VictoriaMetrics/VictoriaMetrics · warning

cannot flush data to client: %w

Error message

cannot flush data to client: %w

What it means

BufferedWriter.Flush propagates an error from the underlying bufio.Writer.Flush when flushing buffered data to the client fails. If the error is a trivial network error (client disconnected), it is swallowed and nil is returned; otherwise bw.err is set and returned to the caller.

Source

Thrown at lib/bufferedwriter/bufferedwriter.go:87

	}
	return n, bw.err
}

// Flush flushes bw to the underlying writer.
//
// Connection close errors are ignored to not trigger on them and to not write to logs, but Write method doesn't ignore
// them since it may lead to an unexpected behaviour (see https://github.com/VictoriaMetrics/VictoriaMetrics/pull/8157)
func (bw *Writer) Flush() error {
	bw.lock.Lock()
	defer bw.lock.Unlock()
	if bw.err != nil {
		if netutil.IsTrivialNetworkError(bw.err) {
			return nil
		}
		return bw.err
	}
	if err := bw.bw.Flush(); err != nil {
		bw.err = fmt.Errorf("cannot flush data to client: %w", err)
		if netutil.IsTrivialNetworkError(err) {
			return nil
		}
	}
	return bw.err
}

// Error returns the first occurred error in bw.
func (bw *Writer) Error() error {
	bw.lock.Lock()
	defer bw.lock.Unlock()
	return bw.err
}

View on GitHub (pinned to 5079fb58f1)

Solutions

1. Read the wrapped cause; trivial network errors are already handled (nil returned), so only real failures reach you
2. Confirm client behavior — reconnects/timeouts on the consumer side are the usual root cause
3. Check network stability, proxies, and keep-alive settings between server and client
4. Add retry at the application layer if the flushed payload can be regenerated

Example fix

// before
if err := bw.Flush(); err != nil { log.Fatal(err) }
// after
if err := bw.Flush(); err != nil {
    if netutil.IsTrivialNetworkError(err) {
        return nil // client disconnected — not a server fault
    }
    return err
}

if err := bw.Flush(); err != nil && !netutil.IsTrivialNetworkError(err) {
    log.Printf("flush failed: %v", err)
}

When it happens

Trigger: Calling bw.Flush() when the underlying writer (network conn) returns a non-trivial error: broken pipe, connection reset by peer, or disk/socket write failure.

Common situations: HTTP client closed the connection before the buffered response finished; TLS renegotiation failures; container/network interruptions mid-stream.

Related errors

• cannot send %d bytes to client: %w
• cannot read response from %q: %w
• cannot download %q from at %s (remote path %q): %w
• cannot read %q at %s (remote dir %q): %w
• cannot download %q from at %s (remote path %q): %w

AI-assisted analysis of VictoriaMetrics/VictoriaMetrics@5079fb58f1 (2026-09-03). Data as JSON: /api/errors/a81a4afae18d8ded. Report an issue: GitHub.