## 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. 1. Use the `docker image history` command to see the layers in the `getting-started` image you created earlier in the tutorial. ```bash docker image history getting-started ``` You should get output that looks something like this (dates/IDs may be different). ```plaintext IMAGE CREATED CREATED BY SIZE COMMENT a78a40cbf866 18 seconds ago /bin/sh -c #(nop) CMD ["node" "/app/src/ind… 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 13 days ago /bin/sh -c #(nop) ENTRYPOINT ["docker-entry… 0B 13 days ago /bin/sh -c #(nop) COPY file:238737301d473041… 116B 13 days ago /bin/sh -c apk add --no-cache --virtual .bui… 5.35MB 13 days ago /bin/sh -c #(nop) ENV YARN_VERSION=1.21.1 0B 13 days ago /bin/sh -c addgroup -g 1000 node && addu… 74.3MB 13 days ago /bin/sh -c #(nop) ENV NODE_VERSION=12.14.1 0B 13 days ago /bin/sh -c #(nop) CMD ["/bin/sh"] 0B 13 days ago /bin/sh -c #(nop) ADD file:e69d441d729412d24… 5.59MB ``` Each 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. 1. You'll notice that several of the lines are truncated. If you add the `--no-trunc` flag, you'll get the full output (yes... funny how you use a truncated flag to get untruncated output, huh?) ```bash 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... ```dockerfile FROM node:12-alpine WORKDIR /app COPY . . RUN yarn install --production CMD ["node", "/app/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? 1. Update the Dockerfile to copy in the `package.json` first, install dependencies, and then copy everything else in. ```dockerfile hl_lines="3 4 5" FROM node:12-alpine WORKDIR /app COPY package.json yarn.lock ./ RUN yarn install --production COPY . . CMD ["node", "/app/src/index.js"] ``` 1. Build a new image using `docker build`. ```bash docker build -t getting-started . ``` You should see output like this... ```plaintext 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", "/app/src/index.js"] ---> Running in 49999f68df8f Removing intermediate container 49999f68df8f ---> e709c03bc597 Successfully built e709c03bc597 Successfully tagged getting-started:latest ``` You'll see that all layers were rebuilt. Perfectly fine since we changed the Dockerfile quite a bit. 1. Now, make a change to the `src/static/index.html` file (like change the `` to say "The Awesome Todo App"). 1. Build the Docker image now using `docker build -t getting-started .` again. This time, your output should look a little different. ```plaintext hl_lines="5 8 11" 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", "/app/src/index.js"] ---> Running in 2be75662c150 Removing intermediate container 2be75662c150 ---> 458e5c6f080c Successfully built 458e5c6f080c Successfully tagged getting-started:latest ``` First 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. ```dockerfile 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? ```dockerfile 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.