F# – Starting an Agent with supervision

Yan Cui

I help clients go faster for less using serverless technologies.

In Erlang, we have the Supervisor behaviour which makes it very easy to provide the means to monitor and restart a whole network of workers and other supervisors based on some configured strategy.

The MailboxProcessor (aka agents) in F# doesn’t come with the same higher-level abstractions (such as Erlang’s gen_server, gen_fsm behaviours) by default, but it’s very easy to mimic some of their capabilities by building on top of the standard F# agents.

Luca Bolognese’s lightweight LAgent framework is a perfect example and even provides interesting possibilities such as hot swapping of code (although still some way off of what’s possible in Erlang in this regard).

Similarly, the following snippet shows how you can start an agent with supervision so that any escaped exceptions (which will crash the agent) are trapped and restarts the agent:

	namespace FSharp.Control
	type Agent<'T> = MailboxProcessor<'T>
	[<AutoOpen>]
	module AgentExt =
	type MailboxProcessor<'T> with
	static member StartSupervised (body : MailboxProcessor<_> -> Async<unit>) =
	let watchdog f x = async {
	while true do
	try
	do! f x
	with exn -> ()
	}
	Agent.Start (fun inbox -> watchdog body inbox)
	/// Usage
	> let agent = Agent<string>.StartSupervised(fun inbox -> async {
	printfn "agent started"
	while true do
	let! msg = inbox.Receive()
	match msg with
	| "kill" -> failwith "killed.."
	| str -> printfn "%s" str
	});;
	val agent : MailboxProcessor<string>
	>
	agent started
	> agent.Post("hello");;
	val it : unit = ()
	>
	hello
	agent.Post("world");;
	world
	val it : unit = ()
	> agent.Post("kill");;
	val it : unit = ()
	>
	agent started
	agent.Post("hello again!");;
	hello again!
	val it : unit = ()

view raw gistfile1.fs hosted with by GitHub