queue

README

Queue

Users often want to call Publish and then move on to other things on the assumption that the message will eventually get to the server (even if the connection is down at the time the message is sent). This example shows how to implement this with autopaho; it sends a large number of QOS1/2 messages and confirms that they are correctly received (using a second connection).

The main reason for writing this example was to load test the library (sending a large number of messages and, potentially, simulating network outages).

MQTT V5

Note that a linked concept is the MQTT V5 "Receive maximum" functionality. This allows the server to tell the client the maximum number of inflight QOS1/2 messages that it will accept. The client then needs to limit inflight messages to comply; this may mean that the client needs to queue messages to avoid exceeding the limit. How this is configured will vary depending upon the server (with Mosquitto its the max_inflight_messages setting and defaults to 20).

Note that, by default, a memory-based queue is used. This example demonstrates storage on disk thus enabling queued messages to survive an application restart.

Running with the docker Mosquitto instance and testing network outages

Use three consoles; one each for:

• Mosquitto
• The example
• Simulating network outages

Mosquitto

Note: Before running this ensure that mosquitto.conf contains persistence true (this is not in the default config and without it tests that include restarting the server will fail).

Change to the autopaho/examples/docker folder and run docker compose up mosquitto this will start Mosquitto and the log will be output. Note that logging significantly impacts performance so if you are testing that edit the autopaho/examples/docker/binds/mosquitto/config/mosquitto.conf and comment out log_type all.

The example

Build and run the example (i.e. go build then queue). Note that queue~.msg files will be written to the current folder.

Simulating Network Outages

Due to the use of QOS1/2 the test app should receive 100% of the messages sent. To confirm that everything is working as anticipated, we can simulate network issues.

There are a variety of options:

• Stop and start the Mosquitto container
• Kill the network
  • Identify the network with docker network ls (probably called docker_paho-test-net)
  • Identify the mosquitto container with docker container ls (probably be docker-mosquitto-1)
  • disconnect: docker network disconnect docker_paho-test-net docker-mosquitto-1
  • reconnect: docker network connect docker_paho-test-net docker-mosquitto-1