TsgcWSCircuitBreaker › Events › OnFallback

OnFallback Event

Fired just before a fallback response is returned while the circuit is Open; the handler may replace the payload.

Syntax

property OnFallback: TsgcCircuitBreakerOnFallback;
// TsgcCircuitBreakerOnFallback = procedure(Sender: TObject; const aKey: string; var aResponse: string) of object

Default Value

—

Remarks

Fires only when Fallback.Enabled is True and the breaker is about to serve a fallback response for a rejected call. The aResponse var parameter arrives pre-populated in this order: the last successful response for the key (when Fallback.UseLastSuccess is True), otherwise Fallback.CachedResponse, otherwise Fallback.CustomMessage. The handler is free to mutate aResponse — for example to inject the current key into a JSON body, or to return a per-tenant maintenance message. The string returned from the handler is what the caller ultimately sees. Runs synchronously on the thread that made the call; keep the handler fast (no network I/O). When no handler is attached, the pre-populated aResponse is returned as-is.

Example

procedure TForm1.CircuitFallback(Sender: TObject; const aKey: string;
  var aResponse: string);
begin
  // Inject the affected host into a JSON body so the caller knows what failed
  aResponse := Format(
    '{"error":"service unavailable","service":"%s","retry_after":30}',
    [aKey]);
end;

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