Example usage of asynquence.gate() to update an array against a redis cache

async.js

// This function takes a contents object with a files array.
// The goal is to cache each file details in redis then replace
// the file's path with a secure, obscured url.

// Finally the contents object is returned with the updated array
// in place of the old one.

// async = require('async')
// uuid = require('node-uuid')
// redis.client = require('node-redis').createClient()

var transform = function( contents, done ) {
	
	// generate a new auth id for the user and store it in redis with a ttl of 1 hour
	
	var authExpiry = 60 * 60 * 1000, // 1 hour
		authId = uuid.v4(), // unique request to ID the user
		authKey = 'file:auth:' + authId;
	
	redis.client.set( authKey, JSON.stringify( {
		userId: req.user.id
	} ) );
	redis.client.expire( authKey, authExpiry );
	
	// loop through the files array, caching each in redis (if it doens't exist)
	// and turning the file path into {authId}/{fileId}

	ASQ()
	.gate.apply(
		null,
		// create an array of segments for the parallel gate()
		contents.files.map(function( file ){
			return function __segment__( done ) {
				// fileIds are cached in redis with the file path as the key
				var lookupKey = 'file:path:' + file.path;
				var sq = ASQ();

				redis.client.get( lookupKey, sq.errfcb() );

				sq.val(function( fileId ){
					if (fileId) {
						// if we found a fileId, it is already in the cache.
						// convert it to a string and use it.
						fileId = fileId.toString();
					} else {
						// if we didn't, create a new id and cache the file details
						fileId = uuid.v4();
						redis.client.set( 'file:id:' + fileId, JSON.stringify(file) );
						redis.client.set( lookupKey, fileId );
					}
					
					// rewrite the path of the file to {authId}/{fileId}
					file.path = authId + '/' + fileId;

					return file;
				})
				.pipe( done );
			};
		})
	)
	.val(function(){
		// `arguments` is an array of all the `file` objects
		contents.files = [].slice.call( arguments );
		done( contents ); // NOTE: not asynquence's `done` :)
	})
	.or( handleError );
}