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In that case there will need to be some kind of locking, which is probably the reason the whitepaper recommends avoiding transactions that span multiple servers: "For multi-partition transactions, one engine distributes and coordinates work plans for the other engines. VoltDB assumes that an application designer can construct a partitioning/cloning scheme and a transaction design that makes a large majority of the transactions local to a single virtual node. "

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I had an idea for handling this situation that exploited the replication functionality. Basically, you prepare for a transaction by migrating the primary copy of all the objects involved to the same node. One would have to avoid deadlocks by sorting and serializing transaction-prep migration blocks. So transactions never span multiple servers, but you get this by possibly dramatically slowing down the time it takes to prepare for such transactions. (With the idea that the application designer is encouraged to avoid such transactions.)

If you do this for every transaction, you'll have more overhead than you'd have if you had just performed distributed consensus. But in principle you're right. It makes sense to put all the primary copies that are often used together on the same node. The problem is that it's hard to decide the ideal placement strategy. VoltDB puts this responsibility into the hands of the developers.

The point is to a) not have to implement distributed consensus and b) require the user to have to design the database for localized transactions if performance is desired.

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