Setup callback hook for base classes

[MeltyBot]

Migrated from GitLab: https://gitlab.com/meltano/sdk/-/issues/247

Originally created by @rabidaudio on 2021-10-20 16:06:02

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Streams and Sinks often need to do some setup before the EL process. For example, a Sink may need to update table schemas in the target database, a Stream may need to open a connection, etc.

Users can put this stuff in constructors but I see a few problems with this:

• the constructors have some complex arguments, and we probably want to discourage users from overriding the base constructor so that we can change the arguments in the future
• it makes testing a bit complicated, as you can't have an instance of the class without it doing setup

My proposal is a simple method (prepare? setup? something to that effect) with a default pass implementation that is called before any messages come in/out. Perhaps also a corresponding teardown method for resources that need to be cleaned up like database connections.

[MeltyBot]

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