@Override
  public Result postAppend(final ObserverContext<? extends RegionCoprocessorEnvironment> e,
    final Append append, final Result result) throws IOException {
    if (ct.incrementAndGet() == 1) {
      Threads.sleep(sleepTime.get());
    }
    return result;
  }
This coproceesor sleep 2s at first increment/append rpc call.
hbase-server/src/test/java/org/apache/hadoop/hbase/client/SleepAtFirstRpcCall.java