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    rest-core 的承諾

在 RubyConf.TW 2011，我介紹了 rest-core，是用 Ruby 寫成的模組化 REST
client 組合包與工具組。後來組合包的東西被抽至 rest-more，於是變成給 REST APIs 使用的模組化 Ruby clients interface。
在 RubyConf.TW 2012，我介紹了 Ruby 上的 concurrent 應用程式伺服器。之後在 rest-core 內，加入我從以上所學到的東西，使它能輕易地發出 concurrent requests。
在不同的使用情況下，我們有時候需要使用 promise、有時候需要使用
callback。這次我將介紹該怎麼利用 promise 來達成以我們所熟習的同步風格，發出
concurrent requests；同時也能繼續用 callback 發出非同步的 concurrent requests。
我也會分享所有噁心的實作細節。可能會很複雜，請有心理準備！
The Promise of rest-core

In RubyConf.TW 2011, I talked about rest-core as
A modular Ruby REST client collection/infrastructure.
Later it's rephrased as Modular Ruby clients interface for REST APIs
because the collection was extracted into rest-more.
In RubyConf.TW 2012, I talked about Concurrent Ruby Application Servers.
Later I tried to take what I've learnt from concurrency to make rest-core
better, so that we could easily make concurrent requests for accessing
data via REST APIs.
This time I'll talk about how we take the advantage of Promise to make
concurrent requests in an old, synchronous fashion, while still being able to
use callbacks for asynchronous requests concurrently. We need both of promises
and callbacks accordingly.
I'll also talk about all the nasty details about implementing them.
This could be tough, be prepared!
Table of Contents


Introduction
The API / Use Cases

Futures / Synchronous / Blocking

Print the Name
Print the Names
Fire and Forget
Fire and Call


Callbacks / Asynchronous / Non-blocking

Fire and Call
Fire and Rescue
Callback Hell?
We Still need Callbacks


Wait! It's not yet done

Client#wait
Client.wait


Promise Chain: Promise#then

Why: It's Not Blocking Here
RC::Github#all


Promise.claim

Forging Response, for middleware RC::Cache


Streaming, the EventSource

SSE, Server-Sent Events
SSE with Rack Hijacking
Firebase Daemon (rest-firebase)


Some Background...

Race Conditions
Mutex (mutual exclusion) (lock)
ConditionVariable (monitor)
WeakRef (weak reference)


Concurrency Model

None / Blocking (For debugging purpose)
Thread Spawn (Default)
Thread Pool (Throttling)
Gotcha!

Weird Backtrace for Exceptions
Shutting down gracefully
Beware of Deadlock!
Beware of Timeout!
Inaccurate Timeout


Wait! Detail

Client.wait
Promise#wait
Promise#fulfilling
Promise#rejecting
Future#method_missing
Promise#yield


Promise Detail

Promise#defer
Promise#protected_yield
Promise#cancel_task


ThreadPool Detail

ThreadPool#defer
ThreadPool#spawn_worker
ThreadPool::Task