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Version: 1.0.0

Tutorial: Learn the Blobber Library

In this tutorial, we'll learn to use blobber as a Go library by building a simple configuration management tool.

By the end, you'll know how to:

  • Create a blobber client
  • Push files to a registry
  • List files in remote images
  • Open and read individual files
  • Pull complete images

Prerequisites​

  • Go 1.21+
  • Access to an OCI registry
  • Docker credentials configured

What We're Building​

We'll create a small program that:

  1. Pushes configuration files to a registry
  2. Lists what's available
  3. Reads specific files on-demand
  4. Downloads everything when needed

Step 1: Create a New Project​

Set up a Go module:

mkdir blobber-example
cd blobber-example
go mod init blobber-example
go get github.com/meigma/blobber@latest

Create main.go:

package main

import (
	"context"
	"fmt"
	"io"
	"log"
	"os"
	"testing/fstest"

	"github.com/meigma/blobber"
)

func main() {
	if err := run(); err != nil {
		log.Fatal(err)
	}
}

func run() error {
	ctx := context.Background()

	// We'll fill this in step by step
	_ = ctx
	return nil
}

Step 2: Create a Client​

Add client creation to the run function:

func run() error {
	ctx := context.Background()

	// Create a client with default settings
	client, err := blobber.NewClient()
	if err != nil {
		return fmt.Errorf("creating client: %w", err)
	}

	fmt.Println("Client created successfully")
	_ = ctx
	return nil
}

Run it:

go run main.go

You should see:

Client created successfully

Step 3: Push Files​

Let's push some in-memory files. Add this after creating the client:

func run() error {
	ctx := context.Background()

	client, err := blobber.NewClient()
	if err != nil {
		return fmt.Errorf("creating client: %w", err)
	}

	// Create an in-memory filesystem
	files := fstest.MapFS{
		"app.yaml": &fstest.MapFile{
			Data: []byte("name: myapp\nversion: 1.0.0\n"),
		},
		"database.yaml": &fstest.MapFile{
			Data: []byte("host: localhost\nport: 5432\n"),
		},
	}

	// Push to registry (replace with your registry)
	ref := "ghcr.io/YOUR_USERNAME/example-config:v1"
	digest, err := client.Push(ctx, ref, files)
	if err != nil {
		return fmt.Errorf("pushing: %w", err)
	}

	fmt.Printf("Pushed: %s\n", digest)
	return nil
}

Run it:

go run main.go

You should see:

Pushed: sha256:abc123...

Step 4: Open and List Files​

Now let's read what we pushed. Replace the run function:

func run() error {
	ctx := context.Background()

	client, err := blobber.NewClient()
	if err != nil {
		return fmt.Errorf("creating client: %w", err)
	}

	ref := "ghcr.io/YOUR_USERNAME/example-config:v1"

	// Open the image for reading
	img, err := client.OpenImage(ctx, ref)
	if err != nil {
		return fmt.Errorf("opening image: %w", err)
	}
	defer img.Close()

	// List all files
	entries, err := img.List()
	if err != nil {
		return fmt.Errorf("listing: %w", err)
	}

	fmt.Println("Files in image:")
	for _, entry := range entries {
		fmt.Printf("  %s (%d bytes)\n", entry.Path(), entry.Size())
	}

	return nil
}

Run it:

go run main.go

You should see:

Files in image:
  app.yaml (26 bytes)
  database.yaml (28 bytes)

Step 5: Read a Specific File​

Let's read one file without downloading everything. Add this before the return nil:

	// Open a specific file
	rc, err := img.Open("app.yaml")
	if err != nil {
		return fmt.Errorf("opening file: %w", err)
	}
	defer rc.Close()

	content, err := io.ReadAll(rc)
	if err != nil {
		return fmt.Errorf("reading file: %w", err)
	}

	fmt.Printf("\nContents of app.yaml:\n%s", content)

Run it:

go run main.go

You should see:

Files in image:
  app.yaml (26 bytes)
  database.yaml (28 bytes)

Contents of app.yaml:
name: myapp
version: 1.0.0

Step 6: Walk All Files​

The Walk method lets you process files with a callback. Replace the list logic:

	// Walk all files
	fmt.Println("Walking files:")
	err = img.Walk(func(path string, d fs.DirEntry, err error) error {
		if err != nil {
			return err
		}
		if !d.IsDir() {
			info, _ := d.Info()
			fmt.Printf("  %s (%d bytes)\n", path, info.Size())
		}
		return nil
	})
	if err != nil {
		return fmt.Errorf("walking: %w", err)
	}

Note: You'll need to add "io/fs" to your imports.

