use NIOSingletons EventLoops/NIOThreadPool instead of spawning new #697
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FranzBusch
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### Motivation: SwiftNIO allows and encourages to precisely manage all operating system resources like file descriptors & threads. That is a very important property of the system but in many places -- especially since Swift Concurrency arrived -- many simpler SwiftNIO programs only require a single, globally shared EventLoopGroup. Often even with just one thread. Long story short: Many, probably most users would happily trade precise control over the threads for not having to pass around `EventLoopGroup`s. Today, many of those users resort to creating (and often leaking) threads because it's simpler. Adding a `.globalSingle` static var which _lazily_ provides singleton `EventLoopGroup`s and `NIOThreadPool`s is IMHO a much better answer. Finally, this aligns SwiftNIO a little more with Apple's SDKs which have a lot of global singletons that hold onto system resources (`Dispatch`'s thread pool, `URLSession.shared`, ...). At least in `Dispatch`'s case the Apple SDKs actually make it impossible to manage the resources, there can only ever be one global pool of threads. That's fine for app development but wouldn't be good enough for certain server use cases, therefore I propose to add `NIOSingleton`s as an _option_ to the user. Confident programmers (especially in libraries) are still free and encouraged to manage all the resources deterministically and explicitly. Companion PRs: - apple/swift-nio-transport-services#180 - swift-server/async-http-client#697 ### Modifications: - Add the `NIOSingletons` type - Add `MultiThreadedEventLoopGroup.singleton` - Add `NIOThreadPool.singleton` ### Result: - Easier use of NIO that requires fewer parameters for users who don't require full control. - Helps with #2142 - Fixes #2472 - Partially addresses #2473
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@FranzBusch @glbrntt @dnadoba This is ready for review now. |
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Lukasa
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| ```swift | ||
| import AsyncHTTPClient | ||
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| let httpClient = HTTPClient(eventLoopGroupProvider: .createNew) | ||
| let httpClient = HTTPClient(eventLoopGroupProvider: .singleton) |
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Is there any reason these examples don't use the zero-element initializer?
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@Lukasa Yes, that's only coming in #705 . #705 does also rewrite all these examples to just use HTTPClient.shared. But I split this work into two parts (maybe misguidedly):
- Introduce the
.singletonEventLoopGroupProvider--> this PR - Introduce
HTTPClient.shared, a completely shared singleton HTTPClient that doesn't need shutdown --> #700 & #701: HTTPClient.shared a globally shared singleton #705
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use apple/swift-nio#2471 and apple/swift-nio-transport-services#180 (once available)
Relevant discussion: apple/swift-nio#2498