If one service needs to connect to another service then I have to add a shared network between them. In that case, the services essentially shared a common namespace regarding DNS. DNS resolution would routinely leak from one service to another and cause outages, e.g if I connect Gitlab container and Redmine container with OpenLDAP container then sometimes Redmine’s nginx container would access Gitlab container instead of Redmine container and Gitlab container would access Redmine’s DB instead of its own DB.

I maintained some workarounds, like starting Gitlab after starting Redmine would work fine but starting them other way round would have this issue. But switching to Kubernetes and replacing the cross-section connections with network policies solved the issue for me.