TwoCommit

Problem

There can be situations when during commit blobbers will have different states, more than 1/3 of bobbers can receive write marker and commit connection, another part of bobbers can stay with previous uncommitted state (broken). If client will decide to repair such allocation, there won't be enough bobbers to restore committed state and there is no way to rollback.

Solution

To provide a way to get back to previous state even if it is committed on part of the network we introduce double commit approach. We add new state of the filesystem pre-committed (in yellow)