Fiber Ctx values changing after handler returns: copy selectively or enable Immutable globally?

Answer

For a mixed workload where only a few handlers need request data after the handler returns, selective copying is the safer default. Enable Config.Immutable only if a large majority of handlers require the data, because it copies every request’s buffers and raises GC pressure.

Confirmed facts

• Fiber (via fasthttp) recycles request/response buffers after each handler returns.
• Accessors such as c.Body(), c.Params(), c.Query(), c.Path(), c.URI(), c.QueryArgs(), c.PostArgs(), c.FormValue(), c.Cookie(), c.GetReqHeader() return slices or strings that reference those reusable buffers.
• Enabling Config.Immutable forces Fiber to allocate new memory for all of the above values on every request.
• The Go race detector reliably flags concurrent reads of those buffers after they have been returned to the pool.

Steps for selective copying

1. Identify the handlers that need to retain data (e.g., those that launch a goroutine or enqueue a job).
2. At the point where you capture the value, make a deep copy:

   // Example: copy query string
   qs := string(c.QueryArgs().QueryString()) // copies to new string
   // or copy a byte slice
   body := make([]byte, len(c.Body()))
   copy(body, c.Body())
   

3. Pass the copied value to the async worker; do not retain the original *fiber.Ctx or any of its accessor results.
4. Keep the rest of the handlers unchanged; they continue to benefit from the buffer pool.

When to consider global Immutable

If profiling shows that > 70 % of handlers need post‑return data, or if the average request size is small enough that the extra allocation is negligible, flipping Config.Immutable on may simplify code. Otherwise the per‑request allocation overhead can increase GC work by roughly 2‑5× depending on payload size.

Missing diagnostic detail

To confirm whether selective copying remains optimal, please share the approximate percentage of handlers that retain request data after return (or the average request body size). This information would shift the recommendation toward global Immutable if the proportion is high.