Support displaying past invocation

[tillrohrmann]

In order to better support workflow use cases, we believe that it is important to store the history of past invocation runs and to make them easily accessible. The assumption is that the completion/failure of invocation runs itself represent valuable information that the user wants to access. Moreover, it allows building support for re-executing individual workflow runs and to investigate potential failure causes.

1. Option: History server

The Restate runtime itself, does not store the history of invocation runs. Instead, one idea could be to offload this task to an external component, the HistoryServer. In order to implement such a HistoryServer, the runtime would need to expose information about ongoing and completed invocations (e.g. via a CDC stream).

2. Option: Cache in Restate server

An alternative w/o an additional dependency is to cache the past invocations in the Restate server. What we need to figure out is which information needs to be retained and for how long.