Why Does My Docker Container Show Intermittent stdout Output?

stdout Buffering in a Docker Container

When running code within a Docker container, users may encounter instances where stdout buffering occurs, causing outputs to be flushed intermittently rather than appearing immediately upon generation. This phenomenon can vary depending on the operating system of the container and the code itself.

In one particular case, the issue manifested when running the "procwrap" program within a Docker container. While the subprocess executed successfully, the output to stdout and log files appeared in bursts, indicating a buffered response. However, running the same code outside of a container resulted in synchronous output.

Investigation and Troubleshooting

Initial observations revealed that the issue was related to stdout buffering. Modifications to Docker or Linux settings (such as using "stdbuf") proved ineffective in resolving the problem.

Solution

To overcome this challenge, modifications were made to the source code of the console application that was being containerized. These modifications forced an immediate file flush after each write operation. This ensured that data was not buffered within the application and was outputted in real time.

Additional Considerations

The specific configuration used to resolve this issue involved custom logging that wrote to "/dev/stdout" and triggered file flushing upon each log write. This allowed the Docker container to display messages immediately through the "docker logs" command.

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