On Fri, Oct 17, 2014 at 1:23 PM, Karim Tawfik <[email protected]> wrote:
> Hi,
>
> I am new to memcached, and try to introduce it on our company as a caching
> layer, but there is a question came up to my mind, how memcached handle the
> consistency of data to all are the same in all clusters.
>
> For example:
> say I have 2 clusters, each have memcached is installed on it, and clients
> started to send requests (e.g.updating some data), how the other memcached
> server would know about such update if it is already caching an old version
> before it got updated.
>
> I am asking this question as i read 2 contradicting statements on the
> website, which are:
>
> Under ==> https://code.google.com/p/memcached/wiki/NewOverview, section.
>
> Servers are Disconnected From Each Other : Memcached servers are generally
> unaware of each other. There is no crosstalk, no syncronization, no
> broadcasting
>
> Under ==> https://code.google.com/p/memcached/wiki/TutorialCachingStory
>
> So again, he takes keys that the Programmer uses and looks for them on his
> memcached servers. 'get this_key' 'get that_key' But each time he does this,
> he only finds each key on one memcached! Now WHY would you do this, he
> thinks? And he puzzles all night. That's silly! Don't you want the keys to
> be on all memcacheds?
>
> "But wait", he thinks "I gave each memcached 1 gigabyte of memory, and that
> means, in total, I can cache three gigabytes of my database, instead of just
> ONE! Oh man, this is great," he thinks. "This'll save me a ton of cash. Brad
> Fitzpatrick, I love your ass!"
>
> Could you please give me the clear directions, if i have incorrect view.
What is it that you think is contradicting here? The client is
configured for a set of servers to use, computes a specific one of
them from a hash of the key, and writes an item to exactly one server.
When any client with the same configuration looks up that same key it
will do the same computation and thus target the same server. Other
keys may go to other servers.
> The last thing is, does memcached get affected by anymeans of replications
> between server?
No, there is only one copy. If that server instance is down, the
client must get the data from the backing persistent storage - and
depending on the client's hashing strategy it can either continue to
fail for whatever percentage of the cache that server handles until it
comes back up, or it can rebalance the storage over the remaining
servers.
--
Les Mikesell
[email protected]
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