add client side filtering recipe

cookbook/sections/data-publishers.adoc

@@ -118,5 +118,187 @@ image::images/data-publishers-wcmp2-validate-response.png[WIS2 GDC online valida
 
 A response will be provided with validation results.
 
+=== Advertise client side filters for data subscriptions in WCMP2 and WNM
 
+A key concept of a WCMP2 record is "actionable links"; this means being able to access a dataset or data granule
+without any further interactions.  For real-time data, a WCMP2 record provides linkages to the WIS2 Global Broker
+via the MQTT protocol.  At its core, MQTT has two key components:
 
+- topic: the topic to subscribe to
+- message payload: the message provided as part of a notification to a given topic
+
+WIS2 defines the WIS2 Topic Hierarchy (WTH) and WIS2 Notification Message (WNM) standards which provide a standards-based
+GeoJSON payload/message.
+
+A typical MQTT link in a WCMP2 document is defined as follows:
+
+.Typical WCMP2 MQTT link
+[source,json]
+----
+{
+  "rel"  : "items",
+  "type" : "application/geo+json",
+  "title": "WIS2 notification service",
+  "href" : "mqtts://example.org",
+  "channel": "cache/a/wis2/ca-eccc-msc/data/core/weather/surface-based-observations/synop"
+}
+----
+
+Given WCMP2, WTH and WNM, a user can subscribe to topics related to data of interest for download and access.
+
+In some cases, a dataset may be organized in a manner which requires additional further "filtering" such that a
+data consumer is only interested in a certain subset of the data granules being advertised by a given WNM.  Some examples include (but are not limited to), where a data consumer may be only be interested in:
+
+- surface weather observations from a certain station, or
+- numerical weather prediction forecast data for a certain timestep or weather parameter
+
+To implement this behaviour, add additional properties to both WCMP2 and WNM as follows:
+
+==== Example: Surface weather observations
+
+.Surface weather observations: WCMP2 MQTT link with additional properties
+[source,json]
+----
+{
+  "rel"  : "items",
+  "type" : "application/geo+json",
+  "title": "Real-time notifications",
+  "href" : "mqtts://globalbroker.meteo.fr:8883",
+  "channel": "cache/a/wis2/ca-eccc-msc/data/core/weather/surface-based-observations/synop",
+  "properties": {
+    "wigos_station_identifier": {
+       "type": "string",
+       "title": "WIGOS station identifier"
+    }
+  }
+}
+----
+
+.Surface weather observations: WNM additional properties
+[source,json]
+----
+{
+  "properties": {
+    "wigos_station_identifier": "0-20000-0-71628"
+}
+----
+
+When implemented by a data producer, a data consumer can:
+
+- subscribe to real-time notifications to the given topic
+- perform client side filtering by against all incoming WNMs with `properties.wigos_station_identifier = "0-20000-0-71628"`
+
+==== Example: Numerical weather prediction based forecast
+
+.Numerical weather prediction: WCMP2 MQTT link with additional properties
+[source,json]
+----
+{
+  "rel"  : "items",
+  "type" : "application/geo+json",
+  "title": "Real-time notifications",
+  "href" : "mqtts://globalbroker.meteo.fr:8883",
+  "channel": "origin/a/wis2/ca-eccc-msc/data/core/weather/prediction/forecast/medium-range/deterministic/global",
+  "properties": {
+    "model_run": {
+       "type": "string",
+       "title": "Model run",
+       "enum": [
+           "00",
+           "12"
+        ],
+        "example": "00"
+    },
+    "forecast_hour": {
+       "type": "string",
+       "title": "Forecast hour",
+        "example": "004"
+    }
+  }
+}
+----
+
+.Numerical weather prediction: WNM additional properties
+[source,json]
+----
+{
+  "properties": {
+    "model_run": "00",
+    "forecast_hour": "004"
+}
+----
+
+A data producer would extend WCMP2 and WNM as follows:
+
+- WCMP2: add a link `properties` object for MQTT links, where each key of the link `properties` object is a https://json-schema.org/understanding-json-schema/reference/object#properties[JSON Schema property definition].
+- WNM: add additional properties (key: value pairs) in the `properties` object as desired
+
+When implemented by a data producer, a data consumer can:
+
+- subscribe to real-time notifications to the given topic
+- perform client side filtering against all incoming WNMs with `properties.model_run = "00" and properties.forecast_hour = "004"`
+
+A sample Python script can be found below.  The script connects to the Météo-France Global Broker, subscribed to weather notifications
+from Environment and Climate Change Canada, Meteorological Service of Canada.  The script then performs client side filtering by
+evaluating (for each WNM) the `properties.wigos_station_identifier` value to match a particular station (`0-20000-0-71628`).
+
+.Sample Python script to perform client side filtering
+[source,python]
+----
+import json
+from paho.mqtt import client as mqtt_client
+
+broker = 'globalbroker.meteo.fr'
+port = 8883
+username = 'everyone'
+password = 'everyone'
+topic = 'cache/a/wis2/ca-eccc-msc/data/core/weather/surface-based-observations/synop'
+
+wsi_to_filter = '0-20000-0-71628'
+
+
+def connect_mqtt() -> mqtt_client:
+    def on_connect(client, userdata, flags, reason_code, properties):
+        if reason_code == 0:
+            print(f'Connected to {broker}')
+        else:
+            print(f'Failed to connect: {reason_code}')
+
+    def on_log(client, userdata, level, message):
+        print("LOG:", message)
+
+    client = mqtt_client.Client(mqtt_client.CallbackAPIVersion.VERSION2,
+                                client_id='s123')
+    client.username_pw_set(username, password)
+    client.on_connect = on_connect
+    client.on_log = on_log
+    client.tls_set(tls_version=2)
+    client.connect(broker, port)
+
+    return client
+
+
+def subscribe(client: mqtt_client):
+    def on_message(client, userdata, message):
+        message_dict = json.loads(message.payload.decode())
+
+        print('Performing client side filtering')
+        wsi = message_dict['properties'].get('wigos_station_identifier')
+
+        if wsi != wsi_to_filter:
+            print(f'Topic: {message.topic}')
+            print(f'Payload: {message.payload.decode()}')
+
+    client.subscribe(topic)
+    client.on_message = on_message
+
+
+def run():
+    client = connect_mqtt()
+    subscribe(client)
+    client.loop_forever()
+
+
+if __name__ == '__main__':
+    run()
+----