[irfansharif]

The "criticism" as it pertains to CockroachDB and off-the-shelf 2PC is less so about 2PC in isolation and more so about layering 2PC on top of consensus groups used to persist records. When any given txn does the "prepare" phase, it lays down markers for a possible upcoming commit. If the 2PC coordinator fails in an inopportune moment, there's a delay between the failure and the markers being cleaned up (whether or not the transaction is "reprocessed" or aborted). The reason why this delay is problematic is because any subsequent transactions that happen upon said markers, they just have to wait for the resolution ("commit"/"aborted") aka it blocks. So clearly recovery must be built into 2PC, i.e. the transaction state itself must be persisted. This is done so in the same way the markers/regular writes are, through consensus. But marking the transaction state as "committed" can only happen once we're guaranteed that all the individual write markers are persisted. Which adds a second round of consensus.