Perlbal::Manual::LoadBalancer(3pm)			User Contributed Perl Documentation			Perlbal::Manual::LoadBalancer(3pm)

NAME

       Perlbal::Manual::LoadBalancer - Using Perlbal as a Load Balancer

   VERSION
       Perlbal 1.78.

   DESCRIPTION
       How to configure a Perlbal Load Balancing service.

   READ ME FIRST
       Please read Perlbal::Manual::Configuration first for a better explanation on how to configure Perlbal. This document will make much more
       sense after reading that.

   Using Perlbal as a Load Balancer
       For a better understanding of how to set up Perbal as a Load Balancer, it should be noted that a Load Balancer and a Reverse Proxy can
       often be the same thing; not always, but often.

       A Load Balancer is a server (or device) that balances requests across a number of servers to spread the load. A Reverse Proxy can still do
       this but also have a number of other features.

       Perlbal as a Reverse Proxy provides features such as buffering content, preserving connections to the backend servers, starting connections
       ahead of time and a high priority queue, among others.

       You could almost say that a Load Balancer is a subset of a Reverse Proxy (it's not, but you could).

       When it comes to Perlbal, the Load Balancer is implemented as a Reverse Proxy without all the extra options, and that's why you set the
       role of a Load Balancer to "reverse_proxy":

	   SET role	       = reverse_proxy

       Simple load balancing

       Let's assume you want to configure two machines to serve your website and you want to let Perlbal decide how to balance the requests. For
       the sake of this exercise let's assume you have two servers at:

	   10.0.0.1:80
	   10.0.0.2:80

       And now you want to use these two machines to serve your website at:

	   10.0.0.3:80

       Here's a sample configuration to make this happen:

	   CREATE POOL mywebsite
	       POOL mywebsite ADD 10.0.0.1:80
	       POOL mywebsite ADD 10.0.0.2:80

	   CREATE SERVICE service_mywebsite
	       SET role 	   = reverse_proxy
	       SET pool 	   = mywebsite
	       SET listen	   = 10.0.0.3:80
	   ENABLE service_mywebsite

       The first line defines a pool of machines called "mywebsite". The second and third lines add your two machines to that pool (note that the
       indentation is not mandatory).

       After that you define a service called "service_mywebsite" with the role "reverse_proxy" set to listen on "10.0.0.3:80" and using the pool
       "mywebsite" to serve the requests.

       The last line is what allows you have several services configured in a file even if they are not currently active (a common scenario is to
       configure everything on the file and then enable/disable services on-the-fly as required; see Perlbal::Manual::Management for more
       information on this process).

       The Load Balancing algorithm

       Perlbal uses a highly efficient load balancing algorithm. It is very effective for distributing dynamic web requests among potentially
       heterogeneous hardware.

       First, backend servers must have their MaxClients (for apache, or equivalent) setting tuned to a reasonable limit. If your hardware can run
       20 requests in parallel before running out of CPU, set MaxClients to 20.

       Next, by default Perlbal will distribute requests randomly. Opening a new connection to any available backend, and issuing the request.

       The proper algorithm is able to be used if "verify_backend", "backend_persist", "backend_persist_cache", and "connect_ahead" are enabled.

	   SET persist_backend	     = on
	   SET verify_backend	     = on
	   SET backend_persist_cache = 5
	   SET connect_ahead	     = 2

       In this configuration, Perlbal will only route client requests to backends that it knows are real processes, instead of the OS listen
       queue. It will attempt to reuse pre-verified backends, and will attempt to create slightly more idle connections than it needs in
       preparation of future requests.

       When you put all this together, it becomes less likely that a client will wait for Perlbal to find an available backend. By setting your
       MaxClients properly, backends are able to serve traffic without getting overwhelmed. If no backends are available, Perlbal will queue them
       internally, rather than overload backends.

       You would want to disable "verify_backend" if you are balancing across image servers, or other extremely lightweight requests.

   SEE ALSO
       Perlbal::Manual::Configuration, Perlbal::Manual::FailOver, Perlbal::Manual::Management, Perlbal::Manual::ReverseProxy.

perl v5.14.2							    2011-01-23					Perlbal::Manual::LoadBalancer(3pm)