HTTP 412 Precondition Failed

What It Means

The HTTP 412 Precondition Failed status code indicates that one or more conditions given in the request headers (like If-Match or If-Unmodified-Since) evaluated to false on the server.

Common Precondition Headers

Example: Optimistic Locking

# Client has ETag "abc123" from previous GET
PUT /api/documents/1 HTTP/1.1
If-Match: "abc123"
Content-Type: application/json

{"title": "Updated Title"}

---

# If document was modified by someone else (ETag changed):
HTTP/1.1 412 Precondition Failed
Content-Type: application/json

{
  "error": "Precondition Failed",
  "message": "Document was modified. Your ETag: abc123, Current: def456"
}

412 vs 409

Implementation

app.put('/api/documents/:id', async (req, res) => {
  const ifMatch = req.headers['if-match'];
  const doc = await Document.findById(req.params.id);

  if (ifMatch && ifMatch !== doc.etag) {
    return res.status(412).json({
      error: 'Precondition Failed',
      message: 'Document was modified by another request',
      currentETag: doc.etag
    });
  }

  // Proceed with update
  doc.title = req.body.title;
  doc.etag = generateNewETag();
  await doc.save();

  res.set('ETag', doc.etag);
  res.json(doc);
});

Use Cases

• Preventing lost updates: Don't overwrite concurrent changes
• Conditional uploads: Only upload if file doesn't exist
• Safe deletes: Only delete if resource is unchanged