Created question in FIWARE Q/A platform on 25-02-2021 at 14:02
Please, ANSWER this question AT https://stackoverflow.com/questions/66371017/getting-empty-values-when-querying-data-from-sth-comet

Question:
Getting empty values when querying data from STH-Comet

Description:
Here is how I have provisioned the service group on the IoT Agent
url = 'http://localhost:4041/iot/services'
headers =

{'Content-Type': "application/json", 'fiware-service': "smartparking_pmr", 'fiware-servicepath': "/mtp"}

data = {
"services": [

{ "apikey": "4jggokgpepnvsb2uv4s40d59ov", "cbroker": "http://orion:1026", "entity_type": "PMR", "resource": "/iot/d" }

]
}

Then I registered the device.
url = 'http://localhost:4041/iot/devices'
headers =

{'Content-Type': "application/json", 'fiware-service': "smartparking_pmr", 'fiware-servicepath': "/mtp"}

data = {
"devices": [
{
"device_id": "711",
"entity_name": "urn:ngsi-ld:Pmr:711",
"entity_type": "PMR",
"timezone": "Europe/Berlin",
"attributes": [

{ "object_id": "s", "name": "SlotStatus", "type": "Integer" }

,

{ "object_id": "lat", "name": "Latitude", "type": "Double" }

,

{ "object_id": "lon", "name": "Longitude", "type": "Double" }

]
}
]
}

Next I make a subscription to the Orion Context Broker.
curl -iX POST \
'http://localhost:1026/v2/subscriptions/' \
-H 'Content-Type: application/json' \
-H 'fiware-service: openiot' \
-H 'fiware-servicepath: /' \
-d '{
"description": "Notify STH-Comet of all smartparking_pmr SlotStatus changes",
"subject": {
"entities": [

{ "idPattern": "Pmr.*" }

],
"condition":

{"attrs": ["SlotStatus"] }

},
"notification": {
"http":

{ "url": "http://sth-comet:8666/notify" }

,
"attrs": [
"SlotStatus"
],
"attrsFormat": "legacy"
}
}'

After I send some measurement from the device.
url = 'http://localhost:7896/iot/d?i=711&k=4jggokgpepnvsb2uv4s40d59ov'
headers =

{'Content-Type': "text/plain", 'fiware-service': "smartparking_pmr", 'fiware-servicepath': "/mtp"}

data = "s|0|lat|13.3903|lon|52.5075"
data_raw = data.encode('utf-8')

res = requests.post(url, data=data_raw, headers=headers)

I checked that the subscription exists into Orion.
curl -X GET
'http://localhost:1026/v2/subscriptions/'
-H 'fiware-service: smartparking_pmr'
-H 'fiware-servicepath: '/mtp"

And that is the case (you can see the last update coming from the device)
[{"id":"6037a3e82e21184c83eeeadd","description":"Notify STH-Comet of all smartparking_pmr SlotStatus changes","status":"active","subject":{"entities":[

{"idPattern":"Pmr.*"}

],"condition":{"attrs":["SlotStatus"]}},"notification":{"timesSent":15,"lastNotification":"2021-02-25T14:35:37.000Z","attrs":["SlotStatus"],"onlyChangedAttrs":false,"attrsFormat":"legacy","http":

{"url":"http://sth-comet:8666/notify"}

,"lastSuccess":"2021-02-25T14:35:37.000Z","lastSuccessCode":200}},{"id":"6037b1742e21184c83eeeade","description":"Notify STH-Comet of all smartparking_pmr SlotStatus changes","status":"active","subject":{"entities":[

{"idPattern":"Pmr.*"}

],"condition":{"attrs":["s"]}},"notification":{"attrs":["s"],"onlyChangedAttrs":false,"attrsFormat":"legacy","http":

{"url":"http://sth-comet:8666/notify"}

}},{"id":"6037b91e072e0f6c2a7d46e2","description":"Notify STH-Comet of all smartparking_pmr SlotStatus changes","status":"active","subject":{"entities":[

{"idPattern":"Pmr.*"}

],"condition":{"attrs":["s"]}},"notification":{"attrs":["s"],"onlyChangedAttrs":false,"attrsFormat":"legacy","http":

{"url":"http://sth-comet:8666/notify"}

}},{"id":"6037b95c072e0f6c2a7d46e3","description":"Notify STH-Comet of all smartparking_pmr SlotStatus changes","status":"active","subject":{"entities":[

