QGIS Server Python Plugins

Warning

Despite our constant efforts, information beyond this line may not be updated for QGIS 3. Refer to https://qgis.org/pyqgis/master for the python API documentation or, give a hand to update the chapters you know about. Thanks.

Python plugins can also run on QGIS Server (see QGIS as OGC Data Server):

• By using the server interface (QgsServerInterface) a Python plugin running on the server can alter the behavior of existing core services (WMS, WFS etc.).

• With the server filter interface (QgsServerFilter) you can change the input parameters, change the generated output or even provide new services.

• With the access control interface (QgsAccessControlFilter) you can apply some access restriction per requests.

Server Filter Plugins architecture

Server python plugins are loaded once when the FCGI application starts. They register one or more QgsServerFilter (from this point, you might find useful a quick look to the server plugins API docs). Each filter should implement at least one of three callbacks:

• requestReady()

• responseComplete()

• sendResponse()

All filters have access to the request/response object (QgsRequestHandler) and can manipulate all its properties (input/output) and raise exceptions (while in a quite particular way as we’ll see below).

Here is a pseudo code showing a typical server session and when the filter’s callbacks are called:

• Get the incoming request

  • create GET/POST/SOAP request handler

  • pass request to an instance of QgsServerInterface

  • call plugins requestReady filters

  • if there is not a response

    • if SERVICE is WMS/WFS/WCS

      • create WMS/WFS/WCS server

        • call server’s executeRequest and possibly call sendResponse plugin filters when streaming output or store the byte stream output and content type in the request handler

    • call plugins responseComplete filters

  • call plugins sendResponse filters

  • request handler output the response

The following paragraphs describe the available callbacks in details.

requestReady

This is called when the request is ready: incoming URL and data have been parsed and before entering the core services (WMS, WFS etc.) switch, this is the point where you can manipulate the input and perform actions like:

• authentication/authorization

• redirects

• add/remove certain parameters (typenames for example)

• raise exceptions

You could even substitute a core service completely by changing SERVICE parameter and hence bypassing the core service completely (not that this make much sense though).

sendResponse

This is called whenever output is sent to FCGI stdout (and from there, to the client), this is normally done after core services have finished their process and after responseComplete hook was called, but in a few cases XML can become so huge that a streaming XML implementation was needed (WFS GetFeature is one of them), in this case, sendResponse is called multiple times before the response is complete (and before responseComplete is called). The obvious consequence is that sendResponse is normally called once but might be exceptionally called multiple times and in that case (and only in that case) it is also called before responseComplete.

sendResponse is the best place for direct manipulation of core service’s output and while responseComplete is typically also an option, sendResponse is the only viable option in case of streaming services.

responseComplete

This is called once when core services (if hit) finish their process and the request is ready to be sent to the client. As discussed above, this is normally called before sendResponse except for streaming services (or other plugin filters) that might have called sendResponse earlier.

responseComplete is the ideal place to provide new services implementation (WPS or custom services) and to perform direct manipulation of the output coming from core services (for example to add a watermark upon a WMS image).

Raising exception from a plugin

Some work has still to be done on this topic: the current implementation can distinguish between handled and unhandled exceptions by setting a QgsRequestHandler property to an instance of QgsMapServiceException, this way the main C++ code can catch handled python exceptions and ignore unhandled exceptions (or better: log them).

This approach basically works but it is not very “pythonic”: a better approach would be to raise exceptions from python code and see them bubbling up into C++ loop for being handled there.

Writing a server plugin

A server plugin is a standard QGIS Python plugin as described in Developing Python Plugins, that just provides an additional (or alternative) interface: a typical QGIS desktop plugin has access to QGIS application through the QgisInterface instance, a server plugin has also access to a QgsServerInterface.

To tell QGIS Server that a plugin has a server interface, a special metadata entry is needed (in metadata.txt)

server=True

The example plugin discussed here (with many more example filters) is available on github: QGIS HelloServer Example Plugin. You could also find more examples at https://github.com/elpaso/qgis3-server-vagrant/tree/master/resources/web/plugins or browsing the QGIS plugins repository.

