Are there any open source tools out there that can provide an ArcGIS REST API compatible interface or similar functionality?
Many of our clients are running ArcGIS software and are asking us for REST API endpoints, but we are trying to stick to free software.
We serve biodiversity data in a mix of polygons and point data with attributes.
The answer is 'yes' and 'no'. Let me explain.
As Bill mentioned, there are plenty of implementations of various OGC standards WFS, WMS, TMS, etc. (see Geoserver, Mapserver, Tilestache, etc).
However, I would guess that when some of your customers ask for this, they may mean ESRI ArcGIS Server REST Endpoints with ESRI-specific services. The answer as of today - is no (although I have seen some people implement successfully very small subsets of these endpoints in their favorite programming language).
You see, sometime back, ESRI was pushing to make these ESRI-specific endpoints an OGC standard which would have caused various open source projects to implement them. Nevertheless, it was a super heated debate since, among other things, was interpreted as having one vendor trying to push a "standard" that competed with other very well defined, largely implemented, OGC standards (e.g. WFS and WMS) that were in use just for the sake of having the vendor put an "OGC approved" label.
So yes, ArcGIS server supports REST endpoints that implement OGC standards and you will find open source implementations of those... and no you will not find complete open source implementations (as of today) of the ESRI-specific ones.
Sure you have options, MapServer, GeoServer, both put up WMS or WFS services that can be used in ArcGIS JS api. You can also create REST services that just push JSON for the points and read the JSON into an array to make points in ArcGIS JS.api. We tend to use GeoJSON direct from a database or just a file as a source to Leaflet JS API and then use the Esri plugin.
There are a few sites that will convert shapefiles to GeoJSON and that works for our polygons. Esri JSON is different from GeoJSON but the same sites will convert then and ArcMap has tools to convert to Esri JSON.
If the data is static the JSON file option is very easy to use and unless your editing on the web it may even be faster then ArcServer REST services. I have an oracle database, that I mine from a PHP page, if you hit the PHP page you just get the data back as GeoJSON. The data is not stored in any spatial field just in a X,Y field. It's simple and works.
There are a lot of options out there.
An Open Source implementation of the ESRI GeoServices Specification can be found in the GitHub FeatureServer repository which is part of the koop project, an initiative of ESRI.
