Preserve last value with Azure Stream Analytics - azure

Question

We are collecting device events via IoT Hub which are then processed with Stream Analytics. We want to generate a status overview containing the last value of every measurement. The status is then written to a CosmosDB output, one document per device.
The simplified query looks like this:
SELECT
device_id as id,
LAST(value) OVER (PARTITION BY device_id LIMIT DURATION(day, 1) WHEN name = 'battery_status') AS battery_status
INTO status
FROM iothub
The resulting document should be (also simplified):
{
"id": "8c03b6cef760",
"battery_status": 95
}
The problem is that not all events contain a battery_status and whenever the last event with battery_status is older than the specified duration, the last value in the CosmosDB document is overwritten with NULL.
What I would need is some construct to omit the value entirely when there is no data and consequently preserve the last value in the output document. Any ideas how I could achieve this?

Answer 1

Currently Azure Stream Analytics does not support partition your output to CosmosDB
for each device.
There are two option to workaround.
You can choose Azure Function to workaround. In azure function you can create a IoT Hub trigger, filter the data with battery_status property ,and then store the data to CosmosDB for each device programmatically.
You can choose Azure Storage Container instead CosmosDB, and then config Azure Storage Container as endpoints and message route in Azure IoT Hub, please refer to IoT Hub Endpoints and this tutorial about how to save IoT hub messages that contain sensor data to your Azure blob storage. In the route config, you can add query string for filtering the data.

Answer 2

If I understand your problem correctly, I just put where condition to filter non battery_status data. You can write multiple queries to process your data, process battery event data separately.
Sample Input
1.
{
"device_id": "8c03b6cef760",
"name": "battery_status",
"value": 67
}
2.
{
"device_id": "8c03b6cef760",
"name": "cellular_connectivity",
"value": 67
}
Output
{
"id": "8c03b6cef760",
"battery_status": 67,
"_rid": "vYYFAIRr5b8LAAAAAAAAAA==",
"_self": "dbs/vYYFAA==/colls/vYYFAIRr5b8=/docs/vYYFAIRr5b8LAAAAAAAAAA==/",
"_etag": "\"8d001092-0000-0000-0000-5b7ffe8e0000\"",
"_attachments": "attachments/",
"_ts": 1535114894
}
ASA Query
SELECT
device_id as id,
LAST(value) OVER (PARTITION BY device_id LIMIT DURATION(day, 1) WHEN name = 'battery_status') AS battery_status
INTO status
FROM iothub
WHERE name = 'battery_status'