> ## Documentation Index
> Fetch the complete documentation index at: https://notes.kodekloud.com/llms.txt
> Use this file to discover all available pages before exploring further.

# Build Cache

> Efficient use of Docker’s build cache can speed up image builds by reusing unchanged layers.

Efficient use of Docker’s build cache can dramatically speed up your image builds. Docker creates a cache layer for each instruction in your Dockerfile. When you rebuild the image, Docker reuses layers whose instructions and contexts haven’t changed, avoiding redundant work.

## How Docker’s Build Cache Works

```dockerfile theme={null}
FROM ubuntu
RUN apt-get update
RUN apt-get install -y python python3-pip
RUN pip3 install flask
COPY app.py /opt/source-code
ENTRYPOINT ["flask", "run"]
```

* Each `RUN`, `COPY`, or `ADD` instruction produces a layer.
* After a successful build, layers are stored in the local cache.
* On subsequent builds, Docker compares:
  1. The instruction itself.
  2. Any files referenced by `COPY`/`ADD`.
* If both match the cached layer, Docker reuses it.
* Any change invalidates that layer **and all subsequent layers**, triggering a rebuild from that point.

### Cache Invalidation Example

Changing the pip install command:

```dockerfile theme={null}
RUN pip3 install flask flask-mysql
```

Invalidates the `pip3 install` layer and everything that follows—earlier layers remain cached. Similarly, updating `app.py` in:

```dockerfile theme={null}
COPY app.py /opt/source-code
```

busts the cache from that layer onward.

***

## Cache Busting with Combined Instructions

Separating `apt-get update` and `apt-get install` can lead to stale package lists:

```dockerfile theme={null}
RUN apt-get update
RUN apt-get install -y python python3-pip python-dev
```

<Callout icon="triangle-alert" color="#FF6B6B">
  Stale package lists may cause installation of outdated or missing packages.
</Callout>

Instead, combine them:

```dockerfile theme={null}
RUN apt-get update && \
    apt-get install -y \
      python \
      python-dev \
      python3-pip
```

* Forces an update immediately before installation.
* Lists packages alphabetically and on separate lines for readability.

<Callout icon="lightbulb" color="#1CB2FE">
  Always include `&& rm -rf /var/lib/apt/lists/*` if you want to reduce image size.
</Callout>

### Version Pinning

Pinning package versions ensures consistent builds across environments:

```dockerfile theme={null}
RUN apt-get update && \
    apt-get install -y \
      python \
      python-dev \
      python3-pip=20.0.2
```

***

## Optimizing Instruction Order

Place instructions that change least frequently at the top of your Dockerfile. This maximizes cache reuse.

| Instruction Type         | Change Frequency | Caching Benefit                          |
| ------------------------ | ---------------- | ---------------------------------------- |
| Base Image & System      | Low              | Cached once unless you change the base   |
| Package Installation     | Low–Medium       | Reused until you add or remove packages  |
| Application Dependencies | Medium           | Rebuilt when dependencies change         |
| Application Code         | High             | Only this layer rebuilds on code changes |

### Example: Optimal Order

```dockerfile theme={null}
FROM ubuntu

# 1. Infrequently changed – cached once
RUN apt-get update && \
    apt-get install -y \
      python \
      python-dev \
      python3-pip=20.0.2

# 2. Dependencies – rebuild when you add/remove libs
RUN pip3 install flask flask-mysql

# 3. Code – fastest iteration on changes
COPY app.py /opt/source-code

ENTRYPOINT ["flask", "run"]
```

By contrast, placing `COPY app.py` first forces Docker to rerun all subsequent layers on every code update, significantly slowing builds.

***

## Further Reading

* [Dockerfile Best Practices](https://docs.docker.com/develop/develop-images/dockerfile_best-practices/)
* [Docker Build Cache Documentation](https://docs.docker.com/engine/reference/commandline/build/#use-build-cache)

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