PubSub

jam:pub-sub brings three key features to Meteor apps:

1. Method-based publish / subscribe
2. Change Streams-based publish / subscribe
3. Subscription caching

Important: This package expects that you'll use the promise-based *Async Meteor collection methods introduced in v2.8.1.

Method-based publish / subscribe

Meteor's traditional publish / subscribe is truly wonderful. However, there is a cost to pushing updates reactively to all connected clients – it's resource intensive and will eventually place limits on your ability to scale your app.

One way to reduce the need for the traditional publish / subscribe is to fetch the data via a Meteor Method but there's a big problem here: the data won't be automatically merged into Minimongo and you completely lose Meteor's magical reactivity. Minimongo is great to work with and makes things easy as the source of truth on the client. Without it, you'll need to create your own stores on the client and essentially duplicate Minimongo.

With jam:pub-sub, you use Meteor.publish.once and the same Meteor.subscribe to have the data fetched via a Meteor Method and merged automatically in Minimongo so you can work with it as you're accustomed to. It also automatically preserves reactivity for the user when they make database writes. Note that these writes will not be broadcast in realtime to all connected clients by design but in many cases you might find that you don't need that feature of Meteor's traditional publish / subscribe.

Change Streams-based publish / subscribe

Alpha

With jam:pub-sub and MongoDB Change Streams, you can preserve Meteor's magical reactivity for all clients while opting out of the traditional publish / subscribe and its use of the oplog. Use Meteor.publish.stream instead of using Meteor.publish and subscribe using the same Meteor.subscribe on the client.

Important: Change Streams will work best when the filter you use can be shared. To that end, if you have a publication that includes a userId, this package will filter out that condition when setting up the Change Stream because it will result in too many unique change streams. As an example, lets say you have this publication:

Meteor.publish.stream('todos', function() {
  return Todos.find({
    $or: [
      { isPrivate: false },
      { owner: this.userId }
    ]
  });
});

When this publication is invoked, it will pull all the Todos that match the filter above and then begin watching a Change Stream with this filter:

  { isPrivate: false }

For this particular filter, it should behave as you'd expect so you wouldn't need to make changes. However, if you have complex filters involving the userId, you'll need to be sure that the behavior you expect remains when using .stream. If it's not meeting your needs, you could split the one publication into two publications, using a .stream with a filter than can be shared and a .once for the userId:

Meteor.publish.stream('todos.public', function() {
  return Todos.find({ isPrivate: false });
});

Meteor.publish.once('todos.owned', function() {
  return Todos.find({ owner: this.userId });
});

The downside by splitting into two is it could result in over-fetching but the data will be merged correctly into Minimongo.

Note: In most cases, you'd likely benefit the most from using Meteor.publish.once anywhere you can and using Meteor.publish.stream only when you really need it and with a filter than can be shared.

Note: If you decide to entirely opt-out of using the traditional Meteor.publish, then you'll also want to disable the oplog entirely — add the disable-oplog package with meteor add disable-oplog.

At the moment, this feature is considered in an alpha state. Based on previous Change Streams experiments by the Meteor Community, it seems that using Change Streams as a wholesale replacement for the traditional publish / subscribe could "just work". However, in practice it may be a "Your Mileage May Vary" type of situation depending on the frequency of writes, number of connected clients, how you model your data, and how you set up the cursors inside of Meteor.publish.stream. With that said, if you're interested in this feature, I'd encourage you to try it out and share your findings.

Subscription caching

Normally, when a user moves between routes or components, the subscriptions will be stopped. When a user is navigating back and forth in your app, each time will result in a re-subscribe which means more spinners, a slower experience, and is generally a waste.

By caching your subscriptions, you can create a better user experience for your users. Since the subscription itself is being cached, the data in Minimongo will be updated in the background until the cacheDuration expires for that subscription at which point it will be stopped and the data will be removed from Minimongo as expected.

Usage

Add the package to your app

meteor add jam:pub-sub

Define a Method-based publication

Define a publication using Meteor.publish.once and subscribe just as you do currently. Meteor.publish.once expects you to return a cursor or an array of cursors just like Meteor.publish.

// server
Meteor.publish.once('notes.all', function() {
  return Notes.find();
});

// client
// Since each view layer (Blaze, React, Svelte, Vue, etc) has a different way of using `Tracker.autorun`, I've omitted it for brevity. You'd subscribe just as you do currently in your view layer of choice.
Meteor.subscribe('notes.all')

// work with the Notes collection in Minimongo as you're accustomed to
Notes.find().fetch();

That's it. By using Meteor.publish.once, it will fetch the data initally and automatically merge it into Minimongo. Any database writes to the Notes collection will be sent reactively to the user that made the write.

