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An authenticated database (ADB) that provides succinct proofs of any value ever associated with a key. Its implementation is based on an Mmr over a log of state-change operations backed by a Journal.
§Terminology
A key in the db either has a value or it doesn’t. Two types of operations can be applied to the db to modify the state of a specific key. A key that has a value can change to one without a value through the delete operation. The update operation gives a key a specific value whether it previously had no value or had a different value.
Keys with values are called active, and an operation is called active if (1) its key is active, (2) it is an update operation, and (3) it is the most recent operation for that key.
In the Any db, it is not possible to prove whether the value of a key is the currently active one, only that it was associated with the key at some point in the past. This type of authenticated database is most useful for applications involving keys that are given values once and cannot be updated after.