memcache distribution

[atarno]

hi folks,

i'm puzzled by memcache related logs i see and need your advice.
i'm running a java app. i use memcache as a holder of some static variables accessed and changed by different modules.
i do it, because as far as i understand such changes will apply to all distributed application instances.
the initialization of the memcache is done within an init() method of a servlet flagged with <load-on-startup>.
i have a following piece of code there:

if(CacheManager.getInstance().getCache("static_vars") == null)
{
  CacheFactory cacheFactory = CacheManager.getInstance().getCacheFactory();
  Cache cache = cacheFactory.createCache(Collections.emptyMap());
  CacheManager.getInstance().registerCache("static_vars", cache);
  cache.put("var1", 0);
  cache.put("var2", 9302);
.....
}

now, i'm watching the logs and i see two requests arriving within 2 seconds. the first one targets an existing instance and everything goes as expected, but the second one triggers a creation of new servlet instance that loads the cache. i guess it happens because another application instance is created. the problem is that i see that in this new instance CacheManager.getInstance().getCache("static_vars") == null. the new instance does not recognize that a memcache with this name already exists and therefore does not use the values that were set there by first instance.

what's my problem here?

thanks in advance.

[atarno]

bump

sorry, but i'm still looking for an answer. i give this question another chance. thanks.

[Brandon Wirtz]

Memcache is not “Shared Memory” It is a cache that is sometimes shared.  Writes to memcache are not guaranteed, and nor are reads.  Cache flushes can happen at any time.  It doesn’t happen often, but I have put things in memcache and tried to read them 6 lines of code later and they weren’t there… You ALWAYS have to Try the read, and then try something else if your value isn’t there.

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[Jeff Schnitzer]

Stop using the JCache interface. It just adds a useless complicated
layer between you and what's really going on. For example,
CacheManager.getCache(String) doesn't do what you think it does.

Use MemcacheServiceFactory and MemcacheService.

Jeff

[atarno]

thanks for your responses.
as i understand there is no way to hold and change a distributable static variable in gae, but to persist it in the datastore on each change?

[Jeff Schnitzer]

You have an interface to the datastore. You have an interface to
memcache. You have local memory. You can use these in any way you
want.

Jeff

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