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Image Layering
Did you know that you can look at what makes up an image? Using the docker image history
command, you can see the command that was used to create each layer within an image.
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Use the
docker image historycommand to see the layers in thegetting-startedimage you created earlier in the tutorial.docker image history getting-startedYou should get output that looks something like this (dates/IDs may be different).
IMAGE CREATED CREATED BY SIZE COMMENT a78a40cbf866 18 seconds ago /bin/sh -c #(nop) CMD ["node" "src/index.j… 0B f1d1808565d6 19 seconds ago /bin/sh -c yarn install --production 85.4MB a2c054d14948 36 seconds ago /bin/sh -c #(nop) COPY dir:5dc710ad87c789593… 198kB 9577ae713121 37 seconds ago /bin/sh -c #(nop) WORKDIR /app 0B b95baba1cfdb 13 days ago /bin/sh -c #(nop) CMD ["node"] 0B <missing> 13 days ago /bin/sh -c #(nop) ENTRYPOINT ["docker-entry… 0B <missing> 13 days ago /bin/sh -c #(nop) COPY file:238737301d473041… 116B <missing> 13 days ago /bin/sh -c apk add --no-cache --virtual .bui… 5.35MB <missing> 13 days ago /bin/sh -c #(nop) ENV YARN_VERSION=1.21.1 0B <missing> 13 days ago /bin/sh -c addgroup -g 1000 node && addu… 74.3MB <missing> 13 days ago /bin/sh -c #(nop) ENV NODE_VERSION=12.14.1 0B <missing> 13 days ago /bin/sh -c #(nop) CMD ["/bin/sh"] 0B <missing> 13 days ago /bin/sh -c #(nop) ADD file:e69d441d729412d24… 5.59MBEach of the lines represents a layer in the image. The display here shows the base at the bottom with the newest layer at the top. Using this, you can also quickly see the size of each layer, helping diagnose large images.
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You'll notice that several of the lines are truncated. If you add the
--no-truncflag, you'll get the full output (yes... funny how you use a truncated flag to get untruncated output, huh?)docker image history --no-trunc getting-started
Layer Caching
Now that you've seen the layering in action, there's an important lesson to learn to help decrease build times for your container images.
Once a layer changes, all downstream layers have to be recreated as well
Let's look at the Dockerfile we were using one more time...
FROM node:12-alpine
WORKDIR /app
COPY . .
RUN yarn install --production
CMD ["node", "src/index.js"]
Going back to the image history output, we see that each command in the Dockerfile becomes a new layer in the image. You might remember that when we made a change to the image, the yarn dependencies had to be reinstalled. Is there a way to fix this? It doesn't make much sense to ship around the same dependencies every time we build, right?
To fix this, we need to restructure our Dockerfile to help support the caching of the dependencies. For Node-based
applications, those dependencies are defined in the package.json file. So, what if we copied only that file in first,
install the dependencies, and then copy in everything else? Then, we only recreate the yarn dependencies if there was
a change to the package.json. Make sense?
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Update the Dockerfile to copy in the
package.jsonfirst, install dependencies, and then copy everything else in.FROM node:12-alpine WORKDIR /app COPY package.json yarn.lock ./ RUN yarn install --production COPY . . CMD ["node", "src/index.js"] -
Create a file named
.dockerignorein the same folder as the Dockerfile with the following contents.node_modules.dockerignorefiles are an easy way to selectively copy only image relevant files. You can read more about this here. In this case, thenode_modulesfolder should be omitted in the secondCOPYstep because otherwise, it would possibly overwrite files which were created by the command in theRUNstep. For further details on why this is recommended for Node.js applications and other best practices, have a look at their guide on Dockerizing a Node.js web app. -
Build a new image using
docker build.docker build -t getting-started .You should see output like this...
