changemate

Change Notification for CouchDB

changemate

Changemate is a change notification service and framework. At present it only supports responding to the _changes feed of a couch database, but will be expanded in time to support other change notification formats.

The couchdb notifier is the first cab off the rank, as it is required for steelmesh. The couchdb notifier hooks into the continuous changes feed for a couchdb instance.

var changemate = require('changemate');
var counter = 0;
var notifier = changemate('<:couch:> http://sidelab.iriscouch.com/seattle_neighbourhood');
 
notifier
  .on('connect', function() {
    console.log('connected');
  })
  .on('change', function(data) {
    console.log('got change id: ' + data.id + ', seq: ' + data.seq + ', counter: ' + (++counter));
  })
  .on('close', function() {
    console.log('disconnected from server');
  });
 

Supported Options:

  • since - the change_sequence to get changes since
  • include_docs - whether or not document fragments should be included with the change data

You can see by running the example above, that the changes for the feed are read from since=0 each time. While this is good for the purpose of the example, it's unlikely to be the desired behaviour in your application.

Changemate does not offer a checkpoint persistence mechanism, but does provide checkpoint information so that you can store that information yourself and use it:

var changemate = require('changemate');
var counter = 0;
var notifier;
var _checkpoint;
 
function _createNotifier() {
  // reset the counter 
  counter = 0;
 
  console.log(_checkpoint);
 
  // create the notifier 
  notifier = changemate(
    '<:couch:> http://sidelab.iriscouch.com/seattle_neighbourhood', // target 
    {}, // options 
    _checkpoint // checkpoint 
  );
 
  notifier.on('change', function(datacheckpoint) {
    // save the checkpoint 
    _checkpoint = checkpoint;
 
    console.log('got change id: ' + data.id + ', seq: ' + data.seq + ', counter: ' + (++counter));
    if (counter >= 100) {
      notifier.close();
      _createNotifier();
    }
  });
}
 
// create the notifier 
_createNotifier();
 

In the example above, we are collecting the checkpoint data that is passed through in the change event. Additionally, we are closing the notifier for every 100 updates, and creating a new notifier with the checkpoint data that has been captured in previous events.

To make the sample above persist across sessions, the checkpoint information could be save to a local JSON file or similar and reloaded the next time the script ran to start from next required update.

Changemate is designed to be a change notification library that supports providing notifications from various sources. There are some excellent (and more mature) libraries that provide similar services for usually just one of the change sources:

Copyright (c) 2014 Damon Oehlman damon.oehlman@gmail.com

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