Issue #12 - The Mechanics of Async Await
The Mechanics of Async Await
Another weekend, another weekend read, this time all about The Mechanics of Async Await
My book Thinking in Distributed Systems is now available. 12 chapters, full of mental models, diagrams, illustrations, and examples, all about distributed systems.
This weekend, I am featuring my own post, The Mechanics of Async Await. The journey from synchronous to asynchronous programming is a shift from sequential to concurrent execution.
We will reason about synchronous and asynchronous programming in terms of pairs of events captured during the execution. We assume that traces contain the following events of interest:
in synchronous programming
invoke functionandreturn value.
in asynchronous programming
invoke functionandreturn promise,await promiseandreturn value.
Synchronous Execution
A synchronous execution consists of a single round trip, one invoke-and-return-value interaction between the caller and callee. The caller suspends its execution until the callee completes its execution and returns a value (Figure 1., left).
Asynchronous Execution & Promises
An asynchronous execution consists of two round trips, one invoke-and-return-promise interaction and one await-and-return-value interaction between the caller and the callee.
A promise can either be interpreted as a representation of the callee or as a representation of a future value. A promise is either pending, indicating that the callee is in progress and has not returned a value, or completed, indicating that the callee terminated and has retuned a value.
If the promise is pending on await, the callers suspends its execution until the callee completes its execution and returns a value (Figure 1., center). If the promise is completed on await, the caller continues with the returned value (Figure 1., right).
The Event Loop
The runtime of async await is referred to as an Event Loop. An Event Loop is a scheduler that allows an asynchronous execution to register interest in a specific event. (In this blog we are only interested in the completion of a promise). Once registered, the execution suspends itself. When a promise completes, the Event Loop resumes the execution.
The remainder of this blog implements async await facilities and an Event Loop in Elixir. Why Elixir? For starters, mapping async await to Elixir is straightforward, making the implementation an excellent illustration. More importantly, mapping async await to Elixir allows us to dispel the common myth that async await is intrinsically non-blocking: by mapping asynchronous executions onto Elixir processes and deliberately blocking the process when awaiting a promise.
Happy Reading
The Mechanics of Async Await
Dominik Tornow
Resonate is pioneering a novel programming model to distributed computing called Distributed Async Await. Distributed Async Await is an extension of traditional async await that goes beyond the boundaries of a single process and makes distributed computing a first-class citizen.
Given that Distributed Async Await is an extension of async await, I figured a blog post exploring the mechanics of functions, promises, and the event loop would be fun.