What is Database transaction?

The classic failure it prevents

An order is written, then the process fails before stock is decremented. Without a transaction you have a paid order and stock that says the item is still available, so it sells again.

Where transactions are required

• Order creation with stock movement
• Payment recording with fulfilment
• Webhook idempotency — recording the event id and doing the work must be atomic
• Anything transferring a value from one row to another

The subtlety

Wrapping only the write in a transaction does not prevent a race. The read that decides whether to write must be inside it too, usually with a lock, or two concurrent requests can both read the same stale value.

This is why the bug survives testing. Sequential manual testing never produces two simultaneous reads, so the code appears correct until the first genuinely busy day.

Where this is covered in depth

A definition can only go so far. E-commerce website development: what the whole build actually involves covers this properly — 7 minutes, free, no email required.

Who wrote this

Anas Bin Masud builds e-commerce sites and does technical SEO for businesses in the UK, Canada and Pakistan. These definitions come from client work rather than from a content brief — where an entry describes a mistake, it is usually one found on a real site. More about how I work.