Why Does Stdout Buffering Occur in Docker Containers?

stdout Buffering in Docker Containers

Problem:

Running a process within a Docker container can result in buffered stdout output, unlike when the process is executed on the host or on macOS. This inconsistent behavior is observed when using Go 1.6.3 and is particularly noticeable in containers based on Debian images.

Analysis:

The issue lies in the stdout buffering behavior of the container. By default, stdout is buffered in Docker containers, meaning that output is collected until a certain limit is reached or a flush is triggered. This can lead to intermittent output when running processes that generate a large amount of data.

Cause:

The stdout buffering is a Linux kernel feature that is inherited by Docker containers. It optimizes system performance by reducing the number of writes to the host filesystem.

Solution:

There are several ways to overcome stdout buffering in Docker containers:

• Use stdbuf: The stdbuf tool can be used to force unbuffered stdout. Add stdbuf -o0 before the command that is causing the buffering issue.
• Set the unbuffer environment variable: Setting the unbuffer environment variable to true will disable buffering for all child processes.
• Modify the source code: If the process being run has a console-based interface, it may be necessary to modify the source code to flush stdout after each write.
• Use a Docker logging driver: Using a Docker logging driver that supports real-time logging can help mitigate the issue.

Example:

In the example provided, the following code snippet can be used to disable stdout buffering:

<code class="go">cmd := exec.Command("ping", "127.0.0.1")
cmd.Stdout = io.MultiWriter(os.Stdout, logWriter)
cmd.Env = append(os.Environ(), "unbuffer=true")
err := cmd.Run()</code>

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