We have seen 2 main issues with the Purgatory.
1. There is no atomic checkAndWatch functionality. So, a client typically first checks whether a request is satisfied or not and then register the watcher. However, by the time the watcher is registered, the registered item could already be satisfied. This item won't be satisfied until the next update happens or the delayed time expires, which means the watched item could be delayed.
2. FetchRequestPurgatory doesn't quite work. This is because the current design tries to incrementally maintain the accumulated bytes ready for fetch. However, this is difficult since the right time to check whether a fetch (for regular consumer) request is satisfied is when the high watermark moves. At that point, it's hard to figure out how many bytes we should incrementally add to each pending fetch request.