Transactions
- Node.js course
- Advanced
- Lesson 34/83
- 5 min
Learn transactions with pg.Client.
Transactions
A transaction runs multiple SQL statements as a single atomic unit. Either all succeed or none do. This is critical for data integrity.
The PostgreSQL transaction pattern: BEGIN → queries → COMMIT (success) or ROLLBACK (error). In the pg module you get a client with pool.connect(), start with client.query("BEGIN"), run the queries, then COMMIT or ROLLBACK, and release the client with client.release().
The pool.connect pattern
For a transaction you get a dedicated client with pool.connect(). This client uses a single connection for the whole transaction; this ensures multiple queries run on the same connection.
The pattern: const client = await pool.connect(); try { await client.query("BEGIN"); await client.query(...); await client.query("COMMIT"); } catch(e) { await client.query("ROLLBACK"); } finally { client.release(); }. The finally block ensures the connection is always returned; this prevents pool exhaustion.
Practice Log the type of pool.connect
Import the Pool class from pg. Create a pool: new Pool({}). Log typeof pool.connect.
Starter code
// import the module
// create a pool
// log the type of connectSample solution
const { Pool } = require("pg");
const pool = new Pool({});
console.log(typeof pool.connect);Expected output
function
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 What happens on error in a transaction?
- COMMIT
- ROLLBACK Correct answer
- BEGIN
- SELECT
Explanation: On error the transaction is rolled back with ROLLBACK.
2 Match each piece to its role
| Term | Meaning |
|---|---|
BEGIN |
Starts a transaction |
COMMIT |
Saves changes |
ROLLBACK |
Undoes changes |
client.release() |
Returns the connection |
3 Complete the method name
const client = await pool.____()const client = await pool.connect()Explanation: pool.connect() gives a client for a transaction.