It helps if your client retries have backoff (but that's not always possible), but you do need to have a way to drop requests much faster than you typically process them and it's nice to have a sensible way to drop requests that are unlikely to be successful. Sometimes you can drop based on time in queue, sometimes you can drop based on percentage, sometimes you can track backend success and drop requests to a failing backend, sometimes you have to just flush the whole queue periodically if overloaded. Facebook has published something about switching from a queue to a stack during heavy load, which can work, although that stack needs bounds, too.