Use Promises in execute context

[syrusakbary]

This PR improves the executor abstraction by using Promises.
At the same time, removes all the usage of the Middlewares as are no longer necessary.

The Promise implementation is based in aplus with a lot of improvements and fixes like Promise.all, promise_for_dict, promise.do_resolve.

The resolvers can now return a raw object or a promise.

Executors

The following executors are available:

• graphql.execution.executors.asyncio.AsyncioExecutor: This executor executes the resolvers in the Python asyncio event loop.
• graphql.execution.executors.asyncio.GeventExecutor: This executor executes the resolvers in the Gevent event loop.
• graphql.execution.executors.asyncio.ProcessExecutor: This executor executes each resolver as a process
• graphql.execution.executors.asyncio.ThreadExecutor: This executor executes each resolver in a Thread.
• graphql.execution.executors.asyncio.SyncExecutor: This executor executes each resolver synchronusly (default).

Usage

You can specify the executor to use via the executor keyword argument in the grapqhl.execution.execute function.

from graphql.execution.execute import execute

execute(schema, ast, executor=SyncExecutor())

If no executor is specified, SyncExecutor will be used by default.

[Globegitter]

@syrusakbary that is great stuff! So if I use a library that returns a Future should I be able to simply wrap that in a promise?

So what I am imagining is using the SyncExecutor but every resolve function that does some IO would return 'immediately' with a Promise. Inside the resolver we are using a request library (GRPC) that returns a Future for any IO calls. This Future I would then wrap in a Promise, which, via .then() will return the Promise with actual resolve result.

Is that something that should work with this implementation?

[jhgg]

Neat!

[jhgg]

Process and thread executor should be able to delegate to a pool (shared or individual pool per execution) of finite size. To prevent large queries from blowing the OS

[syrusakbary]

@Globegitter is not necessarily that, your resolver function could return a Future too and will work!
(But you need to specify the executor as AsyncioExecutor)

[syrusakbary]

But if you want to use Promises it will work too (even with the SyncExecutor)