20. Servidor QGIS e Python

20.1. Introdução

To learn more about QGIS Server, read the QGIS Server Guide/Manual.

O Servidor QGIS é três coisas diferentes:

  1. Biblioteca do Servidor QGIS: uma biblioteca que fornece uma API para criar serviços da web OGC

  2. QGIS Server FCGI: a FCGI binary application qgis_maserv.fcgi that together with a web server implements a set of OGC services (WMS, WFS, WCS etc.) and OGC APIs (WFS3/OAPIF)

  3. QGIS Development Server: a development server binary application qgis_mapserver that implements a set of OGC services (WMS, WFS, WCS etc.) and OGC APIs (WFS3/OAPIF)

Este capítulo do livro de receitas se concentra no primeiro tópico e, ao explicar o uso da API do Servidor QGIS, mostra como é possível usar o Python para estender, aprimorar ou personalizar o comportamento do servidor ou como usar a API do Servidor QGIS para incorporar o servidor QGIS ao outra aplicação.

Existem algumas maneiras diferentes de alterar o comportamento do Servidor QGIS ou estender seus recursos para oferecer novos serviços ou APIs personalizados; estes são os principais cenários que você pode encontrar:

  • EMBEDDING → Usa a API do Servidor QGIS de outro aplicativo Python

  • STANDALONE → Run QGIS Server as a standalone WSGI/HTTP service

  • FILTERS → Aprimora/personaliza o Servidor QGIS com complementos de filtro

  • SERVICES → Adiciona um novo SERVICE

  • OGC APIs → Adiciona um novo OGC API

Embedding and standalone applications require using the QGIS Server Python API directly from another Python script or application. The remaining options are better suited for when you want to add custom features to a standard QGIS Server binary application (FCGI or development server): in this case you’ll need to write a Python plugin for the server application and register your custom filters, services or APIs.

20.2. Noções básicas da API do servidor

As classes fundamentais envolvidas em um aplicativo típico do Servidor QGIS são:

  • QgsServer a instância do servidor (geralmente uma instância única para toda a vida do aplicativo)

  • :class:`QgsServerRequest <qgis.server.QgsServerRequest>`o objeto de solicitação (normalmente recriado em cada solicitação)

  • QgsServer.handleRequest(request, response) processa a solicitação e preenche a resposta

O Servidor QGIS FCGI ou o fluxo de trabalho do servidor de desenvolvimento pode ser resumido da seguinte maneira:

1initialize the QgsApplication
2create the QgsServer
3the main server loop waits forever for client requests:
4    for each incoming request:
5        create a QgsServerRequest request
6        create a QgsServerResponse response
7        call QgsServer.handleRequest(request, response)
8            filter plugins may be executed
9        send the output to the client

Inside the QgsServer.handleRequest(request, response) method the filter plugins callbacks are called and QgsServerRequest and QgsServerResponse are made available to the plugins through the QgsServerInterface class.

Aviso

As classes de servidor QGIS não são seguras para threads, você sempre deve usar um modelo ou contêineres de multiprocessamento ao criar aplicativos escaláveis ​​com base na API do Servidor QGIS.

20.3. Independente ou incorporado

Para servidor autônomo ou aplicativos integrados, você precisará usar as classes de servidor mencionadas acima diretamente, agrupando-as em uma implementação de servidor da Web que gerencia todas as interações do protocolo HTTP com o cliente.

