| META_DATA_URL | `https://<somedomain.com>/<somepath>` | HTTP adress where a JSON metadata file for the messages is provided (by the data source). |
| BASE_DEV_URN | `urn:dev:<company_name>:<device_serial_number>` | To identify each unique device from different parties, we add a code to each message composed of the company name and serial number. Farmmaps uses this code to link devices and data to their owners. |
All settings are required except for the `META_DATA_URL`, this parameter is optional.
A message queue is commonly used to make software programs able to send messages between eachother, and thereby making it easy for data to flow from one program into another. There are many variants of message queues, some popular names are Apache Kafka, MQTT and RabbitMQ.
In message queue systems there is usually one central "hub" called the broker or **"message broker"**. This broker holds all the messages. Usually, the messages are organised in groups called **"topics"**.
Now, there are two things that external services can do.
* An external service (like a sensor) could **publish a message to a certain topic**. For example, a temperature sensor would publish the temperature at a specific time and location to the "temperatureMeasurements" topic.
* An external service can **"subscribe" to this topic** by connecting to the broker. This service will then recieve every temperature measurement.
When the subscribed service temporarily disconnects from the broker, it will not recieve any messages, but the messages will remain stored at the broker. Depending on configuration, messages will be kept longer or shorter, or be deleted after they reach the subscribers but the intent is always to ensure the messages get from the **publisher** to the **subscriber**.
Setting communication between applications up like this makes things a lot more flexible than connecting systems directly and provides a central point for management of all communication.
#### What is protobuf?
To quote the Google Documentation:
> Protocol buffers are Google's language-neutral, platform-neutral, extensible mechanism for serializing structured data – think XML, but smaller, faster, and simpler.
> You define how you want your data to be structured once, then you can use special generated source code to easily write and read your structured data to and from a variety of data streams and using a variety of languages.