haproxy, loadbalancer, distributedsystems,

Understanding Core Concepts of HAProxy as Load Balancer Solution

Shaukat Mahmood Ahmad Shaukat Mahmood Ahmad Follow Mar 25, 2019 · 3 mins read
Understanding Core Concepts of HAProxy as Load Balancer Solution
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HAProxy(High Availibility Proxy) is an open source TCP / HTTP proxy solution to create highly available systems. HAProxy is widely used open source load balancer.

In following sections we will look at some important concepts of HAProxy.

HAProxy ACL (Access Control List)

HAProxy ACL allows us to define rules like pattern matching to forward or block some traffic to specific servers.


acl url_product path_beg /product

Above ACL will match any urls beginning with /product for example http://www.example.com/product/product1 and http://www.example.com/product/product2. Please refer to HAProxy website for detailed documentation of Using ACLs and pattern extraction.

Understating HAProxy Backend

In context of HAProxy, backend is a list of servers that will ultimately receive requests forwarded by HAProxy. Configuration of HAProxy backend is done in backend section of HAProxy configuration file located at /etc/haproxy/haproxy.cfg.

Following is a sample configuration for HAProxy backend named http-backend.

backend http-backend
   balance roundrobin
   server srv1 srv1.example.com:80 check
   server srv2 srv2.example.com:80 check


  • backend http-backend : declares the name of backend as http-backend.
  • balance roundrobin : Specifies roundrobin as the load balancing algorithm.
  • server srv1 srv1.example.com:80 check : declares srv1.example.com:80 as first backend server to receive the requests forwarded to this http-backend.
  • server srv1 srv1.example.com:80 check : declares srv2.example.com:80 as second backend server to receive the requests forwarded to this http-backend.
  • check : check option tells the HAProxy to do health check on backend server.

Understating HAProxy Frontend

How the incoming requests will be forwarded to backends is defined using frontend configuration section of HAProxy.

Following is a sample configuration for HAProxy frontend named http-frontend.

frontend http-frontend
    bind *:8080
    mode http
    default_backend http-backend


  • frontend http-frontend : declares the name of frontend as http-frontend.
  • bind *:8080 : http-frontend will handle network traffic received port 8080 of any () IP assigned to machine where *HAPoxy is installed.
  • mode http : http-frontend will handle only HTTP traffic.
  • default_backend http-backend : defines the default backend as http-backend** for *http-frontend.

Understating HAProxy Load Balancing Algorithms

HAProxy uses Load Balancing Algorithm to select a backend server for load balancing. The core load balancing algorithms supported by HAProxy are roundrobin, static-rr, leastconn, source, uri, url_param, hdr and rdp-cookie. In following sections I will explain concepts and working of some core load balancing algorithms for HAProxy, for detailed documentation please refer to algorithm. section of HAProxy’s official documentation.

roundrobin algorithm for load balancing

roundrobin load balancing algorithm specifies that each backend server will be used in turn as per their weight. The weight of a server is determined dynamically. According to HAProxy documentation, roundrobin algorithm has a limitation of 4095 servers per backend.

static-rr algorithm for load balancing

Like roundrobin algorithm, static-rr load balancing algorithm also uses backend server in turns. As the name specifies this is a static algorithm, which means server weight will not be determined on the fly. static-rr algorithm does not impose any limit of number of servers by backend.

leastconn algorithm for load balancing

When load balancing using leastconn algorithm, server with least connections will receive the next incoming request.

source algorithm for load balancing

The source IP address is hashed and divided by the total weight of the running servers to designate which server will receive the request.

Using source load balancing algorithm, backend server is selected on the basis of a hash of user’s IP address to make sure that user connect to same server.

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That’s it, hope you enjoyed it. You like this article, have any questions or suggestions please let us know in the comments section.

Thanks and Happy Learning!

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Shaukat Mahmood Ahmad
Written by Shaukat Mahmood Ahmad Follow
Hi, I am Shaukat Mahmood Ahmad, the author of SMA's blog and CTO / Co Founder at wizlinx.com