Persisting cached data after removal/eviction using Memcache API compatible store

Question

This question is specifically pertaining to Couchbase, but I believe it would apply to anything with the memcached api.

Lets, say I am creating a client/server chat application, and on my server, I am storing chat session information for each user in a data bucket. Well after the chat session is over, I will remove the session object from the data bucket, but at the same time I also want to persist it to a permanent NoSQL datastore for reporting and analytics purposes. I also want session objects to be persisted upon cache eviction, when sessions timeout, etc.

Is there some sort of "best practice" (or even a function of Couchbase that I am missing) that enables me to do this efficiently and maintaining best possible performance of my in memory caching system?

Answer 1

Using Couchbase Server 2.0, you could setup two buckets (or two separate clusters if you want to separate physical resources). On the session cluster, you'd store JSON documents (the value in the key/value pair), perhaps like the following:

{ 
  "sessionId" : "some-guid",
  "users" : [ "user1", "user2" ],
  "chatData" : [ "message1", "message2"],
  "isActive" : true,
  "timestamp" : [2012, 8, 6, 11, 57, 00]
}

You could then write a Map/Reduce view in the session database that gives you a list of all expired items (note the example below with the meta argument requires a recent build of Couchbase Server 2.0 - not the DP4.

function(doc, meta) {
  if (doc.sessionId && ! doc.isActive) {
    emit(meta.id, null);
  }
}

Then, using whichever Couchbase client library you prefer, you could have a task to query the view, get the items and move them into the analytics cluster (or bucket). So in C# this would look something like:

var view = sessionClient.GetView("sessions", "all_inactive");
foreach(var item in view) 
{
   var doc = sessionClient.Get(item.ItemId);
   analyticsClient.Store(StoreMode.Add, item.ItemId, doc);
   sessionClient.Remove(item.ItemId);
}

If you instead, wanted to use an explicit timestamp or expiry, your view could index based on the timestamp:

function(doc) {
   if (doc.sessionId && ! doc.isActive) {
      emit(timestamp, null);
   }
}

Your task could then query the view by including a startkey to return all documents that have not been touched in x days.

var view = sessionClient.GetView("sessions", "all_inactive").StartKey(new int[] { DateTime.Now.Year, DateTime.Now.Months, DateTime.Now.Days-1);

foreach(var item in view)
{
   var doc = sessionClient.Get(item.ItemId);
   analyticsClient.Store(StoreMode.Add, item.ItemId, doc);
   sessionClient.Remove(item.ItemId);
}

Checkout http://www.couchbase.com/couchbase-server/next for more info on Couchbase Server 2.0 and if you need any clarification on this approach, just let me know on this thread.

-- John

Answer 2

CouchDB storage is (eventually) persistent and without built-in expiry mechanism, so whatever you store in it will remain stored until you remove it - it's not like in Memcached where you can set timeout for stored data. So if you are storing session in CouchDB you will have to remove them on your own when they expire and since it's not an automated mechanism, but something you do on your own there is no reason for you not to save data wherever you want at the same time.

BTH I see no advantage of using Persistent NoSQL over SQL for session storage (and vice versa) - performance of both will be IO bound. Memory only key store or hybrid solution is a whole different story.

As for your problem: move data in you apps session expiry/session close mechanism and/or run a cron job that periodically checks session storage for expired sessions and move the data.