Chapter 50 — Transactions, Consistency & Concurrency

📖 Definitions

• Atomicity in MongoDB — every write to a single document is atomic, including its embedded arrays and sub-documents.
• Multi-document transaction — groups writes across documents/collections so they all commit or all roll back (replica sets since 4.0, sharded clusters since 4.2).
• Write concern — how many replica-set members must acknowledge a write before it counts as successful.
• Read concern — which data a read is allowed to see (e.g. only majority-committed data).
• Read preference — which member (primary or secondaries) a read is sent to.

🔍 Do You Even Need a Transaction?

Good document design avoids most transactions because related data lives in one document.

💻 Code Example — Transaction with the Node.js Driver

import { MongoClient } from "mongodb";

const client = new MongoClient(uri);
const accounts = client.db("bank").collection<{ _id: string; balance: number }>("accounts");
const ledger = client.db("bank").collection("ledger");

export async function transfer(from: string, to: string, amount: number) {
  const session = client.startSession();
  try {
    await session.withTransaction(
      async () => {
        const debit = await accounts.updateOne(
          { _id: from, balance: { $gte: amount } },      // guard: no negative balance
          { $inc: { balance: -amount } },
          { session },
        );
        if (debit.modifiedCount !== 1) throw new Error("Insufficient funds");

        await accounts.updateOne({ _id: to }, { $inc: { balance: amount } }, { session });
        await ledger.insertOne({ from, to, amount, at: new Date() }, { session });
      },
      {
        readConcern: { level: "snapshot" },
        writeConcern: { w: "majority" },
        readPreference: "primary",
      },
    );
  } finally {
    await session.endSession();
  }
}

Key points:

• Every operation inside must pass { session } — forgetting it runs the op outside the transaction.
• withTransaction automatically retries on TransientTransactionError (e.g. write conflict) and retries the commit on UnknownTransactionCommitResult — so the callback must be safe to run more than once (no external side effects like sending emails inside).
• Throwing inside the callback aborts the transaction.

💻 Code Example — Mongoose

await mongoose.connection.transaction(async (session) => {
  const order = await Order.create([{ userId, items, total }], { session });   // create() needs an array with session
  for (const i of items) {
    const res = await Product.updateOne(
      { _id: i.productId, stock: { $gte: i.qty } },
      { $inc: { stock: -i.qty } },
      { session },
    );
    if (res.modifiedCount === 0) throw new AppError(409, `Out of stock: ${i.productId}`);
  }
  return order[0];
});

⚠️ Transaction Limits & Costs

• Requires a replica set or sharded cluster (a standalone mongod can't run them — use a single-node replica set locally).
• Default max runtime 60 seconds (transactionLifetimeLimitSeconds); keep them short.
• Keep them small: modifying many thousands of documents in one transaction puts pressure on the WiredTiger cache.
• Write conflicts: two transactions touching the same document → one gets WriteConflict and is retried.
• Performance cost is real; "if you need transactions everywhere, reconsider your schema".
• Collections and indexes can be created inside transactions since 4.4 (with restrictions).

🔍 Write Concern

db.orders.insertOne(doc, { writeConcern: { w: "majority", j: true, wtimeout: 5000 } });

🔍 Read Concern

🔍 Read Preference

Reading from secondaries scales reads but gives eventual consistency: a user might not see their own write. Use it for analytics/reporting, not "show me what I just saved".

💻 Causal Consistency ("Read Your Own Writes")

const session = client.startSession({ causalConsistency: true });
await users.updateOne({ _id }, { $set: { name: "New" } }, { session, writeConcern: { w: "majority" } });
// Even on a secondary, this read waits until it has seen the write above
const u = await users.findOne({ _id }, { session, readPreference: "secondary", readConcern: { level: "majority" } });

💻 Optimistic Concurrency Control (Lost-Update Prevention)

Problem: two admins open the same product, both edit, last save silently overwrites the first.

// Each document carries a version number
const product = await products.findOne({ _id });                       // { …, version: 7 }

const res = await products.updateOne(
  { _id, version: product.version },                                  // only if nobody changed it
  { $set: { price: newPrice }, $inc: { version: 1 } },
);
if (res.matchedCount === 0) throw new AppError(409, "Product was modified by someone else — reload");

Mongoose has this built in: new Schema({...}, { optimisticConcurrency: true }) uses __v and throws VersionError on save() conflicts.

Pessimistic locking (lock first) is rarely used in MongoDB; you can emulate it with a lockedBy/lockedUntil field set via findOneAndUpdate.

💻 Idempotency for Retries

// Client sends an Idempotency-Key header; a unique index makes duplicates impossible
db.payments.createIndex({ idempotencyKey: 1 }, { unique: true });

try {
  await payments.insertOne({ idempotencyKey: key, amount, userId, status: "pending" });
} catch (e) {
  if ((e as { code?: number }).code === 11000) return payments.findOne({ idempotencyKey: key }); // retry → same result
  throw e;
}

MongoDB drivers also enable retryable writes by default (retryWrites=true) — a single write is retried once after a network error or failover without being applied twice.

🔍 Isolation & Locking (Under the Hood)

• WiredTiger uses document-level concurrency control (MVCC) — writers to different documents don't block each other.
• Outside transactions, a multi-document updateMany is not atomic as a whole; other readers can see it half-applied.
• Inside a transaction, reads see a snapshot; writes are invisible to others until commit.

📊 Is MongoDB ACID?

So yes — MongoDB is ACID compliant for multi-document transactions since 4.0.

🌍 Real-World Impact

• Conditional $inc on stock prevents overselling without any transaction.
• w: "majority" prevents "order disappeared after failover" incidents.
• Optimistic concurrency eliminates silent overwrites in admin panels.

🎯 Likely Interview Questions

1. Are single-document writes atomic? — Yes, always.
2. When do you need a multi-document transaction? — Several documents must change together (transfers, order + stock across docs).
3. What are the requirements and limits of transactions? — Replica set/sharded cluster; 60s default limit; pass the session to every op; keep them small.
4. Why does withTransaction require idempotent callbacks? — It retries on transient errors and unknown commit results.
5. Explain write concern w: 1 vs majority. — Primary-only ack (can be rolled back on failover) vs majority ack (durable across failover).
6. What is read concern majority? — Read only data that can't be rolled back.
7. What are the risks of reading from secondaries? — Stale data due to replication lag; no read-your-own-writes unless causal sessions.
8. How do you prevent lost updates? — Optimistic concurrency with a version field in the filter.
9. How do you prevent overselling? — Conditional atomic update { stock: { $gte: qty } } + $inc.
10. What are retryable writes? — Drivers automatically retry a write once on transient network errors without double-applying.
11. Is MongoDB ACID? — Yes for single docs always, for multi-document transactions since 4.0/4.2.
12. What happens on a write conflict in a transaction? — One transaction aborts with a transient error and is retried.

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