Requiring Modules in CouchDB

Thanks to node.js, using the same code on the server and client is easier than ever. What about running application code in the database? If you have a function that is expensive to compute, but always returns the same result for the same input, then you could get a significant speedup by having your database cache the results of the computation ahead of time. This is especially true if you’re using CouchDB, which uses incremental MapReduce to make view computation very efficient.

Our realtime app at Getable is backed by CouchBase, a derivative of CouchDB. CouchBase and CouchDB share the same replication protocol, but differ in several important ways. One such difference is that CouchBase does not support require, while CouchDB does. One major caveat with CouchDB’s require is that you have to push the modules you want up as design documents. Since forcing humans to resolve dependency trees is outlawed in the Geneva Conventions, we need a better way of doing this.

Enter Browserify’s standalone option. We can use it to create a string of code that is easily prepended to the map function in our views. Browserify’s standalone option takes a string, which is the name of the exported module. Critically, the export will be added to the global object if require is not available, which is the case in CouchBase views. Therefore, if you create a browserify bundle with the {standalone: ‘mymodule’} option and prepend that string to your map function, you will now have global.mymodule available for use. The one gotcha is that the global object does not exist in CouchBase views either, so it must be initialized ahead of the bundle.

Create an entry script that just exports the module you want:

// entry.js
module.exports = require(‘mymodule’)  

Then bundle it with the standalone option and initialize global ahead of time:

// bundler.js
var bundle = new browserify({standalone: ‘mymodule’})  
bundle.bundle(function (err, src) {  
  var prelude = ‘var global={};’ + src.toString()

Now, if you have a map function, you can just insert this prelude right after the function header:

function mapFunction (doc) {  
  // prelude goes here!
  emit(doc._id, global.mymodule(doc))

As an implementation note, we have a small script that uses Function.toString() to manage our design documents. It turns our map functions into strings, searches for the use of application logic, and browserifies the appropriate standalone bundle for each function. It’s less prone to failure than manual updates, and makes the experience just a bit more magical.

The “One Year Later” update: we’ve seen vastly improved performance by pushing these standalone bundles up under the lib key.