What is Back Pressure in Exchange Server 2007

The Back Pressure feature uses 5 (five) stages to control the monitored resources. This means that the services will not be stopped at the first sign of trouble. For each stage certain actions that will be performed when Back pressure identifies a resource bottleneck. If there is no result at the current level and resource utilization is still increasing, the stage level will increase until the system resources come back to normal values. By default the levels are checked using the interval defined in the ResourceMonitoringInterval parameter of the EdgeTransport.exe.config file.

Below is a brief description of each stage. Remember that each stage checks their information against the parameters of the configuration file.

Stage 1The memory utilized by the Edge Transport.exe process is validated. If higher than specified in the configuration file a process called Garbage collection will start. This process checks for unused objects that exist in memory, and removes those unused objects from memory.

Stage 2The number of uncommitted message queue database transactions that exist in memory is validated. If higher than the configured value, an attempt is made to force the message queue database transactions that are in memory to be written in the transaction log files.

Stage 3The utilization levels of all monitored resources are checked against the configuration file for normal levels of utilization. If overutilization persists; the resource with the highest level of utilization isÂacted upon. These actions are different in Edge Transport and Hub Transport, and you can see the action in the following tables:

Resource utilization level	Connections from other Hub Transport servers	Connections from other messaging servers	Store driver connections from Mailbox servers	Pickup directory and Replay directory submission	Internal mail flow
Medium	Allowed	Rejected	Allowed	Rejected	Functional
High	Rejected	Rejected	Rejected	Rejected	Not functional

Hub Transport

Resource utilization level	Connections from Hub Transport servers	Connections from other messaging servers	Pickup directory and Replay directory submission
Medium	Rejected	Rejected	Rejected
High	Rejected	Rejected	Rejected

Edge Transport

Stage 4The memory utilization of the Exchange Transport process is validated against the config file. Even if we restart the Exchange Transport service the messages located in the Submission queue will not be processed automatically when the service starts. Other validation occurs in the message queue database transactions that are kept in memory. If higher than the normal level the following actions will occur: the transport dumpster will be disabled; message delivery to any remote destination that uses a remote delivery queue will be disabled.

Stage 5If the memory utilization of the Exchange Transport process is still at a high or medium level or the memory utilization of all processes exceeds the value defined in the configuration file, the following actions will occur: DNS cache will be flushed from memory; and the message dehydration process occurs.

Configuring Back Pressure…

Now, we know what the Back Pressure feature is let's see how we can manage this feature. All the Back Pressure settings are found in a single file called EdgeTransport.exe.config. This file is located in the subdirectory bin of the Exchange Server 2007 installation folder.