OAB History

Offline Address Books, fondly referred to as OABs, are a critical component in Exchange infrastructure for a long time now. An OAB is used by Microsoft Outlook clients in Cached Exchange Mode for address book lookups when offline. OABs are also critical in reducing the workload on Exchange servers as cached mode Outlook clients will always query the local OAB first.

The OAB has evolved over Exchange releases. The last major overhaul of OAB architecture was in Exchange Server 2007, where we introduced web-distribution of OAB along with CAS server role taking major responsibility of distributing the OAB. But the OAB generation process itself hasn't changed much.

Until now.

With the change in the server role architecture introduced in Exchange Server 2013, we have also changed the way OABs are generated and distributed to clients. Let’s explore the new OAB in Exchange 2013 by comparing it to its predecessors.

Changes in OAB generation

Which Server will generate the OAB?

In all previous Exchange releases, OAB generation was bound to a specific Exchange server by the Server property. When you install the first Exchange mailbox server, setup designates it as the OAB generation server. You can create new OABs as needed. When creating a new OAB, the OAB generation server has to be specified.

OAB in Exchange Server 2010:

Get-OfflineAddressBook "Default Offline Address Book" | fl name,server
 
Name : Default Offline Address Book
Server : MBX1

The disadvantage with this approach was that only one server was configured for OAB generation, and it was a single point of failure. If this server was unavailable for a long period, the OAB generation was affected.

In Exchange 2013, the OAB is generated by each Exchange 2013 Mailbox server(s) that hosts a special type of arbitration mailbox, called organization mailbox. OAB generation is not bound by the Server parameter anymore.

OAB in Exchange Server 2013:

Get-OfflineAddressBook "Default Offline Address Book (Ex2012)" | fl name,server
 
Name : Default Offline Address Book (Ex2012)
Server :

The unbinding of OAB from a specific server allows the same OAB to be generated by multiple Mailbox servers. This new architecture provides greater resiliency in OAB generation.

Which component will generate the OAB?

The Microsoft Exchange System Attendant service was the workhorse responsible for OAB generation in previous Exchange versions. The OAB generation was a scheduled process, i.e. OAB generation would start at the scheduled time configured on the OAB property, irrespective of the work load on the server.

In Exchange 2013, the OABGeneratorAssistant, a mailbox assistant running under the Microsoft Exchange Mailbox Assistants service, generates the OAB. Like most other mailbox assitants, the OABGEnerationAssistant is a throttled process – it runs or pauses according to the workload on the server.

Where are the OAB files stored?

In previous Exchange versions, the OAB generated by the Mailbox server was located in the %ExchangeInstallPath%\ExchangeOAB folder. The folder was shared so the CAS could retrieve the OAB files for distribution to Outlook clients.

In Exchange 2013, the OAB files are generated and stored in the Organization Mailbox first and later copied to the %ExchangeInstallPath%\ClientAccess\OAB\ folder.

Changes in OAB distribution

Exchange 2007 and 2010 supported two methods of OAB distribution: web distribution and Public Folder distribution. Exchange 2013 supports only the web distribution method, so let’s explore the changes in web-distribution method.

The Exchange 2007/2010 CAS pulled the OAB files generated on the respective Mailbox server and stored them locally. The Microsoft Exchange File Distribution Service on the CAS role did the task of pulling OAB files.

This was the flow OAB download from client side:

  1. Outlook receives OAB URL from Autodiscover and reaches a CAS server.
  2. The CAS authenticates the user and serves OAB files from local disk.

Couple of disadvantage with this method:

  1. The OAB download fails if the CAS doesn't have the OAB files locally.
  2. If the File Distribution Service on CAS isn't working, clients will receive stale OAB files or, in other words will not receive the updates.

In Exchange 2013, OAB files are not stored locally on the CAS. CAS 2013 proxies all OAB download requests to the appropriate Exchange 2013 Mailbox server. With this change in the architecture, the Microsoft Exchange File Distribution Service is removed from the CAS role.

In Exchange 2013, this is the flow of OAB download:

    1. Outlook receives OAB URL from Autodiscover and reaches designated CAS 2013 through OAB URL.

The CAS server performs the following actions:

  1. Performs initial authentication for OAB.
  2. Queries Active Directory and determines the closest Organization Mailbox for the requesting user.
  3. Queries Active Directory again to determine the mailbox database hosting the Organization Mailbox.

  4. Queries the Active Manager to determine the mailbox server where the mailbox database is active (mounted).
  5. Proxies the request to the Mailbox server identified in step 4.
  6. Retrieves OAB files and passes them to the client.

This new workflow overcomes the disadvantages of legacy OAB download workflow.

The Organization Mailbox

The Organization Mailbox is a new type of arbitration mailbox introduced with Exchange 2013. The arbitration mailbox with persisted capability OrganizationCapabilityOABGen is referred to as Organization Mailbox. It plays a crucial role in OAB generation, storage and distribution.

Each Exchange Server 2013 mailbox role hosting an Organization Mailbox will generate all Exchange 2013 OAB’s defined in the environment. The OAB is generated in the Organization Mailbox first and later copied to the disk.

Use the following command to identify the Organization mailbox:

Get-Mailbox -Arbitration | where {$_.PersistedCapabilities -like "*oab*"}

Example:

Get-Mailbox -Arbitration | where {$_.PersistedCapabilities -like "*oab*"}
 
Name Alias ServerName ProhibitSendQuota
---- ----- ---------- -----------------
SystemMailbox{bb558c35... SystemMailbox{bb5... mbx1 Unlimited

Storing the OAB files in the Organization Mailbox makes the OAB files more resilient.

Putting it together: A real-life scenario:

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 DAG. 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.

The next article in this series will talk about how to manage the new OAB in Exchange 2013.

Bhalchandra Atre