This article explains how you can manage Varnish backends from command line, check varnish backend health status and mark backends as healthy or sick without restarting or reloading Varnish.
Check Varnish backend health status
To see all backends, there are 2 methods: a debug output and a normalized output.
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$ varnishadm -S /etc/varnish/secret -T localhost:6082 backend.list Backend name Refs Admin Probe backend1(127.0.0.1,,80) 1 probe Sick 0/4 fallback(172.16.80.5,,80) 12 probe Healthy (no probe) $ varnishadm -S /etc/varnish/secret -T localhost:6082 debug.health Backend backend1 is Sick Current states good: 0 threshold: 2 window: 4 Average responsetime of good probes: 0.000000 Oldest Newest ================================================================ ---------------------------------------------------------------- Happy |
The backend.list shows all backends, even those without a probe (= healtcheck) configured.
The debug.health command will show in-depth statistics on the varnish probes that are being executed, including the IPv4 connect state, whether a send/receive has worked and if the response code was HTTP/200.
For instance, a healthy backend will be shown like this, with each state of the check (IPv4, send, receive & HTTP response code) on a seperate line.
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$ varnishadm -S /etc/varnish/secret -T localhost:6082 debug.health Backend backend1 is Healthy Current states good: 5 threshold: 4 window: 5 Average responsetime of good probes: 0.014626 Oldest Newest ================================================================ 4444444444444444444444444444444444444444444444444444444444444444 Good IPv4 XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX Good Xmit RRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRRR Good Recv HHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHH Happy |
Now, to change backend statuses.
Mark a varnish backend as healthy or sick
In order to mark a particular backend as sick or healthy, thus overriding the probe, you can do so like this.
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$ varnishadm -S /etc/varnish/secret -T localhost:6082 backend.set_health backend1 healthy |
The above command will mark the backend named backend1 as healthy. Likewise, you can mark a backend as sick to prevent it from getting traffic.
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$ varnishadm -S /etc/varnish/secret -T localhost:6082 backend.set_health backend1 sick |
If you have multiple Varnish backends and they’re configured in a director to load balance traffic, all traffic should gracefully be sent to the other backend(s). (see the examples in mattiasgeniar/varnish-4.0-configuration-templates)
If you mark a backend explicitly as sick, the backend.list output changes and the admin column removes the ‘probe’ and marks it as ‘sick’ explicitly, indicating it was changed via CLI.
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$ varnishadm -S /etc/varnish/secret -T localhost:6082 backend.list Backend name Refs Admin Probe backend1(127.0.0.1,,80) 1 sick Sick 0/4 fallback(172.16.80.5,,80) 12 probe Healthy (no probe) |
You can also change it back to let Varnish decide the backend health.
Mark the backend as ‘varnish managed’, let probes decide the health
To let Varnish decide the health itself, by using it probes, mark the backend to be auto again:
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$ varnishadm -S /etc/varnish/secret -T localhost:6082 backend.set_health backend1 auto |
So to summarise: the backend.set_healthy command in varnishadm allows you to manipulate the backend health state of varnish backends, overriding the result of a probe.
Useful when you’re trying to gracefully update several backend servers, by marking backends as sick one by one without waiting for the probes to discover that backends are sick. This method allows you to do things gracefully before the update.
