RavenDB 5.2: Rolling index deployment July 12, 2021 Author: Oren Eini, CEO RavenDB
Read the orginal blog post

RavenDB 5.2 introduce a new concept for deploying indexes: Rolling indexes deployment.

Typically, deploying an index to production on a loaded database is something that you do only with great trepidation. There are many horror stories about creating a new index and resulting in the entire system locking down for a long period of time.

RavenDB was specifically designed to address those concerns. Deploying a new index in production is something that you are expected to do. In fact, RavenDB will create indexes for you on the fly as needed, in production.

To be perfectly honest, the two aspects are very closely tied together. The fact that we expect to be able to create an index without disruption of  service feeds into us being able to create indexes on the fly. And creating indexes on the fly ensures that we’ll need to keep being able to create indexes without putting too much load on a running system.

RavenDB limits the amount of CPU time that a new index can consume and will control the amount of memory and I/O that is used by an index to prioritize user queries over background work.

In version 5.2, we have extended this behavior to allow RavenDB further. We now allow users to deploy their own code as part of RavenDB indexes, which make it much harder to control what exactly is going on inside RavenDB during indexing. For example, you may have choose to run something like Parallel.For(), which may use more CPU than RavenDB accounts for. The situation is a bit more complex in the real world, because we need to worry about other factors as well (memory, I/O, CPU and network comes to mind).

Consider what happens if a user does something like this in an index:

RavenDB has no way to control what is actually going on there, and this code will use 1GB of RAM and quite a bit of CPU (over multiple cores) without the ability to control that. This is a somewhat contrived example, I’ll admit (can’t think of any reason you’ll want to do this sort of thing in an index). It is far more common to want to do Machine Learning Predictions in indexes now, which can have similar affects.

When pushing a large number of documents through such an index, such as the scenario of deploying a new index, that can put a lot of strain on the system. Enter: Rolling index deployment.

This is an index deployment mode where RavenDB will not immediately deploy the index to all the nodes. Instead, it will choose the least busy node and get it to run the index. At any time, only a single node in the cluster is going to run the index, and that node is going to be in the back of the line for any other work the cluster has for it. Once that node is completed, RavenDB will select the next node (and reassign work as needed).

The idea is that even if you deploy an index that has a negative impact on the system behavior, you have mitigated the potential impact to a single (hopefully unused) node.

The cost of that, of course, is that the indexes are now going to run in a serial fashion, one node at a time. That means that they will take longer to deploy to the entire system, of course, but the resource utilization is going to be far more stable.