Confused about distributed engine deployments

RE: Confused about distributed engine deployments

Mike Cichy — Mon, 11 Dec 2017 13:42:47 GMT

I would suggest that you read up on this topic. This page has all the articles we have on HA:
community.appian.com/.../backup_3A00_HA_3A00_DR
I would specifically review these pages:
community.appian.com/.../configuring-an-environment-for-high-availability
community.appian.com/.../high-availability-and-disaster-recovery-configurations
community.appian.com/.../hardware-engine-failover-configuration

RE: Confused about distributed engine deployments

adamb940 — Fri, 08 Dec 2017 23:48:15 GMT

Hi Mike, the idea of putting such a component in between the Appian gateway (not sure what "transaction management component" means or if this is the same thing) and the Appian engine, is basically to provide some resilience. The middle component does nothing more than direct traffic, and is not expected to fall over, so has a fixed IP address. If the engine server crashes, we can bring it back up, but it may have a different IP address. Therefore we would have liked to point the Appian gateway to this middle component and then that component redirects the traffic to the actual engine. However from what you have said, it sounds like Appian cannot support this kind of setup,

RE: Confused about distributed engine deployments

Mike Cichy — Fri, 08 Dec 2017 22:15:51 GMT

There shouldn't be any components in between the transaction management components and engines. They will be connecting directly to the specified address and port. So yes, if the host in the topology file does not point to itself (either through IP, FQDN or a mapping) then the processes will not start.

RE: Confused about distributed engine deployments

adamb940 — Fri, 08 Dec 2017 22:03:42 GMT

Hi Mike, yes, this is what I was getting at in my first post "Does this mean the hostname specified in topology needs to exactly match /etc/hostname"?

So from your post I understand it can match "/etc/hostname", the machine's IP address, or an entry in "/etc/hosts".

However, if I have for example a component which sits in front of an engine (and directs traffic to that engine), and put that component's IP address in the "appian-topology.xml", does this mean that Appian will not be able to work out that this server entry is meant to correspond to the server which it is running on?

RE: Confused about distributed engine deployments

Mike Cichy — Fri, 08 Dec 2017 21:54:39 GMT

The server tag. (<server host="machine1" port="8000">)
The value for host can be what you specify in the hosts file, IP or FQDN.

RE: Confused about distributed engine deployments

adamb940 — Fri, 08 Dec 2017 20:25:46 GMT

Yes I am deploying the same appian-topology.xml file on all servers and yes I am using the latest version of Appian (17.4).

I am still not able to get the answer I want from what you have said. So you say "only the engines denoted in the appian-topoloxy.xml file for that server will start on it", but how does it know which engines those are, in other words how does it know which part of the appian-topology.xml to look at to find the engines for "that server"?

RE: Confused about distributed engine deployments

aswinb608 — Fri, 08 Dec 2017 19:18:43 GMT

Hope you deploy the same appian-topology.xml file on all servers...then if you go to each individual server and do start-suite.sh on prior 17 versions or start.sh in 17.x versions....only the engines denoted in the appian-topoloxy.xml file for that server will start on it and you have to do start.sh on all other servers denoted in appian-toplogy.xml to start all the engines....Appian has a system called heart beat which detects all of this...

RE: Confused about distributed engine deployments

adamb940 — Fri, 08 Dec 2017 18:28:37 GMT

Hi Aswin, yes I understand that, but that is discussing it conceptually rather than how it works in practice. So in your example, if you have a machine starting up and that machine is the one which should be running "forums" and "notify", how does it know that it is that machine as opposed to the machine running "process-exec"?

RE: Confused about distributed engine deployments

aswinb608 — Fri, 08 Dec 2017 16:32:39 GMT

In the appian-topoloy.xml file the tabg <server host="machine1" port="500"> and then having <engine name="forum"><engine name="notify"> tells that these two engines must be run on that server and then if you have another server with <engine="process-exec"> then that server will run only process execution..