Sitecore Session Persistence Notes

I’ve neglected this blog of late, being focused on a number of “not easily blogged about” scenarios across several Sitecore projects.  It’s too bad, because the work is very interesting, but it doesn’t lend itself to a page or two write-up with a digestible take-away for the general Sitecore community out there.

I do want to keep in the habit of blogging, though, so I’m going to mention this ongoing discussion I’ve been a part of about session management with regards to Sitecore.  There are a few options for managing HTTP session state with Sitecore covered in https://doc.sitecore.net/sitecore_experience_platform/setting_up_and_maintaining/xdb/session_state/session_state: SQL Server, MongoDB, and Redis.  Those three technologies are really just the tip of the mountain, as implementation details for each can get quite detailed.  For the discerning Sitecore implementation, it can be useful to understand the nuances of each session state provider.  While not an exhaustive look at any one of these solutions, I wanted to post some notes on each one given the current state of Sitecore architecture (June 2017):

SQL Server

This is often the default session provider we gravitate to.  The SQL Server “Boost” script from Sitecore is something we’ve used on implementations (see “Optimize SQL Server performance” on that link), but it is not without it’s rough edges (see our Rackspace write-up on how to alter permissions so TempDB is reliably available across service restarts).

You’ll notice the approach for improving SQL Server performance with session state is all about getting session state “in-memory” to the furthest extent possible.  Remember this when we examine the other two providers below . . .

I will say that, generally speaking, SQL Server is easy to administer as it’s a well-known technology and updating it, scaling it, managing fail-overs, etc is simple compared with the alternatives.  SQL Server has been part of the Windows dev stack for ages, now, so it’s often the default session provider one gravitates to.

MongoDB

With MongoDB serving as the persistence layer for Sitecore’s xDB, it became a fully supported and viable option for HTTP session state with Sitecore at the same time.  The comparative performance between MongoDB and SQL Server is up for debate (Redis too, for that matter!), and it usually comes down to testing based on how the specific implementation is using session with Sitecore etc; I’m not going to hazard any generalizations on relative perf, as that’s not really the point of this post.

Instead, I’d like to point out how MongoDB does not come in just a single flavor.  The two most common flavors, or “storage engines,” are MMAP and WiredTiger, but there are still others designed to serve specific use cases.  Take, for example, the Percona Server for MongoDB hosted by ObjectRocket that has a posted option for the RocksDB storage engine.  RocksDB with MongoDB may not be a great fit for Sitecore session state (RocksDB is tuned for write-heavy work loads — and, in some cases, if you’re making extensive use of TTL indexes for Sitecore then RocksDB fits those scenarios in certain appealing ways), but it does open the door to MongoDB being more than just a one-size-fits-all data repository (read more about RocksDB and it’s Facebook pedigree here).  One MongoDB storage engine option that is easily overlooked is for WiredTiger “in-memory” that will force data to be stored in RAM . . . and this is perfect for HTTP Session State for most Sitecore builds.

In fact, if you consider the SQL Server “boost” approach that uses TempDB to store session state for Sitecore . . . WiredTiger “in-memory” is attacking the problem from the same direction.  Store everything in RAM!  This is why one must be cautious with general comparisons between SQL Server and MongoDB, the devil is always in the details: a far better comparison would be “boosted” SQL Server for Sitecore using TempDB vs MongoDB WiredTiger “in-memory” storage engine.  And note the network latency . . . and the size of the session objects . . . and you’re getting the point, I trust.  To really answer the SQL Server vs MongoDB question for Sitecore sessions, one has to develop a matrix of performance evaluations and level assumptions across the board.  “It depends” is the only honest answer that doesn’t come with a list of caveats.

If you’re curious on this MongoDB topic for your project, go to http://objectrocket.com/docs/mongodb_plans.html and spin up a WT 3.2 storage engine plan for 5 GB of storage (this allows 1.5 GB for RAM).  1.5 GB for RAM is going to be overkill for most small/medium Sitecore implementations — but again, you’ll want to test with your specific session data set to see!  Furthermore, network latency of 10 ms or less is going to help make the most of an ObjectRocket hosted MongoDB service like this — otherwise, the network latency may not make it worth the money.  Let me know if you pursue this with ObjectRocket, as there are some benchmarking measures we want to do but we haven’t had a real implementation to try it out on.  So if you feel like being a guinea pig, please let me know at grant.killian [at] rackspace.com.  It would be great to have real world metrics to prove this all out.

Redis

If the way to get the best session management perf out of SQL Server and MongoDB is to find in-memory solutions, Redis looks like the slam dunk since it’s just an in-memory storage solution.  We find most clients aren’t interested in managing Redis infrastructure, so again a hosted option such as ObjectRocket has appeal.

