add ability to combine Schedules for retries

[zkerriga]

#58

I tried to make proposed syntax:

val policy: RetryPolicy = RetryPolicy.immediate(3).fallbackTo(RetryPolicy.backoffForever(100.millis))

but found fallbackTo method on RetryPolicy a bit misleading, because with current design of retry classes we cannot easily compose different ResultPolicy and onRetry functions. Instead, I decided to make composable just Schedule classes.

I added Combination and CombinationForever subclasses by analogy so we can combine Finite schedules with both Finite and Infinite fallbacks, but I have a feeling that they add an unreasonable complication 🤔 - feel free to reject it

P.S. it's my first opensource request

[kciesielski]

Congratulations on your first contribution! 😁
I added a few minor comments. I'd also like @rucek to take a look. The names Combination and CombinationForever feel like something that could be expressed with better words, but I'm not sure what would do the job. Naming is hard 🤔

[rucek]

Thanks for the contribution @zkerriga! This looks good too me, my only concerns are about the naming, which is indeed hard 😅. I think Combined and Combination are too general, since what we're doing is not a generic combination of Schedules, but a very specific one - a fallback to another Schedule.

I was thinking about renaming Combined to WithFallback, and Combination/CombinationForever to FallingBack/FallingBackForever, respectively. WDYT?

[kciesielski]

FallingBackForever sounds so romantic <3

[adamw]

Since we are bikeshedding on names (the PR looks good, @zkerriga , let's get it merged & released :) ), how about simply andThen instead of fallbackTo? Wouldn't it be more intuitive, i.e., "retry 3 times, and then retry 5 times with a pause". The class representing the schedule could then be named Sequence (as there's a "first" and "second" schedule, that is, order matters)?

[zkerriga]

Thank you for reviews! @adamw, it's an interesting idea about Sequence and andThen method, but I decided to use Fallback naming from @rucek because it seems clearer to me and I don't think that making more than 1 fallback will be a popular thing.

[rucek]

I agree that although andThen and Sequence would make sense, it's the "fallback" naming that communicates the purpose more clearly.