WFS for the masses: adding support for paging and sorting in GeoServer

Today we are going to introduce you our latest contribution to GeoServer, WFS paging and sorting for retrieving features.

First off, let's take a step back and see what sorting and paging support is available in the official OGC protocols:

• Neither WMS 1.1 nor WMS 1.3 (or SLD/SE for that matter), have any ability to order the results so that features are painted in a certain order. If features can be organized in categories filters and FeatureTypeStyle elements can do the trick, but that won't work over continous fields
• Same goes for WPS 1.0, which can return significant amounts of vector data that might be useful to page over
• WFS 1.0 does not support either, WFS 1.1 supports sorting, WFS 2.0 supports sorting and paging via the sortBy and startIndex/maxFeature parameters

The last stable GeoServer release does not support WFS 2.0, and allows for sorting only on DBMS based stores. In our latest contribution to the stable series we removed all limitations concerning WFS paging and sorting support:

• WFS 1.0 and 1.1 can now support sorting on top of each and every store kind, using the sortBy parameter as a vendor extension
• WFS 1.0 and 1.1 support paging on top of each and every store, using the startIndex/maxFeatures parameters

Protocol and paging wise we back-ported the work added on trunk along with WFS 2.0 support, and then we merged in some previous work on generic sorting we did for the aggregating store.

Technically speaking, in case the store does not support sorting natively (e.g., shapefile) we gather the features into an optimized merge/sort algorithm that never keeps more than 1000 features in memory, and uses secondary storage to scale up to larger result sets.

But, enough talking, let's see some examples against the states demo layer, using the CSV as the output format for brevity.

First ten features:

http://localhost:8080/geoserver/topp/owsservice=WFS&version=1.0.0&request=GetFeature

&typeName=topp:states&outputFormat=csv&propertyName=STATE_NAME,PERSONS

&maxFeatures=10

FID,STATE_NAME,PERSONS

states.1,Illinois,11430602

states.2,District of Columbia,606900

states.3,Delaware,666168

states.4,West Virginia,1793477

states.5,Maryland,4781468

states.6,Colorado,3294394

states.7,Kentucky,4551524

states.8,Kansas,2477574

states.9,Virginia,6180651

states.10,Missouri,5117073

The next 10 features:

http://localhost:8080/geoserver/topp/ows?service=WFS&version=1.0.0&request=GetFeature

&typeName=topp:states&outputFormat=csv&propertyName=STATE_NAME,PERSONS

&maxFeatures=10&startIndex=10

FID,STATE_NAME,PERSONS

states.11,Arizona,3665228

states.12,Oklahoma,3145585

states.13,North Carolina,6628629

states.14,Tennessee,4829958

states.15,Texas,17122020

states.16,New Mexico,1379559

states.17,Alabama,4040587

states.18,Mississippi,2573216

states.19,Georgia,6457339

states.20,South Carolina,3486703

The first ten states with most people (sort on PERSONS, descending):

http://localhost:8080/geoserver/topp/ows?service=WFS&version=1.0.0&request=GetFeature

&typeName=topp:states&outputFormat=csv&propertyName=STATE_NAME,PERSONS

&maxFeatures=10&sortBy=PERSONS%20D

FID,STATE_NAME,PERSONS

states.47,California,29760021

states.39,New York,18235907

states.15,Texas,17122020

states.23,Florida,12937926

states.40,Pennsylvania,11881643

states.1,Illinois,11430602

states.48,Ohio,9980887

states.24,Michigan,9295297

states.43,New Jersey,7484736

states.13,North Carolina,6628629

The second page of the above result set:

http://localhost:8080/geoserver/topp/ows?service=WFS&version=1.0.0&request=GetFeature

&typeName=topp:states&outputFormat=csv&propertyName=STATE_NAME,PERSONS

&maxFeatures=10&startIndex=10&sortBy=PERSONS%20D

FID,STATE_NAME,PERSONS

states.19,Georgia,6457339

states.9,Virginia,6180651

states.37,Massachusetts,6016425

states.44,Indiana,5544159

states.10,Missouri,5117073

states.49,Washington,4866692

states.14,Tennessee,4829958

states.30,Wisconsin,4796441

states.5,Maryland,4781468

states.7,Kentucky,4551524

These modifications are already available in the GeoServer 2.1.3 release.

There is more work to be done in this area, ordering features before paiting them in WMS, and paging WPS results would both be nice additions. Interested? Let us know!

The GeoSolutions team,