# MongoServerError: TransactionTooOldForSnapshot: Read timestamp is older than the oldest timestamp in the storage engine

- **ID:** `mongodb/transaction-snapshot-too-old`
- **Domain:** mongodb
- **Category:** runtime_error
- **Verification:** ai_generated
- **Fix Rate:** 85%

## Root Cause

A multi-document transaction attempted to read from a snapshot that has been pruned by the storage engine due to cache pressure or write conflicts.

## Version Compatibility

| Version | Status | Introduced | Deprecated |
|---------|--------|------------|------------|
| mongodb-4.0 | active | — | — |
| mongodb-4.2 | active | — | — |
| mongodb-4.4 | active | — | — |
| mongodb-5.0 | active | — | — |
| mongodb-6.0 | active | — | — |
| mongodb-7.0 | active | — | — |

## Workarounds

1. **Reduce transaction duration by committing quickly. Keep transactions as short as possible; avoid long-running transactions with many operations. Example: session.startTransaction(); await collection1.insertOne(a); await collection2.updateOne(b); await session.commitTransaction();** (85% success)
   ```
   Reduce transaction duration by committing quickly. Keep transactions as short as possible; avoid long-running transactions with many operations. Example: session.startTransaction(); await collection1.insertOne(a); await collection2.updateOne(b); await session.commitTransaction();
   ```
2. **Increase the WiredTiger cache size to retain older snapshots: storage.wiredTiger.engineConfig.cacheSizeGB: 8. Restart mongod.** (70% success)
   ```
   Increase the WiredTiger cache size to retain older snapshots: storage.wiredTiger.engineConfig.cacheSizeGB: 8. Restart mongod.
   ```
3. **If the transaction conflicts with writes, use retry logic with exponential backoff. Example in Python: for i in range(3): try: with client.start_session() as session: session.start_transaction(); ...; session.commit_transaction(); break except pymongo.errors.OperationFailure as e: if 'TransactionTooOldForSnapshot' in str(e): time.sleep(0.5 * (2**i)); continue** (80% success)
   ```
   If the transaction conflicts with writes, use retry logic with exponential backoff. Example in Python: for i in range(3): try: with client.start_session() as session: session.start_transaction(); ...; session.commit_transaction(); break except pymongo.errors.OperationFailure as e: if 'TransactionTooOldForSnapshot' in str(e): time.sleep(0.5 * (2**i)); continue
   ```

## Dead Ends

- **** — This does not affect snapshot pruning; the error is about the read timestamp being too old, not the transaction timing out. (80% fail)
- **** — Snapshot read concern is already the default for transactions; the issue is the snapshot itself being pruned. (70% fail)
- **** — This is not configurable and would cause memory exhaustion. (100% fail)