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One way to achieve this is by configuring each instance of the Spring Cloud Kafka Binder to write its messages to a shared topic. Then, you can create a consumer application that reads from this shared topic and writes the incoming messages to a file.

Here are the steps to set up this approach:

1. Configure the Spring Cloud Kafka Binder instances to write to the shared topic by setting the destination property of each instance to the same value. For example:

spring.cloud.stream.bindings.output.destination=my-shared-topic

1. Create a consumer application that reads from the shared topic and writes the incoming messages to a file. You can use the @StreamListener annotation to consume messages from the shared topic and the FileWriter class to write to a file. Here's an example:

@StreamListener("my-shared-topic")
public void processMessage(String message) {
    try {
        FileWriter writer = new FileWriter("/path/to/file", true);
        writer.write(message + "\n");
        writer.close();
    } catch (IOException e) {
        // Handle exception
    }
}

Note that the true parameter in the FileWriter constructor specifies that the file should be opened in "append" mode, so that new messages are added to the end of the file rather than overwriting its contents.

1. Run multiple instances of the Spring Cloud Kafka Binder application, each configured to write to the shared topic. These instances can be run on different machines or on the same machine using different ports.

2. Start the consumer application to read messages from the shared topic and write them to the file.

With this setup, each instance of the Spring Cloud Kafka Binder writes its messages to the shared topic, which is then consumed by the consumer application and written to the file. As long as the destination name is the same for all binder instances, this approach will work regardless of the number of binder instances that are running.