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Re: [paho-dev] Blocking "wait" in the C async library

Hi Frank,

so for the MQTTAsync API the model is:

   1. call sendMessage()    /* no blocking at all - that is the point of the async API */

   2. get the token back in the onSuccess callback /* admittedly I don't like the timing of this callback currently which is at the same time as deliveryComplete.  I was trying to match the _javascript_ API.  I think this should happen at the same time as does the return from MQTTClient API (see below) which would give you the information to use in getPendingTokens, as soon as it's available.  */

   3. match that token with that in the deliveryComplete callback /* if you want to wait for the QoS 1/2 exchange to complete */

The corresponding model for the MQTTClient API is:

    1. call publish(), get the token back.  /* That is because publish() blocks until the packet is written to the wire, and while the inflight message queue has no space if necessary (inflight message queue is limited to 1 or 10 depending on whether the "reliable" flag is set) */

    2. call waitForCompletion()  /* if you want to wait until the QoS 1/2 exchange is complete */

I always thought waitForCompletion() was an erroneous and unnecessary addition to the Java *async* API.  

Why use the asynchronous API if you don't want to use callbacks?


On 20/05/13 15:01, Frank Pagliughi wrote:
Hey Ian,

It presents a simpler means of synchronization, allows the client to block in an OS-independent way, and mimics the API of the MQTT client libraries in other languages. A wait() function as the sole blocking call is a common pattern in other modern asynchronous libraries: Microsoft Async, C++11 threads/futures. the Paho Java client library, etc. I'm not a GUI guy, I'm an RTOS guy, and do this pattern a lot, where a completed action always signals an OS synchronization object, but also fires an optional callback if the user requested it.

In particular, as I originally mentioned, I'm trying to make the C++ API mimic the Java one as much as possible, like:
mqtt::itoken tok = client.publish("some topic", myMessage);
// Do something else for a while
I can work around this in the C++ library by intercepting the callbacks, but two issues make this a little more complicated due to to the outgoing "send" thread in the C lib:

(1) There doesn't appear to be a way to get the MQTTAsync_token of a message when you send it. And thus it's difficult to tie a token to the message.

(2) Upon return of the send/sendMessage calls, the message has "disappeared" into the queue of the send thread. It doesn't appear in the list of outgoing messages (MQTTAsync_getPendingTokens) until some time later. So it is easy for the app to think that all messages were sent when some have yet to even be queued for transmission.

Thanks for your time,

On 05/20/2013 05:30 AM, Ian Craggs wrote:
A deliberate choice.  Why have one blocking call in an asynchronous API which is designed for GUI use, amongst others?  This is an event-oriented API - use the deliveryComplete callback.  To receive messages with MQTTAsync, you have to use callbacks.  You have to use callbacks to determine the success of API calls. 

For a blocking, simpler to use API, with less use of callbacks, there is MQTTClient.


On 19/05/13 19:06, Frank Pagliughi wrote:
Well, while I'm throwing sand in the works, I might as well continue...

One thing that appears to be "missing" from the C async API is the ability to block a thread on a token while waiting for an action to complete. Something like:
int MQTTAsync_waitForCompletion(MQTTAsync handle, MQTTAsync_token token, int timeout);
This would prevent the app from wasting CPU cycles while spinning on a flag from a callback, and can simplify client apps that don't need the full power of a callback function by eliminating the need for them for basic synchronization. Plus it would allow the user to write more portable client apps, since the library would hide the OS-specific thread functionality.

It appears that the library already wraps condition variables in Linux. Maybe Windows could use a Manual Reset Event to do this?


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