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SAP hana system replication and "pseudo clustering"

former_member182307
Contributor
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Hello,

I have one question mainly to know if the kind of set up I could test was supported by SAP or not.

I have a system in which HDB 3tier system replication is set up.

    

     Node 1 -> Node 2 ( sync )

     Node2 -> Node 3 ( async ) 

Node2 is here in order to provide some HANA "pseudo" HA.

I say pseudo because in this landscape, there is no clustering involved...

Node 3 is here for DR. No clustering involved here neither.

An SAP abap application server is running on top of this, secured by VMWare HA ( for HA ) and Veeam system replication ( for DR ).

Doing some tests, I changed the default hdbuserstore key from  :

KEY DEFAULT

  ENV : Node1:30041

  USER: SAPABAP1

To :

KEY DEFAULT

  ENV : Node1:30041,Node2:30041,Node3:30041

  USER: SAPABAP1

As if I was scaled out , but I'm not scaled out. This is only here in order not to reconfigure the ABAP application server in case any hana node fails.

Testing this set up I could see that things seem to be working fine.

     My test was :

          - Connect to Node 1

          - HDB Stop

          - Connect to Node 2

          - hdbnsutil -sr_takeover


     I checked SM21 on the ABAP system and could see the workprocesses going in and then out of reconnect mode.

     In ST04, I see I'm connected to the Node2 db host.

     I also checked the persistence location seen at HANA level and it went from Node1 to Node2 :


         select count(*) from M_TABLE_PERSISTENCE_LOCATIONS

          where

          schema_name='SAPABAP1' and PERSISTENCE_HOST='Node1'

          COUNT(*)

          0


          select count(*) from M_TABLE_PERSISTENCE_LOCATIONS

          where

          schema_name='SAPABAP1' and PERSISTENCE_HOST='Node2'

    

          COUNT(*)

          98877



     Replication between nodes 2 and 3 is still O.K.

     So everything seems to be O.K.

The advantage I find in this set up is that I don't have to reconfigure the ABAP AS to point to the surviving HDB Node.

It avoids me to restart the AS.

Whenever one of the surviving Hana node is started up ( manually ) , The AS reconnects to it and that's it !

And this would be useful in this situation where, ( don't ask me why ... ) , i'm not allowed to use any clustering software for the hana hosts .

My question is : can I use this kind of setup and be supported ?

What would be the drawbacks you would see in this ?

One I can put on the table is split brain situation if Node 1 is set back in the game while Node 2 is also on.

Although the system sees the host holding the tables is Node2 I wonder if this would still stay consistent no matter the situation


Thanks & BR,

Steve.

Accepted Solutions (1)

Accepted Solutions (1)

Former Member
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Hello Steve,

What you have setup is multitier system replication which is fully supported by SAP as far as I am aware.

Set up Multitier System Replication with hdbnsutil: SAP HANA Administration Guide - SAP Library

Also see page 21 in the following presentation.

https://hcp.sap.com/content/dam/website/saphana/en_us/Technology%20Documents/2_BuildI_HA&DR_Czekalla...

To avoid a "split brain" you really should have a clustering solution in place with STONITH.

Another thing I strongly advise is that the HDB instances you install you should install with virtual addresses and only those virtual addresses should be used by your clients (studio/SAP/browser) to connect. eg: in your hdbuserstore you would add the virtual address (v<sid>xxxxx). That way you can perform some isolation with your DNS by making the original primary virtual address non resolvable to your client.

Hope this helps a touch.

KR,

Amerjit

Answers (2)

Answers (2)

former_member182307
Contributor
0 Kudos

Hi,

Thank you guys for your helpful points.

It helped to clear up a few things.

I do agree with your views : using a clustering software would be the way to go for sure.

But as the Rolling Stones would say :"You can't always get what you want" ! 

Thanks and Best regards,

S.SOUMAH.

anandtigadikar
Employee
Employee
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My 2 cents , as you are doing kind of TDI setup, SAP will support  and bless it, but you have to make sure-

1. During any of the nodes in primary goes down, make sure you have Standby node , so you can have proper functioning of  application on primary site. Also Application should understand, through proper configuration of all the nodes in the landscape.

2. Your application should properly do this take over wrt to HANA nodes configuration, due to which, it should be easy for client to work, without any issues or awareness, if they are working on Primary/Secondary DC.

PS: Above all are my comments only and those needs to be tested & tried on sandbox environment, before proceeding with production systems.

-Anand.