- Sensors are easier to implement.

Cons:

- More than one UR is needed
- If a user deletes all files, and there is no validduration, the storage system must produce a new record with a resourcecapacity of 0 in order for the accounting system to know that the user no longer uses any resources. However this requires state on the storage system, which complicates the record generation significantly, and we cannot assume that this can be implemented on all storage systems
- It is more like monitoring than accounting

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