private ByteBuffer processAsciiCommand(ByteBuffer buffer, Cache cache) {
    CharBuffer flb = getFirstLineBuffer();
    getAsciiDecoder().reset();
    getAsciiDecoder().decode(buffer, flb, false);
    flb.flip();
    String firstLine = getFirstLine();
    String[] firstLineElements = firstLine.split(" ");

    assert "incr".equals(firstLineElements[0]);
    String key = firstLineElements[1];
    String incrByStr = stripNewline(firstLineElements[2]);
    long incrBy = Long.parseLong(incrByStr);
    boolean noReply = firstLineElements.length > 3;

    Region<Object, ValueWrapper> r = getMemcachedRegion(cache);
    String reply = Reply.NOT_FOUND.toString();
    ByteBuffer newVal = ByteBuffer.allocate(8);
    while (true) {
      ValueWrapper oldValWrapper = r.get(key);
      if (oldValWrapper == null) {
        break;
      }
      newVal.clear();
      byte[] oldVal = oldValWrapper.getValue();
      long oldLong = getLongFromByteArray(oldVal);
      long newLong = oldLong + incrBy;
      newVal.putLong(0, newLong);
      ValueWrapper newValWrapper = ValueWrapper.getWrappedValue(newVal.array(), 0/* flags */);
      if (r.replace(key, oldValWrapper, newValWrapper)) {
        reply = newLong + "\r\n";
        break;
      }
    }
    return noReply ? null : asciiCharset.encode(reply);
  }
<code>
incr &lt;key&gt; &lt;value&gt; [noreply]\r\n
</code><br>
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/IncrementCommand.java