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The DuraCloud Mill is the back-end system which handles much of the data processing for the DuraCloud system. The Mill makes use of a set of queues where all work tasks that need to be performed are placed. Each DuraCloud instance puts a task in one of the Mill queues every time any storage action occurs. These tasked are then worked, resulting in updates to the audit and manifest details for each space. This allows the Mill to keep track of all content in the DuraCloud system.

A series of workers perform the jobs outlined in the work tasks. These workers can scale automatically based on the number of items in each queue, allowing the number of workers to increase and decrease as the amount of work to be done changes. The workers communicate with the storage providers, as well as with a database that is used to maintain system state.

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Another type of producer, the Bit Integrity Producer, is used to perform checks for content bit-level integrity. Much like the Duplication Producer, the Bit Integrity Producer adds a task to the task queue for each content item in the DuraCloud system. The workers then execute those tasks by retrieving the content, and verifying that the checksum computed for the content matches that provided by the storage provider and maintained in the DuraCloud manifest.

The DuraCloud manifest and audit cache are maintained in the Mill database. This database is kept up-to-date by the workers as they process audit tasks. The DuraCloud instance consults the Mill database when manifest or audit information is required. The audit cache maintained in the database is transitioned into long-term file-level audit storage after a short period of time.

The capabilities of the DuraCloud Mill are likely to expand over time, as it is designed to be a generic and scalable task processing engine.

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