The use of transactions in applications

Basic Concepts of Transactions

MongoDB introduced support for multi-document transactions starting from version 4.0, marking a significant feature that brings NoSQL databases closer to traditional relational databases. In MongoDB, a transaction is represented as a set of operations that either all succeed or all fail, ensuring data consistency. In distributed environments, MongoDB's transaction implementation is based on the two-phase commit protocol.

The ACID properties of transactions in MongoDB are manifested as:

• Atomicity: All operations within a transaction either complete entirely or do not execute at all.
• Consistency: The database remains in a consistent state before and after the transaction.
• Isolation: Concurrent transactions do not interfere with each other.
• Durability: Modifications to data after a transaction completes are permanent.

Use Cases for Transactions

Transactions are particularly useful in the following typical scenarios:

1. Bank transfers: Requires simultaneous updates to the balances of two accounts.
2. Order creation: Requires creating an order and deducting inventory at the same time.
3. User registration: Requires creating a user record and initializing user settings simultaneously.

// Bank transfer example
const session = db.getMongo().startSession();
session.startTransaction({
    readConcern: { level: 'snapshot' },
    writeConcern: { w: 'majority' }
});

try {
    const accounts = session.getDatabase('bank').accounts;
    accounts.updateOne(
        { _id: 'account1' },
        { $inc: { balance: -100 } }
    );
    accounts.updateOne(
        { _id: 'account2' },
        { $inc: { balance: 100 } }
    );
    session.commitTransaction();
} catch (error) {
    session.abortTransaction();
    throw error;
} finally {
    session.endSession();
}

Transaction API Usage

MongoDB provides a rich set of transaction control APIs, primarily managed through the Session object:

1. Start a transaction: startTransaction()
2. Commit a transaction: commitTransaction()
3. Roll back a transaction: abortTransaction()
4. End a session: endSession()

Transactions can be configured with various options:

const sessionOptions = {
    causalConsistency: true,
    readConcern: { level: 'majority' },
    writeConcern: { w: 'majority', j: true },
    readPreference: 'primary'
};
const session = client.startSession(sessionOptions);

Performance Considerations for Transactions

Using transactions incurs some performance overhead. Keep the following in mind:

1. Keep transaction durations as short as possible.
2. Avoid time-consuming operations within transactions.
3. Set reasonable transaction timeout periods.
4. Consider using optimistic concurrency control to reduce lock conflicts.

// Example of setting a transaction timeout
const session = client.startSession();
session.startTransaction({
    maxTimeMS: 5000  // 5-second timeout
});

Transactions and Replica Sets

When using transactions in a replica set environment, pay special attention to:

1. Transactions can only be executed on the primary node.
2. Configure appropriate writeConcern to ensure data durability.
3. Replica set failover may interrupt transactions.

// Replica set transaction example
const session = client.startSession();
session.startTransaction({
    readConcern: { level: 'majority' },
    writeConcern: { w: 3, wtimeout: 5000 }
});

Transactions and Sharded Clusters

Using transactions in a sharded cluster is more complex:

1. All shards participating in the transaction must run MongoDB 4.2+.
2. Transactions cannot modify the same document across multiple shards.
3. Pay special attention to transaction ID allocation.

// Sharded cluster transaction example
const session = client.startSession();
try {
    session.startTransaction();
    const orders = session.getDatabase('shop').orders;
    const inventory = session.getDatabase('shop').inventory;
    
    // These collections may reside on different shards
    orders.insertOne({...});
    inventory.updateOne({...}, {$inc: {stock: -1}});
    
    session.commitTransaction();
} catch (e) {
    session.abortTransaction();
}

Best Practices for Transactions

Based on real-world project experience, the following best practices are recommended:

1. Encapsulate transactions in a dedicated service layer.
2. Implement automatic retry mechanisms for transient errors.
3. Monitor transaction failure rates and durations.
4. Add explicit logging for transaction operations.

// Example of a transaction with a retry mechanism
async function executeWithRetry(txnFunc, maxRetries = 3) {
    let attempt = 0;
    while (attempt <= maxRetries) {
        const session = client.startSession();
        try {
            session.startTransaction();
            await txnFunc(session);
            await session.commitTransaction();
            return;
        } catch (error) {
            await session.abortTransaction();
            if (error.hasErrorLabel('TransientTransactionError') && 
                attempt < maxRetries) {
                attempt++;
                continue;
            }
            throw error;
        } finally {
            session.endSession();
        }
    }
}

Monitoring and Tuning Transactions

To ensure efficient transaction execution, establish a monitoring system:

1. Key metrics:
   • Transaction execution time
   • Transaction success rate
   • Lock wait time
2. Optimization methods:
   • Adjust transaction isolation levels
   • Optimize document schemas to reduce conflicts
   • Set appropriate read and write concern levels

// Example of transaction performance monitoring
const startTime = Date.now();
const session = client.startSession();
try {
    session.startTransaction();
    // Execute transaction operations
    await session.commitTransaction();
    const duration = Date.now() - startTime;
    metrics.track('txn_duration', duration);
} catch (error) {
    metrics.increment('txn_failure');
    throw error;
}

Limitations and Edge Cases of Transactions

MongoDB transactions have some limitations:

1. The total size of modifications in a single transaction cannot exceed 16MB.
2. Transactions default to a 60-second timeout (configurable).
3. Cannot create or delete collections/databases.
4. Cannot modify shard keys.

Handling special edge cases:

// Example of handling large transactions
async function processLargeTransaction(dataChunks) {
    for (const chunk of dataChunks) {
        await executeWithRetry(async (session) => {
            // Process each data chunk
            await processChunk(chunk, session);
        });
    }
}