Add a "Concept" article on outbound requests

[tpmccallum]

At present, whilst single outbound requests are supported; neither Spin nor Cloud supports being able to run multiple concurrent outgoing requests.

This includes Redis, Postgres, MySQL, Key/Value and HTTP requests.

This could present a limitation to a user who is planning on building their application on Spin/Cloud. The following paragraph is a little more information Re: time/resources/practicability of having multiple concurrent outgoing requests supported.

One plan is to add first-class support for structured concurrency (which is needed for concurrent outgoing requests) to the WASI Component Model and WASI Preview 3. The timeline for such a task to be completed could take at least one year.

In the meantime, if concurrent outgoing requests are a high priority, then we could potentially emulate concurrent outgoing HTTP requests using WASI Preview 1 or 2 polling (a WASI API for waiting for I/O events on multiple handles).

[tpmccallum]

• Explain synchronous vs concurrent; give examples of how traditional languages like Python etc, have ready-built libraries which make concurrency very easy to implement i.e. import concurrent, set max_workers in a thread pool executor and start calling etc.
• Consider mentioning fan-out/fan-in pattern, monitoring, and potentially touching on connection timeouts etc.
• Mention WASI API for waiting for I/O events on multiple handles and potential first-class support for structured concurrency in WASI Component Model and WASI Preview 3.
• Suggest a temporary workaround and perhaps make an example up to demonstrate this in the docs

[dicej]

Also note that as of yesterday, the official wasi-http proposal has support for concurrent outbound requests: WebAssembly/wasi-http#3 (comment), so we'll want to adopt that when it's ready. It still won't reflect the long term vision for asynchronous WASI, but it will be a big step in the right direction.

[quyumkehinde]

This appears to be done - https://developer.fermyon.com/spin/v3/http-outbound

[itowlson]

@quyumkehinde Reopening as the linked page doesn't really cover the topics raised in the issue.