Unreachable broker connection blocks 2nd working connection

[jpwsutton]

migrated from Bugzilla #483296
status CLOSED severity normal in component MQTT-C for 1.2
Reported in version 1.2 on platform PC
Assigned to: Ian Craggs

On 2015-11-30 06:10:40 -0500, Juergen Kosel wrote:

Imagine the following situation:
A MQTT client should connect to more than 1 broker and publish some topics.
But unfortunately one of the brokers is (temporary) unreachable.

Since there is only one MQTTAsync_sendThread for all connections to MQTT brokers,
any publish transaction to a reachable broker is blocked, while the MQTTAsync_sendThread tries to connect to the unreachable one.

And to make things even more worse, the client may
try to connect again, by calling MQTTAsync_connect().
Internally the new connect request is placed into the first position of the command list. So any write request to a well reachable broker would be blocked
again.

On 2015-12-08 02:43:22 -0500, Juergen Kosel wrote:

While view at the code, I had seen that there is only one thread doing all the work.
But I have overseen, that the sockets are opened in none blocking mode.
So attempts to open a connection to a not reachable broker decrease performance, but do not block other connections at all.

Sorry for reporting this false bug.
Juergen

On 2015-12-08 09:04:24 -0500, Ian Craggs wrote:

Hi Jurgen,

thanks for the update. I had been thinking about this, and the only potentially blocking call is getaddrinfo(), to resolve hostnames. This could be improved by having DNS lookups in a separate thread or by using a non-blocking DNS lookup library.

Most of the time, DNS lookups are quick, so you wouldn't see an adverse effect, and I've not had it reported as a problem. But it is a possible improvement that could be made.