Um exemplo mínimo do uso da API do Servidor QGIS (sem a parte HTTP) é a seguir:

 1from qgis.core import QgsApplication
 2from qgis.server import *
 3app = QgsApplication([], False)
 4
 5# Create the server instance, it may be a single one that
 6# is reused on multiple requests
 7server = QgsServer()
 8
 9# Create the request by specifying the full URL and an optional body
10# (for example for POST requests)
11request = QgsBufferServerRequest(
12    'http://localhost:8081/?MAP=/qgis-server/projects/helloworld.qgs' +
13    '&SERVICE=WMS&REQUEST=GetCapabilities')
14
15# Create a response objects
16response = QgsBufferServerResponse()
17
18# Handle the request
19server.handleRequest(request, response)
20
21print(response.headers())
22print(response.body().data().decode('utf8'))
23
24app.exitQgis()

Here is a complete standalone application example developed for the continuous integrations testing on QGIS source code repository, it showcases a wide set of different plugin filters and authentication schemes (not mean for production because they were developed for testing purposes only but still interesting for learning): https://github.com/qgis/QGIS/blob/release-3_22/tests/src/python/qgis_wrapped_server.py

20.4. Complementos do servidor

Os complementos de servidor python são carregados uma vez quando o aplicativo Servidor QGIS é iniciado e podem ser usados ​​para registrar filtros, serviços ou APIs.

A estrutura de um complemento de servidor é muito semelhante à sua contraparte na área de trabalho, um objeto QgsServerInterface é disponibilizado para os complemento e os complemento podem registrar um ou mais filtros, serviços ou APIs personalizados para o registro correspondente usando um dos métodos expostos pela interface do servidor.

20.4.1. Complementos de filtro de servidor

Filters come in three different flavors and they can be instanciated by subclassing one of the classes below and by calling the corresponding method of QgsServerInterface:

Tipo de Filtro

Classe Básica

Registro da QgsServerInterface

I/O

QgsServerFilter

registerFilter()

Controle de Acesso

QgsAccessControlFilter

registerAccessControl()

Cache

QgsServerCacheFilter

registerServerCache()

20.4.1.1. Filtros I/O

I/O filters can modify the server input and output (the request and the response) of the core services (WMS, WFS etc.) allowing to do any kind of manipulation of the services workflow. It is possible for example to restrict the access to selected layers, to inject an XSL stylesheet to the XML response, to add a watermark to a generated WMS image and so on.

From this point, you might find useful a quick look to the server plugins API docs.

Cada filtro deve implementar pelo menos um dos três retornos de chamada:

Todos os filtros têm acesso ao objeto de solicitação/resposta (QgsRequestHandler) e podem manipular todas as suas propriedades (entrada/saída) e gerar exceções (embora de uma maneira bastante específica, como iremos ver abaixo).

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

 1for each incoming request:
 2    create GET/POST request handler
 3    pass request to an instance of QgsServerInterface
 4    call requestReady filters
 5    if there is not a response:
 6        if SERVICE is WMS/WFS/WCS:
 7            create WMS/WFS/WCS service
 8            call service’s executeRequest
 9                possibly call sendResponse for each chunk of bytes
10                sent to the client by a streaming services (WFS)
11        call responseComplete
12        call sendResponse
13    request handler sends the response to the client

The following paragraphs describe the available callbacks in details.

20.4.1.1.1. 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:

  • autenticação/autorização

  • 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).

20.4.1.1.2. sendResponse

This is called whenever any 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 is needed (WFS GetFeature is one of them). In that case, instead of a single call to sendResponse(), the method might be exceptionally called multiple times before the response is complete, 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.

20.4.1.1.3. 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).

20.4.1.2. Raising exceptions 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.

20.4.1.3. Escrevendo um complemento de servidor

A server plugin is a standard QGIS Python plugin as described in Desenvolvendo complementos Python, 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 only access to a QgsServerInterface when it is executed within the QGIS Server application context.

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

server=True

Importante

Only plugins that have the server=True metadata set will be loaded and executed by QGIS Server.

The example plugin discussed here (with many more) is available on github at https://github.com/elpaso/qgis3-server-vagrant/tree/master/resources/web/plugins, a few server plugins are also published in the official QGIS plugins repository.

20.4.1.4. Arquivos de complementos

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

1PYTHON_PLUGINS_PATH/
2  HelloServer/
3    __init__.py    --> *required*
4    HelloServer.py  --> *required*
5    metadata.txt   --> *required*
20.4.1.4.1. __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:

def serverClassFactory(serverIface):
    from .HelloServer import HelloServerServer
    return HelloServerServer(serverIface)
20.4.1.4.2. 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 instances of a QgsServerFilter.

