How to have a primary and a backup server and switch them?

Help!
1

Hi,

I am in a situation where I have a backend with a main server A and a backup server B.
The intention is to have 100% of requests served by server A, and fallback to the server B if A is failing.

But I’d like to be able to switch the roles of the servers. Server B should them serve all the requests and the server A should become the backup i case of B failing.

I don’t want disable server A in HAproxy for it to use server B. because I still want the fallback to be possible.

I’ve found a weird option : having my 2 different configurations and a symlink to one of them to tell wich one HAProxy should use. But it forces me to reload the configuration each time I want to switch, and to maintain 2 different files.

I’ve search in the API and I’ve found my settings I can change (weight…) but nothing about the “backup” attribute of a server.
Also I didn’t find an algorithme which would do this automatically.

Anyone have an idea to solve this use case?

2

Hi Jeremy

There are a few options available, and you already mentioned a few of them:

  • in the config file, use the backup keyword on the backup server and reload HAProxy
  • play with server weight, this can be adjusted at runtime
  • there is a LB algorithm whose name is first. It picks up the first available server in the backend. Just play with runtime api to trigger some other changes
  • have a single server and use the runtime API to update the server’s IP address
  • you can play with health check or agent check to report a server status that HAProxy can use to decide where to route the traffic
3

Hi Baptiste,

Thank you for your time and answer.

in the config file, use the backup keyword on the backup server and reload HAProxy

That’s already what I’m doing and I want to invert the “main”/“backup” statuses.

play with server weight, this can be adjusted at runtime

If I give 100% to one and 0% to the other, there is no fallback anymore, am I right ?

there is a LB algorithm whose name is first. It picks up the first available server in the backend. Just play
with runtime api to trigger some other changes

That is a lead. Thanks.

have a single server and use the runtime API to update the server’s IP address

I need to have a backup if the main one is faulty

you can play with health check or agent check to report a server status that HAProxy can use to decide where to route the traffic

same answer, I need to have a backup

4

If I give 100% to one and 0% to the other, there is no fallback anymore, am I right ?

yes you are correct. Just one thing, this will be applied to new connections. Established connections will keep on being established on the current server.