What is idempotency and how is it applied in web development?

The idempotency of an HTTP method means that performing the same request multiple times with the same parameters will result in the same server state as performing it once. The result can be successful or unsuccessful, but the server's state after execution will not change.

Application in web development:

• GET: Idempotent. Multiple retrievals of a resource do not change its state.
• PUT: Idempotent. Multiple full overwrites of a resource lead to the same final state of the resource.
• DELETE: Idempotent. Multiple deletions of a resource after the first successful deletion do not change the state (the resource remains deleted).
• POST: Not idempotent. Multiple submissions of data can lead to the creation of multiple identical resources or multiple executions of an action.
• PATCH: Not idempotent by default, but can be idempotent if the partial update operation is defined as idempotent.

Idempotency is important for reliability: it allows safe retries of requests in case of network failures without undesirable side effects on the server.

Example of a non-idempotent operation with POST:

// POST /orders - creates a new order
// Repeated calls will create another order
router.post('/orders', (req, res) => {
  const order = new Order(req.body);
  order.save()
    .then(() => res.status(201).send(order))
    .catch(err => res.status(500).send(err));
});

Example of an idempotent operation with PUT:

// PUT /orders/:id - updates an order by id
// Repeated calls will repeatedly update the same order with the same state
router.put('/orders/:id', (req, res) => {
  Order.findByIdAndUpdate(req.params.id, req.body, { new: true })
    .then(order => res.status(200).send(order))
    .catch(err => res.status(500).send(err));
});