Each queue has a suffix of Apriority number, where A1 is the highest priority. The message queues are treated as FIFO (First In, First Out), and new messages are always added to the end of the queue.
The Exchange Mailbox task processes the queues in order of priority. The task scans through each queue, starting with the highest priority. If it finds a message on a queue, it processes the message and then starts the scan again from the highest priority queue. Therefore, queues A2 through A7 will not be processed until queue A1 is empty. When A1 is empty, A2 requests are processed before A3 requests.
However, queues A4 and A5 are special queues that are used only by the archiving schedule. The Exchange Mailbox task processes messages on the A5 queue only during a scheduled archive period. Outside the scheduled periods, messages on these queues are ignored until the next schedule period.
Using performance monitor, you can monitor the changes in the queues to assess the progress of the task.
For example, at the start of a scheduled period, the number of messages on queue A5 rises (to the number of enabled mailboxes on the Exchange server). These are Process Mailbox messages. The Exchange Mailbox task will take the first message off queue A5 and find all the eligible messages in the mailbox and change them to archive pending. A Process Item message is then placed on the Storage Archive queue of the Storage service for each message to be archived.
After the vault store has been backed up, Update Shortcut messages will be placed on queue A1 - which will be processed immediately because the queue has a higher priority.
Queue A3 performs the same function as queue A5, but for an immediate process system. This queue also performs shortcut expiry and deletion. Explicit user archives from the Outlook client extension are placed on queue A2.
Queue A5 will only be processed during a scheduled period, but queues A1 through A3, A6, and A7 will always be processed. If a queue has not been processed for more than 10 minutes, and there are no messages in a higher priority queue, there may be a problem with the task. Check the Enterprise Vault event log on the Exchange Mailbox task computer for any additional information.
Monitoring queue A1 will indicate that a backup has correctly updated shortcuts. However, if A1 is being used during normal use (before a backup), this may indicate a problem with tasks. Check the Enterprise Vault event log for errors.