PCo Cache Mode

[Former Member]

In PCo, my OPC agent instance's Cache Mode is set to Demand. How do I decide if this is the right setting for us?

The article below, "SAP Manufacturing Implementation Architecture," by Salvatore Castro, points out that this and other configuration settings are important to identify, but neither I nor our Basis team was able to figure out the best setting from reading that article or from reading the PCo documentation.

SAP Manufacturing Implementation Architecture:
http://www.sdn.sap.com/irj/scn/go/portal/prtroot/docs/library/uuid/a053fb30-9553-2e10-d78e-af8333fed...

We have:
MII 12.2.4 Build(92) on a Linux server
which connects to
PCo 2.2 (2.203.1195.1058 Patch level 32808) on a Windows 2008 server
which connects to
FactoryTalk Gateway, an OPC DA server (OPC-DA 2.05a) running on the same Windows 2008 server

[Former Member · Accepted solution]

Greg,

Demand Mode is probably your best choice.  In Demand Mode tags are added to the PCo internal cache as they are quieried, or if they are added as Subscription Items to an Agent Instance.  if a tag that is not in the cache is queried from MII, it is retrieved from the data source and added to the  PCo internal cache. Cache mode may just try to read all the defined tags in the data source when the agent instance is started, which may delay the instance startup.  You can test it with both settings in your Dev environment and see if there are any performance differences.

Regards, Steve

[Former Member · Accepted solution]

Thanks for the reply from both of you...

Manoj, I had read the documentation but was not certain how to use that to decide which setting was best.

Steve, thanks for the reply. As you know this was coming from the standpoint of me getting some errors with PCo and rethinking whether I had it configured correctly.

I still am confused by one thing... I get the distinction between demand, cache, and none. I don't understand why "alias" would be in this list. What is it about having an alias that should prevent you from being able to choose the "demand", "cache", or "none" options?

Greg

[Former Member · Accepted solution]

I have another follow-up question.

Has anyone experienced success or problems when setting Cache Mode to NONE?

The reason I'm asking: I am continuing to experience intermittent problems as described in this thread, where tag values from PCo are getting "stuck," not updating when they should. I see caching as a possible problem; hence we might try turning caching off.

Greg

[Former Member · Accepted solution]

Hi Greg,

Cache Mode set to NONE will simply have PCo query the tags directly from the OPC server/PLC for updates, rather than having the OPC Server send DataChanged notifications to PCo for the subscription group defined in the OPC Source Agent.  this can impact performance if you have a large number of tags in the PCo Agent Instance Subscription.

Which OPC Server are you connecting to, and which OPC DA Interface (2.05A, 3.00) is specified for the OPC Source Agent? Are there anly events in the OPC Server logs that correlate with your notice of loss of updates?  Is the OPC Server remote from PCo (so DCOM configuration is required)?

This very unusual behavior for OPC Servers to just stop updating clients.  I have seen this happen before, but only  in relation to a disconnect/reconnect between the PLC and the OPC Server due to network congestion affecting the read requests between the OPC Server and PLC, and only for a relatively short period of time ( minutes or less).

Regards, Steve

[Former Member · Accepted solution]

The OPC server is FactoryTalk Gateway, an OPC-DA 2.05a server. It is running on the same server as PCo.

I see nothing in the OPC server logs that correspond with our loss of updates.

One part of your answer jumps out at me... You say that Cache Mode = DEMAND will have "the OPC Server send DataChanged notifications to PCo for the subscription group defined in the OPC Source Agent."

What do you mean by subscription group? The only thing I see like that in our OPC Source Agent is the list of aliases. Is that what you mean? I did not set up any aliases for our OPC Source Agent, and didn't think this had anything to do with how the cache mode worked.

One of the reasons I didn't create aliases was that I wanted to avoid any extra steps that might be required when adding new tags to our system. I also didn't want to change the tag name, and didn't know if it would improve performance much.

As an additional piece of info, we have at least 500+ tags that we query on our OPC server. I was meaning to ask if this is outside the norm for PCo.

Do we need to set up aliases or "subscription groups"? Or is it a viable option to consider turning off caching, especially considering PCo is on the same machine as the OPC software.

Greg

[Former Member · Accepted solution]

Greg,

Can you browse the FactoryTalk Gateway OPC Server from the following PCo locations:

Source Agent ->Aliases Tab ->Add Alias->Agent->Browse

and

Agent Instance -> Subscription items Tab ->Browse button ->Browse

And successfully navigate the browse tree and see the tags in the OPC Server?

If you have created tags in an Alias in the Source Agent the only option for accessing them from an Agent Instance is to set the Cache mode to Alias.  If you have created Subscription Items for the Agent Instance then the Agent Instance Cache modes of Demand, Cache, and None are selectable.

Note: to browse all the tags in the OPC Server you may need to adjust the Tools->Options->Global Properties->Browsing->Returned Row Size to a number large enough to allow retrieving all of the tags in the OPC Server.

Subscription Group isthe default name for the Source Agent subscription group, is is defined in the OPC Source Agent Settings Tab->Group Settings group-->Name

We have successfully deployed PCo with OPC Servers containing thousands of tags across dozens of Agent Instances. 500 tags for an Agent Instance is not outside of the norm for PCo.

Regards, Steve

[Former Member · Accepted solution]

So just to be clear, I set up no aliases in the source system, and I also set up no subscription items in the agent instance.

Are you suggesting that the cache modes of demand/cache/none are only supposed to be possible if subscription items are set up? I have been using "demand" (and also tried out "none" today), despite having no subscription items.

To answer your question regarding the ability to browse tags in PCo... yes, I am able to browse the tags that way (although I had never done this before).

Regarding the adjustment of "returned row size"... Thank you for telling me this. This explains why we have not been getting the full list of tags when setting up PCoQuery items in MII. We have been getting around this problem by not using the PCoQuery tag browsing tree, but just typing in the exact tag name. I have adjusted the returned rows up to 1000. I can now see the entire list of tags when browsing tags for aliases or subscription items within PCo (for some reason this has not altered the list in MII's PCoQuery tag browser yet).

Does any of this give you any ideas for why we had general success, but intermittent failures?

Will the adjustment of returned row size help with this problem? Will some action be required for MII to receive the newer, fuller list of tags?

Greg

[Former Member]

Hi Gregory,

If you are talking about Agent Instance --> Tag Query Tab--> Cache Mode than below information I found in SAP Plant Connectivity 2.2 Help document may be you already aware of this

Cache Mode

None   With this setting, all data is retrieved directly from the data source for queries. With this setting, performance is slowest.   Demand  With this setting, for a query the cache is first checked to see whether there are tags and metadata. If not, the tags are retrieved from the data source and added to the cache.   Alias  You use this setting if you have previously created an alias for the source system. An alias is a user-defined structure for tags, metadata, and secondary elements. The alias is loaded when the agent starts. See also: Source System: Aliases Tab. Note Using an alias improves performance and is therefore the recommended approach.   Cache  With this setting, all tags are queried from the data source and stored in the cache. Metadata is retrieved when a tag is requested.

http://help.sap.com/saphelp_pco22/helpdata/en/46/a00344d44852b7e10000000a155369/frameset.htm

Hope this help!!

Regards,

Manoj Bilthare