Delay startup till dependencies ready

[karolz-ms]

Problem description

Many services require dependencies (databases, async comm facilities etc.) to be available to do any work. While it is ultimately required for robust services to be resilient when dependencies are missing or misbehaving, starting everything at once and relying on health checks/retries unnecessarily slows down the startup and results in a lot of noise in the logs.

Aspire customers have been using and recommending https://github.com/davidfowl/WaitForDependenciesAspire and experimenting with it (e.g. https://github.com/afscrome/WaitForDependenciesAspire fork) which shows this is an unmet need.

Use cases

• Wait till dependency has successfully started
• Wait till dependency service is ready to serve traffic (endpoint created)
• Wait till dependency is healthy
• Wait till dependency is done running (ran successfully) -- e.g. for database seeding

Related issues

https://github.com/microsoft/usvc/issues/69 (the health check feature) will be required for "wait till dependency is healthy" use case.

[karolz-ms]

Some design questions to consider:

• What in the model constitutes a "dependency"? Can Executables/Containers depend only on other Containers/Executables, or could they also depend on Services (Services being in "Ready" state in particular)?
• To what degree, and how, are we going to model various kinds of dependency checks? E.g. could we express a dependency check as a predicate against resource status? If that seems like a good idea, what is the grammar for these predicates?

[dbreshears]

Moving this to backlog as it was decided in 9.0 timeframe to handle waitfor() within the apphost and so no work for DCP currently.