Building a Pull-Through Cache Registry for Kubernetes Workloads: A Game-Changer for Performance and Security

If you’ve ever run a Kubernetes workload that pulls container images from external registries, you know the pain points: network latency, unnecessary costs, and security risks. But what if you could eliminate these issues with a smart, on-demand solution?

Enter the Pull-Through Cache Registry! 🎉

In this post, I’ll walk you through how to create a pull-through cache registry for your Kubernetes workloads — boosting performance, slashing costs, and securing your images.

Ready? Let’s dive in!

Why Bother with a Pull-Through Cache Registry?

Before we get into the techy details, let's first understand why you need one:

• Reduced Latency: No more waiting for images to pull across multiple network hops. A local registry within your VPC (Virtual Private Cloud) means faster pod startups.

• Cost Savings: Cross-network traffic costs money. By caching images locally, you avoid unnecessary external network calls, saving $$$ — especially when you're in the cloud!

• Security: External registries often have unknown risks. A local pull-through cache allows you to enforce security scans and ensure only trusted images run on your cluster.

Sounds good? Let’s move on.

---

🤖 What Exactly Is a Pull-Through Cache Registry?

Traditional Local Registry:

• You pull images ahead of time.

• You manage the images and run security scans.

• You store the images in your local registry.

Pull-Through Cache Registry:

• Images are pulled on-demand as needed (at pod creation).

• The image is pulled from an external registry, cached locally, scanned for security, and then deployed.

• For subsequent pod deployments, the image is pulled from your local cache, reducing latency and avoiding redundant downloads.

TL;DR: The pull-through cache does all the heavy lifting automatically, on the fly! 🔥

---

🛠️ Setting Up Your Pull-Through Cache Registry

You have two main options:

1. Managed Registry (Cloud Provider):

   • Providers like AWS offer pull-through caching, but it’s not universal (and might not cover every external registry).

2. DIY with Kubernetes:

   • You can create your own pull-through cache registry. This is super flexible and lets you control exactly which registries are used.

Here's how it works:

• Use Kubernetes Mutating Webhook: A webhook intercepts pod creation, checks the image spec, pulls the image if it's not cached locally, and then proceeds with pod creation.

• Security Scanning: Only trusted, scanned images are allowed to run.

---

📜 High-Level Workflow

1. Request Pod Creation: When a new pod is created, the Kubernetes API server triggers the mutating webhook.

2. Image Validation: The webhook checks if the requested image is available in the local cache.

   • If not available, it pulls the image from the external registry, stores it locally, and runs security scans.

   • If already cached, it skips the pull and proceeds directly.

3. Pod Deployment: Once validated and the image is available locally, the pod is deployed.

🧑‍💻 Let’s Build This: The Tech Stack

We’ll create the pull-through cache registry using Golang and Kubernetes.

---

1. Mutating Webhook Server in Go 🖥️

You'll need a mutating webhook that listens for pod creation requests. Here's how to do it:

• Golang + HTTPS Server: The server listens for AdmissionReview requests from the K8s API server.

• Image Handling: The server checks if the image is available in the local registry (e.g., AWS ECR).

  • If not, it pulls from the external registry (Docker Hub, Quay.io, etc.), tags the image, and pushes it to the local registry.

Code Snippet (Go Webhook Server):

func PatchPod(ar *admissionv1.AdmissionReview) *admissionv1.AdmissionResponse {
    // Extract image spec from pod
    image := extractImageFromPod(ar)
    
    // Check if image exists in local registry
    if !checkImageInLocalRegistry(image) {
        pullAndCacheImage(image)
    }

    // Respond back with the patched pod
    return &admissionv1.AdmissionResponse{
        Allowed: true,
        Patch:   patchPodImage(image),
    }
}

You can find the full code here on GitHub.

---

2. Kubernetes Mutating Webhook Configuration 🛠️

Once your webhook server is up and running, configure Kubernetes to call the webhook for every pod creation.

MutatingWebhookConfiguration Example:

apiVersion: admissionregistration.k8s.io/v1
kind: MutatingWebhookConfiguration
metadata:
  name: pullthroughcache-webhook
webhooks:
  - name: pullthroughcache.k8s.local
    clientConfig:
      service:
        name: pullthroughcache-service
        namespace: default
        path: "/mutate"
      caBundle: <CA-Bundle>
    rules:
      - operations: ["CREATE"]
        apiGroups: ["", "apps"]
        apiVersions: ["v1"]
        resources: ["pods"]
    admissionReviewVersions: ["v1"]
    sideEffects: None

This configuration ensures that every time a new pod is created, it triggers your webhook server.

---

3. Docker-in-Docker (DinD) for Image Handling 🐳

Since you’ll be interacting with Docker (pulling and pushing images), you’ll need Docker-in-Docker (DinD). Here’s a Dockerfile template for your pull-through cache server:

FROM docker:dind
RUN apk add --no-cache go git
COPY ./pullthroughcache /app
WORKDIR /app
CMD ["go", "run", "main.go"]

Also, configure your CI pipeline (GitLab, GitHub Actions, etc.) to build and push this Docker image to your repository.

---

📣 Important Considerations

• Security Scanning: Make sure your local registry is set up with repository scanning to reject untrusted or insecure images.

• Scaling: Ensure your webhook server and local registry are highly available and scaled based on traffic.

• External Registries: You can add custom logic to support multiple external registries, making the pull-through cache more flexible.

---

🌟 Wrapping It Up

Building a pull-through cache registry for your Kubernetes workloads gives you a powerful way to enhance performance, reduce costs, and strengthen security. With this setup, you're not just caching images; you're optimizing your Kubernetes operations and automating crucial aspects of container management.

Pro Tip: This setup works wonders when you need to speed up deployments or secure your workloads against untrusted images. Plus, it gives you full control over what runs in your cluster.

So, what are you waiting for? Go ahead and build your own pull-through cache registry and enjoy the benefits! 🚀