The tale of the IsSolrAliveAgent for Sitecore

I had the pleasure of assessing a Sitecore 7 implementation the other day and it made me a bit nostalgic for the good old days — OMS had grown into DMS, all nicely contained in a tidy SQL Server database; barely a whisper of Solr; ItemBuckets were the new kid on the block.

It got me thinking about how some features can evolve over time until the obscure becomes mainstream. One example of this feature evolution, that for some reason has relevance to a variety of customers we’re working with right now, is in how Sitecore maintains connections with Solr (or should I say how Sitecore doesn’t maintain connections?).

Many of us have learned hard lessons that if you have a Solr server and it reboots or experiences an interruption in service, it can mean downtime for your Sitecore site unless you take special measures to guard against it. Sitecore’s initialization process connects to Solr and then holds that connection for the lifetime of the IIS AppPool. If that connection fails for any reason, there wasn’t logic in the Sitecore Solr Provider for a graceful reconnect. At least, that was the case for several iterations of Sitecore’s standard Solr search integration.

A few years ago, a basic approach evolved through the Sitecore developer community to correct the above limitation with Sitecore; it may have come from Sitecore support, but it wasn’t publicized in any way. One could explicitly repeat Sitecore’s initialization logic for Solr in a custom agent, scheduled to run periodically. This was a custom solution and I saw a few iterations of it — mostly a brute force approach. But it worked.

In Sitecore 8.2 update-1,  buried in the Release Notes is this fragment relevant to the story:

Solr

The Sitecore.ContentSearch.SolrProvider, from that point onward, contained a new agent defined in Sitecore.ContentSearch.Solr.DefaultIndexConfiguration.config named IsSolrAliveAgent that would serve as a retryer for Solr if the connectivity was lost.  It was configured to run every 10 minutes for a default implementation.

By the way, 10 minutes is probably too long an interval in my experience — even 1 minute can be too long for a production environment to wait before trying to reconnect a key component such as Solr. Also, if you set the agent to 1 minute, but have a /sitecore/scheduling/frequency value defined as something like 10 minutes, you need to change the frequency value to ensure the IsSolrAliveAgent is executed on the schedule you expect.

What had been a little-known approach to keeping Sitecore connected to Solr across interruptions had made the big time: it was now part of the official Sitecore search provider code!

This first implementation included in the Sitecore code base wasn’t perfect, though, and it was improved upon in subsequent releases of Sitecore. Improvements include better logging . . . more efficient iteration through the search indexes . . . but the general approach remains the same.

At this point, there’s an official patch for the IsSolrAliveAgent that Sitecore makes available at https://github.com/SitecoreSupport/Sitecore.Support.163850.171950/releases/tag/8.2.6.0 and please note the compatibility caveats on that repo. We do have a Sitecore 8.2 update-5 customer making use of the patch without issue, even though it’s not officially listed as compatible for that version — but that could be exceptional in our case, so always perform your own evaluations, tests, etc. This patch addresses a known issue where, if Solr is unavailable during Sitecore initialization, the SwitchOnRebuildSolrCloudSearchIndex indexes are not properly initialized.

It’s interesting — at least to a Sitecore/Solr nerd like me — to decompile and compare all the changes over time to this component; there’s logging and tests and generally better code present in the newest iteration of this IsSolrAliveAgent vs the earlier implementations. Some of the changes are subtle, but the evolution of this IsSolrAliveAgent from the days of “hey, we’ve got this homegrown ten lines of code that reconnects Sitecore to Solr if Solr goes down” is remarkable.

In some ways, it parallels the progress of Sitecore as an entire platform. The CMS became a “personalization platform” which now is adding a Commerce ecosystem. OMS became DMS which became xDB. On we go.

At this point, adding a patch .config file such as the following to your Sitecore project is the state of the art in Sitecore and Solr connectivity . . . but it surely will be improved upon over time:

<configuration xmlns:patch="http://www.sitecore.net/xmlconfig/" xmlns:set="http://www.sitecore.net/xmlconfig/set/">
  <sitecore>
    <scheduling>
      <agent type="Sitecore.ContentSearch.SolrProvider.Agents.IsSolrAliveAgent, Sitecore.ContentSearch.SolrProvider">
        <patch:attribute name="type">Sitecore.Support.ContentSearch.SolrProvider.Agents.IsSolrAliveAgent, Sitecore.Support.163850.171950</patch:attribute>
        <patch:attribute name="interval">00:01:00</patch:attribute>
      </agent>
    </scheduling>
  </sitecore>
</configuration>

 

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Sitecore’s SessionDictionaryData class saves the day

More and more projects are using Azure SQL as the database back-end for Sitecore (so long as they’re running Sitecore 8.2 and newer — if alignment to Sitecore official support guidance is important to you). This sets up a new class of performance considerations around Azure SQL, and I want to share one tuning option we learned while investigating high DTU usage for the Sitecore xDB “ReferenceData” database in a Sitecore 9 PaaS build. We wanted to off-load some of the work this “ReferenceData” database was doing, and investigations into which Azure SQL queries were causing the DTU spikes pointed to INNER JOINs between the ReferenceData.DefinitionMonikers and ReferenceData.Definitions tables.

Sitecore support pointed us in the right direction at this juncture, since the default DictionaryData was using AzureSQL for persistence — we should consider a store more suited to rapid key/value access. If this sounds like a job for Redis, you’d be correct, and fortunately Sitecore has an implementation that’s suited for this type of dictionary access in the Sitecore.Analytics.Data.Dictionaries.DictionaryData.Session.SessionDictionaryData class.

The standard Sitecore pipeline we’re talking about is the getDictionaryDataStorage pipeline and it’s used by Sitecore Analytics to store Device, UserAgent, and other key/value pair lookups. Here’s it’s definition:

The alternative we moved to is to use session state for storing that rapidly requested data,  so we updated the DictionaryData node to instead use the class Sitecore.Analytics.Data.Dictionaries.DictionaryData.Session.SessionDictionaryData. For this Azure PaaS solution, it amounts to using Azure Redis for this work since that’s where the session state is managed. Here’s the new definition:

What this boils down to is the implementation in Sitecore.Analytics.DataAccess.dll of Sitecore.Analytics.DataAccess.Dictionaries.DataStorage.ReferenceDataClientDictionary was shown to be a performance bottleneck for this particular project, so changing to use the Sitecore.Analytics.dll with it’s Sitecore.Analytics.Data.Dictionaries.DictionaryData.Session.SessionDictionaryData aligns the project to a better-fit persistence mechanism.

We considered if we could improve upon this progress by extending the SessionDictionaryData class to be IIS in-memory regardless of the Sitecore session-state configuration; there would be no machine boundary to cross to resolve the (apparently) volatile data. Site visitors would require affinity to a specific AppService host in Azure, though, with this and it’s possible – or even likely — that Sitecore assumes this is shared state across an entire implementation. We talked ourselves out of seriously considering a pure IIS in-memory solution.

I think it’s possible we could improve the performance with the default ReferenceDataClientDictionary by tuning any caches around this analytics data, but I didn’t look into that since time was of the essence for this investigation and the SessionDictionaryData class looked like such a quick win. I may revisit that in the next iteration, however, depending on how this new solution performs over the long term.