The Layerfile Cache

Webapp.io has extended & improved Docker’s caching model for use in CI.

Consider the following Layerfile:

CopiedFROM vm/ubuntu:18.04
COPY . .
RUN sleep 20 && cat file1
RUN sleep 20 && cat file2

In this case, we’ll make snapshots after each line and map which files were read back to the snapshots. This means:

• if you edit any other file than file1 or file2, this entire Layerfile will be skipped.
• if you edit file1, the last two lines will be rerun (40s)
• if you edit file2, only the last line will be rerun (20s)
• if you edit the layerfile, we’ll invalidate the cache at the point of the edit.

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Differences from Docker

Here are the major differences between Layerfiles and Dockerfiles for use in CI:

1. Layerfiles define VMs, not containers - this means you can run anything (including docker) that you could run on a regular cloud server.
2. Running processes are snapshotted and reused. If you start & populate a database, that’ll be included in the layer so that you don’t have to re-run the steps to set up the database for every pipeline.
3. COPY in webapp.io does not invalidate the cache when it runs, instead the files copied are monitored for read/write starting at that point. This means that COPY . . is much more common in Layerfiles than Dockerfiles
4. You can copy files from parent directories (COPY /file1 . or COPY ../.. .) and inherit from other Layerfiles FROM ../../other/Layerfile

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File watching COPY

In most CI providers and in Docker, you need to micromanage cache keys. The following Dockerfile and Layerfile are equivalent because we watch which files are read by each step:

CopiedFROM ubuntu:18.04
COPY package.json package-lock.json ./
RUN npm install
COPY . .
RUN npm run build

CopiedFROM ubuntu:18.04
COPY . .
RUN npm install
RUN npm run build

Instead of micromanaging COPY, you can simply copy the entire repository and we’ll load the bottommost layer from the cache which agrees with a commit’s changes.

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Faster installs: The CACHE directive

Sometimes there are steps which will run repeatedly because their constituent files change often, usually source files. Consider this Layerfile:

CopiedFROM vm/ubuntu:18.04

RUN curl -sS https://dl.yarnpkg.com/debian/pubkey.gpg | sudo apt-key add - && \\
    echo "deb https://dl.yarnpkg.com/debian/ stable main" > /etc/apt/sources.list.d/yarn.list && \\
    curl -fSsL https://deb.nodesource.com/setup_12.x | bash && \\
    apt-get install nodejs yarn

MEMORY 2G
ENV NODE_OPTIONS=--max-old-space-size=8192

COPY package.json ./
CACHE /usr/local/share/.cache/yarn
RUN npm ci

In this case, unless you change package.json, the default webapp.io cache will skip the entire pipeline after every push.

The CACHE directive only acts to speed up the npm ci step in this case.

Note that CACHE will “leak” state across runs, so it might allow one run to break all following ones until someone force-retries without caches. To avoid this problem, only cache stateless directories (which usually contain “cache” in their paths)

Some other examples:

• /var/cache/apt
• /root/.cache/go-build
• ~/.npm ~/.next/cache ~/.yarn/cache