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Notifiers and Subscriptions ​

Notifiers and subscriptions both let a service notify clients of state changes. Specifically, both are abstractions for producing and consuming asynchronous value sequences. They rely on promises to deliver a stream of messages allowing many clients to receive notifications without the originator having to track a subscription list. An object wanting to publish updates to interested clients makes a notifier or a subscription available to them.

In JavaScript, async iterations are manipulated by AsyncGenerators, AsyncIterables, and AsyncIterators. For an introduction to them, see Async iteration and generators.

Distributed Asynchronous Iteration ​

An async iteration is an abstract sequence of values. It consists of zero or more non-final values in a fully ordered sequence, revealed asynchronously over time. In other words, the values have a full ordering, and all consumers see the whole sequence, or a subset of it, in the same order.

The sequence may continue indefinitely or terminate in one of two ways:

  • Finish: The async iteration successfully completes and reports any JavaScript value as a final completion.
  • Fail: The async iteration fails and gives a reported final reason. This should be an error object, but can be any JavaScript value.

Finish and Fail are final values. To avoid confusion, for iteration values in this doc, "final" and "non-final" merely refer to position in an iteration, and not "final" in the sense of the Java keyword or similar.

NotifierKit and SubscriptionKit ​

makeNotifierKit() makes an {updater, notifier} pair, while makeSubscriptionKit() makes a similar {publication, subscription} pair. Each pair’s first element (i.e., updater or publication) produces the async iteration which is then consumed using each pair’s second element (i.e., notifier or subscription).

js
import { makeNotifierKit } from '@agoric/notifier';
import { makeSubscriptionKit } from '@agoric/notifier';
const { updater, notifier } = makeNotifierKit();
const { publication, subscription } = makeSubscriptionKit();

The key difference between the two is

  • notifiers are lossy.
    • While the sequence is ordered, the consumer requests only the current value and so may never see any values that happened between requests.
  • subscriptions are lossless.
    • The consumer will see every value in the sequence.

If your consumers only care about more recent states, use makeNotifierKit(). Notifiers are often appropriate when the iteration represents a changing quantity, like a purse balance, and the consumer is updating a UI that doesn't care about any older and stale non-final values. Notifiers only communicate values at the rate they're consumed, bounded by the network round-trip time, and many non-final values are never communicated if the quantity changes quickly. Notifiers' lossy nature enables this optimization.

notifier and subscription both implement the JavaScript AsyncIterable API to consume the iteration. updater and publication implement the IterationObserver API, as defined by Agoric (JavaScript has no standard for producing iterations). For both pairs, IterationObserver only produces the iteration. AsyncIterable consumes the iteration.

NotifierKit ​

A NotifierKit producer produces iteration values with the updater using the IterationObserver API. Its consumers consume iteration values via the notifier using the AsyncIterable API. Each NotifierKit consumer iteration is a lossy sampling subset of the iteration produced by the producer. Different consumers may see different sampling subsets.

The following properties hold for every sampling subset:

  • Any non-final value from the producer may be missing from the sampling subset.
  • Every non-final value in the sampling subset is a non-final value from the producer (e.g., if "7" is in the subset, then it was in the original producer iteration).
  • Every non-final value in the sampling subset appears in producer order (e.g., If the producer sequence is 1, 3, 8, 5, 9, the sampling subset is guaranteed to not be 8, 1, 5.).
  • The sampling subset has the same termination value as the iteration from the producer.

When a new iteration value is available, either it or a later value promptly becomes available on each sampling subset. In other words, if value 'a' is introduced on the producer end followed a few moments later by 'b', then all clients either promptly see 'a', or won't see it but will promptly see a successor, such as 'b'. If a value is added and nothing else follows for a while, then that value must be distributed promptly to the consumers.

SubscriptionKit ​

Use makeSubscriptionKit() for pub-sub operations, where subscribers should see each published value starting when they subscribe. The producer can be described as the publisher and publishes iteration values with the publication using the IterationObserver API. Subscribers consume the published iteration values with the subscription using the AsyncIterable API. Since each published value is sent to all subscribers, makeSubscriptionKit() generally should not be used with rapidly-produced values.

