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tgext.geo FeatureServer Tutorial

Introduction

FeatureServer is a simple Python-based geographic feature server. It allows you to store geographic vector features in a number of different backends, and interact with them – creating, updating, and deleting – via a REST-based API. It is distributed under a BSD-like open source license.

The text.geo.featureserver module enables easy integration of featureserver into TurboGears2 apps by providing the following:

  • GeoAlchemy Datasource - This allows geographic features to be stored in any of the spatial databases supported by GeoAlchemy.
  • FeatureServer Controller - This creates a new controller that reads the config and makes use of the FeatureServer API to dispatch requests to featureserver.

About This Tutorial

In this tutorial we will create a TG2 app and use the tgext.geo extension to configure, store, manipulate and retreive GIS features in a PostGIS database.

Installation

It is assumed that a fresh virtualenv has been created and TG2 installed following the TurboGears 2.1 Standard Installation. Install tgext.geo using easy_install:

(tg2env)$ easy_install -i http://www.turbogears.org/2.0/downloads/current/index/ tgext.geo

We assume that a PostgreSQL server is installed and ready for use. Install PostGIS and create a new PostGIS enabled database called gis. Refer to the PostGIS docs to achieve this. We also need to install GeoAlchemy and the python db-api for postgres:

(tg2env)$ easy_install GeoAlchemy egenix-mx-base psycopg2

Download and install featureserver from the svn repo:

(tg2env) $ svn co http://svn.featureserver.org/trunk/featureserver featureserver
(tg2env) $ cd featureserver
(tg2env) $ python setup.py install

Creating A New TG2 App

Create a new TG2 app named “TGFeature” with gis capability:

(tg2env)$ paster quickstart TGFeature --geo
(tg2env)$ cd TGFeature
(tg2env)$ python setup.py develop

Model Definition For Features

We assume that we have to model a layer of roads in our application. We open the tgfeature/model/__init__.py file in the package and add the following model definition:

from datetime import datetime
from sqlalchemy import Column, Integer, Unicode, DateTime
from geoalchemy import GeometryColumn, LineString
from geoalchemy import GeometryDDL

class Road(DeclarativeBase):
    __tablename__ = 'roads'
    id = Column(Integer, primary_key=True)
    name = Column(Unicode, nullable=False)
    width = Column(Integer)
    created = Column(DateTime, default=datetime.now())
    geom = GeometryColumn(LineString(2))

GeometryDDL(Road.__table__)

Apart from the standard attributes, we have defined a spatial attribute called geom as a GeometryColumn. We will use this attribute to store geometry values of data type LineString in the database. GeoAlchemy supports other geometry types such as Point, Polygon and Mutiple Geometries. We also pass the dimension of the geometry as a parameter. The Geometry type takes another parameter for the SRID, the Spatial Reference ID. In this case we leave it to its default value of 4326 which means that our geometry values will be expressed in geographic latitude and longitude coordinate system. There is a nice blogpost on SharpGIS that explains the concept of SRID. EPSG:4326 is chosen as the default SRID by PostGIS and other software primarily because it is based on the global ellipsoid (called WGS84) which is not specific to a particular region or continent.

We finally call the GeometryDDL DDL Extension that enables creation and deletion of geometry columns just after and before table create and drop statements respectively. The GeometryColumn, LineString and GeometryDDL must be imported from the geoalchemy package.

Creating Tables In The Database

The database tables can now be created using the setup-app paster command

$ (tg2env) paster setup-app development.ini

In case we need sample data to be inserted during application startup, we must add the sample data into the setup script, i.e. tgfeature/websetup.py prior to running the setup command. Let us add some sample data.

from geoalchemy import WKTSpatialElement
wkt = "LINESTRING(-80.3 38.2, -81.03 38.04, -81.2 37.89)"
road1 = model.Road(name="Peter St", width=6, geom=WKTSpatialElement(wkt))
wkt = "LINESTRING(-79.8 38.5, -80.03 38.2, -80.2 37.89)"
road2 = model.Road(name="George Ave", width=8, geom=WKTSpatialElement(wkt))
model.DBSession.add_all([road1, road2])

FeatureServer Config

Now we need to configure our app by declaring certain parameters under the [app:main] section of the ini file. In this case we use development.ini as we are in development mode right now.

geo.roads.model=tgfeature.model
geo.roads.cls=Road
geo.roads.table=roads
geo.roads.fid=id
geo.roads.geometry=geom

The config parameters use a geo.<layer>.param=value format. This allows additional layers to be defined within the same app as follows:

geo.lakes.model=tgfeature.model
geo.lakes.cls=Lake
geo.lakes.table=lakes
geo.lakes.fid=id
geo.lakes.geometry=geom

In this tutorial, however, we will use only the roads layer.

