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What is HAProxy?
HAProxy is a free, open source high availability solution, providing load balancing and proxying for TCP and HTTP-based applications by spreading 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).
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How to use this image
Since no two users of HAProxy are likely to configure it exactly alike, this image does not come with any default configuration.
Please refer to upstream's excellent (and comprehensive) documentation on the subject of configuring HAProxy for your needs.
It is also worth checking out the examples/ directory from upstream.
Note: Many configuration examples propose to put daemon into the global section to run haproxy as daemon. Do not configure this or the Docker container will exit immediately after launching because the haproxy process would go into the background.
Create a Dockerfile
FROM haproxy:1.5
COPY haproxy.cfg /usr/local/etc/haproxy/haproxy.cfg
Build the container
$ docker build -t my-haproxy .
Test the configuration file
$ docker run -it --rm --name haproxy-syntax-check haproxy:1.5 haproxy -c -f /usr/local/etc/haproxy/haproxy.cfg
Run the container
$ docker run -d --name my-running-haproxy my-haproxy
Directly via bind mount
$ docker run -d --name my-running-haproxy -v /path/to/haproxy.cfg:/usr/local/etc/haproxy/haproxy.cfg:ro haproxy:1.5
Reloading config
If you used a bind mount for the config and have edited your haproxy.cfg file, you can use haproxy's graceful reload feature by sending a SIGHUP to the container:
$ docker kill -s HUP my-running-haproxy
The entrypoint script in the image checks for running the command haproxy and replaces it with haproxy-systemd-wrapper from haproxy upstream which takes care of signal handling to do the graceful reload. Under the hood this uses the -sf option of haproxy so "there are two small windows of a few milliseconds each where it is possible that a few connection failures will be noticed during high loads" (see Stopping and restarting HAProxy).