Step 7: Pull Everything​

To download all files to disk, use the client's Pull method:

	// Pull to local directory
	err = client.Pull(ctx, ref, "./downloaded")
	if err != nil {
		return fmt.Errorf("pulling: %w", err)
	}

	fmt.Println("\nPulled to ./downloaded")

Run it, then check the directory:

go run main.go
ls ./downloaded

Step 8: Handle Errors​

Blobber provides sentinel errors for common cases. Add error handling:

import "errors"

// ...

	img, err := client.OpenImage(ctx, ref)
	if errors.Is(err, blobber.ErrNotFound) {
		return fmt.Errorf("image does not exist: %s", ref)
	}
	if errors.Is(err, blobber.ErrUnauthorized) {
		return fmt.Errorf("authentication failed for: %s", ref)
	}
	if err != nil {
		return fmt.Errorf("opening image: %w", err)
	}

Complete Example​

Here's the full program:

package main

import (
	"context"
	"errors"
	"fmt"
	"io"
	"io/fs"
	"log"
	"testing/fstest"

	"github.com/meigma/blobber"
)

func main() {
	if err := run(); err != nil {
		log.Fatal(err)
	}
}

func run() error {
	ctx := context.Background()

	client, err := blobber.NewClient()
	if err != nil {
		return fmt.Errorf("creating client: %w", err)
	}

	// Create test files
	files := fstest.MapFS{
		"app.yaml": &fstest.MapFile{
			Data: []byte("name: myapp\nversion: 1.0.0\n"),
		},
		"database.yaml": &fstest.MapFile{
			Data: []byte("host: localhost\nport: 5432\n"),
		},
	}

	ref := "ghcr.io/YOUR_USERNAME/example-config:v1"

	// Push
	digest, err := client.Push(ctx, ref, files)
	if err != nil {
		return fmt.Errorf("pushing: %w", err)
	}
	fmt.Printf("Pushed: %s\n\n", digest)

	// Open image
	img, err := client.OpenImage(ctx, ref)
	if errors.Is(err, blobber.ErrNotFound) {
		return fmt.Errorf("image not found: %s", ref)
	}
	if err != nil {
		return fmt.Errorf("opening: %w", err)
	}
	defer img.Close()

	// Walk files
	fmt.Println("Files:")
	err = img.Walk(func(path string, d fs.DirEntry, err error) error {
		if err != nil {
			return err
		}
		if !d.IsDir() {
			info, _ := d.Info()
			fmt.Printf("  %s (%d bytes)\n", path, info.Size())
		}
		return nil
	})
	if err != nil {
		return fmt.Errorf("walking: %w", err)
	}

	// Read one file
	rc, err := img.Open("app.yaml")
	if err != nil {
		return fmt.Errorf("opening file: %w", err)
	}
	defer rc.Close()

	content, err := io.ReadAll(rc)
	if err != nil {
		return fmt.Errorf("reading: %w", err)
	}
	fmt.Printf("\napp.yaml:\n%s", content)

	return nil
}

What We Learned​

  • blobber.NewClient() creates a client with default settings
  • client.Push(ctx, ref, fs) uploads an fs.FS to a registry
  • client.OpenImage(ctx, ref) opens an image for reading
  • img.List() returns file entries
  • img.Open(path) returns a reader for one file
  • img.Walk(fn) walks all files
  • client.Pull(ctx, ref, dir) downloads everything
  • Sentinel errors like ErrNotFound enable precise error handling

Next Steps​