F# Mailbox Processor Out-of-Order Message Processing
Example
You can use Scan or TryScan methods to look for specific messages in the queue and process them regardless of how many messages are before them. Both methods look at the messages in the queue in the order they arrived and will look for a specified message (up until optional timeout). In case there is no such message, TryScan will return None, while Scan will keep waiting until such message arrives or the operation times out.
Let's see it in practice. We want the processor to process RegularOperations when it can, but whenever there is a PriorityOperation, it should be processed as soon as possible, no matter how many other RegularOperations are in the queue.
type Message =
| RegularOperation of string
| PriorityOperation of string
let processor = MailboxProcessor<Message>.Start(fun inbox ->
let rec innerLoop () = async {
let! priorityData = inbox.TryScan(fun msg ->
// If there is a PriorityOperation, retrieve its data.
match msg with
| PriorityOperation data -> Some data
| _ -> None)
match priorityData with
| Some data ->
// Process the data from PriorityOperation.
| None ->
// No PriorityOperation was in the queue at the time, so
// let's fall back to processing all possible messages
let! message = inbox.Receive()
match message with
| RegularOperation data ->
// We have free time, let's process the RegularOperation.
| PriorityOperation data ->
// We did scan the queue, but it might have been empty
// so it is possible that in the meantime a producer
// posted a new message and it is a PriorityOperation.
// And never forget to process next messages.
innerLoop ()
}
innerLoop())