Each QgsServerFilter implements one or more of the following callbacks:

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

 1class HelloFilter(QgsServerFilter):
 2
 3    def __init__(self, serverIface):
 4        super().__init__(serverIface)
 5
 6    def requestReady(self):
 7        QgsMessageLog.logMessage("HelloFilter.requestReady")
 8
 9    def sendResponse(self):
10        QgsMessageLog.logMessage("HelloFilter.sendResponse")
11
12    def responseComplete(self):
13        QgsMessageLog.logMessage("HelloFilter.responseComplete")
14        request = self.serverInterface().requestHandler()
15        params = request.parameterMap()
16        if params.get('SERVICE', '').upper() == 'HELLO':
17            request.clear()
18            request.setResponseHeader('Content-type', 'text/plain')
19            # Note that the content is of type "bytes"
20            request.appendBody(b'HelloServer!')

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

class HelloServerServer:
    def __init__(self, serverIface):
        serverIface.registerFilter(HelloFilter(serverIface), 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:

20.4.1.4.3. 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:

 1class ParamsFilter(QgsServerFilter):
 2
 3    def __init__(self, serverIface):
 4        super(ParamsFilter, self).__init__(serverIface)
 5
 6    def requestReady(self):
 7        request = self.serverInterface().requestHandler()
 8        params = request.parameterMap( )
 9        request.setParameter('TEST_NEW_PARAM', 'ParamsFilter')
10
11    def responseComplete(self):
12        request = self.serverInterface().requestHandler()
13        params = request.parameterMap( )
14        if params.get('TEST_NEW_PARAM') == 'ParamsFilter':
15            QgsMessageLog.logMessage("SUCCESS - ParamsFilter.responseComplete")
16        else:
17            QgsMessageLog.logMessage("FAIL    - ParamsFilter.responseComplete")

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

1 src/core/qgsmessagelog.cpp: 45: (logMessage) [0ms] 2014-12-12T12:39:29 plugin[0] HelloServerServer - loading filter ParamsFilter
2 src/core/qgsmessagelog.cpp: 45: (logMessage) [1ms] 2014-12-12T12:39:29 Server[0] Server plugin HelloServer loaded!
3 src/core/qgsmessagelog.cpp: 45: (logMessage) [0ms] 2014-12-12T12:39:29 Server[0] Server python plugins loaded
4 src/mapserver/qgshttprequesthandler.cpp: 547: (requestStringToParameterMap) [1ms] inserting pair SERVICE // HELLO into the parameter map
5 src/mapserver/qgsserverfilter.cpp: 42: (requestReady) [0ms] QgsServerFilter plugin default requestReady called
6 src/core/qgsmessagelog.cpp: 45: (logMessage) [0ms] 2014-12-12T12:39:29 plugin[0] SUCCESS - ParamsFilter.responseComplete

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 below).

Dica

If you really want to implement a custom service it is recommended to subclass QgsService and register your service on registerFilter() by calling its registerService(service)

20.4.1.4.4. 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:

 1from qgis.server import *
 2from qgis.PyQt.QtCore import *
 3from qgis.PyQt.QtGui import *
 4
 5class WatermarkFilter(QgsServerFilter):
 6
 7    def __init__(self, serverIface):
 8        super().__init__(serverIface)
 9
10    def responseComplete(self):
11        request = self.serverInterface().requestHandler()
12        params = request.parameterMap( )
13        # Do some checks
14        if (params.get('SERVICE').upper() == 'WMS' \
15                and params.get('REQUEST').upper() == 'GETMAP' \
16                and not request.exceptionRaised() ):
17            QgsMessageLog.logMessage("WatermarkFilter.responseComplete: image ready %s" % request.parameter("FORMAT"))
18            # Get the image
19            img = QImage()
20            img.loadFromData(request.body())
21            # Adds the watermark
22            watermark = QImage(os.path.join(os.path.dirname(__file__), 'media/watermark.png'))
23            p = QPainter(img)
24            p.drawImage(QRect( 20, 20, 40, 40), watermark)
25            p.end()
26            ba = QByteArray()
27            buffer = QBuffer(ba)
28            buffer.open(QIODevice.WriteOnly)
29            img.save(buffer, "PNG" if "png" in request.parameter("FORMAT") else "JPG")
30            # Set the body
31            request.clearBody()
32            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 supporting PNG or JPG only.

