Handling Concurrency in SQLite: Best Practices

Learn how SQLite handles multiple people using the database at the same time and how to avoid problems.

In one of our previous blogs, we learned how to handle errors in SQLite—catching problems, tracking mistakes, and using tools to keep your data safe and correct. But what happens when multiple people try to use the same database at the same time?

This can lead to problems, like two people trying to change the same thing simultaneously, which might cause errors. This is called concurrency.

SQLite has ways to handle this and make sure everything works properly. In this blog, we’ll show you how SQLite handles multiple people using the database at the same time and how to avoid issues. If you want to learn more about making your SQLite database work better, check out our SQLite Performance Guide.

What is Concurrency in SQLite?

When many people are using the database at once, SQLite has to decide who gets to change the data first. This is called concurrency control. If two people try to change the same thing at the same time, SQLite needs to stop them from messing things up.

SQLite uses something called a lock to make sure only one person can change the data at a time. This keeps things safe and accurate!

Step 1: Using Transactions to Handle Concurrency

The best way to handle concurrency is by using transactions. A transaction is like a box that holds everything a person wants to do in the database. When a transaction starts, it gets a lock on the data it’s working with, so no one else can change it until the transaction is finished.

Here’s an example of how a transaction works:

BEGIN TRANSACTION;

-- Do some changes to the data here, like adding or updating rows

COMMIT;

Explanation:

• BEGIN TRANSACTION: This starts the transaction and locks the data so no one else can change it.

• COMMIT: When you're done, COMMIT tells SQLite to save everything and unlock the data so others can use it.

If something goes wrong during the transaction, you can use ROLLBACK to undo everything and make sure no mistakes were made:

ROLLBACK;

This ensures that if something goes wrong, nothing is saved, and the database is back to how it was before.

Examples:

1. Transaction with Multiple Queries:

BEGIN TRANSACTION;

UPDATE users SET balance = balance - 100 WHERE user_id = 1;
UPDATE users SET balance = balance + 100 WHERE user_id = 2;

COMMIT;

Explanation: In this example, two users are transferring money to each other. If the transaction is interrupted, the database will not be left in a half-completed state.

2. Transaction with ROLLBACK:

BEGIN TRANSACTION;

INSERT INTO orders (user_id, product_id, quantity) VALUES (1, 3, 2);

-- Something goes wrong, so we roll  back the changes
ROLLBACK;

Explanation: If something goes wrong after inserting the order, we use ROLLBACK to undo all the changes made during the transaction.

Step 2: Avoiding Long Locks

If a lock is held for too long, it can slow down everyone else. Here are a few tips to avoid holding locks for too long:

• Do only necessary work: Don’t keep the transaction open longer than you need. Update just the data you need to, and close the transaction as soon as possible.

• Use smaller transactions: If you can, split large transactions into smaller ones. This helps reduce the time each lock is held.

Examples:

1. Short Transaction for Simple Update:

BEGIN TRANSACTION;

UPDATE books SET available = 0 WHERE book_id = 1;

COMMIT;

Explanation: The transaction only updates the availability of a single book. It’s quick and doesn’t keep the database locked for long.

2. Breaking Large Transaction into Smaller Ones:

BEGIN TRANSACTION;

UPDATE orders SET status = 'Processing' WHERE order_id = 1;

COMMIT;

BEGIN TRANSACTION;

UPDATE orders SET status = 'Shipped' WHERE order_id = 1;

COMMIT;

Explanation: Instead of making one big update, we split it into smaller transactions, each updating a single piece of data. This reduces the time each lock is held.

Step 3: Using WAL Mode for Better Concurrency

SQLite has a special mode called WAL (Write-Ahead Logging). In this mode, SQLite handles concurrency better, especially when multiple people are reading and writing at the same time.

In WAL Mode, the database can allow readers to keep reading data even when someone is writing to the database. This makes things faster because multiple people can work at once.

1. Switching to WAL Mode:

PRAGMA journal_mode = WAL;

Explanation: This command switches the database to WAL Mode, allowing better handling of concurrent read and write operations.

2. Checking Current Journal Mode:

PRAGMA journal_mode;

Explanation: This command checks which journal mode your database is using. If it’s using WAL, it will return wal.

Step 4: Avoiding Deadlocks

A deadlock happens when two people are each waiting for the other to finish before they can continue. This can happen when transactions are waiting for each other to release locks, and they both get stuck.

To avoid deadlocks, follow these tips:

• Access tables in the same order: If two transactions need to work with different tables, make sure they always access the tables in the same order. This avoids conflicts where two transactions are waiting for each other.

• Keep transactions short: The shorter a transaction is, the less likely it will cause a deadlock. Try to do as little work as possible inside a transaction.

Examples:

1. Consistent Table Access Order to Avoid Deadlocks:

BEGIN TRANSACTION;

-- Always update tables in the same order
UPDATE users SET balance = balance - 50 WHERE user_id = 1;
UPDATE inventory SET stock_quantity = stock_quantity - 1 WHERE item_id = 2;

COMMIT;

Explanation: Always update tables in the same order (first users, then inventory) to avoid the risk of deadlocks.

2. Breaking Complex Transactions into Smaller Ones:

BEGIN TRANSACTION;
UPDATE products SET price = price + 10 WHERE product_id = 5;
COMMIT;

BEGIN TRANSACTION;
UPDATE orders SET total = total + 10 WHERE order_id = 1;
COMMIT;

Explanation: Instead of updating multiple tables in one long transaction, break it into smaller transactions to reduce the chance of a deadlock.

Conclusion

Handling concurrency in SQLite is important to keep everything running smoothly when multiple people are using the database at the same time. By using transactions, WAL mode, and avoiding long locks or deadlocks, you can make sure your database stays fast and safe. For more tips on making your SQLite database better, check out our list of SQLite posts here.

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