to a time ({:?}) not greater or equal to the operators input-output summary ({:?}) applied to the capabilities time ({:?})", new_time, path, self.time());
291            }
292        }
293        else {
294            panic!("Attempted to delay a capability for a disconnected output");
295        }
296    }
297
298    /// Transform to an owned capability.
299    ///
300    /// This method produces an owned capability which must be dropped to release the
301    /// capability. Users should take care that these capabilities are only stored for
302    /// as long as they are required, as failing to drop them may result in livelock.
303    ///
304    /// This method panics if the timestamp summary to output zero strictly advances the time.
305    pub fn retain(self) -> Capability<T> {
306        self.retain_for_output(0)
307    }
308
309    /// Transforms to an owned capability for a specific output port.
310    ///
311    /// This method panics if the timestamp summary to `output_port` strictly advances the time.
312    pub fn retain_for_output(self, output_port: usize) -> Capability<T> {
313        use crate::progress::timestamp::PathSummary;
314        let self_time = self.time().clone();
315        if let Some(path) = self.summaries.borrow().get(output_port) {
316            if path.iter().flat_map(|summary| summary.results_in(&self_time)).any(|time| time.less_equal(&self_time)) {
317                Capability::new(self_time, Rc::clone(&self.internal.borrow()[output_port]))
318            }
319            else {
320                panic!("Attempted to retain a time ({:?}) not greater or equal to the operators input-output summary ({:?}) applied to the capabilities time ({:?})", self_time, path, self_time);
321            }
322        }
323        else {
324            panic!("Attempted to retain a capability for a disconnected output");
325        }
326    }
327}
328
329impl<T: Timestamp> Deref for InputCapability<T> {
330    type Target = T;
331
332    fn deref(&self) -> &T {
333        self.time()
334    }
335}
336
337impl<T: Timestamp> Debug for InputCapability<T> {
338    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
339        f.debug_struct("InputCapability")
340            .field("time", self.time())
341            .field("internal", &"...")
342            .finish()
343    }
344}
345
346/// Capability that activates on drop.
347#[derive(Clone, Debug)]
348pub struct ActivateCapability<T: Timestamp> {
349    pub(crate) capability: Capability<T>,
350    pub(crate) address: Rc<[usize]>,
351    pub(crate) activations: Rc<RefCell<Activations>>,
352}
353
354impl<T: Timestamp> CapabilityTrait<T> for ActivateCapability<T> {
355    fn time(&self) -> &T { self.capability.time() }
356    fn valid_for_output(&self, query_buffer: &Rc<RefCell<ChangeBatch<T>>>) -> bool {
357        self.capability.valid_for_output(query_buffer)
358    }
359}
360
361impl<T: Timestamp> ActivateCapability<T> {
362    /// Creates a new activating capability.
363    pub fn new(capability: Capability<T>, address: Rc<[usize]>, activations: Rc<RefCell<Activations>>) -> Self {
364        Self {
365            capability,
366            address,
367            activations,
368        }
369    }
370
371    /// The timestamp associated with this capability.
372    pub fn time(&self) -> &T {
373        self.capability.time()
374    }
375
376    /// Creates a new delayed capability.
377    pub fn delayed(&self, time: &T) -> Self {
378        ActivateCapability {
379            capability: self.capability.delayed(time),
380            address: Rc::clone(&self.address),
381            activations: Rc::clone(&self.activations),
382        }
383    }
384
385    /// Downgrades this capability.
386    pub fn downgrade(&mut self, time: &T) {
387        self.capability.downgrade(time);
388        self.activations.borrow_mut().activate(&self.address);
389    }
390}
391
392impl<T: Timestamp> Drop for ActivateCapability<T> {
393    fn drop(&mut self) {
394        self.activations.borrow_mut().activate(&self.address);
395    }
396}
397
398/// A set of capabilities, for possibly incomparable times.
399#[derive(Clone, Debug)]
400pub struct CapabilitySet<T: Timestamp> {
401    elements: Vec<Capability<T>>,
402}
403
404impl<T: Timestamp> CapabilitySet<T> {
405
406    /// Allocates an empty capability set.
407    pub fn new() -> Self {
408        Self { elements: Vec::new() }
409    }
410
411    /// Allocates an empty capability set with space for `capacity` elements
412    pub fn with_capacity(capacity: usize) -> Self {
413        Self { elements: Vec::with_capacity(capacity) }
414    }
415
416    /// Allocates a capability set containing a single capability.
