Concurrent Set Queue

Question

Maybe this is a silly question, but I cannot seem to find an obvious answer.

I need a concurrent FIFO queue that contains only unique values. Attempting to add a val

Answer 1

I would use a synchronized LinkedHashSet until there was enough justification to consider alternatives. The primary benefit that a more concurrent solution could offer is lock splitting.

The simplest concurrent approach would be a a ConcurrentHashMap (acting as a set) and a ConcurrentLinkedQueue. The ordering of operations would provide the desired constraint. An offer() would first perform a CHM#putIfAbsent() and if successful insert into the CLQ. A poll() would take from the CLQ and then remove it from the CHM. This means that we consider an entry in our queue if it is in the map and the CLQ provides the ordering. The performance could then be adjusted by increasing the map's concurrencyLevel. If you are tolerant to additional racy-ness, then a cheap CHM#get() could act as a reasonable precondition (but it can suffer by being a slightly stale view).