Perlbal::Manual::LoadBUsercContributed Perl DoPerlbal::Manual::LoadBalancer(3)NAMEPerlbal::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(3)
