Specification: http://www.opengeospatial.org/standards/cat

• CSW 3 has now renamed CQL to CommonQL probably to help distinguish it from other CQLs.
• CQL is a subset of the OGC Filter specification.
• Geotools extended CQL with ECQL to get feature parity with OGC Filter 1.1.
• There is now OGC Filter 2.x so potentially Geotools will have to come up with a new ECQL version too.
• Another issue with CQL is it does not have a way to encode its version. The OGC Filter XML can include that kind of information. (We can get around that issue by including some kind of filter type/version information but with XML it is part of the payload.) (edited)
• I am also wondering if we can add this without DDF 3. For example we can add the attribute and the extended Filter visit method can lazy load from that attribute when needed.
• The other issue with CQL/CommonQL is that it is part of the CSW standard and in the past at least has been a subset of OGC Filter. I am not sure if that is still true of CommonQL in CSW 3.
• OGC Filter on the other hand is stand alone and can be used by multiple OGC specifications.
• We can convert OGC Filters to ECQL before showing them to users. We can hide the internal detail if we use XML for an internal representation.