Rob’s Sprint: Idly indexing

During Rob Ashton’s visit to our secret lair, we did some work on hard problems. One of those problems was the issue of index prioritization. As I have discussed before, this is something that isn’t really easy to do, because of the associated IO costs with not indexing properly.

With Rob’s help, we have the defined the following:

• An auto index can be set to idle if it hasn’t been queried for a time.
• An index can be forced to be idle by the user.
• An index that was automatically set to idle will be set to normal on its first query.

What are the implications for that? And idle index will not be indexed by RavenDB during the normal course of things. Only when the database is idle for a period of time (by default, about 10 minutes with no writes) will we actually get it indexing.

Idle indexing will continue indexing as long as there is no other activity that require their resources. When that happens, they will complete their current run and continue to wait for the database to become idle again.

But wait, there is more. In addition to introducing the notion of idle indexes, we have also created another two types of indexes. The first is pretty obvious, the disabled index will use no system resources and will never take part in indexing. This is mostly there so you can manually shut down a single index. For example, maybe it is a very expensive one and you want to stop it while you are doing an import.

More interesting, however, is the concept on an abandoned index. Even idle indexes can take some system resources, so we have added another level beyond that, an abandoned index is one that hasn’t been queried in 72 hours. At that point, RavenDB is going to avoid indexing it even during idle periods. It will still get indexed, but only if there has been a long enough time passed since the last time it was indexed.

Next, we will discuss why this feature was a crucial step in the way to killing temporary indexes.