Jim Cheung

HAProxy

Access Control List (ACL)

In relation to load balancing, ACLs are used to test some condition and perform an action (e.g. select a server, or block a request) based on the test result.

acl url_blog path_beg /blog
## Backend ##

a backend can be defined by:

Frontend

Their definitions are composed of the following components:

Types of Load Balancing

No Load Balancing

Layer 4 Load Balancing

Load balancing this way will forward user traffic based on IP range and port.

Layer 7 Load Balancing

Using layer 7 allows the load balancer to forward requests to different backend servers based on the content of the user's request.

Load Balancing Algorithms

roundrobin

Round Robin selects servers in turns. This is the default algorithm.

leastconn

Selects the server with the least number of connections–it is recommended for longer sessions. Servers in the same backend are also rotated in a round-robin fashion.

source

This selects which server to use based on a hash of the source IP i.e. your user's IP address. This is one method to ensure that a user will connect to the same server.

Sticky Sessions

Some applications require that a user continues to connect to the same backend server. This persistence is achieved through sticky sessions, using the appsession parameter in the backend that requires it.

Health Check

The default health check is to try to establish a TCP connection to the server i.e. it checks if the backend server is listening on the configured IP address and port.

For certain types of backends, like database servers in certain situations, the default health check is insufficient to determine whether a server is still healthy.

Reference

Further Reading