code: @Override
      public void append(Append append) {
          log.trace("Processing append received from client {}", append);
          UUID id = append.getWriterId();
          synchronized (lock) {
              Long lastEventNumber = latestEventNumbers.get(Pair.of(append.getSegment(), id));
              Preconditions.checkState(lastEventNumber != null, "Data from unexpected connection: %s.", id);
              Preconditions.checkState(append.getEventNumber() >= lastEventNumber, "Event was already appended.");
              waitingAppends.put(id, append);
          }
          pauseOrResumeReading();
          performNextWrite();
      }
docs: Append data to the store.
  Because ordering dictates that there only be one outstanding append from a given connection, this is implemented
  by adding the append to a queue.