Search, Sitecore

Avoiding downtime while rebuilding your Lucene search indexes in Sitecore ASP.NET CMS

This is the first blog post I’ve written since my arrival in Sydney and my new job within Sitecore. It will be about a simple yet strong feature often overlooked in the Lucene indexing capabilities of Sitecore 7: how to avoid downtime on your Lucene indexes.
One of the problems I run into occasionally is that a rebuild of an index, e.g. the web database index, will cause features of the site to not work properly. This is due to the fact that the index is taken offline while rebuilding.

But what causes a rebuild?

A rebuild may be triggered manually through the control panel in Sitecore, or by an indexing strategy such as RemoteRebuild (see more here). But full rebuilds might also be triggered by simply publishing too many items from master to web. This is due to the Threshold setting in e.g. the PublishEndStrategy, which will transform many single item indexes to simply one full rebuild if the given threshold is reached (See more here).

How do I avoid downtime then?

Well the solution to the above problem is relatively simple, yet often overlooked: Changing the standard Lucene index handler (the class which actually handles reading and writing to the Lucene index files) from LuceneIndex to SwitchOnRebuildLuceneIndex will remove this problem.

<configuration xmlns:patch="http://www.sitecore.net/xmlconfig/">
  <sitecore>
    <contentSearch>
        <configuration type="Sitecore.ContentSearch.LuceneProvider.LuceneSearchConfiguration, Sitecore.ContentSearch.LuceneProvider">
          <indexes hint="list:AddIndex">
            <index id="sitecore_web_index" type="Sitecore.ContentSearch.LuceneProvider.SwitchOnRebuildLuceneIndex, Sitecore.ContentSearch.LuceneProvider">
              <param desc="name">$(id)</param>
              <param desc="folder">$(id)</param>
<!-- ... -->

The SwitchOnRebuildLuceneIndex maintains two separate indexes on disk, one active and one passive. As a rebuild occurs, it is executed on the passive index and when the rebuild completes, the passive becomes active and the active becomes passive. Thereby, there will be no downtime of the index.

Things to keep in mind

Therefore, a couple of things to keep in mind:

  • Switch to SwitchOnRebuildLuceneIndex if you want to make sure that you have no downtime on your Sitecore search indexes.
  • Make room on disk for maintaining duplicate disks.
  • Always choose your Index update strategies carefully.
  • Different server roles might require different index update strategies.
Standard