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| MQTT and HA |
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| p.cradwick |
 Posted: Thu Nov 27, 2008 1:36 pm Post subject: MQTT and HA |
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Acolyte
Joined: 16 May 2001
Posts: 56
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Hi,
I have an existing (Windows based) broker system (6.0) using MQTT to communicate with mobile clients. The system is working in prod but after a recent scare, the customer would now like to implement an HA solution. Simplistically, my initial thought was to use MQ clusters to get performance benefits as well. This would involve 3 clustered Queue Managers with QM1 feeding QM2 and QM3 each on separate servers. Message order problems would be solved using a tailored workload exit which would ensure that all messages for a specific client went through the same server, and on failure of one server, it would route all messages through the other.
This seems to come unstuck however when considering how MQTT would work with clusters. The mobile clients have to connect to a broker using the Scada port, but the Scada ports of the 2 servers cannot be the same so that on failover, the healthy server cannot communicate with the clients of the failed server. Also, according to the manual, the subscriptions on the failed server cannot be transferred to the healthy server.
It would seem from this that MQ clusters don't work with MQTT and neither would any active/active setup. Is that a fair assessment or have I missed something?
What would you recommend for an HA solution in this scenario? Standard active/passive servers and MSCS?
Peter |
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| fjb_saper |
| Posted: Thu Nov 27, 2008 2:03 pm Post subject: |
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Grand High Poobah
Joined: 18 Nov 2003
Posts: 20756
Location: LI,NY
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Hi Peter,
This should be able to work in an active/active or active/passive scenario.
The point is that the IP address (hope scada uses IP) used by the client is neither IP of the physical servers but a logical IP that resolves to whichever is active. In an active/passive scenario the port number does not change as only one of the attach points is active at any time.
In an active/active scenario the port number cannot be the same for both attach points.
Now any non durable subscriptions would need to be renewed after failover.
So HA should work but failover to another qmgr within MQ cluster? I know way to little about MQTT to be able to answer that one.
Hope it helps some 
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MQ & Broker admin |
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| p.cradwick |
| Posted: Thu Nov 27, 2008 4:10 pm Post subject: MQTT and HA |
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Acolyte
Joined: 16 May 2001
Posts: 56
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Yes Scada does use IP but in an active/active case they would both have to be active and as you say the 2 port numbers would have to be different. Does this mean that there are 2 virtual IPs?
It seeems to me then that the clients would have to connect explicitly to one or other broker as I don't think the port numbers can be virtual, and under failover conditions, all clients on the failed server would have to reconnect and resubscribe to the healthy one.
Do you think that would work?
If so, then maybe this manual intervention is the price to pay to have an active/active MQTT setup and unfortunately the 'seamless' failover that MQ clusters can potentially provide is not available.
I would still like to use MQ clusters in an active/active scenario for performance reasons and will investigate whether it is possible for the clients to perform the reconnect specifying a different port - like at 3am!
Peter |
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| fjb_saper |
| Posted: Thu Nov 27, 2008 9:16 pm Post subject: |
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Grand High Poobah
Joined: 18 Nov 2003
Posts: 20756
Location: LI,NY
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Ideally what you would end up with are 4 IP addresses
- IP1 for HA cluster admin
- IP2 for applications connecting to the HA cluster (virtual IP)
- IP3 for HA server 1
- IP4 for HA server 2
Now in an active/active scenario you would have IP2 port 1 connect to IP3 port 1 and IP2 port 2 connect to IP4 port 2.
In case of failover of server 1 to server 2, IP2 port 1 would then connect to the failedover IP4 port 1.
Like I said, in case of failover all non persistent subscriptions will need to be renewed.
Have fun
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