Mapper Attempt: Write, Commit, or Recompute Diagram

Attempts Are Isolated

Each retry writes under a fresh attempt id, never the failed name
A retry reads only its immutable input split, nothing shared
Two attempts of one task never both look committed

Lineage Makes Rerun Safe

The input split is immutable, so recompute replays the same records
Deterministic mapping makes the rewritten file equal the lost one
No outside side effects means rerun is safe by construction

Checksums Detect

Every map-output file carries a checksum next to its bytes
A mismatch fails the fetch instead of poisoning the reduce
Only the lost mapper reruns: the rest of the job keeps its files