OAB in Exchange Server 2013

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.