Image Server

A high-performance image processing server written in Go. Supports on-demand image resizing, format conversion, and cloud storage integration.

Features

• On-demand image processing - Resize, crop, and convert images via URL parameters
• Multiple output formats - JPEG, WebP, GIF, PNG, HEIC/HEIF (iPhone images)
• Cloud storage - Upload processed images to Amazon S3
• Signed URLs - Secure uploads with HMAC-SHA256 signed URLs (similar to AWS S3 pre-signed URLs)
• Batch processing - Process multiple image sizes in a single request
• Prometheus metrics - Built-in metrics endpoint for monitoring
• Webhooks - Notify external systems when images are uploaded or processed
• Docker support - Ready-to-use Docker image

Quick Start

Using Docker

docker build -t image-server .
docker run -p 7000:7000 -p 7002:7002 image-server

Building from Source

Requires Go 1.21+ and libvips.

# macOS
brew install vips
# Ubuntu/Debian
apt-get install libvips-dev
# Build
go build -o image-server .# Run
./image-server server --port 7000

Server

Uploading Images

Images are uploaded to a namespace. Namespaces group image types (e.g., avatars vs product images may need different sizes).

Upload from URL:

curl -X POST "http://localhost:7000/products?source=https://example.com/image.jpg"

Upload binary data:

curl --data-binary "@./image.jpg" -X POST http://localhost:7000/products

Response:

{
"hash": "6e0072682e66287b662827da75b244a3",
"height": 600,
"width": 800,
"content_type": "image/jpeg"
}

Upload and process immediately:

curl --data-binary "@./image.jpg" -X POST "http://localhost:7000/products?outputs=x300.jpg,x300.webp"

Retrieving Images

Images are accessed via their hash, partitioned into path segments:

GET http://localhost:7000/{namespace}/{hash[0:3]}/{hash[3:6]}/{hash[6:9]}/{hash[9:]}/{dimensions}.{format}

Examples:

# By width (maintains aspect ratio)
GET http://localhost:7000/products/6e0/072/682/e66287b662827da75b244a3/w200.jpg
# Square crop
GET http://localhost:7000/products/6e0/072/682/e66287b662827da75b244a3/x200.jpg
# Specific dimensions (width x height)
GET http://localhost:7000/products/6e0/072/682/e66287b662827da75b244a3/300x200.jpg
# With quality adjustment (1-100)
GET http://localhost:7000/products/6e0/072/682/e66287b662827da75b244a3/x200-q50.jpg
# WebP format
GET http://localhost:7000/products/6e0/072/682/e66287b662827da75b244a3/x200.webp

Image Information

curl http://localhost:7000/products/6e0/072/682/e66287b662827da75b244a3/info.json

Batch Processing

Process multiple sizes for an existing image:

curl -X POST "http://localhost:7000/products/6e0/072/682/e66287b662827da75b244a3/process?outputs=x100.jpg,x200.jpg,x300.webp"

Signed URLs (Authentication)

Secure your image server by requiring signed URLs for uploads (and optionally reads). This works similarly to AWS S3 pre-signed URLs.

Setup

1. Generate a signing secret:

   ./image-server generate-secret > /etc/image-server/secrets.txt

2. Start the server with signature validation:

   ./image-server server \
   --require-signature \
   --signing-secrets-file /etc/image-server/secrets.txt \
   --signature-max-ttl 60

3. Generate signed URLs in your backend application:

   The signature algorithm:

   StringToSign = METHOD + "\n" + PATH + "\n" + EXPIRES_UNIX_TIMESTAMP
   Signature = Base64RawURL(HMAC-SHA256(secret, StringToSign))
   

   URL format:

   POST /namespace?X-Expires=1702156800&X-Path=/namespace&X-Signature=...
   

   Go example:

   import"github.com/image-server/image-server/core/signature"signer:=signature.NewSigner("your-secret", "https://images.example.com")
   url:=signer.SignURL("POST", "/products", 15*time.Minute)

4. Test with the CLI:

   ./image-server sign-url \
   --secret "your-secret" \
   --base-url "https://images.example.com" \
   --method POST \
   --path /products \
   --ttl 15m

Configuration Options

Flag	Description	Default
--require-signature	Enable signature validation for uploads	false
--require-signature-for-reads	Also require signatures for GET requests	false
--signing-secrets-file	Path to file with secrets (one per line)	-
--signature-max-ttl	Maximum allowed TTL in minutes	60

Secret Rotation

The secrets file supports multiple secrets for rotation. Add new secrets to the top of the file:

new-secret-abc123
old-secret-xyz789

The server validates against all secrets, so you can:

1. Add the new secret
2. Update your backend apps to use it
3. Remove the old secret after existing URLs expire

Path-Based Signing

Sign a path prefix to allow uploads to any path under it:

# Sign for entire namespace
./image-server sign-url --secret "..." --path /products --ttl 15m
# Allows: POST /products, POST /products/abc/def/...# Sign for specific path only
./image-server sign-url --secret "..." --path /products/abc/def/ghi/jkl --ttl 15m
# Allows only that exact path

Webhooks

Send HTTP notifications to external systems when images are uploaded or processed. Useful for triggering downstream workflows like OCR, ML pipelines, or cache invalidation.

