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                  Copyright 2004, Danga Interactive, Inc.
                   Copyright 2005-2010, Six Apart, Ltd.

  You can use and redistribute Perlbal under the same terms as Perl itself.



If you have CPAN installed you can install Perlbal from the command line:

    $ cpan Perlbal

See Perlbal::Manual::Install for further information on installing Perlbal,
including instructions for specific operating systems and some
troubleshooting (the file lives under lib/Perlbal/Manual/Install.pod, it is
recommended that you read it using perldoc).


Perlbal is a Perl-based reverse proxy load balancer and web server.

It processes hundreds of millions of requests a day just for LiveJournal,
TypePad and dozens of other high-traffic websites.

Perlbal is a single-threaded event-based server supporting HTTP load
balancing, web serving, and a mix of the two (see below).

Almost everything in Perlbal can be configured or reconfigured on the fly
without needing to restart the software (see Perlbal::Manual::Management).

In this file you'll find:



Perlbal has many features; this is just a short list of some of them:

Role: Reverse Proxy

    * Maintains pool of connected backend connections to reduce turnover

    * Gets list of nodes either from asynchronously monitored node file, or
        from in-server pool objects which you can add/remove nodes from
        using the management interface

    * Intelligent load balancing based on what backend connections are free
        for a new request. No unreliable "weighting" numbers required

    * Can verify (using a quick OPTIONS request) that a backend connection is
        talking to a webserver and not just the kernel's listen queue before
        sending client requests at it. Lower latency for the client

    * Has a high priority queue for sending requests through to backends quickly

          o Uses cookies to determine if a request should go to fast queue

          o Highpri (high priority) plugin supports making requests high
            priority by URI or Host

          o Can specify a relief level to let low priority requests through to
            prevent starvation

    * Can allow X-Forwarded-For (and similar) headers from client based on
        client IP

    * Configurable header management before sending request to backend

    * Internal redirection to file or URL(s)

          o Big one for us; a backend can instruct Perlbal to fetch the user's
            data from a completely separate server and port and URL, 100%
            transparent to the user

          o Can actually give Perlbal a list of URLs to try. Perlbal will find
            one that's alive. Again, the end user sees no redirects happening

          o Can also redirect to a local file, which Perlbal will serve
            non-blocking. See webserver mode below

    * Persistent client connections (configurable)

    * Persistent backend connections (shared by multiple clients;
        no "backend waste") 

Role: Web Server

    * Listen on a port, share from a directory

    * Directory indexing

    * Byte range support (clients can resume downloads)

    * Can have directory index requests fall back to index file list

          o I.e., requests for /foo/ go to /foo/index.html instead

          o Multiple index files supported, tries one at a time until it finds

    * Persistent client connections (configurable)

    * Almost all disk operations are done asynchronously as to not stall the
    event loop

    * Configurable support for storing files (PUT, DELETE support)


    * Great performance "out-of-the-box" (for both small and large sites)

    * 100% asynchronous in all the recommended use cases

    * Lightweight

    * HTTP Header processing (optionally) done in C with
        Perlbal::XS::HTTPHeaders for maximum performance

    * Event-based using epoll or kqueue to avoid the scalability problems of
        not-so-modern systems


Perlbal's management interface provides extremely detailed and powerful
statistics in addition to runtime configuration. For example:

    * CPU usage (user, system)

    * Total requests served across all services

    * Requests service by individual backends

    * Perlbal's uptime

    * All connected sockets (and tons of info about each)

    * Outstanding connections to backends

    * Backends that have recently failed verification

    * Pending backend connections by service

    * Total of all socket states by socket type

    * Size (in seconds and number of connections) of all queues

    * State of reproxy engine (queued requests, outstanding requests,

    * Loaded plugins per service

(All statistics are in machine readable form, easy to parse and write scripts
that check on the status of Perlbal)


Perlbal supports the concept of having per-service (and global) plugins that
can add functionality or override many parts of request handling and behavior.
There are many custom plugins that send new headers to the backends, promote
requests to the fast queue, maintain more detailed statistics, do image
header manipulation, and more.

Writing your own plugins is also easy.

For more information on how plugins work, and a list of known plugins see Perlbal::Manual::Plugins. You may also find them easily on CPAN.


Perlbal's documentation is split into several sections under

Perlbal::Manual provides the index for the manual:

    perldoc Perlbal::Manual

Individual sections can be viewed in the same manner:

    perldoc Perlbal::Manual::Configuration
    perldoc Perlbal::Manual::LoadBalancer
    perldoc Perlbal::Manual::Plugins

If you're interested in the internals of the Perlbal:

    perldoc Perlbal::Manual::Internals

The documentation is relatively new (December 2010) and was mostly written
or gathered by Bruno Martins and José Castro under a TPF grant. You can read
more about it at and


Feel free to ask us questions on the mailing list:

There are also the old Perlbal List Archives for postings until June 2008:


You may find information on how to contribute under

The source code currently resides in


Perlbal was originally written by Brad Fitzpatrick and counts with the help
and contributions from many other people.

See Perlbal::Manual::Credits for details.


Copyright 2004, Danga Interactive, Inc. Copyright 2005-2010, Six Apart, Ltd.

You can use and redistribute Perlbal under the same terms as Perl itself.


Perl HTTP Load Balancer






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