Message Queuing Message Log File
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What is an MQ file?
An MQ file is a Queueing Message log (QMLog) file created by Microsoft Message Queuing (MSMQ), a messaging protocol implemented in Windows for asynchronous communication between applications. It contains a log of the processing that occurs in a single queue or multiple queues. MQ files may grow up to 4 MB in size, then another MQ file is generated by MSMQ.
MSMQ allows applications that are running on separate servers and at different times to communicate with each other. The "Queuing" aspect allows messages to be stored in a queue in the event that an application receiving a message from a sending application is currently inaccessible. When the application becomes accessible again, the messages waiting in the queue are then sent to the application. An example implementation of MSMQ is in eCommerce where sales need to be processed by different applications running on different networks.
You will most likely only encounter an MSMQ MQ file if you are a developer of a system that utilizes MSMQ and are exploring the MSMQ\Storage folder. This folder also contains .LG1 and .LG2 files that store information about the state of an MQ file while MSMQ is writing information to it.
NOTE: The MSMQ messaging protocol was developed by Microsoft and introduced in Windows NT 4 and Windows 95.