Plugin files

Here’s the directory structure of our example server plugin

PYTHON_PLUGINS_PATH/
  HelloServer/
    __init__.py    --> *required*
    HelloServer.py  --> *required*
    metadata.txt   --> *required*

__init__.py

This file is required by Python’s import system. Also, QGIS Server requires that this file contains a serverClassFactory() function, which is called when the plugin gets loaded into QGIS Server when the server starts. It receives reference to instance of QgsServerInterface and must return instance of your plugin’s class. This is how the example plugin __init__.py looks like:

# -*- coding: utf-8 -*-

def serverClassFactory(serverIface):
    from HelloServer import HelloServerServer
    return HelloServerServer(serverIface)

HelloServer.py

This is where the magic happens and this is how magic looks like: (e.g. HelloServer.py)

A server plugin typically consists in one or more callbacks packed into objects called QgsServerFilter.

Each QgsServerFilter implements one or more of the following callbacks:

• requestReady()

• responseComplete()

• sendResponse()

The following example implements a minimal filter which prints HelloServer! in case the SERVICE parameter equals to “HELLO”:

from qgis.server import *
from qgis.core import *

class HelloFilter(QgsServerFilter):

    def __init__(self, serverIface):
        super(HelloFilter, self).__init__(serverIface)

    def responseComplete(self):
        request = self.serverInterface().requestHandler()
        params = request.parameterMap()
        if params.get('SERVICE', '').upper() == 'HELLO':
            request.clearHeaders()
            request.setHeader('Content-type', 'text/plain')
            request.clearBody()
            request.appendBody('HelloServer!')

The filters must be registered into the serverIface as in the following example:

class HelloServerServer:
    def __init__(self, serverIface):
        # Save reference to the QGIS server interface
        self.serverIface = serverIface
        serverIface.registerFilter( HelloFilter, 100 )

The second parameter of registerFilter sets a priority which defines the order for the callbacks with the same name (the lower priority is invoked first).

By using the three callbacks, plugins can manipulate the input and/or the output of the server in many different ways. In every moment, the plugin instance has access to the QgsRequestHandler through the QgsServerInterface. The QgsRequestHandler class has plenty of methods that can be used to alter the input parameters before entering the core processing of the server (by using requestReady()) or after the request has been processed by the core services (by using sendResponse()).

The following examples cover some common use cases:

Modifying the input

The example plugin contains a test example that changes input parameters coming from the query string, in this example a new parameter is injected into the (already parsed) parameterMap, this parameter is then visible by core services (WMS etc.), at the end of core services processing we check that the parameter is still there:

from qgis.server import *
from qgis.core import *

class ParamsFilter(QgsServerFilter):

    def __init__(self, serverIface):
        super(ParamsFilter, self).__init__(serverIface)

    def requestReady(self):
        request = self.serverInterface().requestHandler()
        params = request.parameterMap( )
        request.setParameter('TEST_NEW_PARAM', 'ParamsFilter')

    def responseComplete(self):
        request = self.serverInterface().requestHandler()
        params = request.parameterMap( )
        if params.get('TEST_NEW_PARAM') == 'ParamsFilter':
            QgsMessageLog.logMessage("SUCCESS - ParamsFilter.responseComplete", 'plugin', QgsMessageLog.INFO)
        else:
            QgsMessageLog.logMessage("FAIL    - ParamsFilter.responseComplete", 'plugin', QgsMessageLog.CRITICAL)

