**Accumulator**: Write-only shared variable; workers add; driver reads. For counters. Fault-tolerant via lineage. **Broadcast**: Read-only; sent to all workers; avoids shuffling small lookup. Cached on executors. **Why**: Accumulators for diagnostics (not control flow—retries double-count). Broadcast for lookups <100MB. **Scalability**: Large broadcast = executor OOM....
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