In IoT projects, the same question always comes up: “Where do we store our telemetry data?”. As usual, the answer to that question is not straightforward. We have seen all kinds of solutions in the wild:
- save directly to a relational database (SQL Server, MySQL, …)
- save to a data lake and/or SQL
- save to Cosmos DB or similar (e.g. MongoDB)
- save to Azure Table Storage or similar
- save to Time Series Insights
Saving the data to a relational database is often tempting. It fits in existing operational practices and it is easy to extract, transform and visualize the data. In practice, I often recommend against this approach except in the simplest of use cases. The reason is clear: these databases are not optimized for fast ingestion of time-series data. Instead, you should use a time-series database which is optimized for fast ingest and efficient processing of time-series data.
TimescaleDB
TimescaleDB is a an open-source time-series databases optimized for fast ingest even when the amount of data stored becomes large. It does not stand on its own, as it runs on PostgreSQL as an extension. Note that you can store time-series in a regular table or as a TimescaleDB hypertable. The graphic below (from this post), shows the difference:
The difference is clear. With a regular table, the insert rate is lowered dramatically when the amount of data becomes large.
The TimescaleDB extension can easily be installed on Azure Database for PostgreSQL. Let’s see how that goes shall we?
Installing TimescaleDB
To create an Azure Database for PostgreSQL instance, I will use the Azure CLI with the db-up extension:
az postgres up -g RESOURCEGROUP -s SERVERNAME -d DBNAME -u USER -p PASSWORD
The server name you provide should result in a unique URL for your database (e.g. servername.postgres.database.azure.com).
Tip: do not use admin as the user name 👍
When the server has been provisioned, modify the server confguration for the TimescaleDB extension:
az postgres server configuration set --resource-group RESOURCEGROUP ––server-name SERVERNAME --name shared_preload_libraries --value timescaledb
Now you need to actually install the extension. Install pgAdmin and issue the following query:
CREATE EXTENSION IF NOT EXISTS timescaledb CASCADE;
In pgAdmin, you should see extra schemas for TimescaleDB:
Creating a hypertable
A hypertable uses partitioning to optimize writing and reading time-series data. Creating such a table is straightforward. You start with a regular table:
CREATE TABLE conditions (
time TIMESTAMPTZ NOT NULL,
location TEXT NOT NULL,
temperature DOUBLE PRECISION NULL,
humidity DOUBLE PRECISION NULL );
Next, convert to a hypertable:
SELECT create_hypertable('conditions', 'time');
The above command partitions the data by time, using the values in the time column. By default, the time interval for partitioning is set to 7 days, starting from version 0.11.0 of TimescaleDB. You can override this by setting chunk_time_interval when creating the hypertable. You should make sure that the chunk belonging to the most recent interval can fit into memory. According to best practices, such a chunk should not use more than 25% of main memory.
Now that we have the hypertable, we can write time-series data to it. One advantage of being built on top of a relational database such as PostgreSQL is that you can use standard SQL INSERT INTO statements. For example:
const query = 'insert into conditions(time, device, temperature, humidity) values(NOW(),\'' + eventHubMessage.device + '\',' + + eventHubMessage.temperature + ',' + eventHubMessage.humidity + ');';
The example above is from an Azure Function we will look at in a moment. In extracts values from a message received via IoT Hub and inserts them into the hypertable via an INSERT INTO query.
Let’s take a look at the Azure Function next.
Azure Function: from IoT Hub to the Hypertable
The Azure Function is kept bare bones to focus on the essentials. Note that you will need to open the console and install the pg module with the following command:
npm install pg
The image below shows the Azure Function (based on this although the article does not use a hypertable and stores the telemetry as JSON).
Naturally, the Azure Function above requires an Azure Event Hubs trigger. In this case, event hub cardinality was set to One. More information here. Note that you should NOT use the NOW() function to set the time. It’s only used here for demo purposes. Instead, you should take the timestamp sent by the device or the time the data was queued at the Event Hub!
Naturally, you will also need an IoT Hub where you send your data. In this case, I created a standard IoT Hub and used the IoT Hub Visual Studio Code extension to generate code (1 in image below) to send sample messages. I modified the code somewhat to include the device name (2 in image below):
Now we can run the code (saved as sender.js) with:
node sender.js
Note: do not forget to first run npm install azure-iot-device
Data is being sent:
Data processed by the Azure Function as viewed in Application Insights Live Metrics stream:
With only one device sending data, there isn’t that much to do! In pgAdmin, you should see connections from at least one of the Azure Function hosts that are active:
Note: I encountered some issues with ECONNRESET errors under higher load; take a look at this post which runs the same function on a Linux Consumption Plan
Querying the data
TimescaleDB, with help from PostgreSQL, has a rich query language especially when compared to some other offerings. Yes, I am looking at you Cosmos DB! 😉 Below are some examples (based on the documentation at https://docs.timescale.com/v1.2/using-timescaledb/reading-data:
SELECT COUNT(*) FROM conditions WHERE time > NOW() - interval '1 minute';
The above query simply counts the messages in the last minute. Notice the flexibility in expressing the time which is what we want from time-series databases.
SELECT time_bucket('1 minute', time) AS one_min, device, COUNT(*), MAX(temperature) AS max_temp, MAX(humidity) AS max_hum FROM conditions WHERE time > NOW() - interval '10 minutes' GROUP BY one_min, device ORDER BY one_min DESC, max_temp DESC;
The above query displays the following result:
Conclusion
When dealing with time-series data, it is often beneficial to use a time-series database. They are optimized to ingest time-series data at high speed and greater efficiency than general purpose SQL or NoSQL databases. The fact that TimescaleDB is built on PostgreSQL means that it can take advantage of the flexibility and stability of PostgreSQL. Although there are many other time-series databases, TimescaleDB is easy to use when coupled with PaaS (platform-as-a-service) PostgreSQL offerings such as Azure Database for PostgreSQL.
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