xmpl - remote key/value store healthz revisited


Changing approach to establish the healthz of a KVStore::Remote object for xmpl. With rationale. This post is part of a series.

In previous post xmpl - remote key/value store we looked into the implementaton of the KVStore::Remote class, which acts as a proxy for a remote instance of xmpl.

The is_healthy() method described there is always successful:

sub is_healthy { return 1 }

This was the explanation:

This might seem counterintuitive. Why not ask the remote also in this case, or raise something if a timeout occurs?

The answer lies in how this is_healthy method will be eventually consumed, i.e. possibly by a Kubernetes liveness probe. We don’t want Kubernetes to evict our pod(s) for the frontend every time the backend is unavailable… so we just ignore it and always flag that things are fine.

After pondering harder on this matter, I think this is not the right approach. The class, and in general the program, should provide faithful information about its state; if it’s not able to access the real key-value store, it should complain much like the KVStore::OnFile class does.

On the other hand, how this information will be used needs to be aware of it. If it’s in Kubernetes and it does not need to undergo an eviction-recreation cycle when the test goes wrong… well, then there should be no Liveness test. It’s at that point that the information should be dropped.

So… the method is now changed as follows:

sub is_healthy ($self) { eval { $self->ua->head($self->url) } }

i.e. it just checks that the query does not time out, although it disregards the outcome. One step at a time.

While at it, I also decided to set some limits in the Mojo::UserAgent object:

sub ua ($self) {
   state $ua = Mojo::UserAgent->new(
      max_redirects   => 5,
      connect_timeout => 2,

In this way, a failing remote instance will not make the current instance wait for too long.

If you want to look at the complete code for the class… head to it!

Stay safe!

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