20.4.1.5. Access control filters

Access control filters gives the developer a fine-grained control over which layers, features and attributes can be accessed, the following callbacks can be implemented in an access control filter:

20.4.1.5.1. Arquivos de complementos

Here’s the directory structure of our example plugin:

1PYTHON_PLUGINS_PATH/
2  MyAccessControl/
3    __init__.py    --> *required*
4    AccessControl.py  --> *required*
5    metadata.txt   --> *required*
20.4.1.5.2. __init__.py

Este arquivo é requerido pelo sistema de importação do Python. Como para todos os complementos de servidor QGIS, este arquivo contém uma função serverClassFactory(), chamada quando o complemento é carregado no QGIS Server na inicialização. Ele recebe uma referência a uma instância de QgsServerInterface e deve retornar uma instância da classe do seu complemento. É assim que o exemplo do complemento __init__.py se parece com:

def serverClassFactory(serverIface):
    from MyAccessControl.AccessControl import AccessControlServer
    return AccessControlServer(serverIface)
20.4.1.5.3. AccessControl.py
 1class AccessControlFilter(QgsAccessControlFilter):
 2
 3    def __init__(self, server_iface):
 4        super().__init__(server_iface)
 5
 6    def layerFilterExpression(self, layer):
 7        """ Return an additional expression filter """
 8        return super().layerFilterExpression(layer)
 9
10    def layerFilterSubsetString(self, layer):
11        """ Return an additional subset string (typically SQL) filter """
12        return super().layerFilterSubsetString(layer)
13
14    def layerPermissions(self, layer):
15        """ Return the layer rights """
16        return super().layerPermissions(layer)
17
18    def authorizedLayerAttributes(self, layer, attributes):
19        """ Return the authorised layer attributes """
20        return super().authorizedLayerAttributes(layer, attributes)
21
22    def allowToEdit(self, layer, feature):
23        """ Are we authorised to modify the following geometry """
24        return super().allowToEdit(layer, feature)
25
26    def cacheKey(self):
27        return super().cacheKey()
28
29class AccessControlServer:
30
31   def __init__(self, serverIface):
32      """ Register AccessControlFilter """
33      serverIface.registerAccessControl(AccessControlFilter(serverIface), 100)

This example gives a full access for everybody.

É papel do complemento saber quem está conectado.

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

20.4.1.5.4. layerFilterExpression

Usado para adicionar uma Expressão para limitar os resultados, por exemplo:

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

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

20.4.1.5.5. 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”.

20.4.1.5.6. 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.

Exemplo:

1def layerPermissions(self, layer):
2    rights = QgsAccessControlFilter.LayerPermissions()
3    rights.canRead = True
4    rights.canInsert = rights.canUpdate = rights.canDelete = False
5    return rights

To limit everything on read only access.

20.4.1.5.7. authorizedLayerAttributes

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

The argument attribute return the current set of visible attributes.

Exemplo:

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

To hide the ‘role’ attribute.

20.4.1.5.8. allowToEdit

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

It is used in the WFS-Transaction protocol.

Exemplo:

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.

20.4.1.5.9. 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.

20.4.2. Custom services

In QGIS Server, core services such as WMS, WFS and WCS are implemented as subclasses of QgsService.

To implemented a new service that will be executed when the query string parameter SERVICE matches the service name, you can implemented your own QgsService and register your service on the serviceRegistry() by calling its registerService(service).

