How does Meteor receive updates to the results of a MongoDB query?

Question

I asked a question a few months ago, to which Meteor seems to have the answer.

Which, if any, of the NoSQL databases can provide stream of *changes* to a query result set?

How does Meteor receive updates to the results of a MongoDB query?

Thanks,

Chris.

Answer 1

You want query.observe() for this. Say you have a Posts collection with a tags field, and you want to get notified when a post with the important tag is added.

http://docs.meteor.com/#observe

// collection of posts that includes array of tags
var Posts = new Meteor.Collection('posts');

// DB cursor to find all posts with 'important' in the tags array.
var cursor = Posts.find({tags: 'important'});

// watch the cursor for changes
var handle = cursor.observe({
  added: function (post) { ... }, // run when post is added
  changed: function (post) { ... } // run when post is changed
  removed: function (post) { ... } // run when post is removed
});

You can run this code on the client, if you want to do something in each browser when a post changes. Or you can run this on the server, if you want to say send an email to the team when an important post is added.

Note that added and removed refer to the query, not the document. If you have an existing post document and run

Posts.update(my_post_id, {$addToSet: {tags: 'important'}});

this will trigger the 'added' callback, since the post is getting added to the query result.

Answer 2

Currently, Meteor really works well with one instance/process. In such case all queries are going through this instance and it can broadcast it back to other clients. Additional, it polls MongoDB every 10s for changes to the database which were done by outside queries. They are plans for 1.0 to improve the scalability and hopefully allow multiple instances to inform each one about changes.

DerbyJS on the other hand is using Redis PubSub.