MQTT Vs REST API: Handling Multiple Numbers Differently?

Have you ever encountered a situation where sending SMS messages to multiple recipients works flawlessly with one method but fails with another? This is a common issue when dealing with different APIs and protocols. In this article, we'll dive into a specific scenario involving MQTT and REST APIs, exploring why the handling of multiple phone numbers can vary between these two approaches. Understanding these differences is crucial for developers and system administrators who rely on these technologies for notifications and messaging.

Understanding the Issue: MQTT and REST API Differences

The core issue arises from how MQTT and REST APIs interpret and process data, particularly when it comes to recipient formatting. MQTT (Message Queuing Telemetry Transport) is a lightweight messaging protocol ideal for IoT devices and real-time applications. It operates on a publish-subscribe model, where devices publish messages to topics, and subscribers receive those messages. REST (Representational State Transfer), on the other hand, is an architectural style for building web services. REST APIs use standard HTTP methods (GET, POST, PUT, DELETE) to interact with resources.

When sending SMS messages, the format of the recipient's phone numbers is critical. The original poster, PavelVe, discovered that using formats like +3********35,+3********48, 3********35,3********48, or 04*******35,04*******48 with MQTT resulted in errors, and the SMS messages were not sent. However, when using the REST API with the notify.sms_gateway service in Home Assistant, the same formats worked perfectly fine. This discrepancy highlights the different ways these systems parse and validate input data.

Key Differences in Data Handling

• MQTT's Strict Formatting: MQTT often requires a specific format for phone numbers, and it might not automatically handle comma-separated lists or variations in international prefixes. This strictness is partly due to MQTT's focus on efficiency and simplicity, which means less flexibility in data parsing.
• REST API's Flexibility: REST APIs, especially those designed for services like SMS gateways, are typically more flexible in handling different number formats. They often include built-in parsing and validation logic to accommodate various input styles, making it easier to send messages to multiple recipients.

Diving Deeper into MQTT Behavior

To further understand why MQTT might fail with multiple phone numbers, let's examine the provided MQTT action configuration:

action: mqtt.publish
data:
  topic: homeassistant/sensor/sms_gateway/send
  payload: |-
    {{
      {
        "number": sms_number,
        "text": sms_text
      } | tojson
    }}
enabled: false

This configuration publishes a JSON payload to an MQTT topic. The payload includes the recipient's number (sms_number) and the message text (sms_text). The crucial part here is how sms_number is being formatted before it's included in the JSON payload.

Potential Issues in MQTT Configuration

1. String Interpolation: If sms_number contains a comma-separated string of phone numbers, the MQTT broker might interpret this as a single, invalid phone number. MQTT doesn't inherently know how to split this string into individual numbers.
2. JSON Payload Structure: The JSON payload expects a single number field. If you pass a string with multiple numbers, the receiving end (the SMS gateway) may not know how to parse it correctly. It expects a single phone number, not a list.
3. Error Handling: MQTT clients and brokers may not provide detailed error messages when a message fails to send due to formatting issues. This lack of feedback can make it challenging to diagnose the problem.

To successfully send SMS messages to multiple recipients via MQTT, you would likely need to modify the configuration to send individual messages for each number or format the payload in a way that the receiving service understands (e.g., an array of numbers). This might involve more complex scripting or additional logic within your Home Assistant setup.

Examining REST API Functionality

Now, let's consider the REST API action configuration:

action: notify.sms_gateway
data:
  target: "{{ sms_number }}"
  message: "\"{{sms_text}}\""

This configuration uses the notify.sms_gateway service, which likely interacts with an SMS gateway via a REST API. The key part here is the target field, which contains the recipient's phone number(s).

REST API's Handling of Multiple Numbers

1. Service-Level Parsing: The notify.sms_gateway service probably includes built-in logic to parse the target string. It can identify comma-separated phone numbers and process them individually.
2. API Flexibility: REST APIs often allow for more flexible data inputs. The SMS gateway API may be designed to accept a string of comma-separated numbers and handle the splitting and sending internally.
3. Abstraction: The notify.sms_gateway service acts as an abstraction layer, shielding you from the complexities of the underlying API. It takes care of formatting the requests correctly and handling responses, making it easier to send messages to multiple recipients.

The REST API approach simplifies the process because the service handles the parsing and delivery logic. You don't need to worry about the intricacies of formatting the payload; the service takes care of it for you.

