One of the more interesting use cases I've seen over the past year is a request for a "Snap To" processor in GeoEvent. Unfortunately, this requested operation is a relatively costly operation to perform (timewise) and doesn't lend itself to high event throughputs. But if you are still interested in a Snap To processor and willing to accept some performance loss, the following methodology might be useful. The solution involves using a Feature Service's Query method using a spatial operator. As I said, the call to the featuer service does take some time (in my testing it averaged about 95 ms round trip) but this solution doesn't require a large amount of GeoFences to be loaded into memory so it isn't limited by the road network size.
Spatial Query Processor
First, you will need the Spatial Query Processor located here:
Attached to this post is a document that describes the process of preparing the road network data and publshing it as a feature service, creating your GeoEvent Service, and using the Spatial Query Processor.
As always, leave a comment if you have questions, issues, or enhancement requests.