Datasheet

EXCHANGE SERVER ARCHITECTURE 15
checked to ensure that they are in the correct sequence. Once this is verified, the replication ser-
vice copies the log file (E0000000001.LOG) to the target log file directory. The Information Store
service then replays the transactions found in the E0000000001.LOG file and the transactions
are committed to the passive copy of the database.
At any given time, the most out-of-sync passive copy of the database will be approximately
15 minutes. The 15-minute lag time would be in a worst-case scenario such as in the dead of
night when there is absolutely no activity on the mailbox database. During a normal workday
in which users are actually using the database, the passive copy of the database will be no more
than a few minutes behind.
If a database is dismounted or the Information Store service is stopped, the data is all com-
mitted to the active database and the log files are pulled over to the servers that hold a passive
copy of the database. If the administrator has to manually switch over to the passive copy of
the database, the passive copy should be completely synchronized with the active copy of the
database.
Mailbox Database Mobility
Exchange 2010 introduces the concept of database mobility. Database mobility is a set of
technologies and features that allow a mailbox database to be replicated to more than one
Exchange server in an organization and that database to be brought online if the active copy of
the database is no longer available. High availability is no longer tied to a specific server but
rather to individual databases.
A mailbox database can be replicated to any Exchange 2010 Mailbox server within the same
DAG. The DAG is a collection of one to 16 Exchange 2010 Mailbox servers that can be config-
ured to host a set of databases. The DAG is the boundary of database replication and can span
multiple Active Directory sites and geographic locations.
Figure 1.6 shows a simplified example of a DAG. This group has three Exchange Mailbox
servers as members and each of the servers has a single ‘‘active’’ mailbox. The server in Tokyo
has an active mailbox database called Executives, but a copy of this database is replicated to the
Denver and Honolulu servers. The database can be replicated to one or more servers in the DAG.
In the event of a failure on the Tokyo Mailbox server or a problem with the Executives
database on the Tokyo Mailbox server, the database on either the Denver or the Honolulu
server will be made active and users will be redirected to the new ‘‘active’’ location.
Database mobility replaces the SCR, CCR, LCR, and single-copy cluster features that were
available in previous versions of Exchange.
High Availability and Resiliency
XYZZY Cor poration has their headquarters office in South Florida as well as regional offices
on the East Coast and in Colorado. The Colorado office has a small data center. Most data
services are handled in the Florida office. In recent years, the South Florida office has had
several instances where they had to close the office and shut down their data center because
of hurricanes. This means not only does the South Florida office lose Exchange services but it
also loses all users in the eastern United States.