murder

Murder by Larry Gadea [e-mail] and Matt Freels [e-mail]

Copyright 2010 Twitter Inc.

DESCRIPTION

Murder is a method of using Bittorrent to distribute files to a large amount of servers within a production environment. This allows for scaleable 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.

For an intro video, see: Twitter - Murder Bittorrent Deploy System

QUICK START

For the impatient, gem install murder and add these lines to your Capfile:

require 'murder'

set :deploy_via, :murder
after 'deploy:setup', 'murder:distribute_files'
before 'murder:start_seeding', 'murder:start_tracker'
after 'murder:stop_seeding', 'murder:stop_tracker'

HOW IT WORKS

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. 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.

CONFIGURATION AND USAGE

Murder integrates with Capistrano. The most simple way to use it is as a deploy strategy, by setting :deploy_via to :murder. By default, murder makes the same assumptions that cap makes. All servers without :no_release => true will act as peers. Additionally, murder will automatically use the first peer as both tracker and seeder. you may redefine the tracker, seeder and peer roles yourself to change these defaults, for instance, if you want to set up a dedicated tracker.

All involved servers must have python installed and the related murder support files (BitTornado, etc.). To upload the support files to the tracker, seeder, and peers, run:

cap murder:distribute_files

By default, these will go in shared/murder in your apps deploy directory. Override this by setting the variable remote_murder_path. For convenience, you can add an after hook to run this on deploy:setup:

after 'deploy:setup', 'murder:distribute_files'

Before deploying, you must start the tracker:

cap murder:start_tracker

To have this happen automatically during a deploy, add the following hooks:

before 'murder:start_seeding', 'murder:start_tracker'
after 'murder:stop_seeding', 'murder:stop_tracker'

At this point you should be able to deploy normally:

cap deploy

MANUAL USAGE (murder without a deploy strategy)

Murder can also be used as a general mechanism to distribute files across a generic set of servers. To do so create a Capfile, require murder, and manually define roles:

require 'rubygems'
require 'murder'

set :remote_murder_path, '/opt/local/murder' # or some other directory

role :peer, 'host1', 'host2', 'host3', 'host4', 'host5', host6', host7'
role :seeder, 'host1'
role :tracker, 'host1'

To distribute a directory of files, first make sure that murder is set up on all hosts, then manually run the murder cap tasks:

1. Start the tracker:

    cap murder:start_tracker
   

2. Create a torrent from a directory of files on the seeder, and start seeding:

    scp -r ./files host1:~/files
    cap murder:create_torrent tag="Deploy1" files_path="~/files"
    cap murder:start_seeding tag="Deploy1"
   

3. Distribute the torrent to all peers:

    cap murder:peer tag="Deploy1" destination_path="/tmp"
   

4. Stop the seeder and tracker:

    cap murder:stop_seeding tag="Deploy1"
    cap murder:stop_tracker
   

When this finishes, all peers will have the files in /tmp/Deploy1

TASK REFERENCE

distribute_files: SCPs a compressed version of all files from ./dist (the python Bittorrent library and custom scripts) to all server. The entire directory is sent, regardless of the role of each individual server. The path on the server is specified by remote_murder_path and will be cleared prior to transferring files over.

start_tracker: Starts the Bittorrent tracker (essentially a mini-web-server) listening on port 8998.

stop_tracker: If the Bittorrent tracker is running, this will kill the process. Note that if it is not running you will receive an error.

create_torrent: Compresses the directory specified by the passed-in argument 'files_path' and creates a .torrent file identified by the 'tag' argument. Be sure to use the same 'tag' value with any following commands. Any .git directories will be skipped. Once completed, the .torrent will be downloaded to your local /tmp/TAG.tgz.torrent.

download_torrent: Although not necessary to run, if the file from create_torrent was lost, you can redownload it from the seeder using this task. You must specify a valid 'tag' argument.

start_seeding: Will cause the seeder machine to connect to the tracker and start seeding. The ip address returned by the 'host' bash command will be announced to the tracker. The server will not stop seeding until the stop_seeding task is called. You must specify a valid 'tag' argument (which identifies the .torrent in /tmp to use)

stop_seeding: If the seeder is currently seeding, this will kill the process. Note that if it is not running, you will receive an error. If a peer was downloading from this seed, the peer will find another host to receive any remaining data. You must specify a valid 'tag' argument.

stop_all_seeding: Identical to stop_seeding, except this will kill all seeding processes. No 'tag' argument is needed.

peer: Instructs all the peer servers to connect to the tracker and start download and spreading pieces and files amongst themselves. You must specify a valid 'tag' argument. Once the download is complete on a server, that server will fork the download process and seed for 30 seconds while returning control to Capistrano. Cap will then extract the files to the passed in 'destination_path' argument to destination_path/TAG/*. To not create this tag named directory, pass in the 'no_tag_directory=1' argument. If the directory is empty, this command will fail. To clean it, pass in the 'unsafe_please_delete=1' argument. The compressed tgz in /tmp is never removed. When this task completes, all files have been transferred and moved into the requested directory.

stop_all_peering: Sometimes peers can go on forever (usually because of an error). This command will forcibly kill all "murder_client.py peer" commands that are running.

CONFIG REFERENCE

Variables

default_tag: A tag name to use by default such that a tag parameter doesn't need to be manually entered on every task. Not recommended to be used since files will be overwritten.

default_seeder_files_path: A path on the seeder's file system where the files to be distributed are stored.

default_destination_path: A path on the peers' file system where the files that were distributed should be decompressed into.

remote_murder_path: A path where murder will look for its support files on each host. cap murder:distribute_files will upload murder support files here.

Roles

tracker: Host on which to run the BitTorrent tracker

seeder: Host which will be the source of the files to be distributed via BitTorrent

peers: All hosts to which files should be distributed