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zc.resumelb 0.7.1

Résumé-based WSGI load balancer

This package provides a load balancer for WSGI applications that sorts requests into request classes and assigns requests of a given class to the same workers.

The load balancer can benefit you if you have an application that:

  • has too much load (or is too slow) to be handled by a single process,
  • has a working set that is too large to fit in the caches used by your process, and
  • there is a way to classify requests so that there is little overlap in the working sets of the various classes.

If what's above applies to you (or if you're curious), read on.

Architecture

An application deployed using the load balancer consistes of one or more load balancers, and multiple workers. Web requests come into the load balancers, are converted to WSGI environments and requests, in environment form, are handed over to workers over long-lived multi-plexed connections.

Workers compute résumés, which are dictionaries mapping request classes to scores, which are average requests per second. Workers send load balancers their résumés periodically, and when load balancers connect to them.

Multiple load balancers can be used for redundancy or load distribution. Résumés are managed by workers to assure that load balancer's have the same information about worker skills.

Status

The current version of the load-balancer should be considered experimental. We're currently testing it in production.

The documentation is a bit thin, but there are extensive doctests.

Request Classification

You need to provide a request-classification function that takes a WSGI environment and returns a request class string.

Two classifiers are built-in:

host
The host classifier uses HTTP Host header values, normalized by removing leading "www." prefixes, if present.
re_classifier

A general classifier (factory) that applies a regular expression with a class group to an environment value.

For example, to use the first step in a request URL path, you'd use the following request-classifier option to one of the load-balancer scripts described below:

-r 'zc.resumelb.lb:re_classifier("PATH_INFO",r"/(?P<class>[^/]+)")'

Deployment

Deploying the load balancer requires deploying each of the workers, and deploying the load balancer(s) itself. The workers are deployed much like any WSGI stack. The workers serve as WSGI servers, even though they don't accept HTTP requests directly.

There are two built-in strategies for deploying applications, depending on whether you're willing to drink some ZooKeeper kool-aid.

If you use ZooKeeper, workers can bind to ephemeral ports and register them with ZooKeeper. The load balancer monitors ZooKeeper and adds and removes workers to it's pool as worker processes are worker processes are started and stopped.

Basic deployment

The basic deployment is the easiest to get up and running quickly.

Basic worker deployment

In the basic deployment, you deploy each worker as you would any WSGI application. A Paste Deployment server runner is provided by the paste.server_runner main entry point. The runner accepts parameters:

use egg:zc.resumelb
This selects the basic worker runner.
address HOST:PORT
The address to listen on, in the form HOST:PORT
history SIZE
Roughly, the number of requests to consider when computing a worker's résumé. This defaults to 9999.
max_skill_age SIZE

The maximum number of requests without a request in a request class before a request class is dropped from a worker's résumé.

If not specified, this defaults to 10 times the history.

threads NTHREADS
If specified with a number greater than zero, then a thread pool of the given size is used to call the underlying WSGI stack.
resume_file PATH
The path to a résumé file. Periodically, the worker's résumé is saved to this file and the file is read on startup to initialize the worker's résumé.
tracelog LOGGER
Request trace logging and specify the name of the Python logger to use.

Basic load-balancer deployment

The load balancer is a program that should be run with a daemonizer, like zdaemon, or supervisor. It get's it's configuration by way of command-line arguments. Run it with -h to get a list of options.

The basic load-balancer is provided by the resumelb script provided by the package.

Basic Example

Here's a sample paste.ini file defining a WSGI stack:

[app:main]
use = egg:bobo
bobo_resources = zc.resumelb.tests

[server:main]
use = egg:zc.resumelb
address = 127.0.0.1:8000

And here's a load-balancer command you'd use with this worker:

resumelb -LINFO -a :8080 127.0.0.1:8000

In this example, the load balancer listens on port 8080 and connects to the worker on port 8000.

ZooKeeper-based deployment

In a ZooKeeper-based deployment, workers register with ZooKeeper and the load balancer gets worker addresses from ZooKeeper. As workers are started and stopped, they're automatically added to and removed from the load-balancer pool. In addition, most configuration parameters are read from ZooKeeper and are updated at run time when they are changed in ZooKeeper. To learn more about ZooKeeper and how to build and maintain a ZooKeeper tree, see http://pypi.python.org/pypi/zc.zk.

ZooKeeoper-based worker deployment

As with the basic deployment, you deploy ZooKeeoper-based workers as servers in a WSGI stack. A Paste Deployment server runner is provided by the paste.server_runner zk entry point. The runner accepts parameters:

use egg:zc.resumelb#zk
This selects the ZooKeeoper-based worker runner.
zookeeper CONNECTION
A ZooKeeoper connection string.
path PATH
The path to a ZooKeeper node where the worker should get configuration and register it's address. The node should have a providers subnode where address is is published.
address HOST:PORT
The address to listen on, in the form HOST:PORT
threads NTHREADS
If specified with a number greater than zero, then a thread pool of the given size is used to call the underlying WSGI stack.
resume_file PATH
The path to a résumé file. Periodically, the worker's résumé is saved to this file and the file is read on startup to initialize the worker's résumé.
tracelog LOGGER
Request trace logging and specify the name of the Python logger to use.

