Layer With Modification Time (LWMT) JIB Extension

Contents

1. Purpose
2. Usage
3. Example
4. Interesting reads
5. Requirements
6. Contributing
7. License

Purpose

JIB's default layer packaging process causes all files to have their timestamps set to 1970-01-01T00:00:01Z. This is caused due to JIB plugin's reproducibility feature and is a good practice for most use cases, but causes problems for our public-facing web services, because it messes up Last-Modified headers. When hosting resources, Spring MVC can read file modification timestamp and use it to answer to client's If-Modified-Since header (works with bundled archives as well). Without this information, browser caching can work incorrectly or less efficiently.

While modification time can be set using a plugin property filesModificationTime, it has some drawbacks:

• it can be set only in all-or-nothing principle, forcing you to choose between seamless browser caching support and Docker layer caching. Usually we only need one or two files/libraries to have modified time set.
• it can not be set to take the current time, but rather needs to be passed the time as a property. Additionally, it expects a certain time format which not all projects use. In order to support this we would need to introduce new Maven profiles.

This extension aims to achieve the best of both worlds, by enabling users to define which files they want to move from standard layers (that have modification time set to JIB's default) to a special layer that has last modified time set to build time.

This extension is opinionated in a way that it only allows moving the files into a single new layer. Keep in mind that changing modification time also changes the layer hash and makes it irreproducible. Therefore, number of files affected by this extension should be kept to a minimum and only include the files which are a part of the project.

Usage

The plugin will:

• Remove files matching the filter from layers and put them in a final layer called layerWithModificationTime
• Remove an original layer if it is empty after the files were moved

Here is an example of adding this plugin to a project's plugin management:

<build>
    <pluginManagement>
        <plugins>
            <plugin>
                <groupId>com.google.cloud.tools</groupId>
                <artifactId>jib-maven-plugin</artifactId>
                <version>${jib-maven-plugin.version}</version>
                <dependencies>
                    <dependency>
                        <groupId>com.infobip.jib</groupId>
                        <artifactId>jib-layer-with-modification-time-extension-maven</artifactId>
                        <version>${jib-layer-with-modification-time-extension-maven.version}</version>
                    </dependency>
                </dependencies>
                <executions>
                    <execution>
                        <phase>package</phase>
                        <goals>
                            <goal>dockerBuild</goal>
                        </goals>
                    </execution>
                </executions>
                <configuration>
                    <pluginExtensions combine.children="append">
                        <pluginExtension>
                            <implementation>
                                com.infobip.jib.extension.lwmt.LayerWithModificationTimeJibExtension
                            </implementation>
                            <configuration implementation="com.infobip.jib.extension.lwmt.Configuration">
                                <filters>
                                    <!-- The following are just examples, make sure to adjust the value for your project -->
                                    <filter>**/resources/**</filter>
                                    <filter>**/static/**</filter>
                                    <filter>**/*${project.parent.artifactId}*.jar</filter>
                                </filters>
                            </configuration>
                        </pluginExtension>
                    </pluginExtensions>
                </configuration>
            </plugin>
        </plugins>
    </pluginManagement>
...
</build>

Example

If we had a layers organized like this:

- layer1
    - a.file
    - b.file
    - c.file
- layer2
    - d.file
    - e.file
- layer3
    - f.file

and we ran the plugin with filter **/a.file, we'd end up with:

- layer1
    - b.file
    - c.file
- layer2
    - d.file
    - e.file
- layer3
    - f.file
- layerWithModificationTime
    - a.file

while running the plugin with filter **/f.file would result in:

- layer1
    - a.file
    - b.file
    - c.file
- layer2
    - d.file
    - e.file
- layerWithModificationTime
    - f.file

Interesting reads

• JIB FAQ on reproducible builds
• Reproducible builds
• GitHub discussion

Requirements:

• Java 11
• Maven

Contributing

If you have an idea for a new feature or want to report a bug please use the issue tracker.

Pull requests are welcome!

License

This library is licensed under the Apache License, Version 2.0.