PLC4X Kafka Connect "Connection String format is incorrect" with S7 and Modbus in Docker

[i7pacheco]

Hi everyone,

I'm working on integrating PLC4X (version 0.13.0-SNAPSHOT) as a Kafka Connect source connector for industrial data collection. I’m running everything in Docker and have built a fat JAR that includes all drivers (S7, Modbus, etc.). However, I’m consistently hitting the following issues:

Problems Encountered

1. Connection String Format Errors

• When I use what the documentation states is the correct format, e.g.:
  • s7://192.168.172.51?remote-rack=0&remote-slot=0
  • modbus-tcp://192.168.172.51:502
• I get errors like:

  Connection String format is incorrect sources.machineA.connectionString=s7://192.168.172.51?remote-rack=0&remote-slot=0
  Connection String format is incorrect sources.machineA.connectionString=modbus-tcp://192.168.172.51:502
  

• I have verified there are no trailing spaces and the JSON is properly formatted.

2. Driver Registration

• The logs show some drivers (e.g., Allen Bradley) being registered, but not always Modbus or S7.
• My fat JAR includes all drivers (plc4j-driver-all), and I have checked the contents to confirm.
• The JAR is placed in its own subdirectory in the Kafka Connect plugins directory.

3. Plugin Directory and Classloader

• The JAR is deployed in its own folder under /plugins/plc4x/ as recommended.
• I have restarted Kafka Connect after every change.
• There are no duplicate or old versions of PLC4X in the plugins directory.

4. Version Consistency

• All PLC4X artifacts are 0.13.0-SNAPSHOT.
• The error persists whether I use the default port or specify it explicitly.

What I’ve Tried

• Both the new (modbus-tcp://ip:port) and old (modbus-tcp:tcp://ip:port) connection string formats.
• Minimal JSON configs with only the required fields.
• Cleaning and rebuilding the JAR, redeploying, and restarting Kafka Connect.
• Reviewing logs for driver registration (sometimes missing for Modbus/S7).
• Ensuring no invisible characters or formatting issues in the config.
• Running in both standalone and distributed Kafka Connect modes.

Background

• Running in Docker containers.
• Using Notepad++ and JSON tools for config validation.
• Experience with Telegraf and other data collection tools in similar environments.

Questions for the Community

• Is there a known issue with driver registration or connection string parsing in PLC4X 0.13.0-SNAPSHOT for Kafka Connect?
• Are there additional steps needed to ensure all drivers are loaded and recognized by the connector?
• Is there a way to force or verify driver registration at runtime?
• Are there any logs or debug flags that could help diagnose why a driver is not being registered or why the connection string is rejected?
• Has anyone successfully used the modbus-tcp:// or s7:// connection strings with this version in Kafka Connect, and if so, could you share a working config and deployment structure?

Any help or suggestions would be greatly appreciated! If you need more logs, my config, or Docker setup details, I can provide them.

Thanks in advance!

Let me know if you want to add more technical details or logs, or if you want this tailored for a specific repository or issue tracker.

[chrisdutz]

Are you basing your work on the existing, but old Kafka Connect adapter in the plc4x-extras repo, or is this a completely new approach? But I think you are using the existing one.

With fat jars there always is a problem on how they are created. While you can simply merge the driver code without problems, as all drivers are located in separate packages, there usually are issues if you create a fat jar with multiple drivers when it comes to the resource files PLC4X uses to locate drivers. Here all jars have the same file "META-INF/services/org.apache.plc4x.java.api.PlcDriver" ... I think per default either the first file with that name is used and the others are dropped. This results in PLC4X only finding one driver.

You should have a look at your fat jar and inspect this file and double check all drivers are listed.

There is however an option to have the plugins merge the resource files, but you need to use plugins for the plugins for that. See here:
https://github.com/apache/plc4x-extras/blob/d46b95746f3fd3c1735fc51a86471100c49128f5/plc4j/examples/pom.xml#L67

Hope that helps.

[i7pacheco]

Hello @chrisdutz , you were right, I had to add manually the drivers on the file. Only the first was being listed. Thanks!