code: private static Journal getStorage(ServletContext context,
        UploadImageParam params) throws IOException {
      final Journal journal = JournalNodeHttpServer
          .getJournalFromContextIfExists(context, params.journalId);
      if (journal == null) {
        throwIOException("Journal: " + params.journalId + " does not exist");
      }
      final JNStorage storage = journal.getImageStorage();
  
      // check namespace metadata
      storage.checkConsistentNamespace(params.getStorageInfo());
  
      // check if this is the active writer
      if (!journal.checkWriterEpoch(params.epoch)) {
        throwIOException("This is not the active writer");
      }
      return journal;
    }
docs: Get storage object from underlying journal node, given request parameters.
  If yournal does not exist, an exception will be thrown. If the journal
  metadata does not match the request metadata, we also fail.