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HTTP 409 Conflict

⚠️ HTTP 409 Conflict

? Quick Overview

HTTP 409 Conflict indicates that the request could not be completed because it conflicts with the current state of the resource on the server.

? Key Concepts

  • Client request is valid but causes a logical conflict
  • Common in REST APIs with concurrent updates
  • Usually related to versioning or duplicate data

? Syntax / Theory

A 409 response is typically returned when the server detects that applying the request would violate a constraint, such as updating stale data or creating a duplicate record.

? Code Example(s)

? View Code Example 
// Express.js example returning HTTP 409 Conflict
res.status(409).json({
message: "Conflict: Resource already exists"
});

? Live Output / Explanation

Server Response

The server responds with status code 409 and a message explaining the conflict so the client can resolve it.

? Interactive Example

? View Code Example 
// Simulated client-side conflict check
function createUser(username) {
if (username === "admin") {
console.log("409 Conflict: Username already taken");
}
}
createUser("admin");

? Live Simulator: User Registration

Try to register a new user. Existing users: admin, root.

 

? Use Cases

  • Preventing duplicate user registrations
  • Optimistic locking in database updates
  • Handling concurrent edits in collaborative apps

✅ Tips & Best Practices

  • Always include a clear error message with 409 responses
  • Use versioning or ETags to avoid conflicts
  • Let clients retry with updated data

? Try It Yourself

  • Simulate a duplicate entry in your API and return 409
  • Handle 409 errors gracefully on the frontend
  • Log conflict details for debugging