[infinispan-dev] HotRod?? [ISPN-29 and a "custom protocol"]

[Manik Surtani]

On 10 Dec 2009, at 15:24, Galder Zamarreno wrote:

> 
> 
> On 12/10/2009 03:15 PM, Manik Surtani wrote:
>> 
>> On 10 Dec 2009, at 12:23, Galder Zamarreno wrote:
>>>>> Also, it might be wised to implement operations like replaceIfEquals
>>>>> using with a cas long, like memcached does
>>>>> (http://code.google.com/p/memcached/wiki/MemcacheBinaryProtocol), rather
>>>>> than sending the value cos that would be more network efficient. The old
>>>>> value could be much more longer.
>>>> 
>>>> Not sure I understand.  So we have 2 replace methods (thanks to ConcurrentMap) - replace(k, v) which succeeds if k is mapped to something, and replace(k, v1, v2) which succeeds if k is mapped to v1.  From what I see, only the former is supported by memcached's binary protocol and not the latter.  Are you suggesting supporting the latter in HotRod?
>>> 
>>> I'm suggesting supporting both actually.
>>> 
>>> replace(k, v) which succeeds if k is mapped to something - that's the
>>> same as memcached's replace command. I plan to support that.
>>> 
>>> replace(k, v1, v2) is what memcached does with 'cas' but instead of
>>> passing the old value, it passes the cas id, which is 8 bytes at the
>>> most. The old value could be a lot longer. In ConcurrentMap, replace
>>> works by passing the old value.
>> 
>> Sure, and that's in the same VM so you pass by reference.  Across the wire, this may involve serializing the entire (old) value which may be quite large.
>> 
>>> I think, to save network space, I think we should implement it the way
>>> memcached does. IOW, whenever something gets stored, it's stored with a
>>> cas id and this can later be used to replace a value, only of the cas is
>>> the one as we've passed.
>> 
>> You get this cas id back as a return value?  Which you can then store and use in future 'replace' commands?  Is this it?
> 
> Yeah, u can get the cas id via gets command.
> 
>> 
>>> In the txt protocol, I implemented the cas id using System.nanoTime().
>> 
>> Is this cas id meant to be unique?  If so, System.nanoTime() may not be ...
> 
> Not over time. You want to be unique from the minute you retrieved it 
> until you use it and for operations on that particular key.
> 
> The cas Id is simply used for comparison, to check whether someone else 
> has changed the entry since you retrieved it. System.nanoTime() gives 
> you precisely that, anyone that modifies that entry in that machine will 
> definitely have a different nano time. Also, since nano time is based 
> off an arbitrary time, the chances that a different machine will produce 
> the same nano time when modifying that very same key are almost very 
> very small.

Improbable but certainly not impossible, even in 1 machine.

Why not just use what we use for 'versioning' internally - object references?  System.identityHashCode(), for example?

Cheers
--
Manik Surtani
manik at jboss.org
Lead, Infinispan
Lead, JBoss Cache
http://www.infinispan.org
http://www.jbosscache.org