Using The FeatureServerController

We can now import and mount the FeatureServer Controller inside our root controller.

from tgfeature.model import DBSession
from tgext.geo.featureserver import FeatureServerController

class RootController(BaseController):
    ....
    roads = FeatureServerController("roads", DBSession)

We pass two parameters here. The first one being the layer name. This must be the same as layer name used in development.ini. The second parameter is the sqlalchemy session. In case we were using the lakes layer too, as shown in the sample config, we would create two controller instances as:

class RootController(BaseController):
    ....
    roads = FeatureServerController("roads", DBSession)
    lakes = FeatureServerController("lakes", DBSession)

Testing The Server Using Curl

We are now ready to start and test out new geo-enabled TG2 app. Start the server in development mode by running:

$(tg2env) paster serve --reload development.ini

Note the –reload option. This tells the server to reload the app whenever there is a change in any of the package files that are in its dependency chain. Now we will open up a new command window and test the server using the curl utility.

# Request the features in GeoJSON format (default)
$ curl http://localhost:8080/roads/all.json
or simply
$ curl http://localhost:8080/roads
{"crs": null, "type": "FeatureCollection", .... long GeoJSON output

# Request the features in GML format
$ curl http://localhost:8080/8080/roads/all.gml
<wfs:FeatureCollection
    xmlns:fs="http://example.com/featureserver
    ....   long GML output

# Request the features in KML format
$ curl http://localhost:8080/roads/all.kml
<?xml version="1.0" encoding="UTF-8"?>
    <kml xmlns="http://earth.google.com/kml/2.0"
    ....  long KML output

Now lets create a new feature using curl. Store the following json data in a new file postdata.json:

{"features": [{
    "geometry": {
        "type": "LineString",
        "coordinates": [[-88.913933292993605, 42.508280299363101],
                        [-88.8203027197452, 42.598566923566899],
                        [-88.738375968152894, 42.723965012738901],
                        [-88.611305904458604, 42.968073292993601],
                        [-88.365525649681501, 43.140286668789798]
        ]
    },
    "type": "Feature",
    "id": 10,
    "properties": {"name": "Broad Ave", "width": 10}
}]}

Create a POST request using this data and send it to the server.

$(tg2env) curl -d @postdata.json http://localhost:8080/roads/create.json

This creates a new feature and returns back the features in json format. To modify the feature edit the postdata.json file and change the properties. Lets change the name property from Broad Ave to Narrow St and the width property from 10 to 4. The modify url should include the feature id as shown below:

$(tg2env)  curl -d @postdata.json http://localhost:8080/roads/3.json

The data can be requested in JSON, GML, KML and ATOM formats by using the apprpriate suffix, i.e. 3.json, 3.gml, 3.kml or 3.atom respectively. JSON is the default content type resturned by featureserver, so using it without any suffix (e.g. roads/3) returns data in GeoJSON format. For deleting the feature simply send a DELETE request with the feature id in the url:

$(tg2env) curl -X DELETE http://localhost:8080/roads/3.json

An OpenLayers Application Using FeatureServer

The server is now ready to be accessed by client applications for storing, manipulating and deleting featues. OpenLayers is an open source javascript web mapping application. It is quite mature and is under active development. To develop an OpenLayers web application using featureserver the developer is strongly recommended to have a look at the demo application available with the featureserver source code. Copy the demo app (index.html inside featureserver source code directory) to the public folder under a different name:

$(tg2env) cp /path/to/featureserversource/index.html tgfeature/public/demo.html
$(tg2env) cp /path/to/featureserversource/json.html tgfeature/public/
$(tg2env) cp /path/to/featureserversource/kml.html tgfeature/public/

Now modify these files to change the following:

* change all references to featureserver.cgi to '' (null)
* change all references to scribble to 'roads' (layer)

Point your browser to http://localhost:8080/demo.html. You should now be able to view, create and modify features using featureserver running inside your TG2 app.

Adding Authentication and Authorization

TG2 supports authentication and authorization using the repoze.who and repoze.what packages along with other packages in these namespaces. A TG2 app created using the authentication and authorization option (default) has these packages already included and configured as WSGI middleware.

By default TG2 uses SQLAlchemy based authentication and authorization, where the user credentials and authorization roles / permissions are maintained in database tables. There are plugins available to support other authentication mechanisms such as LDAP based auth, OpenID based auth, etc. Refer to the Authentication and Authorization docs for details.

At the moment only controller wide authorization control is available in tgext.geo. In order to have authorization, pass a repoze.what authorization predicate as an additional parameter to FeatureServerController:

from pylons.i18n import ugettext as _, lazy_ugettext as l_
from repoze.what import predicates
from tgext.geo.featureserver import FeatureServerController

class RootController(BaseController):

    allow_only = predicates.has_permission('feature',
                msg=l_('Only for people with "feature" permission'))
    roads = FeatureServerController("roads", DBSession, allow_only)

Now we must go to the admin interface and define a new permission called “feature”. Once defined, this permission must be granted to groups and/or users to whom this new controller is now restricted.