This is an extract of what you see in the log file:

 src/core/qgsmessagelog.cpp: 45: (logMessage) [0ms] 2014-12-12T12:39:29 plugin[0] HelloServerServer - loading filter ParamsFilter
 src/core/qgsmessagelog.cpp: 45: (logMessage) [1ms] 2014-12-12T12:39:29 Server[0] Server plugin HelloServer loaded!
 src/core/qgsmessagelog.cpp: 45: (logMessage) [0ms] 2014-12-12T12:39:29 Server[0] Server python plugins loaded
 src/mapserver/qgsgetrequesthandler.cpp: 35: (parseInput) [0ms] query string is: SERVICE=HELLO&request=GetOutput
 src/mapserver/qgshttprequesthandler.cpp: 547: (requestStringToParameterMap) [1ms] inserting pair SERVICE // HELLO into the parameter map
 src/mapserver/qgshttprequesthandler.cpp: 547: (requestStringToParameterMap) [0ms] inserting pair REQUEST // GetOutput into the parameter map
 src/mapserver/qgsserverfilter.cpp: 42: (requestReady) [0ms] QgsServerFilter plugin default requestReady called
 src/core/qgsmessagelog.cpp: 45: (logMessage) [0ms] 2014-12-12T12:39:29 plugin[0] HelloFilter.requestReady
 src/mapserver/qgis_map_serv.cpp: 235: (configPath) [0ms] Using default configuration file path: /home/xxx/apps/bin/admin.sld
 src/mapserver/qgshttprequesthandler.cpp: 49: (setHttpResponse) [0ms] Checking byte array is ok to set...
 src/mapserver/qgshttprequesthandler.cpp: 59: (setHttpResponse) [0ms] Byte array looks good, setting response...
 src/core/qgsmessagelog.cpp: 45: (logMessage) [0ms] 2014-12-12T12:39:29 plugin[0] HelloFilter.responseComplete
 src/core/qgsmessagelog.cpp: 45: (logMessage) [0ms] 2014-12-12T12:39:29 plugin[0] SUCCESS - ParamsFilter.responseComplete
 src/core/qgsmessagelog.cpp: 45: (logMessage) [0ms] 2014-12-12T12:39:29 plugin[0] RemoteConsoleFilter.responseComplete
 src/mapserver/qgshttprequesthandler.cpp: 158: (sendResponse) [0ms] Sending HTTP response
 src/core/qgsmessagelog.cpp: 45: (logMessage) [0ms] 2014-12-12T12:39:29 plugin[0] HelloFilter.sendResponse

On the highlighted line the “SUCCESS” string indicates that the plugin passed the test.

The same technique can be exploited to use a custom service instead of a core one: you could for example skip a WFS SERVICE request or any other core request just by changing the SERVICE parameter to something different and the core service will be skipped, then you can inject your custom results into the output and send them to the client (this is explained here below).

Modifying or replacing the output

The watermark filter example shows how to replace the WMS output with a new image obtained by adding a watermark image on the top of the WMS image generated by the WMS core service:

import os

from qgis.server import *
from qgis.core import *
from qgis.PyQt.QtCore import *
from qgis.PyQt.QtGui import *


class WatermarkFilter(QgsServerFilter):

    def __init__(self, serverIface):
        super(WatermarkFilter, self).__init__(serverIface)

    def responseComplete(self):
        request = self.serverInterface().requestHandler()
        params = request.parameterMap( )
        # Do some checks
        if (request.parameter('SERVICE').upper() == 'WMS' \
                and request.parameter('REQUEST').upper() == 'GETMAP' \
                and not request.exceptionRaised() ):
            QgsMessageLog.logMessage("WatermarkFilter.responseComplete: image ready {}".format(request.infoFormat()), 'plugin', QgsMessageLog.INFO)
            # Get the image
            img = QImage()
            img.loadFromData(request.body())
            # Adds the watermark
            watermark = QImage(os.path.join(os.path.dirname(__file__), 'media/watermark.png'))
            p = QPainter(img)
            p.drawImage(QRect( 20, 20, 40, 40), watermark)
            p.end()
            ba = QByteArray()
            buffer = QBuffer(ba)
            buffer.open(QIODevice.WriteOnly)
            img.save(buffer, "PNG")
            # Set the body
            request.clearBody()
            request.appendBody(ba)

In this example the SERVICE parameter value is checked and if the incoming request is a WMS GETMAP and no exceptions have been set by a previously executed plugin or by the core service (WMS in this case), the WMS generated image is retrieved from the output buffer and the watermark image is added. The final step is to clear the output buffer and replace it with the newly generated image. Please note that in a real-world situation we should also check for the requested image type instead of returning PNG in any case.

