@Override
  public ByteBuffer processCommand(RequestReader request, Protocol protocol, Cache cache) {
    if (protocol == Protocol.ASCII) {
      return processAsciiCommand(request.getRequest(), cache);
    }
    return processBinaryProtocol(request, cache);
  }
decr <key> <value> [noreply]\r\n

value is the amount by which the client wants to increase/decrease the item. It is a decimal
representation of a 64-bit unsigned integer.

The data for the item is treated as decimal representation of a 64-bit unsigned integer. Also,
the item must already exist for incr/decr to work; these commands won't pretend that a
non-existent key exists with value 0; instead, they will fail.
geode-memcached/src/main/java/org/apache/geode/internal/memcached/commands/DecrementCommand.java