The Container Image Registry: How We Store and Distribute Images

The Container Image Registry: How We Store and Distribute Images

Every time you deploy on Deployxa, the platform builds a container image, stores it in the image registry, and distributes it to the container runtime. The image registry is the backbone of the deployment system: it stores every version of your app, enables fast rollbacks (by pulling a previous image), and shares build layers across deployments (for faster builds). Here is how the image registry works and why it matters.

The direct answer is that Deployxa's container image registry is a private registry that stores Docker images for every deployment. Each deployment produces a new image (tagged with the Git commit hash), which is stored in the registry. When you roll back, the platform pulls the previous image from the registry, which is fast (because the image is already stored). The registry also shares layers across deployments, which means subsequent builds are faster (because unchanged layers are reused). For more on the deployment process, see our article on how we built the CI/CD pipeline.

What the Image Registry Does

The container image registry performs the following functions:

1. Store images

Every deployment produces a Docker image, which is stored in the registry. The image is tagged with the Git commit hash (e.g., my-app:abc123), which means every version is uniquely identifiable.

2. Share layers

Docker images are built in layers (e.g., base image, dependencies, source code, build output). The registry stores layers separately, which means unchanged layers are shared across images. This makes subsequent builds faster (because unchanged layers are reused) and reduces storage (because unchanged layers are stored once).

3. Enable fast rollbacks

When you roll back, the platform pulls the previous image from the registry. Because the image is already stored (and likely cached on the host), the rollback is fast (seconds, not minutes).

4. Garbage collect old images

The registry periodically garbage collects old images (e.g., images older than 30 days), to prevent unlimited storage growth.

Step-by-Step: How an Image Is Built, Stored, and Distributed

Step 1: Build

The build stage compiles your code and produces a Docker image (via the zero-config engine's internal Dockerfile). The image is built in layers: base image, dependencies, source code, build output.

Step 2: Push to registry

The built image is pushed to the registry, tagged with the Git commit hash. The registry stores the image's layers, reusing layers that are already stored (from previous builds).

Step 3: Pull to host

The container runtime pulls the image from the registry to the host where the container will run. Because layers are cached on the host (from previous pulls), only changed layers are downloaded.

Step 4: Start container

The container runtime starts the container from the pulled image. The container is now running.

Step 5: Rollback (if needed)

If you roll back, the platform pulls the previous image from the registry. Because the image is already stored (and likely cached on the host), the rollback is fast.

Common Pitfalls and Troubleshooting

The first pitfall is large images. If the image is large (e.g., 1GB+), the pull is slow, which delays deployments. The fix is to use multi-stage builds (which produce smaller images) and to use minimal base images (e.g., node:20-alpine instead of node:20). The second pitfall is not garbage collecting old images. If old images are not garbage collected, the registry's storage grows indefinitely. The fix is to set a retention period (e.g., 30 days) and to garbage collect old images. The third pitfall is layer caching not working. If the Dockerfile is not optimized (e.g., COPY . . before RUN npm install), the layer cache is invalidated on every build, which makes builds slow. The fix is to order Dockerfile instructions from least frequently changed (base image, dependencies) to most frequently changed (source code). The fourth pitfall is registry downtime. If the registry is down, images cannot be pushed or pulled, which blocks deployments and rollbacks. The fix is to make the registry highly available (e.g., via replication). The fifth pitfall is security. If the registry is not secured, unauthorized users might pull or push images. The fix is to secure the registry (e.g., via authentication and encryption).

Advanced Image Registry Patterns

Beyond the basics, the container image registry supports several advanced patterns. The first is multi-architecture images. For apps that need to run on multiple architectures (e.g., x86_64 for production, ARM64 for development on Apple Silicon), the registry can store multi-architecture images (via Docker manifest lists). This means a single image tag (e.g., my-app:latest) works on both architectures, with the registry automatically serving the correct architecture-specific image.

The second pattern is image signing. For security-sensitive apps, images can be signed (via tools like cosign or Docker Content Trust), which means the container runtime can verify the image's authenticity before running it. This prevents man-in-the-middle attacks where an attacker substitutes a malicious image for the legitimate one.

The third pattern is image scanning. Images can be scanned for vulnerabilities (via tools like Trivy, Snyk Container, or Clair) before they are deployed. If a critical vulnerability is found, the deployment can be blocked until the vulnerability is fixed. For more on security scanning, see our article on the agentic security scanning pipeline.

The fourth pattern is image replication. For apps that need to run in multiple regions, the registry can replicate images to regional registries, which means the image is always close to the container runtime. This reduces the pull time for cross-region deployments. For more on multi-region deployment, see our article on Deployxa vs Fly.io.

The fifth pattern is image lifecycle management. The registry can automatically garbage collect old images (e.g., images older than 30 days, or images from deleted branches), which prevents unlimited storage growth. The lifecycle policy can be configured per app (e.g., keep the last 10 deployments, delete older ones).

How the Registry Integrates with the Rollback System

The image registry is the backbone of the rollback system. When you roll back, the platform pulls the previous image from the registry and starts a new container from it. Because the image is already stored in the registry (and likely cached on the host), the rollback is fast (seconds, not minutes). Without the registry, rollbacks would require rebuilding the previous version from source, which would take minutes and might fail (if the source code has changed). With the registry, rollbacks are instant and reliable, because the exact image that was running before is available. For more on rollbacks, see our article on Traefik v3 dynamic routing and the agentic blue/green deployment pipeline.

Lessons Learned

Building the image registry taught us several lessons. First, layer caching is the biggest performance win. By sharing layers across images (e.g., the base image layer, the dependencies layer), subsequent builds and pulls are dramatically faster. Without layer caching, every build would download the full image, which would be slow. Second, garbage collection is essential. Without garbage collection, the registry's storage grows indefinitely, which is expensive and slow. With garbage collection, old images are automatically deleted, which keeps the storage lean. Third, image security is critical. Images contain your app's code and dependencies, which means a compromised image can compromise your app. Signing and scanning images prevents this. Fourth, the registry needs to be highly available. If the registry is down, deployments and rollbacks are blocked, which means the registry is a single point of failure. The fix is to make the registry highly available (via replication). Fifth, the registry needs to be fast. If pulling an image is slow, deployments and rollbacks are slow. The fix is to use a registry that is close to the container runtime (same data center) and to cache images on the host. For more on Deployxa's engineering, see our articles on how we built the CI/CD pipeline and the build cache architecture.

Conclusion: A Fast, Reliable Image Registry

Deployxa's container image registry stores every version of your app, enables fast rollbacks, and shares layers for faster builds. By understanding how the registry works, you can optimize your builds (via layer caching) and your rollbacks (via stored images). For more on Deployxa's engineering, see our articles on how we built the CI/CD pipeline and the build cache architecture. Learn about how we handle DDoS protection and the Git integration system in our companion articles. Explore our free developer tools. Try Deployxa Drop for an instant live preview.