Summary:
Buck (our build system) currently starts multiple packager instances for each target and may build multiple targets in parallel. This means we're paying startup costs and are duplicating the work. This enables us to start one instance of the packager and connect to it via socket to do all the work that needs to be done.
The way this is structured:
1. SocketServer: A server that listens on a socket path that is generated based on the server options
2. SocketClient: Interfaces with the server and exposes the operations that we support as methods
3. SocketInterface: Integration point and responsible for forking off the server
Summary:
The cache is only used for JSTransformer at the moment. We're doing IO and some computation to get each module's name, whether is a haste or node module and it's dependencies. This work happens on startup so by caching this value we shouldbe able to reduce the start up time. Lets promote the Cache to the Packager level to be able to use it by any of the components of the packager. For now, on this diff we'll start using it to cache the mentioned fields.
Also we had to introduce the concept of fields in the cache as manually merging the date we had for each path is not possible as we're using promisses all around. With the new API, each field is a promise.
@amasad and I did some manual testing to measure the impact of this change and looks like it's saves 1 second when building the haste map (which represents 50% of the time). Overall this reduces 1 second of start up time which was currently about 8s on my mac book pro.
