Service client

[benblack86]

Behold, the new service client MVP. It looks like this:

val hostManager = HostManager()
hostManager.addHost("localhost", 6379)
hostManager.addHost("localhost", 6379)
hostManager.addHost("localhost", 6379)

val redisClient = Redis.futureClient(hostManager)

The default behavior is to try every host with exponential backoff. However, you could configure something different, for example:

val hostManager = HostManager(
  LBRetryPolicy(
    replacement = Random,
    backoff = LinearBackoff(1.second),
    attempts = MaxAttempts(3)
  )
)

The possibilities are endless.

It's missing some implementation details and tests, but my main concern is around removing a client from the host manager. When a client is removed, it is disconnected, meaning that any requests that are currently using it will be affected. Maybe this is fine (making the assumption retries is turned on), maybe we should disconnect after a few seconds, or maybe the disconnect should only happen when we know for certain that no request is using it (not sure how feasible that is).

Thoughts?

@DanSimon @amotamed @aliyakamercan @nsauro @dxuhuang @jlbelmonte

[codecov-io]

Codecov Report

Merging #610 into develop will decrease coverage by 1.16%. The diff coverage is 4.61%.

@@             Coverage Diff             @@
##           develop     #610      +/-   ##
===========================================
- Coverage    84.88%   83.72%   -1.17%     
===========================================
  Files           99      101       +2     
  Lines         4452     4515      +63     
  Branches       361      369       +8     
===========================================
+ Hits          3779     3780       +1     
- Misses         673      735      +62

Impacted Files	Coverage Δ
.../src/main/scala/colossus/service/HostManager.scala	0% <0%> (ø)
...ain/scala/colossus/service/LoadBalanceClient.scala	0% <0%> (ø)
...s/src/main/scala/colossus/service/ServiceDSL.scala	83.67% <0%> (-1.75%)	:arrow_down:
...lossus/src/main/scala/colossus/service/Async.scala	26.08% <20%> (-23.92%)	:arrow_down:

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[benblack86]

Going to close this and open after some changes. The problem with the HostManager concept is that it is hard to configure through config. You would probably need to namespace the host manager and then have a separate section for client config and host manager config, which will be confusing for a colossus user.