High Availability and Site Resilience

Si vis pacem, para bellum = if you wish for peace, prepare for war"

database file *.edb file and transaction logs 1 MB in size to provide consistent storage

emails are written to transaction logs then committed to database






continuous replications

database is initially copied and then log files are shipped and replayed constantly to keep the database up-to-date

system 1 is active and passive copy on one or more system.  trans logs shipped and replayed into it.


database may not in sync and data lost during a crash

passive can be set to lag changes from Active to help preventing active corrupted database corrupting the passive database .  ie transaction logs are x hours behind the active/passive database copy.



database availability group

uses continuous replication.  allows up to 16 mailbox servers that hosts a set of replicated databases.

if active database fails, automatic failover to a passive database using Active Manager an Exchange component running inside Exchange Replication service.

Creating a DAG is 3 step process

• create a DAG (15 Character name)

• add mailbox servers to the DAG (up to 16)

• determine passive copies of your active database(s)
• lag database (optional)

 split brain syndrom - active and passive becomes "active" thinking the other is down.

Wintess server and witness directory

DAg witness server and directory are only used for quorum (referee) purposes when you have an even number of DAG members.  witness server can't be a member of the DAG as well.

before becoming active, it checks with witness server which confirms the outage.

in 2008 R2, manually create in AD
and register within subsystem

blank creates default path is created in system drive folder "agfilesharewitnesses"

Add member servers

exchange uses Windows Failover clustering services

 up to 16 member server can be added

static IP can be used for avalability group IP

copy database to passive

sepcify where to copy to.  activation preference is overridden by Active Manager if needed.

"seeding" - copying initial copy.

updates are then done through continuous repilcation

summary of DAG protected vs regular database.






index state is healthy