What does durability mean?

Durability is a transaction property guaranteeing that once a result is committed, it won't be lost, even if a failure occurs: a power outage, a server crash, a process restart, or a system restart.

#What exactly is guaranteed

• data that was successfully committed will definitely reach durable storage (disk, a log, WAL/oplog, and so on)
• after a restart, the database can restore the committed state
• the data can no longer roll back, because the transaction is finished

#How this is achieved technically

Different DBMSs implement Durability through:

• operation logs (a Write-Ahead Log, an oplog)
• fsync / filesystem journaling
• replication (in distributed systems)
• checkpoints

The gist is the same: before confirming a transaction, the system guarantees the changes are written to durable storage, not just held in memory.

#Example

If a transaction transferred money between accounts and returned commit OK, then:

• even if the server crashes instantly, a second later
• after a restart, the data will be in the state it was in at the moment of commit

#The short definition

Durability = "after a commit, the data won't disappear or roll back, the system guarantees it's preserved".

This property closes off the last risk: losing or rolling back changes that were already accepted.