Sitecore relies on the StackExchange.Redis assembly, which doesn’t support Redis Sentinel — it’s a bit of a saga at https://github.com/StackExchange/StackExchange.Redis/pull/406;  therefore there’s not a great high availability story with the self-hosted Redis and Sitecore right now.  How concerned one should be with HA of fairly transient HTTP Session State for Sitecore, however, is an open question.  I usually wouldn’t worry about it too much.  Honestly, Redis is a technology that we’re just now starting to get really serious about at Rackspace so our sophistication in this space will improve dramatically in the months to come.  Between Azure Redis and all the Sitecore PaaS movement we’re seeing, it’s become a key player in a lot of Sitecore architectures.

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Sitecore and TTL Index Heresy for MongoDB

Background

Part of our role at Rackspace is to be pro-active in tuning and optimizing Sitecore implementations for our customers.  Sometimes, the “optimizations” come more in terms of cost-savings instead of performance benefits, and sometimes there is a correlation.

Here’s an optimization that is more oriented to cost-savings.  We have customers using MongoDB for both private and shared session state in Sitecore, as well as xDB data collections; they’re using a hosted MongoDB service through ObjectRocket which provides provisioning elasticity, good performance, and access to really strong MongoDB pros.  We noticed old HTTP session data accumulating in the session collections for one customer, and couldn’t find an obvious explanation.  These session records shouldn’t be leaking through the MongoDB clean-up logic that’s part of the Sitecore API (Sitecore.SessionProvider.MongoDB.MongoSessionStateProvider), but we wanted to correct it and understand why.

The catch was, the customer’s development had several higher priority issues they were working on, and sifting through a root cause analysis of something like this could take some time.  In the interim, we addressed how to clean out the old session data from MongoDB — if the customer was using SQL Server for sessions state, a SQL Server agent deleting expired sessions could be easily employed . . . but that isn’t an option for MongoDB.

Research

At Rackspace, we began evaluating options for a MongoDB equivalent to the “clean up old session records” process and so I started by reviewing everything I could find about data retention for MongoDB and Sitecore.  It turns out there isn’t much.  There isn’t mention of data clean-up in the Sitecore documentation on shared and private session for MongoDB.  There isn’t an explicit <agent> configured anywhere, either.  With Reflector, I started looking through the Sitecore.SessionProvider.MongoDB.MongoSessionStateProvider class and I didn’t see any logic related to data clean-up.

I did finally find some success, however, in reviewing the Sitecore.SessionProvider.MongoDB.MongoSessionStateProvider class that extends the abstract Sitecore.SessionProvider.SitecoreSessionStateStoreProvider class.

The abstract class uses a timer (controlled by the pollingInterval set for the provider); it runs OnProcessExpiredItems . . . and then GetExpiredItemExclusive . . . and eventually RemoveItem for that record.  That finally calls through to the MongoDB API with the delete query:

RemoveItemThis was all helpful information to the broader team working to determine why the records were remaining in MongoDB, but we needed a quick non-invasive solution.

TTL Indexes

MongoDB supports “Time to Live (TTL) Indexes” which purge data based on time rules.  Data older than 1 week, for example, could automatically be removed with this index type.  That’s surely acceptable for sessions state records that leak through the Sitecore API.  We coordinated with the ObjectRocket team and are setting the customer up with TTL Indexes instead of the default; this should dramatically reduce the data storage footprint.

Heresy

While pursuing this effort on behalf of session state management, I realized this could be an intriguing solution to data retention challenges with Sitecore’s xDB.  Using MongoDB TTL indexes for the xDB collections would prevent that data from growing out of control.  Set a TTL value of 180 days, for example, and make use of just the most recent 6 months of user activity as part of content personalization, profiling, etc.  Of course, one sacrifices the value of the old data if one expires it after a set time.  Remember, I’m acknowledging this is heresy! 🙂

I really wonder, though, how many Sitecore implementations intend to store all the xDB data indefinitely and have a strategy for making use of all that old, ever-accumulating, data?  I think the promise of xDB analytics-based presentation rules is far greater than the reality.  I see many organizations lacking a cohesive plan for what data to gather into xDB, and how to effectively make use of it.

I think TTL Indexes for MongoDB would be a good safety net for those companies still sorting out their path with xDB and analytics, without having to bank massive volumes of data in MongoDB during the maturation process.

One final note: since conserving disk space is a priority, performing a weekly data compaction for MongoDB is a good idea.  This fully reclaims the space for all the expired documents.