How Does OAB work in Exchange 2013 DAG environment

I have gone through many articles about OAB generation in Exchange 2013 inlcuding some of below mentioned.

http://blogs.technet.com/b/exchange/archive/2012/10/26/oab-in-exchange-server-2013.aspx

http://clintboessen.blogspot.in/2014/03/how-oab-distribution-works-in-exchange.html

But haven't got the answer i needed, my enviroment is a 3 node DAG (2013) multirole

I had issue with OAB download recently, and while troubleshooting i noticed that on each server OAB location

E:\Microsoft\Exchange Server\V15\ClientAccess\OAB\a8954b91-39b9-4922-a4f1-1bda59e42af4

it showed me a different last modified date. and the server which holds the MBX and OAB arbitration mailbox is only has the recent updated files and other 2 nodes have very old files. Hence wondering does this look normal.

Should not all 3 nodes should have the same file update version as when the database is failed over to other node there should not be OAB issue.

August 1st, 2015 2:21am

Well i was able to find out the answer :) 

The following scenario puts together the critical points we learned so far:

  1. MBX1 and MBX2 are Exchange 2013 Mailbox servers and member of a <acronym style="border-bottom-width:1px;border-bottom-style:dotted;border-bottom-color:black;cursor:help;" title="Database Availability Group">DAG</acronym>. CAS1 is an Exchange 2013 CAS.
  2. The organization mailbox is present on mailbox database DB1. DB1 has copies on MBX1 and MBX2.
  3. DB1 is currently active on MBX1.
  4. The Microsoft Exchange Mailbox Assistants service on MBX1 will generate the OAB.
  5. The OAB will be first generated in the organization mailbox and later copied to disk of MBX1. At this point, MBX2 is not playing any role in OAB generation.
  6. An Outlook client tries to download OAB, and reaches CAS1 through OAB URL.
  7. CAS1 queries Active Manager and finds out database hosting organization mailbox (DB1) is active on MBX1.
  8. CAS1 proxies the OAB download request to MBX1 and serves the files back to the client.
  9. At this point, MBX1 goes down due to power failure and DB1 is activated on the server MBX2.
  10. CAS1 receives another request for OAB download, queries the Active Manager again and this time proxies the request to MBX2, as DB1 is now active on MBX2.
  11. MBX2 extracts OAB files present in the organization mailbox to the disk, to ensure latest files are served to the client.
  12. MBX1 comes back online, but DB1 remains active on MBX2.
  13. At next OAB generation work cycle, the Microsoft Exchange Mailbox Assistants service on MBX2 will generate the OAB.

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August 1st, 2015 4:16am

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