Deploying to one server can be done with a single script doing a bunch of ssh/scp commands. If you have a few more servers, you can run it in a loop sequentially or fork the processes to do them in parallel. At some point though, it will get unmanageable especially if you have the challenge of updating multiple datacenters at the same time. So how does a company like twitter do their releases ?
Murder is a method of using Bittorrent to distribute files to a large amount of servers within a production environment. This allows for scalable and fast deploys in environments of hundreds to tens of thousands of servers where centralized distribution systems wouldn't otherwise function. A "Murder" is normally used to refer to a flock of crows, which in this case applies to a bunch of servers doing something. In order to do a Murder transfer, there are several components required to be set up beforehand -- many the result of BitTorrent nature of the system. Murder is based on BitTornado. - A torrent tracker. This tracker, started by running the 'murder_tracker.py' script, runs a self-contained server on one machine. Although technically this is still a centralized system (everyone relying on this tracker), the communication between this server and the rest is minimal and normally acceptable. To keep things simple tracker-less distribution (DHT) is currently not supported. The tracker is actually just a mini-httpd that hosts a /announce path which the Bittorrent clients update their state onto. - A seeder. This is the server which has the files that you'd like to deploy onto all other servers. For Twitter, this is the server that did the git diff. The files are placed into a directory that a torrent gets created from. Murder will tgz up the directory and create a .torrent file (a very small file containing basic hash information about the tgz file). This .torrent file lets the peers know what they're downloading. The tracker keeps track of which .torrent files are currently being distributed. Once a Murder transfer is started, the seeder will be the first server many machines go to to get pieces. These pieces will then be distributed in a tree-fashion to the rest of the network, but without necessarily getting the parts from the seeder. - Peers. This is the group of servers (hundreds to tens of thousands) which will be receiving the files and distributing the pieces amongst themselves. Once a peer is done downloading the entire tgz file, it will continue seeding for a while to prevent a hotspot effect on the seeder. MURDER TRANSFER PROCESS ----------------------- 1. Configure the list of servers and general settings in config.rb. (one time) 2. Distribute the Murder files to all your servers: (one time) cap murder:distribute_files 3. Start the tracker: (one time) cap murder:start_tracker 4. Create a torrent file from a remote directory of files (on seeder): cap murder:create_torrent tag="Deploy20100101" files_path="~/files" 5. Start seeding the files: cap murder:start_seeding tag="Deploy20100101" 6. Distribute the files to all servers: cap murder:peer tag="Deploy20100101" destination_path="/tmp/out" Once completed, all files will be in /tmp/out/Deploy20091015/ on all servers.
More info here