Mongoose.js transactions

Question

I know MongoDB doesn't support transactions as relational databases do, but I still wonder how to achieve atomicity for several operations. Hunting around the web, I see people mentioning Transactions without Transactions. Reading through the slides, I am still not clear how to implement that with Mongoose.js.

Take this code snippet for example:

player.save(callback1);
story.save(callback2);

How do I implement callback1 and callback2 so that they either succeed together or fail together?

Answer 1

If you really must have transactions across multiple documents types (in separate collections) the means to achieve this is with a single table that stores actions to take.

db.actions.insert( { actions: [{collection: 'players', _id: 'p1', update: {$set : {name : 'bob'} } },             {collection: 'stories', _id: 's1', update: {$set : {location: 'library'} } }], completed: false }, callback); 

This insert is atomic, and all done at once. You then can perform the commands in the 'actions' collection and mark them as complete or delete them as you complete them, calling your original callback when they are all completed. This only works if your actions processing loop is the only thing updating the db. Of course you'll have to stop using mongoose, but the sooner you do that the better you'll be anyway.

Answer 2

This question is quite old but for anyone who stumbles upon this page, you could use fawn. It's an npm package that solves this exact problem. Disclosure: I wrote it

Say you have two bank accounts, one belongs to John Smith and the other belongs to Broke Individual. You would like to transfer $20 from John Smith to Broke Individual. Assuming all first name and last name pairs are unique, this might look like:

var Fawn = require("fawn");
var task = Fawn.Task()

//assuming "Accounts" is the Accounts collection 
task.update("Accounts", {firstName: "John", lastName: "Smith"}, {$inc: {balance: -20}})
  .update("Accounts", {firstName: "Broke", lastName: "Individual"}, {$inc: {balance: 20}})
  .run()
  .then(function(){
    //update is complete 
  })
  .catch(function(err){
    // Everything has been rolled back. 

    //log the error which caused the failure 
    console.log(err);
  });

Caveat: tasks are currently not isolated(working on that) so, technically, it's possible for two tasks to retrieve and edit the same document just because that's how MongoDB works.

It's really just a generic implementation of the two phase commit example on the tutorial site: https://docs.mongodb.com/manual/tutorial/perform-two-phase-commits/