Communication
Choose the interaction before choosing its transport. A call waits for an answer; an event lets another service react later. Either can fail, and neither makes a multi-service operation atomic.
For calls, the catalog describes contracts while local registration and remote resolvers decide where those calls execute. In-process transport is the built-in path for services mounted on the same server. Use the transport matrix to check which network protocols and streaming shapes your deployment can expose.
Events adds a different boundary: the adapter, delivery policy and subscriber determine when a reaction is accepted and what happens after a failure.