How do database transactions work in Django?

Question

IT Interview Practice · Accepted Answer

A **database transaction** groups multiple operations so they either **all succeed or all fail together** (atomicity) — preventing partial, inconsistent updates. Django provides `transaction.atomic` to wrap operations in a transaction, ensuring data integrity for multi-step operations.

## The problem transactions solve

```python
# ❌ WITHOUT a transaction — partial failure leaves inconsistent data
def transfer_money(from_account, to_account, amount):
    from_account.balance -= amount
    from_account.save()                  # ✓ succeeds
    # ...if the server crashes HERE, money vanished from one account...
    to_account.balance += amount
    to_account.save()                    # ✗ never runs → money LOST!
```

Without a transaction, if the second operation fails, the first has already committed — leaving the database in a broken, inconsistent state (money debited but not credited).

## transaction.atomic — all-or-nothing

```python
from django.db import transaction

# as a context manager
def transfer_money(from_account, to_account, amount):
    with transaction.atomic():           # everything inside is ONE transaction
        from_account.balance -= amount
        from_account.save()
        to_account.balance += amount
        to_account.save()
    # if ANY operation raises an exception, the WHOLE block is ROLLED BACK
    # → either both saves happen, or neither does (atomicity)
```

```python
# or as a decorator on a whole function/view
@transaction.atomic
def create_order(request):
    order = Order.objects.create(...)
    for item in items:
        OrderItem.objects.create(order=order, ...)
    # all-or-nothing: a failure anywhere rolls back the entire order creation
```

Inside `atomic()`, if any operation raises an exception, Django **rolls back** every change in the block — so the database is never left half-updated. On success, all changes commit together.

## Nested atomic blocks (savepoints)

```python
with transaction.atomic():               # outer transaction
    do_something()
    try:
        with transaction.atomic():       # inner = a SAVEPOINT
            risky_operation()
    except Exception:
        pass                              # inner rolls back to the savepoint; outer continues
```

Nested `atomic` blocks use savepoints — the inner block can roll back independently without aborting the whole outer transaction.

## Important details

```text
✓ Catch exceptions OUTSIDE the atomic block (catching inside can break rollback handling)
✓ select_for_update() locks rows within a transaction (prevent race conditions on updates)
    with transaction.atomic():
        account = Account.objects.select_for_update().get(pk=id)  # row locked until commit
✓ ATOMIC_REQUESTS setting wraps EVERY request in a transaction (per-database)
✓ Keep transactions SHORT — long transactions hold locks, hurting concurrency
```

## Why it matters

Database transactions are essential for **data integrity** whenever an operation involves multiple related database changes that must succeed or fail as a unit — and understanding `transaction.atomic` is important knowledge for building correct, reliable applications.

The core problem transactions solve is **partial failure**: without atomicity, if a multi-step operation fails midway (a crash, an exception, a constraint violation), the database is left in an inconsistent state — the classic money-transfer example (debited from one account but not credited to another) illustrates how this corrupts data and can have serious consequences. `transaction.atomic` provides the guarantee that operations are **all-or-nothing** (rolling back the entire block on any failure), which is necessary for any operation spanning multiple writes — creating an order with its line items, transferring funds, updating related records — to maintain a consistent database.

Understanding how to use atomic blocks (as a context manager or decorator), nested transactions with savepoints, and the important details — catching exceptions *outside* the block, using **`select_for_update()`** to lock rows and prevent race conditions in concurrent updates, and keeping transactions short to avoid holding locks — is practical knowledge for building robust applications.

Data consistency is a fundamental requirement of reliable systems, and transactions are the mechanism that ensures it for multi-step database operations, making this an important topic that distinguishes developers who write correct, integrity-preserving code from those whose applications can silently corrupt data under failure conditions or concurrent access — a frequently-relevant concern in any application handling important, related data changes.

The problem transactions solve

transaction.atomic — all-or-nothing

Nested atomic blocks (savepoints)

Important details

Why it matters

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