| 1 | ### Understand the alert |
| 2 | |
| 3 | This alert monitors the average number of failed HAProxy backends over the last 10 seconds. If you receive this alert in a critical state, it means that one or more HAProxy backends are inaccessible or offline. |
| 4 | |
| 5 | HAProxy is a reverse-proxy that provides high availability, load balancing, and proxying for TCP and HTTP-based applications. A backend in HAProxy is a set of servers that receive forwarded requests and are defined in the backend section of the configuration. |
| 6 | |
| 7 | ### Troubleshoot the alert |
| 8 | |
| 9 | - Check the HAProxy configuration file for errors |
| 10 | |
| 11 | Making changes in the configuration file may introduce errors. Always validate the correctness of the configuration file. In most Linux distros, you can run the following check: |
| 12 | |
| 13 | ``` |
| 14 | haproxy -c -f /etc/haproxy/haproxy.cfg |
| 15 | ``` |
| 16 | |
| 17 | - Check the HAProxy service for errors |
| 18 | |
| 19 | 1. Use `journalctl` and inspect the log: |
| 20 | |
| 21 | ``` |
| 22 | journalctl -u haproxy.service --reverse |
| 23 | ``` |
| 24 | |
| 25 | - Check the HAProxy log |
| 26 | |
| 27 | 1. By default, HAProxy logs under `/var/log/haproxy.log`: |
| 28 | |
| 29 | ``` |
| 30 | cat /var/log/haproxy.log | grep 'emerg\|alert\|crit\|err\|warning\|notice' |
| 31 | ``` |
| 32 | |
| 33 | You can also search for log messages with `info` and `debug` tags. |
| 34 | |
| 35 | - Investigate the backend servers |
| 36 | |
| 37 | 1. Verify that the backend servers are online and accepting connections. |
| 38 | 2. Check the backend server logs for any errors or issues. |
| 39 | 3. Ensure that firewall rules or security groups are not blocking traffic from HAProxy to the backend servers. |
| 40 | |
| 41 | - Review the HAProxy load balancing algorithm and configuration |
| 42 | |
| 43 | 1. Analyze the load balancing algorithm used in the configuration to ensure it is suitable for your setup. |
| 44 | 2. Check for any misconfigurations, such as incorrect server addresses, ports, or weights. |
| 45 | |
| 46 | ### Useful resources |
| 47 | |
| 48 | 1. [The Four Essential Sections of an HAProxy Configuration](https://www.haproxy.com/blog/the-four-essential-sections-of-an-haproxy-configuration/) |
| 49 | 2. [HAProxy Explained in DigitalOcean](https://www.digitalocean.com/community/tutorials/an-introduction-to-haproxy-and-load-balancing-concepts) |