Only allow one message of a particular type in queue at a time

[SteveChandler]

I have an actor for a serial device. I want to be able to queue up any number of messages except for one. Periodically I put a "send update" message in the queue. If there is one already in there then I don't want to allow another until the one already in the queue has been processed. I doubt this is possible. Does anyone have any advice on how to solve this problem?

[justACS]

You're right; what you want isn't possible in the current framework.  What is the larger problem you are trying to solve?

[David_Staab · Accepted solution]

Turn Update Msg into a Reply Msg, then use "Send Msg and Wait for Response.vi". Probably a bad idea, but it's there for you.

Better idea: Define an Update Was Processed Msg and have your sender keep track of whether an update is still hanging in the pipe. Requires a little state information (a single Boolean) in your sender, but it keeps the actors asynchronous.

[onnodb]

Would a Time-Delayed Send Message Once VI help in this case?

[David_Staab]

The behavior can't be restricted by the framework itself, but it can definitely be restricted by considerate actor and message design.

[SteveChandler]

David_Staab wrote:

Turn Update Msg into a Reply Msg, then use "Send Msg and Wait for Response.vi". Probably a bad idea, but it's there for you.

Better idea: Define an Update Was Processed Msg and have your sender keep track of whether an update is still hanging in the pipe. Requires a little state information (a single Boolean) in your sender, but it keeps the actors asynchronous.

Even if the message was a wait for response, wouldn't others get queued up behind it? I was thinking that I would have to add something like an "update was processed". I think that is probably the best way to go.

onnodb, I will take a look at the time delayed send once.

What I am doing now is sending my update status message from the timeout event of an event structure. Depending on what commands are sent to the queue, it might take more than the timeout time to complete. This causes multiple "send status" messages to get queued up.

[komorbela]

I would probably make an intermediary actor that receives and passes the messages from both sides. It can pass the first "send update" and wait for reply. Until no reply received, it shoul just not pass the "send update" messages. When the reply arrives it passes it further and sets it's status to pass the next "send update".

[drjdpowell]

Timestamp the messages?  Allow multiple "send update" messages to be enqueued, but have the "Do" method do nothing if the timestamp isn't greater than the time the last update was sent.

[jlokanis]

Why would you want to only have on in the queue at a time?  Perhaps the reasonaing behind your need will help us offer an alternative.

In a slightly related case, if you were trying to do the opposite and send multiple messages but only act on the freshest data, I would use a notifier as your data type and then have the sender update the data in the notifer so that your processor would only see the latest status when it processes the first message.

[SteveChandler]

David_Staab wrote:

Turn Update Msg into a Reply Msg, then use "Send Msg and Wait for Response.vi". Probably a bad idea, but it's there for you.

Better idea: Define an Update Was Processed Msg and have your sender keep track of whether an update is still hanging in the pipe. Requires a little state information (a single Boolean) in your sender, but it keeps the actors asynchronous.

Thanks, I implemented the "Better idea"