Programming: Atomicity

Atomicity is when an operation is guaranteed to happen all at once, or not at all.
ex: a database transaction, or an programming language atomic type.

Strategies

Copy-on-Write

A mv operation in a filesystem is generally cheap/atomic (change pointer).

• write your fullchangeset to a temp directory
• only if the write is successful, mv it to the desired location

A variation on this strategy is sometimes used by firmware.

• Two separate firmware installs are persisted
• When flashing new firmware, the non-used copy is updated
• If it succeeds, the new firmware copy is used (and the old becomes the new update target). otherwise the old copy is used.

Some filesystems (like zfs) do this under the hood.

Double-Write Buffer

Databases like MySQL's innodb use a double-write buffer to guarantee write atomicity.

• In addition to the final datastore, we keep a rolling buffer (the double-write buffer) with a short history of writes.
• Before writing to the datastore, the buffer is written to
• Immediately after writing the buffer, we write to the datastore

When the program is started up, some mechanism is used to verify that the info is sane.

• if the buffer write failed, the (old) datastore value is used
• if the datastore write failed, the (new) buffer write value is copied to the datastore and used