Skip to main content

Distributed Async Await

The concept of Distributed Async Await a.k.a Durable Executions is an emerging concept. However, an accurate and concise definition or description or articulation is not yet available.

💀 Warning 💀

Do not fall for the trap of reasoning only in terms of reliability, failure tolerance, or failure transparency. While enhanced reliability, failure tolerance, or failure transparency are attractive attributed and enhanced reliability, failure tolerance, or failure transparency are an enabling mechanism, they are not the essence of Distributed Async Await

Programming Model​

Here is an example of the programming model:

function foo(n : number) Promise<void>{

  console.log("span 1");

  await bar(n);

  console.log("span 2");

}

Function Promise Relationship

Async Await is based on functions and promises:

Functions: Represent the fundamental unit of computation.

Promises: Representations of future values, represent the fundamental unit of coordination.

Function and promise state machine

We can conceptualize a function execution as a sequence of steps delineated by await statements:

  • When an execution starts, the execution runs until reaching an await expression.
  • When reaching an await expression, the execution is suspended until the associated promise completes.
    • If the promise is resolved, the execution resumes with the value of the promise.
    • If the promise is rejected, an error is thrown.

There are two key relationships between downstream function executions, promises, and upstream function executions:

Invocation introduces a relationship between a downstream execution, a promise, and an upstream execution, so that the downstream execution creates the promise and the promise creates the upstream execution.

Await introduces a relationship between a downstream execution, a promise, and an upstream execution, so that the downstream execution can only resume after the upstream execution completes the promise.

Invocation and Await Relationship

Durable Async Await is based on Durable Functions and Durable Promises, that is, functions and promises that persist their identity and their state in durable storage.

TBD (including Continuation, Serializable Continuation)

Sync vs Async Await vs Async Promise Await

How does Durable Async Await work?​

Describing how Durable Async Await works is much easier than describing what Durable Async Await is or what Durable Async Await does.

Mechanics

Interleave a function execution with save points, enabling the execution to resume its latest save point in case the execution terminates prematurely before returning.

Restart

In the event of premature termination, the runtime restarts the execution and deduplicates on previously completed Durable Promises to continue

Restore

In the event of premature termination, the runtime restores the execution to continue