Terminology used in Exchange Server 2013 High Availability
Active Manager
An internal Exchange component which runs inside the Microsoft Exchange Replication service that's responsible for failure monitoring and corrective action through failover within a database availability group (DAG).
Primary Active Manager (PAM)
PAM is the Active Manager role in a DAG that decides which copies will be active and passive. PAM is responsible for getting topology change notifications and reacting to server failures. The DAG member that holds the PAM role is always the member that currently owns the cluster quorum resource (default cluster group). If the server that owns the cluster quorum resource fails, the PAM role automatically moves to a surviving server that takes ownership of the cluster quorum resource. In addition, if you need to take the server that hosts the cluster quorum resource offline for maintenance or an upgrade, you must first move the PAM to another server in the DAG.
Standby Active Manager (SAM)
The SAM provides information on which server hosts the active copy of a mailbox database to other components of Exchange that are running an Active Manager client component (for example, Client Access or Transport services). The SAM detects failures of local databases and the local Information Store. It reacts to failures by asking the PAM to initiate a failover (if the database is replicated).
AutoDatabaseMountDial
A property setting of a Mailbox server that determines whether a passive database copy will automatically mount as the new active copy, based on the number of log files missing by the copy being mounted.
Loseless - The database does not automatically mount until all logs generated on the active device are copied to the passive device.
Good Availability - The database automatically mounts immediately after a failover if the queue length is less than or equal to 6.
Best Availability - The database automatically mounts immediately after a failover if the queue length is less than or equal to 12.
Continuous replication - block mode
In block mode, as each update is written to the active database copy's active log buffer, it's also shipped to a log buffer on each of the passive mailbox copies in block mode. When the log buffer is full, each database copy builds, inspects, and creates the next log file in the generation sequence.
Continuous replication - file mode
In file mode, closed transaction log files are pushed from the active database copy to one or more passive database copies.
Database availability group
A group of up to 16 Exchange 2013 Mailbox servers that hosts a set of replicated databases.
Lagged mailbox database copy
A passive mailbox database copy that has a log replay lag time greater than zero.
Safety Net
Formerly known as transport dumpster, this is a feature of the transport service that stores a copy of all messages for X days. The default setting is 2 days.
Shadow redundancy
A transport server feature that provides redundancy for messages for the entire time they're in transit.
Attempt Copy Last Logs (ACLL)
Process which tries to copy any missing log files from the server that hosted the active database copy prior to the failure or switchover.
Datacenter Activation Coordination (DAC) mode
DAC mode is used to control the database mount on startup behavior of a DAG. This control is designed to prevent split brain from occurring at the database level during a datacenter switchback. Split brain, also known as split brain syndrome, is a condition that results in a database copying being mounted as an active copy on two members of the same DAG that are unable to communicate. Split brain is prevented using DAC mode because DAC mode requires DAG members to obtain permission to mount databases before they can be mounted.
For example, consider a scenario where a primary datacenter contains two DAG members and the witness server, and a second datacenter contains two other DAG members. In this scenario, the DAG is not in DAC mode. The primary datacenter loses power, so you activate the DAG in the second datacenter. Eventually power to the primary datacenter is restored, and the DAG members in the primary datacenter, which had quorum before the power failure, will start up and mount their databases. Because the primary datacenter was restored without network connectivity to the second datacenter, and because the DAG was not in DAC mode, the active databases within the DAG entered a split brain condition.
TruncationLagTime
This parameter specifies the amount of time that the Microsoft Exchange Replication service should wait before truncating log files that have replayed into a copy of the database. The time period begins after the log has been successfully replayed into the copy of the database. The format for this parameter is (Days.Hours:Minutes:Seconds). The default setting for this value is 0 seconds. The maximum allowable setting for this value is 14 days. The minimum allowable setting is 0 seconds. Setting the value for truncation lag time to 0 turns off log truncation delay.
SeedingPostponed
This parameter specifies that the task shouldn't automatically seed the database copy on the specified Mailbox server. This option is typically used when you intend to seed a new mailbox database copy by using an existing passive copy of the database (for example, adding a second copy of a specific database to a remote location).
Auto Reseed
AutoReseed is designed to automatically restore database redundancy after a disk failure by using spare disks that have been provisioned on the system.