Sending build context to Docker daemon 219.1kB Step 1/6 : FROM node:12-alpine ---> b0dc3a5e5e9e Step 2/6 : WORKDIR /app ---> Using cache ---> 9577ae713121 Step 3/6 : COPY package.json yarn.lock ./ ---> bd5306f49fc8 Step 4/6 : RUN yarn install --production ---> Running in d53a06c9e4c2 yarn install v1.17.3 [1/4] Resolving packages... [2/4] Fetching packages... info fsevents@1.2.9: The platform "linux" is incompatible with this module. info "fsevents@1.2.9" is an optional dependency and failed compatibility check. Excluding it from installation. [3/4] Linking dependencies... [4/4] Building fresh packages... Done in 10.89s. Removing intermediate container d53a06c9e4c2 ---> 4e68fbc2d704 Step 5/6 : COPY . . ---> a239a11f68d8 Step 6/6 : CMD ["node", "src/index.js"] ---> Running in 49999f68df8f Removing intermediate container 49999f68df8f ---> e709c03bc597 Successfully built e709c03bc597 Successfully tagged getting-started:latestYou'll see that all layers were rebuilt. Perfectly fine since we changed the Dockerfile quite a bit.
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Now, make a change to the
src/static/index.htmlfile (like change the<title>to say "The Awesome Todo App"). -
Build the Docker image now using
docker build -t getting-started .again. This time, your output should look a little different.Sending build context to Docker daemon 219.1kB Step 1/6 : FROM node:12-alpine ---> b0dc3a5e5e9e Step 2/6 : WORKDIR /app ---> Using cache ---> 9577ae713121 Step 3/6 : COPY package.json yarn.lock ./ ---> Using cache ---> bd5306f49fc8 Step 4/6 : RUN yarn install --production ---> Using cache ---> 4e68fbc2d704 Step 5/6 : COPY . . ---> cccde25a3d9a Step 6/6 : CMD ["node", "src/index.js"] ---> Running in 2be75662c150 Removing intermediate container 2be75662c150 ---> 458e5c6f080c Successfully built 458e5c6f080c Successfully tagged getting-started:latestFirst off, you should notice that the build was MUCH faster! And, you'll see that steps 1-4 all have
Using cache. So, hooray! We're using the build cache. Pushing and pulling this image and updates to it will be much faster as well. Hooray!
Multi-Stage Builds
While we're not going to dive into it too much in this tutorial, multi-stage builds are an incredibly powerful tool to help use multiple stages to create an image. There are several advantages for them:
- Separate build-time dependencies from runtime dependencies
- Reduce overall image size by shipping only what your app needs to run
Maven/Tomcat Example
When building Java-based applications, a JDK is needed to compile the source code to Java bytecode. However, that JDK isn't needed in production. Also, you might be using tools like Maven or Gradle to help build the app. Those also aren't needed in our final image. Multi-stage builds help.
FROM maven AS build
WORKDIR /app
COPY . .
RUN mvn package
FROM tomcat
COPY --from=build /app/target/file.war /usr/local/tomcat/webapps
In this example, we use one stage (called build) to perform the actual Java build using Maven. In the second
stage (starting at FROM tomcat), we copy in files from the build stage. The final image is only the last stage
being created (which can be overridden using the --target flag).
React Example
When building React applications, we need a Node environment to compile the JS code (typically JSX), SASS stylesheets, and more into static HTML, JS, and CSS. If we aren't doing server-side rendering, we don't even need a Node environment for our production build. Why not ship the static resources in a static nginx container?
FROM node:12 AS build
WORKDIR /app
COPY package* yarn.lock ./
RUN yarn install
COPY public ./public
COPY src ./src
RUN yarn run build
FROM nginx:alpine
COPY --from=build /app/build /usr/share/nginx/html
Here, we are using a node:12 image to perform the build (maximizing layer caching) and then copying the output
into an nginx container. Cool, huh?
Recap
By understanding a little bit about how images are structured, we can build images faster and ship fewer changes. Multi-stage builds also help us reduce overall image size and increase final container security by separating build-time dependencies from runtime dependencies.