Here is an example of a custom service named CUSTOM:

 1from qgis.server import QgsService
 2from qgis.core import QgsMessageLog
 3
 4class CustomServiceService(QgsService):
 5
 6    def __init__(self):
 7        QgsService.__init__(self)
 8
 9    def name(self):
10        return "CUSTOM"
11
12    def version(self):
13        return "1.0.0"
14
15    def executeRequest(self, request, response, project):
16        response.setStatusCode(200)
17        QgsMessageLog.logMessage('Custom service executeRequest')
18        response.write("Custom service executeRequest")
19
20
21class CustomService():
22
23    def __init__(self, serverIface):
24        serverIface.serviceRegistry().registerService(CustomServiceService())

20.4.3. Custom APIs

In QGIS Server, core OGC APIs such OAPIF (aka WFS3) are implemented as collections of QgsServerOgcApiHandler subclasses that are registered to an instance of QgsServerOgcApi (or it’s parent class QgsServerApi).

To implemented a new API that will be executed when the url path matches a certain URL, you can implemented your own QgsServerOgcApiHandler instances, add them to an QgsServerOgcApi and register the API on the serviceRegistry() by calling its registerApi(api).

Here is an example of a custom API that will be executed when the URL contains /customapi:

 1import json
 2import os
 3
 4from qgis.PyQt.QtCore import QBuffer, QIODevice, QTextStream, QRegularExpression
 5from qgis.server import (
 6    QgsServiceRegistry,
 7    QgsService,
 8    QgsServerFilter,
 9    QgsServerOgcApi,
10    QgsServerQueryStringParameter,
11    QgsServerOgcApiHandler,
12)
13
14from qgis.core import (
15    QgsMessageLog,
16    QgsJsonExporter,
17    QgsCircle,
18    QgsFeature,
19    QgsPoint,
20    QgsGeometry,
21)
22
23
24class CustomApiHandler(QgsServerOgcApiHandler):
25
26    def __init__(self):
27        super(CustomApiHandler, self).__init__()
28        self.setContentTypes([QgsServerOgcApi.HTML, QgsServerOgcApi.JSON])
29
30    def path(self):
31        return QRegularExpression("/customapi")
32
33    def operationId(self):
34        return "CustomApiXYCircle"
35
36    def summary(self):
37        return "Creates a circle around a point"
38
39    def description(self):
40        return "Creates a circle around a point"
41
42    def linkTitle(self):
43        return "Custom Api XY Circle"
44
45    def linkType(self):
46        return QgsServerOgcApi.data
47
48    def handleRequest(self, context):
49        """Simple Circle"""
50
51        values = self.values(context)
52        x = values['x']
53        y = values['y']
54        r = values['r']
55        f = QgsFeature()
56        f.setAttributes([x, y, r])
57        f.setGeometry(QgsCircle(QgsPoint(x, y), r).toCircularString())
58        exporter = QgsJsonExporter()
59        self.write(json.loads(exporter.exportFeature(f)), context)
60
61    def templatePath(self, context):
62        # The template path is used to serve HTML content
63        return os.path.join(os.path.dirname(__file__), 'circle.html')
64
65    def parameters(self, context):
66        return [QgsServerQueryStringParameter('x', True, QgsServerQueryStringParameter.Type.Double, 'X coordinate'),
67                QgsServerQueryStringParameter(
68                    'y', True, QgsServerQueryStringParameter.Type.Double, 'Y coordinate'),
69                QgsServerQueryStringParameter('r', True, QgsServerQueryStringParameter.Type.Double, 'radius')]
70
71
72class CustomApi():
73
74    def __init__(self, serverIface):
75        api = QgsServerOgcApi(serverIface, '/customapi',
76                            'custom api', 'a custom api', '1.1')
77        handler = CustomApiHandler()
78        api.registerHandler(handler)
79        serverIface.serviceRegistry().registerApi(api)