Concurrent programming with Agents
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This document discusses building concurrent programs using agents. It introduces agents and describes how they exchange messages and react in a reactive way. Inside an agent's brain, the document explains how agents can have a single state or multiple states implemented as a state machine. Agents communicate through messages and events to decouple them. Reusable agents like a bulking agent and blocking queue agent are presented. The document concludes with discussing how to organize large networks of agents using patterns like worker agents, layered networks, and pipeline processing.
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Concurrent programming with Agents
- 1. Concurrent programming with Agents Tomáš Petříček http://tomasp.net/blog http://twitter.com/tomaspetricek
- 2. Introducing agents Inside agent’s brain Inter-agent communication Reusable agents Intelligence networks
- 6. Agent-based concurrency Programs compose from agents Instead of functions or objects Agents exchange messages Receive message and react Reactive system Handle inputs while running Emit results while running
- 8. Functional loop with single state Single state Same reaction to all messages Implemented using recursive loop What if reaction depends on the state? What if agent cannot handle some message? Immutable state Maintained as parameter of recursive function Can use mutable collections for performance
- 9. Hiding agent’s brain Accessing agent directly Exposes implementation details Users can call wrong methods (e.g. Receive) Encapsulating agent Agent as a private filed Add methods for all (public) messages Expose asynchronous calls first
- 10. Example Encapsulating agent into an object Exposing chat room via HTTP
- 11. Hiding agent’s brain Asynchronous calls from F# Synchronous calls from F# (optional) Asynchronous calls from C# (task based) member x.AsyncGetContent(?timeout) = agent.PostAndAsyncReply(GetContent, ?timeout=timeout) member x.GetContent(?timeout) = agent.PostAndReply(GetContent, ?timeout=timeout) member x.GetContentAsync() = Async.StartAsTask(agent.PostAndAsyncReply(GetContent)) member x.GetContentAsync(cancellationToken) = Async.StartAsTask(agent.PostAndAsyncReply(GetContent), cancellationToken=cancellationToken)
- 12. Introducing agents Inside agent’s brain Inter-agent communication Reusable agents Intelligence networks
- 13. Inside agent’s brain Single state Accept and react to all messages Example: Twitter status agent with pause Resume Pause d Running Resume Status Pause Multiple states Agent implements a state machine
- 14. Example Reading statuses from Twitter Pausing the stream using blocking agent
- 15. State and state transitions Accepting all messages Asynchronously Receive and use pattern matching Waiting for specific message Other messages stay in the queue Writing message handling using Scan let rec state = agent.Scan(function | Message -> Some <| async { handleMessage() return! newState } | _ -> None )
- 16. Introducing agents Inside agent’s brain Inter-agent communication Reusable agents Intelligence networks
- 17. Inter-agent communication Direct links between agents Complicates reusability and reconfiguration Decoupling using events Expose event instead of sending message We used events in the previous example!
- 18. Types of agent’s members Send message to the agent Members of type: 'T -> unit Notifications from the agent Exposed as events or observables: IObservable<'T> No synchronization and no thread guarantees Send and wait for a reply Uses asynchronous reply channels from F# library Takes arguments and returns result: 'T -> Async<'R>
- 19. Connecting agents Sending message in response to notification Alternatively, Observable.subscribe supports removal Connecting agents with asynchronous actions Create and start asynchronous workflow source.Notification |> Observable.add target.Action async { while true do let! value = source.AsyncAction() do! target.Action(value) } |> Async.Start
- 20. Introducing agents Inside agent’s brain Inter-agent communication Reusable agents Intelligence networks
- 21. Reusable agents I. Aggregating messages into bulks Bulk specified number of messages Emit bulk after timeout Uses of bulking agent Grouping data for further processing Writing live data to a database new BulkingAgent : int -> int -> BulkingAgent member Enqueue : 'T -> unit member BulkProduced : Event<'T[]>
- 22. Example Bulk processing of Twitter statuses A look at the BulkingAgent implementation
- 23. Reusable agents II. Blocking queue with limited size Block reader when queue is empty Block adder when queue is full Uses of blocking queue agent The producer consumer pattern Immediate buffer in pipeline processing new BlockingQueueAgent : int -> BlockingQueueAgent member AsyncGet : unit -> Async<'T> member AsyncAdd : 'T -> Async<unit>
- 24. Introducing agents Inside agent’s brain Inter-agent communication Reusable agents Intelligence networks
- 25. Managing intelligence network Lots of things going on! How to keep a big picture? Using loosely coupled connections Agents don’t reference each other directly Common ways of organizing agents Worker agent – Single agent does all the work Layered network – Agent uses agents from lower level Pipeline processing – Step-by-step processing
- 26. Pipeline processing Values processed in multiple steps Worker takes value, processes it, and sends it Worker is blocked when source is empty Worker is blocked when target is full Steps of the pipeline run in parallel
- 28. Summary Why use agent-based concurrency? Easy to understand reactive applications Elegant implementation of concurrent patterns How to write an agent-based application? State machine using recursive functions Encapsulate and provide communication points (send and send & reply methods and notifications) Use reusable agents for recurring patterns
- 29. Questions? Email: tomas@tomasp.net Blog: http://tomasp.net/blog Twitter: http://twitter.com/tomaspetricek My book: http://functional-programming.net