It is possible to improve concurrency here, I'm not sure that #4049 is relevant, but principally because indeed it's a case of single writer - multiple readers on an `ArrayList` + `idForNull`.
But just removing synchronization is not good because ArrayList data structure itself is racy. When a writer performs `ArrayList.add()` that reallocates underlying array a reader may intercept and read `null` for some old id, or get an IndexOutOfBoundsException. `ArrayList` should be replaced with a manually crafted ArrayList-like data structure, probably making use of CAS and CoW principles, that avoids this type of race. [ Full content available at: https://github.com/apache/incubator-druid/issues/6322 ] This message was relayed via gitbox.apache.org for devnull@infra.apache.org