{"idPattern":"Pmr.*"}

],"condition":{"attrs":["SlotStatus"]}},"notification":{"timesSent":1,"lastNotification":"2021-02-25T14:51:08.000Z","attrs":["SlotStatus"],"onlyChangedAttrs":false,"attrsFormat":"legacy","http":

{"url":"http://sth-comet:8666/notify"}

,"lastSuccess":"2021-02-25T14:51:08.000Z","lastSuccessCode":400}},{"id":"6037b95f072e0f6c2a7d46e4","description":"Notify STH-Comet of all smartparking_pmr SlotStatus changes","status":"active","subject":{"entities":[

{"idPattern":"Pmr.*"}

],"condition":{"attrs":["SlotStatus"]}},"notification":{"timesSent":1,"lastNotification":"2021-02-25T14:51:11.000Z","attrs":["SlotStatus"],"onlyChangedAttrs":false,"attrsFormat":"legacy","http":

{"url":"http://sth-comet:8666/notify"}

,"lastSuccess":"2021-02-25T14:51:11.000Z","lastSuccessCode":400}}]

Finally I want to query data from STH-Comet.
curl "http://localhost:8666/STH/v1/contextEntities/type/PMR/id/Pmr:711/attributes/SlotStatus?lastN=10" --header "fiware-service: smartparking_pmr" --header "fiware-servicepath: /mtp"

But I get empty values for whatever reason.
{"contextResponses":[{"contextElement":{"attributes":[

{"name":"SlotStatus","values":[]}

],"id":"Pmr:711","isPattern":false,"type":"PMR"},"statusCode":{"code":"200","reasonPhrase":"OK"}}]}

Here are the logs from the STH-Comet console.
time=2021-02-25T14:56:57.921Z | lvl=DEBUG | corr=2dd3ef29-df47-4520-9ee3-c0dfccd5a51b | trans=2dd3ef29-df47-4520-9ee3-c0dfccd5a51b | op=OPER_STH_GET | from=n/a | srv=smartparking_pmr | subsrv=/mtp | comp=STH | msg=GET /STH/v1/contextEntities/type/PMR/id/Pmr:711/attributes/SlotStatus?lastN=10

time=2021-02-25T14:56:57.921Z | lvl=DEBUG | corr=2dd3ef29-df47-4520-9ee3-c0dfccd5a51b | trans=2dd3ef29-df47-4520-9ee3-c0dfccd5a51b | op=OPER_STH_GET | from=n/a | srv=smartparking_pmr | subsrv=/mtp | comp=STH | msg=Getting access to the raw data collection for retrieval...

time=2021-02-25T14:56:57.924Z | lvl=WARN | corr=2dd3ef29-df47-4520-9ee3-c0dfccd5a51b | trans=2dd3ef29-df47-4520-9ee3-c0dfccd5a51b | op=OPER_STH_GET | from=n/a | srv=smartparking_pmr | subsrv=/mtp | comp=STH | msg=Error when getting the raw data collection for retrieval (the collection 'null' may not exist)

time=2021-02-25T14:56:57.924Z | lvl=DEBUG | corr=2dd3ef29-df47-4520-9ee3-c0dfccd5a51b | trans=2dd3ef29-df47-4520-9ee3-c0dfccd5a51b | op=OPER_STH_GET | from=n/a | srv=smartparking_pmr | subsrv=/mtp | comp=STH | msg=Responding with no points

This is strange since I can see that the measurement has been recorded by querying Orion.
curl -v http://localhost:1026/v2/entities/urn:ngsi-ld:Pmr:711 --header "Fiware-Service: smartparking_pmr" --header "Fiware-ServicePath: /mtp"

Which gives:
{"id":"urn:ngsi-ld:Pmr:711","type":"PMR","Latitude":{"type":"Double","value":"13.3903","metadata":{"TimeInstant":

{"type":"DateTime","value":"2021-02-25T14:35:35.743Z"}

}},"Longitude":{"type":"Double","value":"52.5075","metadata":{"TimeInstant":

{"type":"DateTime","value":"2021-02-25T14:35:35.743Z"}

}},"SlotStatus":{"type":"Integer","value":"0","metadata":{"TimeInstant":

{"type":"DateTime","value":"2021-02-25T14:35:35.743Z"}

}},"TimeInstant":{"type":"DateTime","value":"2021-02-25T14:35:35.743Z","metadata":{}}}

Could you please help me out? Thanks in advance.
Regards,