An iteration’s suffix subset is defined by its starting point in the original iteration, which can be a non-final value or a termination. The suffix subset has exactly the original iteration’s members from its starting point to and including its termination. For example, if the original is { 2 5 9 13 Fail } with Fail as the termination, the subset from a starting point of 9 is { 9 13 Fail }.

When a new value becomes available on the original iteration, it promptly becomes available on every suffix subset whose starting point is at or before that value. So if the original is { 2 5 9 13 Fail } and 9 is published, 9 promptly becomes available to any suffix subset with a starting point of 2, 5, or 9. It does not become available to any subset starting at 13 or Fail.

Each subscription is an AsyncIterable that produces any number of AsyncIterators. These AsyncIterators are SubsciptionIterators which also have a subscribe() method. Calling a subscribe() method makes a Subscription whose starting point is that of the SubscriptionIterator's current position.

Methods ​

The updater and publication both have the same three methods:

  • updateState(state)
    • Supplies and sends out a new state to consumers. All active Promises produced by getUpdateSince() are resolved to the next record.
  • finish(finalState)
    • Closes the stream of state changes and supplies a final state value to consumers.
  • fail(reason)
    • Closes the stream of state changes, indicates a failure to finish satisfactorily, and supplies a reason for the failure to consumers. Does not provide a next state. Instead, it causes the Promise to be rejected with the reason, signalling that the monitored object encountered an error condition.

The notifier has an additional method that the subscription does not:

  • getUpdateSince(previousUpdateCount)
    • Returns a promise for the next published value as a { value, updateCount } record, using an optional previousUpdateCount to communicate the last obtained value. value is the new value from the publisher. updateCount is a piece of data for use with getUpdateSince to request notification the next time there's a new value. If the state becomes final (e.g., when a Zoe seat exits), updateCount will be undefined. If there's an error, the promise for the record is rejected and no further values can exist.
    • If getUpdateSince is called without previousUpdateCount, or with any previousUpdateCount other than the most recent one, it returns a promise already resolved to the current state. If called with the most recent updateCount, it returns a promise for the next record, which is resolved on the next state change.

Notifiers and Subscriptions in Zoe ​

Zoe provides updates on the state of seats within a contract. The updates from Zoe indicate changes to seat allocation and seat exits. These are available from E(userSeat).getNotifier() and zcfSeat.getNotifier(), each of which provide a long-lived notifier object associated with a particular seat. ZCFSeats are available within contracts while UserSeats are accessible from the REPL, deploy scripts, and other code outside contracts. There are no equivalent getSubscription() or getUpdater() methods on the seats.

Zoe's updates for an offer show the current allocation that will be paid if the contract completes without further changes.

Individual contracts can use notifiers and subscriptions to provide updates giving current prices or other contract-specific details.

The following methods use or return notifiers.

Examples ​

Subscription Example ​

Let’s look at a subscription example. We have three characters: Paula the publisher, and Alice and Bob the subscribers. While Alice and Bob both consume Paula's published iteration, they use different tools to do so.

First we create a publication/subscription pair with makeSubscriptionKit(). Paula publishes an iteration with the non-final sequence 'a', 'b' and 'done' as its completion value.

js
const { publication, subscription } = makeSubscriptionKit();
// Paula the publisher says
publication.updateState('a');
publication.updateState('b');
publication.finish('done');

Remember, a SubscriptionKit is lossless. It conveys all of an async iteration’s non-final values, as well as the final value.

You can use the JavaScript AsyncIterable API directly, but both the JavaScript for-await-of syntax and the observeIteration adaptor are more convenient. Here, Alice uses the former, and then Bob uses the latter.

