Why Promise.all in MongoDB Transactions Leads to Undefined Behavior

Summary

• MongoDB transactions require sequential execution of operations within a session.
• Promise.all and similar constructs cause parallel execution, violating MongoDB’s transaction isolation constraints.
• This pattern can appear to work but risks data corruption and undefined behavior under load.

Root Cause

• MongoDB’s transaction protocol requires operations to happen sequentially for:
  • Predictable lock acquisition order (optimistic locking prevents deadlocks via conflict detection).
  • Ensuring isolatable atomicity (serializable isolation guarantee).
• Session affinity: All operations in a transaction must flow sequentially through the same TCP connection/session.
• Driver compatibility: Parallelized operations break Mongoose’s session-handling logic, causing state corruption.

Why This Happens in Real Systems

• Developers misinterpret transactions as independent of operation-ordering.
• Optimizing for performance by parallelizing “independent” I/O (e.g., creating unrelated documents).
• Legacy patterns from non-transactional databases applied to MongoDB.
• Lack of immediate errors during development obscures the issue until scale increases.

Real-World Impact

• Write conflicts/partial execution: Parallel operations may deadlock internally or leave documents partially committed.
• Session corruption: Multiple parallel writes overload Mongoose’s session-tracking mechanism.
• Data inconsistency: Conflicts bypass MongoDB’s conflict detection, breaking transaction guarantees.
• Heisenbugs: Intermittent failures under production loads, hard to reproduce locally.
• Unreliable rollbacks: Failed parallel operations may corrupt session state, preventing clean abort.

Example or Code

Problem Code (flawed parallel execution)

javascript
await session.withTransaction(async () => {
await Driver.create(driverData, { session });
await Promise.all([ // ❌ Unsafe parallelization
Document.create({ …doc1 }, { session }),
Document.create({ …doc2 }, { session })
]);
});

Resolved Code (sequential execution)

javascript
await session.withTransaction(async () => {
await Driver.create(driverData, { session });
await Document.create({ …doc1 }, { session }); // ✅ Sequential
await Document.create({ …doc2 }, { session });
});

How Senior Engineers Fix It

• Enforce sequential execution: Replace Promise.all with explicit chained await calls.
• Batch writes: Use MongoDB’s bulk operations where possible (e.g., Model.collection.insertMany with session).
• Refactor granularly: Move non-transactional operations outside the transaction block.
• Monitor retries: Implement explicit retry logic for transaction aborts due to conflicts.
• Documentation review: Validate transaction patterns against MongoDB’s explicit parallelism restrictions.

Why Juniors Miss It

• Testing gaps: Issues appear only under concurrency, which is undersimulated locally.
• “Works on my machine” mentality: Lack of symptoms in low-load environments hides risks.
• Misplaced optimization instinct: Prematurely parallelize without verifying atomicity guarantees.
• Superficial reading: Overlooking nuances in MongoDB/Mongoose docs about session-specific restrictions.
• False independence assumption: Believing writes to unrelated documents naturally parallelize.