Important: when naming your publications be sure to include the collection name(s) in it. This is generally common practice and this package relies on that convention. If you don't do this and you're caching the subscription, Minimongo data may be unexpectedly removed or retained when the subscription stops. It's recommended that you follow this convention for all publications including Meteor.publish. Here are some examples of including the collection name in the publication name:

// the name you assign inside Mongo.Collection should be in your publication name(s), in this example 'notes'
const Notes = new Mongo.Collection('notes')

// as long as it appears somewhere in your publication name, you're good to go. here are some examples:
Meteor.publish.once('notes');
Meteor.publish.once('notes.all');
Meteor.publish.once('notes/single');
Meteor.publish.once('somethingAndNotes');

It also works just as you'd expect for an array of cursors:

// server
Meteor.publish.once('notes.todos.all', function() {
  return [Notes.find(), Todos.find()];
});

// client
Meteor.subscribe('notes.todos.all');

// work with the Notes collection in Minimongo as you're accustomed to
Notes.find().fetch();

// work with the Todos collection in Minimongo as you're accusomted to
Todos.find().fetch();

Inside Meteor.publish.once, this.userId and this.added can still be used. The added document will be included in the final result data. The rest of the low-level publish API will be disregarded, as they no longer fit into the context of a Method-based data fetch.

Meteor.publish.once('notes.all', function() {
  // ... //
  const userId = this.userId;

  this.added('notes', _id, fields);
  // ... //
  return Notes.find();
})

Define a Change Streams-based publication

Define a publication using Meteor.publish.stream and subscribe just as you do currently. Meteor.publish.stream expects you to return a cursor or an array of cursors just like Meteor.publish.

// server
Meteor.publish.stream('notes.all', function() {
  return Notes.find();
});

// client
// Since each view layer (Blaze, React, Svelte, Vue, etc) has a different way of using `Tracker.autorun`, I've omitted it for brevity. You'd subscribe just as you do currently in your view layer of choice.
Meteor.subscribe('notes.all')

// work with the Notes collection in Minimongo as you're accustomed to
Notes.find().fetch();

That's it. By using Meteor.publish.stream, any database writes to the Notes collection will be sent reactively to all connected clients just as with Meteor.publish.

Setting the maxPoolSize for Change Streams

maxPoolSize defaults to 100 which may not need adjusting. If you need to adjust it, you can set it in Meteor.settings like this:

{
  //...//
  "packages": {
    "mongo": {
      "options": {
        "maxPoolSize": 200 // or whatever is appropriate for your application
      }
    }
  }
  // ... //
}

Turn on subscription caching

With jam:pub-sub, you can enable subscription caching globally or at a per-subscription level. Subscription caching is turned off by default to preserve the current behavior in Meteor. Any subscription can be cached, regardless of how it's published.

To enable subscription caching globally for every subscription:

// put this in a file that's imported on the client at a minimum. it can be used isomorphically but the configuration only applies to the client.
import { PubSub } from 'meteor/jam:pub-sub';

PubSub.configure({
  cache: true // defaults to false
});

The global cacheDuration is set to 60 seconds by default. This is from when the subscription was originally set to be stopped, i.e. when the component housing the subscription was destroyed because the user navigated away. If the user comes right back, then the cache will be used. If they don't, after 60 seconds, the subscription cache will be removed. If you want to change the global cacheDuration, change it with a value in seconds:

import { PubSub } from 'meteor/jam:pub-sub';

PubSub.configure({
  cacheDuration: 5 * 60 // sets the cacheDuration to 5 minutes. defaults to 1 min
});

You can also configure cache and cacheDuration for each individual subscription when you use Meteor.subscribe. For example:

Meteor.subscribe('todos.single', _id, { cacheDuration: 30 }) // caches for 30 seconds, overriding the global default
Meteor.subscribe('notes.all', { cache: true }) // turns caching on, overriding the global default, and uses the global default cacheDuration

Note: the rest of the Meteor.subscribe API (e.g. onStop, onReady) works just as you'd expect.

Note: Because the data will remain in Minimongo while the subscription is cached, you should be mindful of your Minimongo .find selectors. Be sure to use specific selectors to .find the data you need for that particular subscription. This is generally considered best practice so this is mainly a helpful reminder.

Clearing the cache

Each individual subcription will be automatically removed from the cache when its cacheDuration elapses.

Though it shouldn't be necessary, you can programmatically clear all cached subscriptions:

import { PubSub } from 'meteor/jam:pub-sub';

PubSub.clearCache();