Subscriber Alice consumes the iteration using the for-await-of loop. She can see the non-final values and whether the iteration completes or fails. She can see a failure reason, but the for-await-of syntax does not let her see the completion value 'done'. She can write code that only executes after the loop finishes, but the code won’t know what the completion value actually was “done”, “completed”, or something else. This is a limitation of JavaScript's iteration, whether asynchronous or synchronous (as consumed by a for-of loop).

js
const consume = async subscription => {
  try {
    for await (const val of subscription) {
      console.log('non-final-value', val);
    }
    console.log('the iteration finished');
  } catch (reason) {
    console.log('the iteration failed', reason);
  }
};
consume(subscription);
// Eventually prints:
// non-final-value a
// non-final-value b
// the iteration finished

Subscriber Bob consumes using the (observeIteration(asyncIterableP, iterationObserver) adaptor.

js
const observer = harden({
  updateState: val => console.log('non-final-value', val),
  finish: completion => console.log('finished', completion),
  fail: reason => console.log('failed', reason)
});
observeIteration(subscription, observer);
// Eventually prints:
// non-final-value a
// non-final-value b
// finished done

Notifier Example ​

A NotifierKit is a lossy conveyor of non-final values, but does losslessly convey termination. Let's say the subscription example above started with the following instead of makeSubscriptionKit().

js
const { updater, notifier } = makeNotifierKit();

If we then renamed publication to updater and subscription to notifier in the rest of the example, the code would still be correct and work. However, when using a notifier, either Alice or Bob may have missed either or both of the non-final values due to NotifierKit's lossy nature.

Distributed Operation ​

Either makeNotifierKit() or makeSubscriptionKit() can be used in a multicast manner with good distributed systems properties, where there is only one producing site but any number of consuming sites. The producer is not vulnerable to the consumers; they cannot cause the kit to malfunction or prevent the code producing values from making progress. The consumers cannot cause each other to hang or miss values.

For distributed operations, all the iteration values—non-final values, successful completion value, failure reason—must be Passable, which means they're values that can somehow be passed between vats. The rest of this doc assumes all these values are Passable.

The makeNotifierKit() or makeSubscriptionKit() call makes the notifier/updater or publication/subscription pair on the producer's site. As a result, both the iterationObserver and the initial asyncIterable are on the producer's site. If Producer Paula sends Consumer Bob the asyncIterable, Bob receives a possibly remote reference to it. Producers and their Consumers can be remote from each other.

Bob's example code above is still correct if he uses this reference directly, since observeIteration only needs its first argument to be a reference of some sort to an AsyncIterable conveying Passable values. This reference may be a local AsyncIterable, a remote presence of an AsyncIterable, or a local or remote promise for an AsyncIterable. observeIteration only sends it eventual messages using E() and so doesn't care about these differences.

However, Bob’s code is sub-optimal. Its distributed systems properties are not terrible, but Bob does better using getSharableSubscriptionInternals() (provided by SubscriptionKit). This lets Bob make a local AsyncIterable that coordinates better with producer Paula's IterationObserver.

Subscriber Alice's code, located in the Subscription Example section above, is less forgiving. She's using JavaScript's for-await-of loop which requires a local AsyncIterable. It cannot handle a remote reference to an AsyncIterable at Paula's site. Alice has to make an AsyncIterable at her site by using getSharableSubsciptionInternals(). She can replace her call to consume(subscription) with:

js
import { makeSubscription } from '@agoric/notifier';

const localSubscription = makeSubscription(
  E(subscription).getSharableSubsciptionInternals()
);
consume(localSubscription);

The code above uses a SubscriptionKit. NotifierKits have a similar pair of a getSharableNotifierInternals() method and a makeNotifier. However, this technique requires that Alice know what kind of a possibly remote AsyncIterable she has, and she must have the required making function code locally available.

Alternatively, Alice can generically mirror any possibly remote AsyncIterable by making a new local pair and plugging them together with observeIteration.

js
const { publication: adapterPublication, subscription: adapterSubscription } =
  makeSubscriptionKit();
observeIteration(subscription, adapterPublication);
consume(adapterSubscription);

This works when subscription is a reference to a AsyncIterable. If Alice only needs to consume in a lossy manner, she can use the makeNotifierKit() method instead, which still works independently of what kind of AsyncIterable subscription is a reference to.

Summary ​

Data producers have to decide between lossless and lossy publication. If consumers only care about the most recent state, then use makeNotifierKit(). This is often appropriate when the iteration represents a changing quantity. If you want to support consumers that need to see all values, then use makeSubscriptionKit().

Consumers can choose different ways of processing the data. In all cases, the publisher doesn't need to know the consumers, and the consumers can't interfere with the producer or each other.

© 2024 Agoric Systems Operating Company. All Rights Reserved.