Why the Discrepancy Matters

The different behavior between MQTT and REST APIs can lead to confusion and frustration if not understood. Imagine setting up a critical notification system that relies on sending SMS messages to multiple people. If you use MQTT without proper formatting, those messages might not go through, potentially leading to missed alerts and communication breakdowns.

Real-World Implications

• Emergency Notifications: In a smart home setup, you might want to send emergency alerts (e.g., fire alarm, intrusion) to multiple family members. If MQTT fails to deliver these messages, the consequences could be severe.
• System Monitoring: For system administrators, SMS notifications are often used to alert them of critical issues. If the notification system can't handle multiple recipients, it limits its effectiveness.
• Automation Scenarios: Many home automation scenarios involve sending notifications to several people (e.g., a package delivery alert). Consistent and reliable delivery is crucial for these automations to work as expected.

Best Practices for Handling Multiple Numbers

To avoid issues with multiple phone numbers, consider these best practices:

• Understand API Requirements: Always check the documentation for the MQTT broker, SMS gateway API, or any other service you're using. Pay close attention to the required format for phone numbers.
• Use Service Abstractions: When possible, leverage higher-level services (like notify.sms_gateway) that handle the complexities of API interactions. These services often provide built-in support for multiple recipients.
• Implement Error Handling: Add error handling to your configurations to catch and log failures. This can help you identify issues quickly and prevent critical messages from being missed.
• Test Thoroughly: Always test your notification systems with multiple recipients to ensure they work as expected. This includes testing different number formats and edge cases.

Solutions and Workarounds

If you encounter issues with MQTT and multiple phone numbers, here are some potential solutions and workarounds:

1. Send Individual Messages: Instead of sending a single message with multiple numbers, send individual messages for each recipient. This might require looping through the list of numbers and publishing a separate MQTT message for each one.
2. Format Payload as Array: Some SMS gateway APIs might accept an array of phone numbers in the JSON payload. Check the API documentation to see if this is supported and format your payload accordingly.
3. Use a Scripting Component: Employ a scripting component (e.g., a Python script in Home Assistant) to handle the formatting and sending of messages. This gives you more control over the process and allows you to implement custom logic.
4. Switch to REST API: If MQTT is proving too cumbersome for your needs, consider using the REST API directly or through a service abstraction like notify.sms_gateway.
5. Pre-process the Numbers: Before sending the message, pre-process the sms_number variable to split the comma-separated string into individual numbers. You can then iterate over these numbers and send a separate MQTT message for each.

Example: Sending Individual Messages via MQTT

Here’s an example of how you might send individual messages via MQTT using a scripting component in Home Assistant:

automation:
  - alias: "Send SMS to Multiple Numbers via MQTT"
    trigger:
      ...
    action:
      - service: python_script.send_sms_mqtt
        data_template:
          numbers: "{{ sms_number }}"
          text: "{{ sms_text }}"

And the corresponding Python script (python_script.send_sms_mqtt.py):

numbers = data.get('numbers').split(',')
text = data.get('text')

for number in numbers:
    payload = {"number": number.strip(), "text": text}
    hass.services.call('mqtt', 'publish',
                        {"topic": 'homeassistant/sensor/sms_gateway/send',
                         "payload": json.dumps(payload)})

This script splits the comma-separated string of numbers, iterates over them, and sends a separate MQTT message for each one. This approach ensures that each recipient receives the message, even if the SMS gateway doesn't support multiple numbers in a single request.

Conclusion: Choosing the Right Approach

In conclusion, the different behavior between MQTT and REST APIs when handling multiple phone numbers stems from their architectural differences and how they parse and process data. MQTT's stricter formatting requirements can lead to issues if not properly addressed, while REST APIs often provide more flexibility and built-in support for multiple recipients. Understanding these nuances is crucial for building reliable notification systems.

When designing your system, consider the specific requirements of your application, the capabilities of the APIs you're using, and the trade-offs between simplicity and flexibility. By following best practices and implementing appropriate workarounds, you can ensure that your SMS messages reach all intended recipients, regardless of the underlying technology.

For more information on MQTT and REST APIs, consider exploring resources like the MQTT official website and the REST API tutorial. These resources can provide a deeper understanding of these technologies and help you make informed decisions when building your systems.