Introduction
Sequelize is a popular Node.js ORM for managing relational databases like PostgreSQL, MySQL, MariaDB, SQLite, and MSSQL. One of its powerful features is the ability to handle transactions. Transactions are crucial in ensuring data integrity, allowing multiple database operations to be treated as a single unit of work, and are either all completed successfully or rolled back on error. In this guide, we’ll explore how to work with transactions in Sequelize, starting from the basics and moving to more advanced topics.
Getting Started with Transactions
To begin using transactions in Sequelize, you first need to understand the concept of a transaction. A transaction encapsulates a sequence of database operations, which are written to the database only if all operations succeed. If any operation within the transaction fails, the transaction can be rolled back, leaving the database in its previous state.
To create a transaction in Sequelize, you use the sequelize.transaction() method. This method optionally accepts a configuration object and returns a promise that resolves to a transaction object which you should pass to your queries to execute them within the transaction’s context.
const { Sequelize } = require('sequelize');
const sequelize = new Sequelize('database', 'username', 'password', {
dialect: 'postgres'
});
sequelize.transaction().then(transaction => {
// here you execute queries within the transaction
// your logic here...
transaction.commit().then(() => {
console.log('Transaction committed.');
}).catch(err => {
console.error('Error during commit:', err);
transaction.rollback();
});
}).catch(err => {
console.error('Transaction failed:', err);
});Managing Transactions
One essential aspect of transactions is error handling. When a query inside a transaction fails, Sequelize will not automatically roll back the transaction. Instead, you should include error handling logic to rollback manually when an error occurs.
To simplify this process, Sequelize provides a way to automatically handle rollbacks using a callback. If you pass a callback to the sequelize.transaction() method, Sequelize will automatically rollback the transaction if an error is thrown within the callback.
sequelize.transaction(async (t) => {
try {
// your transactional queries here, passing the transaction object 't'.
} catch (error) {
// Sequelize will automatically perform a rollback in case of errors.
throw error;
}
}).then(result => {
// Transaction has been committed
}).catch(err => {
// Something went wrong
});Transaction Isolation Levels
In addition to handling the transaction flow, Sequelize allows you to specify the isolation level of a transaction. The isolation level determines how the changes made by one transaction affect other transactions and vice versa. Sequelize supports various isolation levels, such as READ UNCOMMITTED, READ COMMITTED, REPEATABLE READ, and SERIALIZABLE.
sequelize.transaction({ isolationLevel: Sequelize.Transaction.ISOLATION_LEVELS.SERIALIZABLE }, async (t) => {
// Your transactional operations here.
}).then(result => {
// Transaction has been committed
}).catch(err => {
// Handle errors
});Nested Transactions
Transactions can be nested within each other. This allows for modularization of database operations into separate functions that can be independently committed or rolled back, while still participating in a larger transaction if necessary. Sequelize handles nested transactions by creating savepoints.
To create a nested transaction, simply call sequelize.transaction() within an already existing transaction. If the outer transaction is rolled back, it will also roll back the inner transactions. However, if an inner transaction is rolled back, it won’t affect the outer transaction.
sequelize.transaction(async (outerTransaction) => {
// Outer transaction
sequelize.transaction({ transaction: outerTransaction }, async (innerTransaction) => {
// Inner transaction
});
}).catch(err => {
// Handle errors
});Advanced Transaction Patterns
As your application grows in complexity, you may find yourself requiring more advanced transaction patterns. For instance, you might need to manage transactions across several model files or services. In such cases, you can use Sequelize’s Transaction CLS (Continuation Local Storage) to automatically pass transaction objects across async calls.
Another advanced pattern is the ability to retry transactions. If a transaction conflicts with another one (e.g., due to concurrent access to the same row in the database), you can set up retry logic so that Sequelize will automatically retry the transaction a configurable number of times before failing.
Summary
In this guide, we’ve covered the basics of handling transactions in Sequelize—from simple transactions to advanced transaction patterns such as nested transactions, isolation levels, and automated retries. Understanding these concepts is fundamental to ensuring data integrity and building robust applications with Sequelize.
Keep in mind that the use of transactions can influence your application’s performance due to the overhead they introduce, so use them judiciously and always optimize based on your application’s specific requirements.
Transactions provide a safety net for your data operations, allowing you to maintain consistency even in the face of errors. With Sequelize’s support for transactions, you have the tools necessary to manage complex data interaction scenarios confidently and securely in your Node.js applications.