Access control plugin

Plugin files

Here’s the directory structure of our example server plugin:

PYTHON_PLUGINS_PATH/
  MyAccessControl/
    __init__.py    --> *required*
    AccessControl.py  --> *required*
    metadata.txt   --> *required*

__init__.py

This file is required by Python’s import system. As for all QGIS server plugins, this file contains a serverClassFactory() function, which is called when the plugin gets loaded into QGIS Server at startup. It receives a reference to an instance of QgsServerInterface and must return an instance of your plugin’s class. This is how the example plugin __init__.py looks like:

# -*- coding: utf-8 -*-

def serverClassFactory(serverIface):
    from MyAccessControl.AccessControl import AccessControl
    return AccessControl(serverIface)

AccessControl.py

class AccessControl(QgsAccessControlFilter):

    def __init__(self, server_iface):
        super(QgsAccessControlFilter, self).__init__(server_iface)

    def layerFilterExpression(self, layer):
        """ Return an additional expression filter """
        return super(QgsAccessControlFilter, self).layerFilterExpression(layer)

    def layerFilterSubsetString(self, layer):
        """ Return an additional subset string (typically SQL) filter """
        return super(QgsAccessControlFilter, self).layerFilterSubsetString(layer)

    def layerPermissions(self, layer):
        """ Return the layer rights """
        return super(QgsAccessControlFilter, self).layerPermissions(layer)

    def authorizedLayerAttributes(self, layer, attributes):
        """ Return the authorised layer attributes """
        return super(QgsAccessControlFilter, self).authorizedLayerAttributes(layer, attributes)

    def allowToEdit(self, layer, feature):
        """ Are we authorise to modify the following geometry """
        return super(QgsAccessControlFilter, self).allowToEdit(layer, feature)

    def cacheKey(self):
        return super(QgsAccessControlFilter, self).cacheKey()

This example gives a full access for everybody.

It’s the role of the plugin to know who is logged on.

On all those methods we have the layer on argument to be able to customise the restriction per layer.

layerFilterExpression

Used to add an Expression to limit the results, e.g.:

def layerFilterExpression(self, layer):
    return "$role = 'user'"

To limit on feature where the attribute role is equals to “user”.

layerFilterSubsetString

Same than the previous but use the SubsetString (executed in the database)

def layerFilterSubsetString(self, layer):
    return "role = 'user'"

To limit on feature where the attribute role is equals to “user”.

layerPermissions

Limit the access to the layer.

Return an object of type LayerPermissions, which has the properties:

• canRead to see it in the GetCapabilities and have read access.

• canInsert to be able to insert a new feature.

• canUpdate to be able to update a feature.

• canDelete to be able to delete a feature.

Example:

def layerPermissions(self, layer):
    rights = QgsAccessControlFilter.LayerPermissions()
    rights.canRead = True
    rights.canRead = rights.canInsert = rights.canUpdate = rights.canDelete = False
    return rights

To limit everything on read only access.

authorizedLayerAttributes

Used to limit the visibility of a specific subset of attribute.

The argument attribute return the current set of visible attributes.

Example:

def authorizedLayerAttributes(self, layer, attributes):
    return [a for a in attributes if a != "role"]

To hide the ‘role’ attribute.

allowToEdit

This is used to limit the editing on a subset of features.

It is used in the WFS-Transaction protocol.

Example:

def allowToEdit(self, layer, feature):
    return feature.attribute('role') == 'user'

To be able to edit only feature that has the attribute role with the value user.

cacheKey

QGIS server maintain a cache of the capabilities then to have a cache per role you can return the role in this method. Or return None to completely disable the cache.