From looking at the samples, this is what I would do. There might be better ways, but this should get you going:
- Add the data to ArcMap using a simple renderer (no classification)
- Publish as a dynamic map service, but enable dynamic layers.
- Consume the dynamic map service within an app built in the Esri JS API.
- Make use of GenerateRenderer to update the renderer on the service.
You have the option to draw this either server side or within a feature layer. I would recommend doing it server side if lots of features or complex geometries involved.
- Within this you can make use of a UniqueValueRenderer to get all the current values from the field that you want to symbolise by.
I have done something similar for symbolising a live feed and it works fast and has been stable for awhile now.
I would encourage you to work through some of the samples and read the help topics around this area.. Take a look at the latest (at time of writing) 3.3 JS API from Esri. If your data was changing more frequently, I would suggest taking a look at how you can generate a renderer using a function.