Explanation of HAProxy Stats Page Backend Queue Values

haproxy

As shown in the image below, I have a HAProxy backend with 2 servers which have the following settings:

maxconn 64 check inter 5s fastinter 2s downinter 2s

My question regards the Queue column. How can I specify the limit of the backend queue? And why does the Backend at the bottom, show the value of 11 when api-1 and api-2 show 22 and 18 respectively? What does the number 11 represent in my case?

Best Answer

Question 1/
As mentioned by Willy in this thread: https://www.mail-archive.com/haproxy@formilux.org/msg21521.html , you can't specify maxqueue for a backend but you can do it for backend's servers.

First let's make it clear about queues: There is a per-backend queue and a per-server queue. Connections with a server persistence go to the server (in question) queue and other connections go to the backend queue.

When a server maxqueue is reached, the HAProxy doc http://cbonte.github.io/haproxy-dconv/1.7/configuration.html#5.2-maxqueue says that

If this limit is reached, next requests will be redispatched to other servers instead of indefinitely waiting to be served. This will break persistence but may allow people to quickly re-log in when the server they try to connect to is dying.

So next requests won't be lost in this case.

It would be different if a supposed backend maxqueue is reached and clients may get errors in return. Thus, To avoid this, there is no backend maxqueue implementation but you still can achieve the same behavior using a rule in the backend like this:

backend foo
      tcp-request content reject if { queue ge 100 }

Question 2/
The column Max Queue tells the Maximum Queue size reached since the last HAProxy reload.
So 11 is the Maximum Queue size reached by your backend. This is different from the Max Queue size of your servers because, as said earlier, backend queue and servers queues are separated.

Related Solutions

Use backups if unavailable (not just down)

The correct way is to add an ACL in the frontend which checks the amount of connections on the server, and then makes a decision based on that.

The config below will check the "monitor_conns" frontend and if there are 500 or more connections, they will be sent to the "backups" backend, otherwise they'll go to the "regular" backend.

Here's an untested example:

frontend monitor_conns
  bind *:80
  acl too_many_conns fe_conn 500
  use_backend backups if too_many_conns
  default_backend regular

backend backups
  ... your config
  server backupsrv 192.168.0.101:80 check port 80 maxconn 1000 inter 1s rise 1 fall 1

backend regular
  ... your config
  server regularsrv 192.168.0.100:80 check port 80 maxconn 500 inter 1s rise 1 fall 1

It's just an example, but it should give you an idea on how to proceed.

Haproxy maxconn meaning

It sets the maximum number of concurrent connections. Each connection lasts as long as that request (or, with keep-alive, that set of requests).

So, yes, if all 10,000 users are making a request at the exact same time, then a maxconn setting of 4000 will limit that.

Additionally, most modern browsers open more than one TCP connection when they have a long list of resources to request - this multiplexing allows for resources to load more quickly in general, but increases the effective number of connections that your load balancer sees.

Best Answer

Related Solutions

Use backups if unavailable (not just down)

Haproxy maxconn meaning

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