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Redundancy: What is the standby buffer and what is its purpose?
I have come across the term "standby buffer" several times in the past but I had to find out that I hardly know anything about it. Neither I understand how it works.
Apparently, the standby server with the highest available rate amongst all standby servers in an associaction is in so-called "active standby" mode. It maintains a buffer with values that can be subscribed by clients. This is confusing to me. According to my prior knowledge clients always subscribe to the active server. I understand that this mechanism shall optimize performance upon switchover of the active server but currently I do not see the benefit of it. As well I do not really understand the role of standby servers in an association. Until now I thought that it would be running almost idle.
Posted : 06/11/2012 3:54 pm
Ok let me clarify.
1. You must make the difference between Active Server and Active Server!
- Active Server: Data Acquisition point of view. Server that is getting the data from the device (i.e. Single Active Server Association). For my explanations I will use Active Server (DA).
- Active Server: Client/Server connection point of view. A client is refreshed by a Server. This Server is the Active server. For my explanations I will use Active Server (CLT).
2. In a Single Server Assoc, the StandBy server is not idle. It is a "client like". It is refreshed by the Active Server (DA)
3. Whatever the Association type, 1 Client is ALWAYS connected to both Servers. It ALWAYS receives the data from both Servers. But, it takes in account the data from only one. This one is the Active Server (CLT).
4. Who is the Active Server (CLT)?
- In a Single Active Association the Active Server (CLT) is ALWAYS the Active Server (DA).
- In a Multiple Active Association the Active Server (CLT) is the first Server replying the first Client connection request. Note: You can change it by Scada Basic.
5. So, what's the benefits?
- The Client is refreshed by both Servers therefore, when it switches from Server A to B, it has all data subscribed and refreshed already. Then the switch over is very very fast (almost instantly)!
- The side effect is: The traffic on Network is doubled.
Now, I hope it's more clear for you.
Posted : 12/11/2012 11:09 am