Settings in this section concern projections that are running on the server. Read more about projections here.
Server-side projections impact the performance of the EventStoreDB server. For example, some standard system projections like Category or Event Type projections produce new (link) events that are stored in the database in addition to the original event. This effectively doubles or triples the number of events appended and therefore creates pressure on the IO of the server node. We often call this effect "write amplification".
RunProjections option tells the server if you want to run all projections, only system projections or no projections at all. Hence that the
StartSystemProjections setting has no effect on custom projections.
The option accepts three values:
When the option value is set to
None, the projections subsystem of EventStoreDB will be completely disabled and the Projections menu in the Admin UI will be disabled.
By using the
System value for this option, you can instruct the server to enable system projections when the server starts. However, system projections will only start if the
StartStandardProjections option is set to
true. When the
RunProjections option value is
All) but the
StartSystemProjections option value is
false, system projections will be enabled but not start. You can start them later manually via the Admin UI or via an API call.
Finally, you can set
All and it will enable both system and custom projections.
None, all projections are disabled by default.
Accepted values are
The primary reason for increasing the number of projection threads is projections which perform a large amount of CPU-bound processing. Projections are always eventually consistent - if there is a mismatch between egress from the database log and processing speed of projections, the window across which the latest events have not been processed promptly may increase. Too many projection threads can end up with increased context switching and memory use, since a V8 engine is created per thread.
There are three primary influences over projections lagging:
- Large number of writes, outpacing the ability of the engine to process them in a timely fashion.
- Projections which perform a lot of CPU-bound work (heavy calculations).
- Projections which result in a high system write amplification factor, especially with latent disks.
ProjectionThreads option to adjust the number of threads dedicated to projections.
Fault out of order projections
It is possible that in some cases a projection would get an unexpected event version. It won't get an event that precedes the last processed event, such a situation is very unlikely. But, it might get the next event that doesn't satisfy the
N+1 condition for the event number. The projection expects to get an event number
5 after processing the event number
4, but eventually it might get an event number
7 because events
6 got deleted and scavenged.
The projections engine can keep track of the latest processed event for each projection. It allows projections to guarantee ordered handling of events. By default, the projections engine ignore ordering failures like described above. You can force out of order projections to fail by setting the