Runtimes

Runtimes are execution environments that host code and provide operational behavior.

Examples include language runtimes, web runtimes, actor runtimes, workflow runtimes, durable execution engines, serverless runtimes, job processors, stream processors, and application frameworks.

Different runtimes realize observers differently. An actor runtime may emphasize identity, placement, supervision, and serialized message handling. An HTTP host may emphasize request pipelines, routing, middleware, and short-lived handlers.

Some runtimes associate an observer with an OS thread and call stack. Green-thread, fiber, coroutine, task, or async runtimes relax that association: the observer follows the logical execution context governed by a scheduler, even when execution resumes on different OS threads.

Runtimes realize scheduling through mechanisms such as thread schedulers, event loops, work-stealing pools, actor dispatchers, continuation queues, timer services, and workflow activation queues. These mechanisms select execution opportunity; they do not automatically supply domain authority, message-delivery guarantees, consistency, or one shared fairness property across layers.

A runtime fairness claim should identify the schedulable unit and failure assumptions. Fair CPU scheduling does not imply fair actor activation, fair mailbox admission, eventual network delivery, or fair workflow retry. Arbitration at queues, locks, and asynchronous boundaries can introduce additional ordering and progress constraints.

This is where blocking and non-blocking must be separated carefully. A logical operation may wait for an asynchronous result while the runtime does not block the physical thread. Callback-, continuation-, task-, fiber-, or actor-based runtimes can suspend the logical continuation and resume it later, preserving the semantic wait without tying up the underlying thread.

The same semantic model can be preserved across runtimes when observer, entity, event, command, state, and boundary meanings are kept explicit.