cluster-workers

This is a simple client/master/worker system for distributing Python jobs in a cluster. It is my third attempt (after clusterfutures and execnet-futures) to build a comfortable system for running huge, parallel jobs in Python.

Architecture

As with something like Hadoop, there are three distinct machine roles in cluster-workers. Workers run jobs; you typically run one of these per core or per machine. The client produces the jobs; it's your main program that you're aiming to offload work from. The master maintains connections to the workers and the client; it is responsible for routing jobs and responses between the other nodes.

It's certainly possible for multiple clients to share the same master, but fairness enforcement is currently pretty simplistic (FIFO). The distinction between the master and the client is primarily to avoid needing to spin up new workers for every new task you want to run. This way, you can allocate nodes to do your work and leave them running while you run various client programs. You can add and remove workers at your leisure whether a client is running or not.

The advantage over clusterfutures is that the cluster management infrastructure is not involved with the inner loop. On a SLURM cluster, for example, each new call would require a new SLURM job. If the cluster failed to start one job, clusterfutures had no way of knowing this and would wait forever for the job to complete. With cluster-workers, SLURM is only involved "offline" and cannot hold up the actual task execution.

Using

Documentation is currently sparse. But take a look at the square.py example in the examples directory for a quick introduction. Here's the gist of how things work:

Start a master process with python -m cw.master.
Start lots of workers with python -m cw.worker [HOST]. Provide the hostname of the master (or omit it if the master is on the same host).
In your client program, start a ClientThread. The constructor takes a callback function and the master hostname (the default is again localhost). Call the submit method to send jobs and wait for callbacks.

There's also a ClusterExecutor class that lets you use Python's concurrent.futures module as a more convenient way to start jobs. This may be untenable, however, if your task has a lot of jobs and a lot of data because of the way futures must persist in memory.

A Consistent Environment

The infrastructure tries to make it as seamless as possible to execute code remotely by replicating your local environment on the remote machine. Specifically:

You can send a closure; captured values will be included in the remote context (thanks to the old PiCloud library).
The code is executed on the worker using the same working directory as the client. (This assumes the remote machines have a shared filesystem.)
The Python search path, sys.path, is extended on the worker to match the client's. (Again, this assumes a shared filesystem.) This makes it possible to, for example, run tasks that use libraries inside of a virtualenv.

Using With SLURM

SLURM is a cluster management system for Linux. This package contains a few niceties for running jobs on a SLURM cluster.

The slurm.py script lets you run SLURM jobs for your master and workers. Just run ./slurm.py -n NWORKERS start to kick off a master job and a bunch of worker jobs.

Then, use cw.slurm.master_host() in your client programs to automatically find the host of the master to connect to. Pass this to the ClientThread constructor.

Using Locally on an SMP

For testing and small jobs, you may want to run a cluster-workers program on a single multiprocessor machine. The included mp.py script works like slurm.py but starts the master and workers on the local machine.

Using with Docker

Slurm mode now has the option to run cluster workers as Docker containers. Use the following command-line arguments to specify how to run:

--docker-image <image/tag>: the name of the docker image to use (if unspecified, docker will not be invoked)
--docker-args "...": arguments to pass to docker run. In particular, you must mount the shared filesystem that cluster-workers uses for logging output.

The Docker image must have python and cluster-workers installed, and must specify the python executable as its "entrypoint". For an example, see this Dockerfile.

Note that when run in a cluster environment, it may take some time for each node to pull the specified docker image the first time, so the worker jobs may take a while to fire up. But subsequent runs should be quite quick.

Author

By Adrian Sampson. Code is made available under the MIT license.

jonasc/cluster-workers