publication-client
v3.2.4
Published
A client for a publication-server
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publication-client
This module provides a client side module for subscribing to publications from a publication-server.
Installation
$ npm install publication-client
Usage
See usage examples.
Requirements
publication-client
is an ES6 module, so you most likely want to use a bundler
such as rollup. It also depends on
underscore
, so make sure that the project using publication-client
can
resolve underscore
, this normally means either:
npm install
ing the module and using rollup-plugin-node-resolve- if you already have underscore somewhere else locally in your build, you can make an alias to is using rollup-plugin-alias
Initialization
The only required field for creating a client is the host to connect to as a
URL. This means that if you have the publication server mounted at
wss://sub.domain.tld/websocket
we would provide https://sub.domain.tld
as
the URL param. Also note that you can provide query params to be passed as
part of the provided connect URL (i.e. https://sub.domain.tld?foo=bar
).
import PublicationClient from 'publication-client';
var client = new PublicationClient(`https://testing.domain.com?foo=bar`);
Checking connection state
The publication client emits a connected
event once it has successfully
connected.
client.once('connected', () => {
console.log('connected successfully!');
});
It also emits disconnected
event as soon as it becomes disconnected, and
will emit a connected
event if it is able to successfully connect to the
publication server again.
client.on('disconnected', () => {
console.log('Oh no! Our connection is gone!');
client.once('connected', () => {
console.log('Phew! We have a new connection again!');
});
});
Subscribing
To subscribe to a publication, simply provide the publication name and any parameters for it. The subscription will not begin until the connection has been successfully connected.
client.subscribe('hello', {
actor: 'world'
});
Subscriptions also can be queried for 'readiness'. They can return a Promise
via the whenReady
function, and also emit a ready
event.
client.subscribe('foo').whenReady().then(() => {
console.log('subscription is ready!');
});
Likewise, if a subscription fails during initialization, the Promise returned
from whenReady
will reject with the responsible error and a nosub
event
will also be emitted.
client.subscribe('noSuchSub').whenReady().catch((err) => {
console.log(`Subscription failed with err: ${err.message}`);
});
Waiting on initial subscriptions to load
whenReady
is provided as a convenience function if you want to be able to
wait for a single subscription or for multiple subscriptions to complete
before performing some action. For example:
var sub0 = client.subscribe('sub0'),
sub1 = client.subscribe('sub1'),
sub2 = client.subscribe('sub2');
Promise.all(_.invoke([sub0, sub1, sub2], 'whenReady')).then(() => {
console.log('Our initial subscriptions are all ready!');
});
Subscription "readiness" after disconnection
When the client reconnects to the server after being disconnected, the
subscription will reset - meaning that whenReady
can be queried once again
to know when the subscription is ready.
The reason we do this is to allow a consumer to know about the updated state of a subscription after a connection has been broken/down for a long period of time (i.e. a user closed their laptop and re-opened it later). This is necessary because if a connection is down for a significant period of time, we could have missed a very large number of additions, changes and removals to any local collections that our subscription populates. As such, all local collections are cleared of any documents upon reconnection (so that we can ensure that they are then updated with the appropriate state). Another benefit of this approach is that any pre-exising reactive queries on local collection do not need to be re-created, they will still function as desired.
Note that similarly to how a subscription emits ready
when it is ready, after
the client has been disconnected and a subscription is ready after
re-subscription, it will again emit a ready
event.
Subscription errors after initialization
After a subscription has been initialized, errors may still occur server side.
The subscription may be listened to in order to receive these events which are
emitted as nosub
events (the naming comes from Meteor which this publication
system is based off of, see this documentation for more detail).
let helloSub = client.subscribe('hello');
helloSub.on('nosub', (err) => {
console.log(`the subscription experienced an error: ${err.message}`);
});
Querying collections
The publication client also manages all collections created by the subscribed publications. Retrieving a collection is as simple as:
var collection = client.getCollection('baz');
We can then find
documents of interest and listen for changes that we're
interested in. To find documents and retrieve them:
var docs = client.getCollection('baz').find({
_id: 'hello'
}).fetch();
Note that currently find only supports direct matching (as in the example above) and the $elemMatch operator for matchin objects inside of arrays.
To listen for changes that match a provided query:
client.getCollection('baz').find({
_id: 'foo'
}).on('added', (id, fields) => {
// `fields` contains all fields of the document excluding the `_id` field.
console.log(`added a new document: ${Object.assign({}, {id}, fields)}`);
}).on('changed', (id, changes) => {
// `changes` contains the changed fields with their new values.
// If a field was removed from the document then it will be present in `changes`
// with a value of `undefined`.
console.log(`document with id ${id} has changes: ${changes}`);
}).on('removed', (id) => {
console.log(`removed document with id: ${id}`);
});
Note that the handlers for these events are the same as those for Meteor's Mongo.Cursor.observeChanges (specifically the added, changed and removed events).
Closing the connection
If for whatever need, you need to close the publication connection, simply use
the stop()
method. Note that once you do this you, you'll need to recreate an
entirely new publication-client to reconnect.
client.stop();