protected void checkOpen() throws IOException {
    if (server.isAborted()) {
      throw new RegionServerAbortedException("Server " + server.getServerName() + " aborting");
    }
    if (server.isStopped()) {
      throw new RegionServerStoppedException("Server " + server.getServerName() + " stopping");
    }
    if (!server.isDataFileSystemOk()) {
      throw new RegionServerStoppedException("File system not available");
    }
    if (!server.isOnline()) {
      throw new ServerNotRunningYetException(
        "Server " + server.getServerName() + " is not running yet");
    }
  }
Called to verify that this server is up and running.
hbase-server/src/main/java/org/apache/hadoop/hbase/regionserver/RSRpcServices.java