At a thousand foot view, the functional difference between ArcGIS GeoEvent Server and ArcGIS Velocity is as follows:
- GeoEvent Server is on-premise (Enterprise) whereas Velocity is SaaS (Online). You have to manage the system in which GeoEvent Server runs. The opposite is true for Velocity. Getting started with Velocity is as simple as reaching a URL.
- While GeoEvent Server and Velocity both offer real-time analytic capabilities, Velocity also offers big data analytic capabilities.
- GeoEvent Server can be extended a variety of ways beyond what is available out of the box. It has a java-based SDK for building custom connectors & processors. Custom connectors can also be configured with out-of-the-box transports and adapters. Lastly, GeoEvent Server has the GeoEvent Gallery where pre-built add-on connectors and processors can be downloaded. Velocity is not as directly extensible due to the fact that its a SaaS. Extensibility with Velocity is largely achieved with features such as the gRPC feed type for data ingestion, or the use of Arcade expressions in various tools (such as calculate fields, map fields, and more) to push the boundaries of what those tools can do.
- Velocity has built-in high availability and disaster recovery. GeoEvent Server does not quite.
- Data stored by Velocity is stored in the cloud.
See How is ArcGIS Velocity different from ArcGIS GeoEvent Server or ArcGIS GeoAnalytics Server?
Recommending one over another comes down thinking through a few questions such as 1) do you want or need to manage software on-premise? This includes wanting or needing to manage stored data on-premise vs. the cloud. 2) What type of analysis are you trying to do? Are you interested in asking questions and deriving results in real-time, or do you have the need for post-processing of historical / batch data to derive trends and other answers over time? 3) Do you need to be able to extend your solution with some sort of custom ingestion or geoprocessing? Or can the analysis be achieved with what's offered out of the box?
There's a lot more subtle nuances that can said about both products so its definitely worth checking out the documentation for both. Based on the information I presented, it might seem like GeoEvent Server is the definitive way to go for building your own custom geoprocessing tools, and while this might be true, you could just as well find that because Velocity supports the Arcade language and GeoEvent Server does not, Velocity might be the better solution for custom geoprocessing. This is just one example but I hope it serves as something to think about when looking through both offerings.
