metropolis/pkg/event/event.go

// Copyright 2020 The Monogon Project Authors.
//
// SPDX-License-Identifier: Apache-2.0
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//     http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.

// Package event defines and implements Event Values, a mechanism in which
// multiple consumers can watch a value for updates in a reliable way.
//
// Values currently are kept in memory (see: MemoryValue), but a future
// implementation might exist for other storage backends, eg. etcd.
//
// Background and intended use
//
// The Event Value library is intended to be used within Metropolis'
// supervisor-based runnables to communicate state changes to other runnables,
// while permitting both sides to restart if needed. It grew out of multiple
// codebases reimplementing an ad-hoc observer pattern, and from the
// realization that implementing all possible edge cases of such patterns is
// non-trivial and subject to programming errors. As such, it was turned into a
// self-standing library.
//
// Why not just channels?
//
// Plain channels have multiple deficiencies for this usecase:
//  - Strict FIFO behaviour: all values sent to a channel must be received, and
//    historic and newest data must be treated in the same way. This means that
//    a consumer of state changes must process all updates to the value as if
//    they are the newest, and unable to skip rapid updates when a system is
//    slowly settling due to a cascading state change.
//  - Implementation overhead: implementing an observer
//    registration/unregistration pattern is prone to programming bugs,
//    especially for features like always first sending the current state to a
//    new observer.
//  - Strict buffer size: due to their FIFO nature and the possibility of
//    consumers not receiving actively, channels would have to buffer all
//    existing updates, requiring some arbitrary best-guess channel buffer
//    sizing that would still not prevent blocking writes or data loss in a
//    worst case scenario.
//
// Or, in other words: Go channels are a synchronization primitive, not a
// ready-made solution to this problem. The Event Value implementation in fact
// extensively uses Go channels within its implementation as a building block.
//
// Why not just condition variables (sync.Cond)?
//
// Go's condition variable implementation doesn't fully address our needs
// either:
// - No context/canceling support: once a condition is being Wait()ed on,
//   this cannot be interrupted. This is especially painful and unwieldy when
//   dealing with context-heavy code, such as Metropolis.
// - Spartan API: expecting users to plainly use sync.Cond is risky, as the API
//   is fairly low-level.
// - No solution for late consumers: late consumers (ones that missed the value
//   being set by a producer) would still have to implement logic in order to
//   find out such a value, as sync.Cond only supports what amounts to
//   edge-level triggers as part of its Broadcast/Signal system.
//
// It would be possible to implement MemoryValue using a sync.Cond internally,
// but such an implementation would likely be more complex than the current
// implementation based on channels and mutexes, as it would have to work
// around issues like lack of canceling, etc.
//
// Type safety
//
// The Value/Watcher interfaces are, unfortunately, implemented using
// interface{}. There was an attempt to use Go's existing generic types facility
// (interfaces) to solve this problem. However, with Type Parameters likely soon
// appearing in mainline Go, this was not a priority, as that will fully solve
// this problem without requiring mental gymnastics. For now, users of this
// library will have to write some boilerplate code to allow consumers/watchers
// to access the data in a a typesafe manner without assertions. See
// ExampleValue_full for one possible approach to this.
package event

import (
	"context"
)

// A Value is an 'Event Value', some piece of data that can be updated ('Set')
// by Producers and retrieved by Consumers.
type Value interface {
	// Set updates the Value to the given data. It is safe to call this from
	// multiple goroutines, including concurrently.
	//
	// Any time Set is called, any consumers performing a Watch on this Value
	// will be notified with the new data - even if the Set data is the same as
	// the one that was already stored.
	//
	// A Value will initially have no data set. This 'no data' state is seen by
	// consumers by the first .Get() call on the Watcher blocking until data is Set.
	//
	// All updates will be serialized in an arbitrary order - if multiple
	// producers wish to perform concurrent actions to update the Value partially,
	// this should be negotiated and serialized externally by the producers.
	Set(val interface{})

	// Watch retrieves a Watcher that keeps track on the version of the data
	// contained within the Value that was last seen by a consumer. Once a
	// Watcher is retrieved, it can be used to then get the actual data stored
	// within the Value, and to reliably retrieve updates to it without having
	// to poll for changes.
	Watch() Watcher
}

// A Watcher keeps track of the last version of data seen by a consumer for a
// given Value. Each consumer should use an own Watcher instance, and it is not
// safe to use this type concurrently. However, it is safe to move/copy it
// across different goroutines, as long as no two goroutines access it
// simultaneously.
type Watcher interface {
	// Get blocks until a Value's data is available:
	//  - On first use of a Watcher, Get will return the data contained in the
	//    value at the time of calling .Watch(), or block if no data has been
	//    .Set() on it yet. If a value has been Set() since the the initial
	//    creation of the Watch() but before Get() is called for the first
	//    time, the first Get() call will immediately return the new value.
	//  - On subsequent uses of a Watcher, Get will block until the given Value
	//    has been Set with new data. This does not necessarily mean that the
	//    new data is different - consumers should always perform their own
	//    checks on whether the update is relevant to them (ie., the data has
	//    changed in a significant way), unless specified otherwise by a Value
	//    publisher.
	//
	// Get() will always return the current newest data that has been Set() on
	// the Value, and not a full log of historical events. This is geared
	// towards event values where consumers only care about changes to data
	// since last retrieval, not every value that has been Set along the way.
	// Thus, consumers need not make sure that they actively .Get() on a
	// watcher all the times.
	//
	// If the context is canceled before data is available to be returned, the
	// context's error will be returned. However, the Watcher will still need to be
	// Closed, as it is still fully functional after the context has been canceled.
	//
	// Concurrent requests to Get result in an error. The reasoning to return
	// an error instead of attempting to serialize the requests is that any
	// concurrent access from multiple goroutines would cause a desync in the
	// next usage of the Watcher. For example:
	//   1) w.Get() (in G0) and w.Get(G1) start. They both block waiting for an
	//      initial value.
	//   2) v.Set(0)
	//   3) w.Get() in G0 returns 0,
	//   4) v.Set(1)
	//   4) w.Get() in G1 returns 1,
	// This would cause G0 and G1 to become desynchronized between eachother
	// (both have different value data) and subsequent updates will also
	// continue skipping some updates.
	// If multiple goroutines need to access the Value, they should each use
	// their own Watcher.
	Get(context.Context) (interface{}, error)

	// Close must be called if the Watcher is not going to be used anymore -
	// otherwise, a goroutine will leak.
	Close() error
}