Transport Dumpster feature in Different Version of Exchange Server

To better understanding for myself and to compare it with the different version of exchange I have put them all together.

Exchange 2007

Transport Dumpster feature was introduced on the Hub Transport role in Exchange 2007

It was only enabled for continuous cluster replication (CCR) and local continuous replication (LCR). 

Messages were retained in the transport dumpster until the administrator-defined time limit or size limit is reached

Get-TransportConfig |fl *Dumpster*

MaxDumpsterSizePerStorageGroup (E2K7) 18 Days Default

MaxDumpsterTime 7 Days

Exchange 2010

It now works with Database availability groups (DAGs), When a message is sent from the hub transport server to a DAG, the message is stored in the transport dumpster so in case of failure, messages can still be sent to users

The transport dumpsters now receive feedback from the replication pipeline to determine which messages have been delivered and replicated. As a message goes through Hub Transport servers on its way to a replicated mailbox database in a DAG, a copy is kept in the transport queue (mail.que) until the replication pipeline has notified the Hub Transport server that the transaction logs representing the message have been successfully replicated to and inspected by all copies of the mailbox database. After the logs have been replicated to and inspected by all database copies, they are truncated from the transport dumpster. This keeps the transport dumpster queues smaller by maintaining only copies of messages whose transactions logs haven't yet been replicated.

In the event of a lossy failover (where the server or database goes offline due to some reason), the “MSExchangeRepl” (MSExchange replication service) will set the “DumpsterRedeliveryRequired” Attribute to True for the database which just became Active after the failover. This is done with the help of the “LastLogInspected” marker which is set on every database. Hub servers will then redeliver the missing messages from the mail.que file depending upon the MaxDumpsterSizePerDatabase or MaxDumpsterTime whichever is latest.

Presently active settings can be viewed using the following command

Get-TransportConfig |fl *Dumpster*

Exchange 2013

The transport dumpster has been revamped in Exchange 2013 and is now known as the Safety Net.

Similarities between Safety Net and Transport Dumpster

• Safety Net is a queue that's associated with the Transport service on a Mailbox server. This queue stores copies of messages that were successfully processed by the server.
• You can specify how long Safety Net stores copies of the successfully processed messages before they expire and are automatically deleted. The default is 2 days

Here's how Safety Net is different in Exchange 2013:

• Safety Net doesn't require DAGs. For Mailbox servers that don't belong to a DAGs, Safety Net stores copies of the delivered messages on other Mailbox servers in the local Active Directory site.
• Safety Net itself is now redundant, and is no longer a single point of failure. This introduces the concept of the Primary Safety Net and the Shadow Safety Net. If the Primary Safety Net is unavailable for more than 12 hours, resubmit requests become shadow resubmit requests, and messages are re-delivered from the Shadow Safety Net.
• Safety Net takes over some responsibility from shadow redundancy in DAG environments. Shadow redundancy doesn't need to keep another copy of the delivered message in a shadow queue while it waits for the delivered message to replicate to the passive copies of mailbox database on the other Mailbox servers in the DAG. The copy of the delivered message is already stored in Safety Net, so the message can be resubmitted from Safety Net if necessary.
• In Exchange 2013, transport high availability is more than just a best effort for message redundancy. Exchange 2013 attempts to guarantee message redundancy.

• You can't specify a maximum size limit for Safety Net. You can only specify how long Safety Net stores messages before they're automatically deleted.

Message re-submissions 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. For more information about Active Manager, see Active Manager.