# Docker build cache


When you build the same Docker image multiple times, knowing how to optimize
the build cache is a great tool for making sure the builds run fast.

## How the build cache works

Understanding Docker's build cache helps you write better Dockerfiles that
result in faster builds.

The following example shows a small Dockerfile for a program written in C.

```dockerfile
# syntax=docker/dockerfile:1
FROM ubuntu:latest

RUN apt-get update && apt-get install -y build-essentials
COPY main.c Makefile /src/
WORKDIR /src/
RUN make build
```

Each instruction in this Dockerfile translates to a layer in your final image.
You can think of image layers as a stack, with each layer adding more content
on top of the layers that came before it:

![Image layer diagram](/images/cache-stack.png)

Whenever a layer changes, that layer will need to be re-built. For example,
suppose you make a change to your program in the `main.c` file. After this
change, the `COPY` command will have to run again in order for those changes to
appear in the image. In other words, Docker will invalidate the cache for this
layer.

If a layer changes, all other layers that come after it are also affected. When
the layer with the `COPY` command gets invalidated, all layers that follow will
need to run again, too:

![Image layer diagram, showing cache invalidation](/images/cache-stack-invalidated.png)

And that's the Docker build cache in a nutshell. Once a layer changes, then all
downstream layers need to be rebuilt as well. Even if they wouldn't build
anything differently, they still need to re-run.

## Other resources

For more information on using cache to do efficient builds, see:

- [Cache invalidation](/build/cache/invalidation/)
- [Optimize build cache](/build/cache/optimization/)
- [Garbage collection](/build/cache/garbage-collection/)
- [Cache storage backends](/build/backends/)