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0 points__alexs12y ago0 comments

Sounds a bit like SET ISOLATION LEVEL SERIALIZABLE?

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chongli12y ago· 2 in thread

No, it's far more elegant than that. Datomic makes the entire database an immutable, persistent data structure. This means that anyone can grab the database as a value -- at any point in time, past or present -- and never have it change behind their back. There are no read transactions whatsoever and clients have the power to conduct their queries offline. This lets you easily scale read operations without limit. The only bottleneck in the system is a dedicated server known as the transactor which handles the coordination of all updates.

AlisdairO12y ago

Looks interesting. The docs make it sound like a temporal triple store which you can (lazily) make replicas of.

How is consistency handling performed? Does it rely on knowing my snapshot version and checking for write conflicts (a la oracle SERIALIZED level), or is it based on explicit locking?

chongli12y ago

There are no snapshots (or you could say that everything is a snapshot); you can get a value of the database from any moment in time, stretching all the way back to the creation of the database. All transactions are performed against a stable view of the database (its value at a moment in time) and either succeed or fail entirely (they are atomic). Once a transaction is validated and sent to storage the new data can be communicated to all clients and cached indefinitely; it will never change.

Datomic has no notion of mutation/destruction. Data is stored in the form of facts which are asserted or retracted. Retracted data can still be retrieved from an earlier moment in time, so it isn't really gone. In many ways, it has a lot in common with DVCSs such as git (though it has only a single authority with commit access: the transactor).

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