ZooKeeper-based load-balancer deployment

The load balancer is a program that should be run with a daemonizer, like zdaemon, or supervisor. It get's it's configuration by way of command-line arguments. Run it with -h to get a list of options.

ZooKeeper Example

Here's a sample paste.ini file defining a WSGI stack:

[app:main]
use = egg:bobo
bobo_resources = zc.resumelb.tests

[server:main]
use = egg:zc.resumelb#zk
zookeeper = 127.0.0.1:2181
path = /lb/workers

And here's a load-balancer command you'd use with this worker:

zkresumelb -LINFO 127.0.0.1:2181 /lb

The above example assumes you have a ZooKeeper server running on port 2181 and that it includes a tree that looks like:

/lb
  /providers
  /workers
    /providers

See http://pypi.python.org/pypi/zc.zk to learn more about building and maintaining ZooKeeper trees.

Change History

0.7.1 (2012-10-17)

  • Fixed: When used with ZooKeeper, a load balancer could end up with multiple connections to the same worker due to ZooKeeper "flapping". (ZooKeeper might report that workers had gone away and come back without the workers actually going away.)
  • Fixed: When using single-version mode, flapping between versions could cause worker and book backlogs to be computed concorrectly, causing assertion errors.
  • In single-version mode, log version changes.

0.7.0 (2012-07-05)

  • Added support in the load balancer for applications that can't have multiple worker versions. You can upgrade workers gradually. Workers with the new version will be ignored until they're in the majority, at which time the lb will stop using workers with the old version.

0.6.2 (2012-06-15)

  • Fixed: a lack of socket timeout could cause requests to leak.

0.6.0 (2012-05-11)

  • Added a command-line script to fetch lb status data, assuming you're using the ZooKeeper-aware load-balancer script and have requested a status server. (Also updated the status output to show request start times as integer seconds.)

0.5.2 (2012-05-09)

  • Fixed: Temporary files created when buffering data in the load balancers weren't closed explicitly. Generally, they were closed through garbage collection, but in certain situations, their numbers could build quickly, leading to file-descriptor exhaustion.
  • Fixed: Tracelog 'I' records didn't always contain input length information.
  • Fixed: Tracelog 'I' records were only included when using thread pools.

0.5.1 (2012-05-07)

  • Fixed: Worker resume data wasn't initialized correctly when no parameters are passed to the constructor and when reading a resume file, causing resmes not not to update.
  • Fixed: worker errors were written to standard out rather than being logged.
  • Fixed: Poorly-behaved WSGI applications that fail to catch errors caused requests to hang rather than return 500 responses.

0.5.0 (2012-05-03)

  • Changed the way tracelog records are identified to reflect lb request numbers. Records are disambiguated by including an lb identifier as a prefix. For example "1.22" indicated request number 22 from lb 1.
  • When defining workers that register with ZooKeeper, you can now supply a description in the paste.ini file that shows up in ZooKeeper. While the pid alone provides enough information to find a worker, often a description (e.g. instance name or path) can make it easier.

0.4.0 (2012-04-27)

  • Change the load-balancing algorithm to take backlogs of underutilized workers into account to allow a lower variance parameter to be used, which allows new workers to be better utilized.
  • Changed the load-balancing algorithm to try just a little bit harder to keep work with skilled workers by not penalizing workers for their first outstanding request. (In other words, when adjusting worker scrores chacking a maximum backlog, we subtract 1 from the worker's backlog if it's non-zero.
  • The status server provided when using ZooKeeper now listens on a unix-domain socket.
  • The status server provided when using ZooKeeper now includes the start time of the oldest request for each worker, to be used for monitoring.
  • Fixed: Workers buffered large request bodies in memory. Now large request bodies are buffered to disk.
  • Internal optimizations, especially writh regard to handling large request and response bodies.

0.3.0 (2012-03-28)

Changed the way the zkresumelb (load-balancer program that works with ZooKeeper) handles access logs. Now, you pass a Python logging logger name. If you don't pass anything, then nothing will be logged.

0.2.0 (2012-03-27)

  • There's a new API for getting worker résumés, typically from monitoring code:

    >>> import zc.resume.worker
    >>> print zc.resume.worker.get_resume(addr)
    

    This is useful both for getting a worker's résumé and for making sure that the worker is accepting load-balancer connections.

    There's also a scriot version of this:

    bin/get-worker-resume 192.168.24.60:33161
    
  • When using ZooKeeper, you can request an lb status server. The address gets registered with ZooKeeper. When you connect to it, you get back a json string containing the overall lb backlog and addresses and backlogs of each worker.

  • The update settings methods were changed to revert settings to default when not provided. This is especially important when used with ZooKeeper, so you can look at a tree and know what settings are without knowing the change history.

  • Added graceful load-balancer and worker shutdown on SIGTERM.

  • Fixed: trace log request ids weren't assigned correctly when using multiple load balancers.

  • Added packaging meta data to help find gevent 1.0b1 (which is at http://code.google.com/p/gevent/downloads/list)

  • Updated the API for application trace logging to match that of zc.zservertracelog, mainly to get database logging for ZTK applications.

0.1.0 (2012-03-09)

Initial release.

 
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