RavenDB allows you to tune, per transaction, what level of safety you want your changes to have. At the most basic level, you can select between a single node transaction and a cluster wide transaction.
We get questions from customers about the usage scenario for each mode. It seems obvious that we really want to always have the highest level of safety for our data, so why not make sure that all the data is using cluster wide transactions at all times?
I like to answer this question with the lottery example. In the lottery example, there are two very distinct phases for the system. First, you record lottery tickets as they are purchased. Second, you run the actual lottery and select the winning numbers. (Yes, I know that this isn’t exactly how it works, bear with me for the sake of a clear example).
While I’m recording purchased lottery ticket, I always want to succeed in recording my writes. Even if there is a network failure of some kind, I never want to lose a write. It is fine if only one node will accept this write, since it will propagate the data to the rest off the cluster once communication is restored. In this case, you can use the single node transaction mode, and rely on RavenDB replication to distribute the data to the rest of the cluster. This is also the most scalable approach, since we can operate on each node separately.
However, for selecting the winning numbers (and tickets), you never want to have any doubt or the possibility of concurrency issues with that. In this case, I want to ensure that there is just one lottery winner selection transaction that actually commit, and for those purposes, I’m going to use the cluster transaction mode. In this way, we ensure that a quorum of the cluster will confirm the transaction for it to go through. This is the right thing to do for high value, low frequency operations.
We also have additional settings available, beyond the single node / full cluster quorum, which is write to a single node and wait for the transaction to propagate the write to some additional nodes. I don’t really have a good analogy for this use case using the lottery example, though. Can you think of one?