Every asynchronous agent has an associated executor.
An agent's executor determines how the agent's completion handlers are
queued and ultimately run.
Example uses of executors include:
-
Coordinating a group of asynchronous agents that operate on shared
data structures, ensuring that the agents' completion handlers never
run concurrently.
-
Ensuring that agents are run on specified execution resource (e.g.
a CPU) that is proximal to data or an event source (e.g. a NIC).
-
Denoting a group of related agents, and so enabling dynamic thread
pools to make smarter scheduling decisions (such as moving the agents
between execution resources as a unit).
-
Queuing all completion handlers to run on a GUI application thread,
so that they may safely update user interface elements.
-
Returning an asynchronous operation's default executor as-is, to run
completion handlers as close as possible to the event that triggered
the operation's completion.
-
Adapting an asynchronous operation's default executor, to run code
before and after every completion handler, such as logging, user authorisation,
or exception handling.
-
Specifying a priority for an asynchronous agent and its completion
handlers.
The asynchronous operations within an asynchronous agent use the agent's
associated executor to:
-
Track the existence of the work that the asynchronous operation represents,
while the operation is outstanding.
-
Enqueue the completion handler for execution on completion of an operation.
-
Ensure that completion handlers do not run re-entrantly, if doing so
might lead to inadvertent recursion and stack overflow.
Thus, an asynchronous agent's associated executor represents a policy of
how, where, and when the agent should run, specified as a cross-cutting
concern to the code that makes up the agent.
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