Function Executions

The fundamental unit of computation — lifecycle, long-running semantics, and the logical/physical distinction.

On an execution level, a system is a collection of executions; each execution has a unique identity. An execution has a well-defined lifecycle, represented by the lifecycle events invoke and return, that governs its participation in the system:

invoke marks the initialization of the execution and represents its entry into the system.

return marks the termination of the execution and represents its exit from the system.

An execution is bound to a single process. An execution cannot emit events outside its (invoke, return) interval. As a consequence, an execution cannot be restarted. Instead, a new execution must be created that may be considered the logical successor or the logical equivalent of the terminated execution.

An execution may raise events such as send or receive events only within its (invoke, return) interval.

Long-running executions#

The term long running is not a well-defined term in the context of distributed systems — "long" is relative.

This system model argues that you should not think about "long" in terms of time. Instead, long refers to the potential for a logical execution to span multiple physical locations. An execution is long-running if it potentially runs across multiple processes.

Physical vs logical#

Distributed Async Await makes the distinction between physical executions and logical executions.

One logical execution consists of multiple concurrent and distributed physical executions — physical executions may execute independently and on different resources.

For example, logical execution le1 starts as physical execution pe1 on physical process p1. Physical process p1 then experiences a crash failure. Logical execution le1 is then resumed as physical execution pe2 on physical process p2.

Physical executions never "restart". Each physical execution is distinct. A logical execution can contain many physical executions, each one starting from the beginning and attempting to make progress. The logical execution completes when a physical execution reaches the return statement of the function.

Concurrency#

The defining characteristic of concurrency is non-deterministic partial order; to mitigate the challenges of partial order, we leverage coordination to ensure consistency.

Distribution#

The defining characteristic of distribution is non-deterministic partial failure; to mitigate the challenges of partial failure, we leverage recovery to ensure completion.

Distribution across space#

An execution is distributed across space when the same logical execution spans multiple physical executions concurrently — at the same point in time.

Distribution across time#

An execution is distributed across time when the same logical execution spans multiple physical executions sequentially — at different points in time.