417    ///
418    /// # Examples
419    /// ```
420    /// use std::collections::HashMap;
421    /// use timely::dataflow::{
422    ///     operators::{ToStream, generic::Operator},
423    ///     channels::pact::Pipeline,
424    /// };
425    /// use timely::dataflow::operators::CapabilitySet;
426    ///
427    /// timely::example(|scope| {
428    ///     vec![()].into_iter().to_stream(scope)
429    ///         .unary_frontier(Pipeline, "example", |default_cap, _info| {
430    ///             let mut cap = CapabilitySet::from_elem(default_cap);
431    ///             move |input, output| {
432    ///                 cap.downgrade(&input.frontier().frontier());
433    ///                 while let Some((time, data)) = input.next() {
434    ///                 }
435    ///                 let a_cap = cap.first();
436    ///                 if let Some(a_cap) = a_cap.as_ref() {
437    ///                     output.session(a_cap).give(());
438    ///                 }
439    ///             }
440    ///         })
441    ///         .container::<Vec<_>>();
442    /// });
443    /// ```
444    pub fn from_elem(cap: Capability<T>) -> Self {
445        Self { elements: vec![cap] }
446    }
447
448    /// Inserts `capability` into the set, discarding redundant capabilities.
449    pub fn insert(&mut self, capability: Capability<T>) {
450        if !self.elements.iter().any(|c| c.less_equal(&capability)) {
451            self.elements.retain(|c| !capability.less_equal(c));
452            self.elements.push(capability);
453        }
454    }
455
456    /// Creates a new capability to send data at `time`.
457    ///
458    /// This method panics if there does not exist a capability in `self.elements` less or equal to `time`.
459    pub fn delayed(&self, time: &T) -> Capability<T> {
460        /// Makes the panic branch cold & outlined to decrease code bloat & give
461        /// the inner function the best chance possible of being inlined with
462        /// minimal code bloat
463        #[cold]
464        #[inline(never)]
465        fn delayed_panic(invalid_time: &dyn Debug) -> ! {
466            // Formatting & panic machinery is relatively expensive in terms of code bloat, so
467            // we outline it
468            panic!(
469                "failed to create a delayed capability, the current set does not \
470                have an element less than or equal to {:?}",
471                invalid_time,
472            )
473        }
474
475        self.try_delayed(time)
476            .unwrap_or_else(|| delayed_panic(time))
477    }
478
479    /// Attempts to create a new capability to send data at `time`.
480    ///
481    /// Returns [`None`] if there does not exist a capability in `self.elements` less or equal to `time`.
482    pub fn try_delayed(&self, time: &T) -> Option<Capability<T>> {
483        self.elements
484            .iter()
485            .find(|capability| capability.time().less_equal(time))
486            .and_then(|capability| capability.try_delayed(time))
487    }
488
489    /// Downgrades the set of capabilities to correspond with the times in `frontier`.
490    ///
491    /// This method panics if any element of `frontier` is not greater or equal to some element of `self.elements`.
492    pub fn downgrade<B, F>(&mut self, frontier: F)
493    where
494        B: borrow::Borrow<T>,
495        F: IntoIterator<Item = B>,
496    {
497        /// Makes the panic branch cold & outlined to decrease code bloat & give
498        /// the inner function the best chance possible of being inlined with
499        /// minimal code bloat
500        #[cold]
501        #[inline(never)]
502        fn downgrade_panic() -> ! {
503            // Formatting & panic machinery is relatively expensive in terms of code bloat, so
504            // we outline it
505            panic!(
506                "Attempted to downgrade a CapabilitySet with a frontier containing an element \
507                that was not beyond an element within the set"
508            )
509        }
510
511        self.try_downgrade(frontier)
512            .unwrap_or_else(|_| downgrade_panic())
513    }
514
515    /// Attempts to downgrade the set of capabilities to correspond with the times in `frontier`.
516    ///
517    /// Returns [`None`] if any element of `frontier` is not greater or equal to some element of `self.elements`.
518    ///
519    /// **Warning**: If an error is returned the capability set may be in an inconsistent state and can easily
520    /// cause logic errors within the program if not properly handled.
521    ///
522    pub fn try_downgrade<B, F>(&mut self, frontier: F) -> Result<(), DowngradeError>
523    where
524        B: borrow::Borrow<T>,
525        F: IntoIterator<Item = B>,
526    {
527        let count = self.elements.len();
528        for time in frontier.into_iter() {
529            let capability = self.try_delayed(time.borrow()).ok_or(DowngradeError(()))?;
530            self.elements.push(capability);
531        }
532        self.elements.drain(..count);
533
534        Ok(())
535    }
536}
537
538impl<T> From<Vec<Capability<T>>> for CapabilitySet<T>
539where
540    T: Timestamp,
541{
542    fn from(capabilities: Vec<Capability<T>>) -> Self {
543        let mut this = Self::with_capacity(capabilities.len());
544        for capability in capabilities {
545            this.insert(capability);
546        }
547
548        this
549    }
550}
551
552impl<T: Timestamp> Default for CapabilitySet<T> {
553    fn default() -> Self {
554        Self::new()
555    }
556}
557
558impl<T: Timestamp> Deref for CapabilitySet<T> {
559    type Target=[Capability<T>];
560
561    fn deref(&self) -> &[Capability<T>] {
562        &self.elements
563    }
564}
timely/dataflow/operators/capability.rs

timely/dataflow/operators/
capability.rs
