Architecting a Hybrid Messaging Strategy with Microsoft Exchange 2013 : Application Architectural Overview : 6.1 Mailbox Servers and Database Distribution
   
6.1 Mailbox Servers and Database Distribution
Given the functional requirements, Rainpole.com’s IT organization has determined the number of mailbox servers and the mailbox database distribution. Microsoft advises customers that each copy of a database be hosted on a separate mailbox server. Therefore, Rainpole.com needs eight mailbox servers across two DAGs to support the three highly available database copies, and one lagged copy per DAG (one copy per server).
The following table illustrates the mailbox database distribution among the required mailbox servers for one of the two DAGs. The second DAG is the mirror image of the first DAG, with mailbox database 1 (DB 1) having copies one and two at the on-premises data center, and the third and the lagged copy at the VMware Cloud Provider data center facility. Other mailbox databases are to be distributed in a similar manner, balancing the workload evenly across all mailbox servers.
Table 1. Database Distribution Among Servers DAG 1
On-Premises DC
Active Server
Server 1 DB Copy
Server 2 DB Copy
 
VMware Cloud Provider Program DC
 
Active Server
Server 3 DB Copy
Server 4 DB Copy
DB 1
Server 1
1
2
DB 1
 
Server 3
3
Lag
DB 2
Server 2
2
1
DB 2
 
Server 4
Lag
3
DB 3
Server 1
1
2
DB 3
 
Server 3
3
Lag
DB 4
Server 2
2
1
DB 4
 
Server 4
Lag
3
DB 5
Server 1
1
2
DB 5
 
Server 3
3
Lag
DB 6
Server 2
2
1
DB 6
 
Server 4
Lag
3
…
 
 
 
...
 
 
 
 
DB 18
Server 2
2
1
DB 18
 
Server 4
Lag
3
1 High Availability Database Copy 1 (Active)
2 High Availability Database Copy 2 (Passive)
3 High Availability Database Copy 3 (Passive)
4 Lagged Database Copy
 
Figure 3. Database Availability Group Distribution Across Mailbox Servers
 
The Microsoft Exchange 2013 messaging system design enables the organization to withstand up to two mailbox server failures without loss of data. For instance, if mailbox server 2 fails, the passive copies (number 2) for each database hosted by mailbox server 2 will activate on mailbox server 1. If mailbox server 1 then also fails, the third database copy hosted at the VMware Cloud Provider facility can be activated by the operations team.
With two mailbox servers at each site hosting active mailboxes (12,500 users per site), the entire population of 25,000 users is divided equally among the four servers (two servers per DAG) resulting in 6,250 users per server during normal operational conditions, with no failed servers. With a single mailbox server failure, a secondary mailbox server is required to handle the workload generated by 12,500 users, and therefore needs to be sized accordingly. This application architecture for Microsoft Exchange 2013 provides highly available database copies and a high level of data protection and redundancy.