Designing Thread-Aware and Async-Aware APIs¶

Page Maps¶

graph LR
  family["Python Programming"]
  program["Python Object-Oriented Programming"]
  section["Time Scheduling Concurrency Boundaries"]
  page["Designing Thread-Aware and Async-Aware APIs"]
  capstone["Capstone evidence"]

  family --> program --> section --> page
  page -.applies in.-> capstone

flowchart LR
  orient["Orient on the page map"] --> read["Read the main claim and examples"]
  read --> inspect["Inspect the related code, proof, or capstone surface"]
  inspect --> verify["Run or review the verification path"]
  verify --> apply["Apply the idea back to the module and capstone"]

Read the first diagram as a placement map: this page is one concept inside its parent module, not a detached essay, and the capstone is the pressure test for whether the idea holds. Read the second diagram as the working rhythm for the page: name the problem, study the example, identify the boundary, then carry one review question forward.

Why this lesson matters¶

An API is concurrency-aware whether you documented it or not.

If callers can use an object from multiple threads or async tasks, they will form assumptions about:

whether calls can overlap safely
whether a method blocks
whether it must be awaited
whether results depend on attachment to a loop or coordinator

If the API does not make those expectations clear, users learn them by failure.

Concurrency awareness is part of the contract¶

A public API should help callers understand:

can this object be called concurrently?
must calls stay on one thread or one loop?
is this surface synchronous, asynchronous, or both through a deliberate bridge?
where does blocking happen?

Those are not implementation trivia. They affect correctness and operability.

Avoid widening the whole surface by default¶

One risky pattern is to take one concurrency concern and spread it through every method.

For example:

one async integration need makes every method async
one shared-state concern makes every consumer manage locks

A stronger approach is to keep concurrency-specific behavior near the boundary that owns it and keep the rest of the API as narrow and explicit as possible.

Blocking behavior should be visible¶

Callers need to know if a method:

performs only in-memory work
may block on I/O
may wait on a queue or lock
requires an event loop context

Without that, "simple" method calls become hidden coordination points.

That hidden cost is often more damaging than an explicit boundary method would have been.

Do not confuse availability of use with safety of use¶

Just because an API can technically be called from many threads or tasks does not mean it is safe to do so.

A good contract may explicitly say:

single-threaded only
caller must serialize access
safe for concurrent reads but not concurrent mutation
async-safe only through one supervisor boundary

These are useful constraints. Hiding them helps nobody.

Common mistakes¶

documenting nothing about blocking or concurrency semantics
exposing the same method to many concurrency modes without a clear ownership story
turning every surface async because one adapter needed async integration
leaking lock or event-loop management outward to callers unnecessarily
implying thread safety through silence

These mistakes make the API broader and less trustworthy than it needs to be.

Review checklist¶

Question	Good sign
can callers tell whether the API is sync, async, or boundary-bridged?	yes
is blocking or waiting behavior visible?	yes
are concurrency guarantees and limits stated explicitly?	yes
does the API avoid leaking unnecessary runtime coordination outward?	yes

Capstone connection¶

Use the capstone runtime and service boundaries to ask:

which surfaces should remain plain synchronous domain APIs?
which surfaces may honestly become async-aware?
where should callers be told about blocking, serialization, or supervisor requirements?

Those questions keep runtime truth attached to the API contract.

Exit check¶

Leave this lesson only when you can do all of these:

explain why concurrency expectations are part of an API contract
identify one case where blocking behavior must be visible to the caller
point to one capstone surface that should stay narrow instead of becoming async-aware everywhere