Thank you for the prompt reply it is a very helpful article. But it
also raises some questions.
Lets use the example from that article.
Cache A contains values 1,4
Cache B contains values 2
Cache C contains values 3
Questions 1-) If cache A does down, would cache B will now have 4,1and
2? If so this is not equally distributed. cache B contains 3 values
(4,1,2) and Cache C has only one value (3). Lets assume if cache A
actually contained thousands of values, shifting these values to only
cache B will truly unbalance the distribution.
Questions-2) Lets use the same example as above for starters.
Cache A contains values 1,4
Cache B contains values 2
Cache C contains values 3
Lets say because of a network issue Cache A was not accessible but the
server is still up. Client will probably move values to cache B. Now
cache B has values 4,1,2. Lets say the content of values 1,4 changed.
Cache B has the new content for values. NOW the network problem is
fixed and Cache A again available. Will client get the values 1 and 4
from Cache A or Cache B? If it gets it from cache A, these values will
be stale because new content for these values are now in Cache B.
On Feb 23, 5:49 pm, Matt Ingenthron <[email protected]> wrote:
> mcevikce wrote:
> > I understand that memcached servers do not communicate with each
> > other. I also understand that consistent hashing algorithm in client
> > libraries endure that data is distributed evenly between the servers.
> > What I am not clear about is:
> > Scenario 1:
> > Three server nodes running. key1 is stores on server #1. Server #1
> > goes down.
> > 1-) Client makes a request on key1 stored on failed server. My
> > assumption is that cache will get a miss and value for key1 will be
> > retrieved from DB and stored in server #2.
> > 2-) Now dead server is brought up. Client makes the request on a
> > key1 again. Does client get the key1 from server #2 then store it back
> > into server #1? How does re balancing occur?
>
> > Scenario 2:
> > Three nodes running. Another node is added. How does memcached
> > redistribute values to new node? I know that servers themselves do not
> > communicate with each other.
>
> The data isn't so much redistributed as it is re-populated from the
> primary source to the new location in the consistent hash. The old
> items in memory in the old positions will eventually expire or LRU out.
>
> The key here is that the clients all have a consistent view and shared
> hashing algorithm pointing to the same servers. If you add a new node,
> for some portion of keys there will initally be misses, but then things
> will balance out later.
>
> Have a look
> at:http://www.last.fm/user/RJ/journal/2007/04/10/rz_libketama_-_a_consis...http://weblogs.java.net/blog/2007/11/27/consistent-hashing
>
> Hope that helps,
>
> - Matt