Active Manager
An internal Exchange component which runs inside the Microsoft Exchange Replication service that's responsible for failure monitoring and corrective action through failover within a database availability group (DAG).
Primary Active Manager (PAM)
PAM is the Active Manager role in a DAG that decides which copies will be active and passive. PAM is responsible for getting topology change notifications and reacting to server failures. The DAG member that holds the PAM role is always the member that currently owns the cluster quorum resource (default cluster group). If the server that owns the cluster quorum resource fails, the PAM role automatically moves to a surviving server that takes ownership of the cluster quorum resource. In addition, if you need to take the server that hosts the cluster quorum resource offline for maintenance or an upgrade, you must first move the PAM to another server in the DAG.
Standby Active Manager (SAM)
The SAM provides information on which server hosts the active copy of a mailbox database to other components of Exchange that are running an Active Manager client component (for example, Client Access or Transport services). The SAM detects failures of local databases and the local Information Store. It reacts to failures by asking the PAM to initiate a failover (if the database is replicated).
AutoDatabaseMountDial
A property setting of a Mailbox server that determines whether a passive database copy will automatically mount as the new active copy, based on the number of log files missing by the copy being mounted.
Loseless - The database does not automatically mount until all logs generated on the active device are copied to the passive device.
Good Availability - The database automatically mounts immediately after a failover if the queue length is less than or equal to 6.
Best Availability - The database automatically mounts immediately after a failover if the queue length is less than or equal to 12.
Continuous replication - block mode
In block mode, as each update is written to the active database copy's active log buffer, it's also shipped to a log buffer on each of the passive mailbox copies in block mode. When the log buffer is full, each database copy builds, inspects, and creates the next log file in the generation sequence.
Continuous replication - file mode
In file mode, closed transaction log files are pushed from the active database copy to one or more passive database copies.
Database availability group
A group of up to 16 Exchange 2013 Mailbox servers that hosts a set of replicated databases.
Lagged mailbox database copy
A passive mailbox database copy that has a log replay lag time greater than zero.
Safety Net
Formerly known as transport dumpster, this is a feature of the transport service that stores a copy of all messages for X days. The default setting is 2 days.
Shadow redundancy
A transport server feature that provides redundancy for messages for the entire time they're in transit.
Attempt Copy Last Logs (ACLL)
Process which tries to copy any missing log files from the server that hosted the active database copy prior to the failure or switchover.
Datacenter Activation Coordination (DAC) mode
DAC mode is used to control the database mount on startup behavior of a DAG. This control is designed to prevent split brain from occurring at the database level during a datacenter switchback. Split brain, also known as split brain syndrome, is a condition that results in a database copying being mounted as an active copy on two members of the same DAG that are unable to communicate. Split brain is prevented using DAC mode because DAC mode requires DAG members to obtain permission to mount databases before they can be mounted.
For example, consider a scenario where a primary datacenter contains two DAG members and the witness server, and a second datacenter contains two other DAG members. In this scenario, the DAG is not in DAC mode. The primary datacenter loses power, so you activate the DAG in the second datacenter. Eventually power to the primary datacenter is restored, and the DAG members in the primary datacenter, which had quorum before the power failure, will start up and mount their databases. Because the primary datacenter was restored without network connectivity to the second datacenter, and because the DAG was not in DAC mode, the active databases within the DAG entered a split brain condition.
TruncationLagTime
This parameter specifies the amount of time that the Microsoft Exchange Replication service should wait before truncating log files that have replayed into a copy of the database. The time period begins after the log has been successfully replayed into the copy of the database. The format for this parameter is (Days.Hours:Minutes:Seconds). The default setting for this value is 0 seconds. The maximum allowable setting for this value is 14 days. The minimum allowable setting is 0 seconds. Setting the value for truncation lag time to 0 turns off log truncation delay.
SeedingPostponed
This parameter specifies that the task shouldn't automatically seed the database copy on the specified Mailbox server. This option is typically used when you intend to seed a new mailbox database copy by using an existing passive copy of the database (for example, adding a second copy of a specific database to a remote location).
Auto Reseed
AutoReseed is designed to automatically restore database redundancy after a disk failure by using spare disks that have been provisioned on the system.
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