Is it still possible for a transaction that involves read and write operations against the replicated database to commit?

Question

I have a question that I couldn't find although I read the whole book. Consider a distributed system in which the database is replicated over five servers. At one point, the network between the replicated servers makes three of servers isolated from the remaining two. Is it still possible for a transaction that involves read and write operations against the replicated database to commit? Motivate

I would appreciate if you could answer this question

Answer 1

This is a bit general question. When I rephrase your question you ask what happens when there a split brain occurs in the distributed database.

And answer is it depends ;-)

Really. It depends what type of the distributed database you work with. From a high level perspective it depends what trade-off from the CAP the database chose - is the database CA or CP system? (I use this differentiation for sake of brevity, see https://martin.kleppmann.com/2015/05/11/please-stop-calling-databases-cp-or-ap.html)

If it's a CA system then for sure you can read and write to any partition. When the split brain is resolved then the database has some recovery tooling that mends the partitions back to a consistent state. Or the database may left this responsibility for the user that will get set of possible values and he has to decide which one is correct.

If it's a CP system then we can say that it depends if you work with the partition which consist the 3 servers of 5 (you work with the majority). Then you may read and write. If your client connects to the minor partition (2 servers of 5) then you probably are not permitted to read and write. But it depends on the consistency you requires. If you requires lineralizability then the client can do neither reads nor writes.