private ByteBuffer processBinaryProtocol(RequestReader request, Cache cache) {
    ByteBuffer buffer = request.getRequest();
    int extrasLength = buffer.get(EXTRAS_LENGTH_INDEX);
    final KeyWrapper key = getKey(buffer, HEADER_LENGTH + extrasLength);

    long decrBy = buffer.getLong(HEADER_LENGTH);
    long initialVal = buffer.getLong(HEADER_LENGTH + LONG_LENGTH);
    int expiration = buffer.getInt(HEADER_LENGTH + LONG_LENGTH + LONG_LENGTH);

    final Region<Object, ValueWrapper> r = getMemcachedRegion(cache);
    ByteBuffer newVal = ByteBuffer.allocate(8);
    boolean notFound = false;
    ValueWrapper newValWrapper = null;

    try {
      while (true) {
        ValueWrapper oldValWrapper = r.get(key);
        if (oldValWrapper == null) {
          if (expiration == -1) {
            notFound = true;
          } else {
            newVal.putLong(0, initialVal);
            newValWrapper = ValueWrapper.getWrappedValue(newVal.array(), 0/* flags */);
            r.put(key, newValWrapper);
          }
          break;
        }
        byte[] oldVal = oldValWrapper.getValue();
        long oldLong = getLongFromByteArray(oldVal);
        long newLong = oldLong - decrBy;
        if (newLong < 0) {
          newLong = 0;
        }
        newVal.putLong(0, newLong);
        newValWrapper = ValueWrapper.getWrappedValue(newVal.array(), 0/* flags */);
        if (r.replace(key, oldValWrapper, newValWrapper)) {
          break;
        }
      }
    } catch (Exception e) {
      return handleBinaryException(key, request, request.getResponse(), "decrement", e);
    }

    if (expiration > 0) {
      StorageCommand.getExpiryExecutor().schedule(() -> {
        r.destroy(key);
      }, expiration, TimeUnit.SECONDS);
    }

    if (getLogger().fineEnabled()) {
      getLogger().fine("decr:key:" + key + " decrBy:" + decrBy + " initVal:" + initialVal + " exp:"
          + expiration + " notFound:" + notFound);
    }

    ByteBuffer response = null;
    if (notFound) {
      response = request.getResponse();
      response.putShort(POSITION_RESPONSE_STATUS, ResponseStatus.KEY_NOT_FOUND.asShort());
    } else {
      if (isQuiet()) {
        return null;
      }
      response = request.getResponse(HEADER_LENGTH + LONG_LENGTH);
      response.putInt(TOTAL_BODY_LENGTH_INDEX, LONG_LENGTH);
      response.putLong(HEADER_LENGTH, newVal.getLong(0));
      response.putLong(POSITION_CAS, newValWrapper.getVersion());
    }
    return response;
  }
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