{"id":"mr-combiner-is-map-side-only","text":"The combiner runs inside `_run_mapper` on each mapper's output for a single partition independently; it never sees data from other mappers or other partitions.","truth_value":"IN","source":"entries/2026/05/29/mapreduce-framework-mapreduce.md","source_url":"","source_hash":"","justifications":[],"dependents":[],"metadata":{"example":"if self._combiner:\n    combined = self._combiner(key, values)\n    # applied after map, before shuffle to reduce"},"explanation":{"steps":[{"node":"mr-combiner-is-map-side-only","truth_value":"IN","reason":"premise"}]}}