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What is HAProxy

HAProxy is free, open source software that provides a high availability load balancer and proxy server for TCP and HTTP-based applications that spreads requests across multiple servers. It is written in C and has a reputation for being fast and efficient (in terms of processor and memory usage)

HAProxy can also be setup with varnish for ssl termination to allow https connections to use an http backed with varnish. This allows your sites to be faster while being secure.

HAProxy is used by a number of high-profile websites including GoDaddy, GitHub, Bitbucket, Stack Overflow, Reddit, Speedtest.net, Tumblr, Twitter and Tuenti.

Interesting Fact about HAProxy

Servers equipped with a dual-core Opteron or Xeon processor generally achieve between 15000 and 40000 hits per second, and have no trouble saturating a 2 Gbit/sec connection under Linux.

HAProxy Terminology

There are many terms and concepts that are important when discussing load balancing and proxying. We will go over commonly used terms in the following sub-sections.

Before we get into the basic types of load balancing, we will talk about ACLs, backends, and frontends.

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. Use of ACLs allows flexible network traffic forwarding based on a variety of factors like pattern-matching and the number of connections to a backend, for example.

Example of an ACL:

acl url_blog path_beg /blog

This ACL is matched if the path of a user's request begins with /blog. This would match a request of http://yourdomain.com/blog/blog-entry-1, for example.

For a detailed guide on ACL usage, check out the HAProxy Configuration Manual.


A backend is a set of servers that receives forwarded requests. Backends are defined in the backend section of the HAProxy configuration. In its most basic form, a backend can be defined by:

  1. which load balance algorithm to use
  2. a list of servers and ports

A backend can contain one or many servers in it--generally speaking, adding more servers to your backend will increase your potential load capacity by spreading the load over multiple servers. Increase reliability is also achieved through this manner, in case some of your backend servers become unavailable.

Here is an example of a two backend configuration, web-backend and blog-backend with two web servers in each, listening on port 80:

backend web-backend
   balance roundrobin
   server web1 web1.yourdomain.com:80 check
   server web2 web2.yourdomain.com:80 check
backend blog-backend
   balance roundrobin
   mode http
   server blog1 blog1.yourdomain.com:80 check
   server blog1 blog1.yourdomain.com:80 check

balance roundrobin line specifies the load balancing algorithm, which is detailed in the Load Balancing Algorithms section.

mode http specifies that layer 7 proxying will be used, which is explained in Types of Load Balancing section.

The check option at the end of the server directives specifies that health checks should be performed on those backend servers.


A frontend defines how requests should be forwarded to backends. Frontends are defined in the frontend section of the HAProxy configuration. Their definitions are composed of the following components:

  1. a set of IP addresses and a port (e.g., *:443, etc.)
  2. ACLs
  3. use_backend rules, which define which backends to use depending on which ACL conditions are matched, and/or a default_backend rule that handles every other case

A frontend can be configured to various types of network traffic, as explained in the next section.

Types of Load Balancing

Now that we have an understanding of the basic components that are used in load balancing, let's get into the basic types of load balancing.

No Load Balancing

In this example, the user connects directly to your web server, at yourdomain.com and there is no load balancing. If your single web server goes down, the user will no longer be able to access your web server. Additionally, if many users are trying to access your server simultaneously and it is unable to handle the load, they may have a slow experience or they may not be able to connect at all.

Layer 4 Load Balancing

The simplest way to load balance network traffic to multiple servers is to use layer 4 (transport layer) load balancing. Load balancing this way will forward user traffic based on IP range and port (i.e. if a request comes in for http://yourdomain.com/anything, the traffic will be forwarded to the backend that handles all the requests for yourdomain.com on port 80).

The user accesses the load balancer, which forwards the user's request to the web-backend group of backend servers. Whichever backend server is selected will respond directly to the user's request. Generally, all of the servers in the web-backend should be serving identical content--otherwise the user might receive inconsistent content. Note that both web servers connect to the same database server.

Layer 7 Load Balancing

Another, more complex way to load balance network traffic is to use layer 7 (application layer) 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. This mode of load balancing allows you to run multiple web application servers under the same domain and port.

In this example, if a user requests yourdomain.com/blog, they are forwarded to the blog backend, which is a set of servers that run a blog application. Other requests are forwarded to web-backend, which might be running another application. Both backends use the same database server, in this example.

A snippet of the example frontend configuration would look like this:

frontend http
  bind *:80
  mode http

  acl url_blog path_beg /blog
  use_backend blog-backend if url_blog

  default_backend web-backend

This configures a frontend named http, which handles all incoming traffic on port 80.

acl url_blog path_beg /blog matches a request if the path of the user's request begins with /blog.

use_backend blog-backend if url_blog uses the ACL to proxy the traffic to blog-backend.

default_backend web-backend specifies that all other traffic will be forwarded to web-backend.

Load Balancing Algorithms

The load balancing algorithm that is used determines which server, in a backend, will be selected when load balancing. HAProxy offers several options for algorithms. In addition to the load balancing algorithm, servers can be assigned a weight parameter to manipulate how frequently the server is selected, compared to other servers.

Because HAProxy provides so many load balancing algorithms, we will only describe a few of them here. See the HAProxy Configuration Manual for a complete list of algorithms.

A few of the commonly used algorithms are as follows:


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


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.


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

HAProxy uses health checks to determine if a backend server is available to process requests. This avoids having to manually remove a server from the backend if it becomes unavailable. 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.

If a server fails a health check, and therefore is unable to serve requests, it is automatically disabled in the backend i.e. traffic will not be forwarded to it until it becomes healthy again. If all servers in a backend fail, the service will become unavailable until at least one of those backend servers becomes healthy again.

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.

Lastest Version HAProxy

HAProxy 1.9 Has Arrived

HAProxy Technologies is proud to announce the release of HAProxy 1.9. This release brings a native HTTP representation (HTX) powering end-to-end HTTP/2 support and paving the way for future innovations such as HTTP/3 (QUIC). It also contains improvements to buffers and connection management including connection pooling to backends, threading optimizations, updates to the Runtime API, and much more.