private void validateOpcode(DataInputStream dis, byte expect) throws IOException {
    byte opCode = dis.readByte();
    if (opCode != expect) {
      if (logger.isTraceEnabled(LogMarker.PERSIST_RECOVERY_VERBOSE)) {
        logger.trace(LogMarker.PERSIST_RECOVERY_VERBOSE, "expected opcode id absent: {}", expect);
      }
      throw new IllegalStateException();
    }
  }
This map is used during recover to keep track of keys that are skipped. Later modify records in
the same oplog may use this map to retrieve the correct key.
geode-core/src/main/java/org/apache/geode/internal/cache/Oplog.java