The Reliable, High Performance TCP/HTTP Load Balancer
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February, 14th, 2023 : CVE-2023-25725 fixed!
December, 1st, 2022 : HAProxy 2.7.0 release
HAProxy is a free, very fast and reliable reverse-proxy offering high availability, load balancing, and proxying for TCP and HTTP-based applications. It is particularly suited for very high traffic web sites and powers a significant portion of the world's most visited ones. Over the years it has become the de-facto standard opensource load balancer, is now shipped with most mainstream Linux distributions, and is often deployed by default in cloud platforms. Since it does not advertise itself, we only know it's used when the admins report it :-)
The HAProxy core team maintains multiple versions in parallel. Since version 1.8, two major version are emitted every year. The first digit usually indicates a breaking change (config format etc) but in practice rarely changes. The second digit indicates new features. Both constitute a branch. One extra number appears after these digits to indicate the bug fix release.
The core team deploys a lot of efforts backporting fixes to older releases while being extremely careful not to break anything. For this reason, it is really important to stay up to date within one branch, i.e. having the highest possible number on the last digits.
Branches with an even number are called "LTS" (for "long term support") and area maintained for 5 years after their release. During this time they will receive fixes for bugs that are discovered after the release. These branches are aimed at general users who seek extreme stability and do not want to qualify a new version too often but still want to receive fixes.
Branches with an odd number are only called "stable", they're aimed at highly skilled users who prefer to upgrade often to benefit from modern features, and who are also able to roll back in case of problem. These versions are maintained between 12 and 18 months. The duration is short and purposely not strict so that the maintenance cycle is decided with users based on feedback, and so that these versions do not end up in embedded products. It may happen that a few features are backported to these version if there is some reasonable demand and the operation is considered riskless enough.
Everyone used to dealing with production knows that it's difficult to upgrade components in field when one has to plan and advertise upwards of any operation. For this reason, the HAProxy core team doesn't insist on users to upgrade, will not ask someone to switch to a new branch (unless they ask for a feature that is part of that other branch), but will often ask the user to re-check with the latest version of their branch before reporting a problem, because nobody likes to troubleshoot a problem a second time. It's often suggested to use the versions that come with the operating system when it follows the official maintenance cycle, and depending on the expected level of stability or exposure, some users may want to update as soon as an update is available while others may prefer to wait a few weeks to a month to be sure the update is reliable enough for them.
The currently supported versions are :
this test run on AWS ARM-based Graviton2, HAProxy scales very well with threads and was shown to be able to reach 2 million requests/s over SSL and 100 Gbps for forwarded traffic. This is made possible thanks to its event-driven architecture that allows to react extremely quickly to I/O events, its parallelism on SMP machines provided by light multi-threading, a task scheduler that permanently composes between low-latency and high throughput, and generally speaking a permanent quest of resource savings at every single architecture layer. These efforts tend to cost a bit in development time but are immediately valued by users who are able to reduce their number of machines upgrade after upgrade. For the vast majority of common loads, the HAProxy process is simply not noticed, which tends to make its users forget it, sometimes resulting in questions regarding extremely old versions. Please consult this section for more information on the architecture details and some performance test results.
HAProxy is first known for being extremely robust. The core team developers tend to be irritated by certain bugs they fix, but this is because their job is to see them all. Most users report having never ever faced any single crash and claim that HAProxy is the most solid part of their infrastructure. Finding machines with HAProxy processes being up for more than 3 years is not exceptional at all!
All this is not an accident, though. A lot of efforts are made in that direction, to provide excellent observability on what is happening, and an amazing number of protections against bad behaviors. HAProxy is built with many checks for unacceptable situations (impossible conditions, endless loops, etc) that in other products might result in service outages or data corruption, but in HAProxy will immediately result in a crash with a dump of the problem. This rigor pays off since most users have never faced such an issue, thanks to the few who faced them and provided useful reports allowing to fix the problem early.
The development process also encourages quality, with a long term maintenance cycle: versions are maintained for 5 years by the same developers who code the new features. This encourages them to write high quality code and commit messages that correspond to the highest standards. A regression testing suite is used and run along development by all developers and before merging code, as well as after on a wide variety of platforms thanks to the continuous integration (CI) system.
The principle of "eating one's dog's food" applies here as well: haproxy.org runs on the latest development release. This usually helps spot a bug or two per major version before it hits a release. But in addition it maintains a permanent pressure on the development team to release something they're confident in.
The program having been designed from its early age to be extremely conservative on resource usage, a significant number of settings are calculated at startup time and enforce many limits on number of sockets, connections, streams etc, guaranteeing that any processing that was started will complete.
Security is a very important concern when deploying a software load balancer, because it runs at the edge and takes all the dirty traffic. It is possible to harden the OS, to limit the number of open ports and accessible services, but the load balancer itself stays exposed. The unified and non-fantasist coding style aims at avoiding common traps when writing or reviewing code. Some high standards are sought when it comes to dealing with unvalidated data. Non-portable functions and those having unreliable behaviors are avoided or replaced. Input data gets sanitized very early in the lower layers. Resource usage is carefully controlled. Dangling pointers are forbidden in the code via careful release functions. These standards already help eliminate a great deal of uncertainty in the code itself.
Since zero-bug is not reasonable, the product embarks a number of defensive measures, such as chroot, privilege drops, fork prevention, strict protocol validation, checks for impossible states and detailed traces in case of violation detection, etc. All these usually result in an attempt to exploit a real bug in a failure or possibly a crash. These measures have to be purposely disabled by the user using sufficiently evocative commands so that the reason for doing so has to be regularly questioned.
Here you will find a quick access to downloadable contents by type and version. Just click on the desired format to access the content in that format.
Please note that official docs are the pure-text ones and directly come from the project, except for the Lua reference manual that is maintained by Thierry Fournier. The HTML versions are direct translations from the text version automatically performed by Cyril Bonté's excellent documentation converter, dconv. A TeX-oriented variant able to produce PDFs was also created by Pavel Lang for versions 1.4 and 1.5 but it is not maintained anymore.
If you think you don't have the time and skills to setup and maintain a free load balancer, or if you're seeking for commercial support to satisfy your customers or your boss, you have the following options :
Feel free to contact us for any questions or comments :
Some people regularly ask if it is possible to send donations, so I have set up a Paypal account for this. Click here if you want to donate.
An IRC channel for HAProxy has been opened on Libera.Chat: