What is Transport Dumpster
MaxDumpsterSizePerDatabase : Defines the size available for each storage group on the Hub Transport server. The recommendation is that this be set to 1.5 times the maximum message size limit within your environment. The default value for this setting is 18 MB.
MaxDumpsterTime : Defines the length of time that a message remains within the transport dumpster if the dumpster size limit is not reached. The default is seven days.
If either the time or size limit is reached, messages are removed from the transport dumpster by order of first in, first out. we can run the following command to see the current settings:
Changes in Exchange 2010
In Exchange 2010, the transport dumpster will remove the message when it got replicated to all database copies. This keeps the transport dumpster queue smaller by maintaining only copies of messages whose transactions logs haven't yet been replicated.
Changes in Exchange 2013
1. Transport dumpster is now “Safety Net”
2. It doesn’t require replicated mailbox databases. It will work for the mailbox databases which is not part of DAG and also with PF databases.
3. Safety Net itself is not redundant to avoid single point of failure. It will have a shadow safety Net and resubmits messages when the primary safety Net is not responding for 12 hrs.
How Safety Net works
1. Safety Net works closely with Shadow redundancy.
2. The Primary Safety Net exists on the Mailbox server that held the primary message before the message was successfully processed by the Transport service.
3. After the primary server processes the primary message, the message is moved from the active queue into the Primary Safety Net on the same server.
4. The Shadow Safety Net exists on the Mailbox server that held the shadow message.
5. After the shadow server determines the primary server has successfully processed the primary message, the shadow server moves the shadow message from the shadow queue into the Shadow Safety Net on the same server.
Message resubmission from Safety Net
Message resubmissions from Safety Net are initiated by the Active Manager component of the Microsoft Exchange Replication service that manages DAGs and mailbox database copies. No manual actions are required to resubmit messages from Safety Net.
After the automatic or manual failover of a mailbox database in a DAG.
After you active a lagged copy of a mailbox database.
Note : Main requirement for successful resubmission from Safety Net for a lagged copy is the amount of time messages are stored in Safety Net must be greater than or equal to the lag time of lagged copy of the mailbox database.
Message resubmission from Shadow Safety Net
Scenario 1
1. Active Manager requests a resubmission of messages from Safety Net for a mailbox database for the time interval 5:00 to 9:00. However, the Mailbox server that holds the Primary Safety Net has crashed due to a hardware failure. Active Manager repeatedly tries to contact the Primary Safety Net for 12 hours.
2. After 12 hours, Active Manager sends a broadcast message to the Transport service on all Mailbox servers in the transport high availability boundary looking for other Safety Nets that contain messages for the target mailbox database for the time interval 5:00 to 9:00. The Shadow Safety Net responds are resubmits messages for the mailbox database for the time interval 5:00 to 9:00.
Scenario 2
1. The queue database on Mailbox server that holds the Primary Safety Net is corrupt, and a new queue database is created at 7:00. All of the primary messages stored in the Primary Safety Net from 1:00 to 7:00 are lost, but the server is able to store copies of successfully delivered messages in Safety Net starting at 7:00.
2. Active Manager requests a resubmission of messages from Safety Net for a mailbox database for the time interval 1:00 to 9:00.
3. The Primary Safety Net resubmits messages for the time interval 7:00 to 9:00.
4. The Primary Safety Net sends a broadcast message to the Transport service on all Mailbox servers in the transport high availability boundary looking for other Safety Nets that contain messages for the target mailbox database for the time interval 1:00 to 7:00 for which the Primary Safety Net has no message. The Shadow Safety Net generates a second resubmit request on behalf of the Primary Safety Net for resubmitting the shadow messages for the target mailbox database for the time interval 1:00 to 7:00.
Important
1. All delivery status notifications (DSNs) and non-delivery reports (NDRs) are suppressed for Safety Net resubmits.
2. Users removed from a distribution group may not receive a resubmitted message when the Shadow Safety Net resubmits the message. For example, a message is sent to a group containing User A and User B, and both recipients receive the message. User B is subsequently removed from the group. Later, a resubmit request from Primary Safety Net is made for the mailbox database that holds User B's mailbox. However, the Primary Safety Net is unavailable for more than 12 hours, so the Shadow Safety Net server responds and resubmits the affected message. During resubmission when the distribution group is expanded, User B isn't a member of the group, and won't receive a copy of the resubmitted message.
3. With the same logic new Users added to a distribution group may receive an old resubmitted message when the Shadow Safety Net resubmits the message.
4. By default Safety Net keeps the messages for 2 days. There is no size limit like in the previous exchange servers. When we run Get-TransportConfig we can still see the MaxDumpsterSizePerDatabase and MaxDumpsterTime parameters. Both these parameters are only used by Exchange 2010 and not 2013.
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