Setup

./image-server server \
--webhook-url "https://api.example.com/webhooks/images" \
--webhook-secret "$(./image-server generate-secret)" \
--webhook-timeout 10 \
--webhook-events "uploaded,batch_complete"

Events

Event	Trigger	Use Case
uploaded	Original image uploaded to storage	Start processing pipeline
processed	Each image variant processed	Cache warming
failed	Processing error	Alerting
batch_complete	All variants done (or already existed)	Trigger downstream workflow

By default, all events are sent. Use --webhook-events to filter.

Payload Format

{
"event": "image.uploaded",
"timestamp": "2025-12-09T15:30:00Z",
"data": {
"namespace": "products",
"hash": "6e0072682e66287b662827da75b244a3",
"width": 1920,
"height": 1080,
"content_type": "image/jpeg",
"remote_url": "https://cdn.example.com/products/6e0/072/.../original"
}
}

For image.processed:

{
"event": "image.processed",
"timestamp": "2025-12-09T15:30:01Z",
"data": {
"namespace": "products",
"hash": "6e0072682e66287b662827da75b244a3",
"filename": "x300.webp",
"format": "webp",
"width": 300,
"height": 169,
"quality": 75,
"remote_url": "https://cdn.example.com/products/6e0/072/.../x300.webp"
}
}

Security

Webhooks are signed with HMAC-SHA256. Verify the signature in your handler:

Headers:

X-Webhook-Signature: sha256=<hex-encoded-hmac>
X-Webhook-Timestamp: 1702135800

Verification (Python):

importhmacimporthashlibdefverify_webhook(secret: str, timestamp: str, body: bytes, signature: str) ->bool:
expected="sha256="+hmac.new(
secret.encode(),
f"{timestamp}.{body.decode()}".encode(),
hashlib.sha256
).hexdigest()
returnhmac.compare_digest(expected, signature)
# In your handler:ifnotverify_webhook(SECRET, request.headers["X-Webhook-Timestamp"],
request.body, request.headers["X-Webhook-Signature"]):
return401

Verification (Go):

funcverifyWebhook(secret, timestampstring, body []byte, signaturestring) bool {
h:=hmac.New(sha256.New, []byte(secret))
h.Write([]byte(fmt.Sprintf("%s.%s", timestamp, string(body))))
expected:="sha256="+hex.EncodeToString(h.Sum(nil))
returnhmac.Equal([]byte(expected), []byte(signature))
}

Configuration

Flag	Description	Default
--webhook-url	Endpoint URL (enables webhooks)	-
--webhook-secret	HMAC signing secret	-
--webhook-timeout	HTTP timeout in seconds	10
--webhook-events	Events to send (comma-separated)	all

Reliability

• Webhooks are sent asynchronously (non-blocking)
• Failed deliveries retry up to 3 times with exponential backoff (1s, 4s)
• Webhook failures don't affect image processing

Cloud Storage (S3)

./image-server server \
--uploader s3 \
--aws_access_key_id $AWS_ACCESS_KEY_ID \
--aws_secret_key $AWS_SECRET_KEY \
--aws_bucket $AWS_BUCKET \
--aws_region us-west-1 \
--remote_base_path "images/" \
--remote_base_url "https://cdn.example.com"

Server Configuration

Flag	Description	Default
--port	Server port	7000
--listen	Listen address	127.0.0.1
--local_base_path	Local image storage directory	public
--extensions	Allowed file extensions	jpg,gif,webp
--maximum_width	Maximum output width	1000
--default_quality	Default JPEG/WebP quality	75
--outputs	Default output formats	-
--uploader	Storage backend (s3 or noop)	auto
--uploader_concurrency	Parallel upload workers	10
--processor_concurrency	Parallel processing workers	4
--http_timeout	HTTP request timeout (seconds)	5
--max_file_age	Local file cleanup age (minutes)	30

Admin Server

A separate admin server runs on port 7002 with health and metrics endpoints:

Endpoint	Description
/probe/ready	Readiness check
/probe/live	Liveness check
/metrics	Prometheus metrics

CLI Commands

Process images locally

./image-server cli /path/to/images --outputs "x300.jpg,x300.webp"

Generate signing secret

./image-server generate-secret
./image-server generate-secret --length 64 --count 3

Generate signed URL

./image-server sign-url --secret "..." --path /namespace --ttl 15m

Version

./image-server version

Monitoring

Prometheus Metrics

Available at http://localhost:7002/metrics

Statsd

./image-server server --enable_statsd --statsd_host 127.0.0.1 --statsd_port 8125

Events:

• image_server.image_request - Image processed and uploaded
• image_server.image_request.{format} - By format (jpg, webp, etc.)
• image_server.image_request_fail - Processing failed
• image_server.original_downloaded - Original fetched from source
• image_server.original_unavailable - Original not found (404)

Profiling

./image-server server --profile
# pprof available at http://localhost:6060

Development

Running locally

# Without S3
make dev-server
# With S3export AWS_ACCESS_KEY_ID=...
export AWS_SECRET_KEY=...
export AWS_BUCKET=...
export AWS_REGION=...
export IMG_REMOTE_BASE_PATH=...
export IMG_REMOTE_BASE_URL=...
make dev-server-s3

Tests

make test# or
go test ./...

Building

make build
# Creates binaries in bin/ for multiple platforms

Error Handling

Status	Description
401	Invalid or missing signature (when signatures required)
404	Image not found
400	Invalid request parameters

License

MIT