@ThreadSafe public final class PipeContainer extends java.lang.ObjectWrites happen in a serialized manner with
putPipeListIfAbsent(Pair, int). This ensures that each key is initialized exactly once, and never updated.
Writes and reads for the same key never occur concurrently with no problem, because (1) write never updates, and (2) read happens only after the write.
Reads can happen concurrently with no problem.