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

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.