Building Better Services
========================

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This package provides methods to deal with the common needs of services talking to other services, like timeouts and retries, treating these as cross-cutting concerns that aren't tied to a specific transport like HTTP.

Examples
========

Batching
--------

In a social network there may be another service that returns user records, if this social network is really busy it would likely be more efficient to capture multiple calls and dispatch them as one multi-get call.

The batchingService function creates both single and multi-get calls (hence the "fmap snd" below), which accumulate requests over a user defined window in time and dispatch them once that window has passed.

```haskell
data User = User Int String

makeUser :: Int -> User
makeUser userId = User userId ("User " ++ (show userId))

userService :: S.HashSet Int -> IO (M.HashMap Int User)
userService request = do
  threadDelay 500
  return $ M.fromList $ fmap (\r -> (r, makeUser r)) $ S.toList request

batchedUserService :: IO (S.HashSet Int -> IO (M.HashMap Int User))
batchedUserService = fmap snd $ batchingService defaultBatchingOptions userService
```


Retries
-------

Often we want to retry service calls if they've failed because it might've been a transient error that subsequently succeeds.

```haskell
failingService :: IORef Int -> Int -> IO Int
failingService ref request = do
  counter <- atomicModifyIORef' ref (\c -> (c + 1, c + 1))
  if counter `mod` 3 == 0 then fail "Bang!" else return (request * 2)

notSoFailingService :: IO (Int -> IO Int)
notSoFailingService = do
  ref <- liftIO $ newIORef 0
  return $ retryingService defaultRetryOptions $ failingService ref
```

Caching
-------

```haskell
serviceThatNeedsCaching :: Int -> IO Int
serviceThatNeedsCaching request = do
  putStrLn "Doing Something Expensive!"
  return (request * 2)

cachedService :: IO (Int -> IO Int)
cachedService = do
  cache <- newAtomicLRU $ Just 100
  return $ cacheWithLRU cache serviceThatNeedsCaching
```