At my previous job we used docker swarm for a cluster of a dozen machines. Does not have all the fancy features of k8s but gets the job done to automate deployments
The major argument is that "if it's wrong, nothing bad will happen, because it will break in runtime". Isn't it better to break before things are in production, like at compile time?
Solr is a great tool, we use it at my job to index documents and never had any problem with it. We choosed it over elasticsearch because it seems simpler to setup and administer.
Why is 100% coverage stupid? I agree that there could be configuration or data classes that does not make sense to test, so these could be ignored by the coverage tool. Is it still stupid to aim for 100% coverage in the rest of the relevant not-ignored code?
I don't use XMPP since the Google talk era, how are the available servers nowadays? Does they support federation, meaning that users from different servers can talk to each other?
k8s is very powerfull tough complex speccialy if you manage the cluster yourself. Docker swarm is a simpler alternative that provides all the features required by the majority of use cases.