Note: This library is under active development and the API may change.

AshScylla

An Ash Framework data layer for ScyllaDB/Apache Cassandra

Quick Start • Features • Documentation • Contributing • License

Overview

AshScylla enables you to use ScyllaDB or Apache Cassandra as a persistence layer for your Ash Framework resources. It implements the Ash.DataLayer behaviour using Xandra (a native Elixir CQL driver) to communicate via CQL (Cassandra Query Language).

Key Benefits

• Seamless Ash Integration: Use familiar Ash resources, actions, and queries
• ScyllaDB Performance: Leverage ScyllaDB's high-performance, low-latency architecture
• Cassandra Compatibility: Works with Apache Cassandra and ScyllaDB
• Rich Feature Set: TTL, consistency levels, secondary indexes, materialized views, batch operations

Quick Start

Prerequisites

• Elixir 1.17+
• Running ScyllaDB or Cassandra instance
• Basic knowledge of Ash Framework

Installation

Add ash_scylla to your dependencies in mix.exs:

def deps do
  [
    {:ash_scylla, "~> 0.6.0"}
  ]
end

Minimal Setup

1. Configure a Repo:

# lib/my_app/repo.ex
defmodule MyApp.Repo do
  use AshScylla.Repo,
    otp_app: :my_app
end

2. Configure the Repo in config/config.exs:

config :my_app, MyApp.Repo,
  nodes: ["127.0.0.1:9042"],
  keyspace: "my_app_dev",
  pool_size: 10

3. Add the Repo to your supervision tree:

# lib/my_app/application.ex
children = [
  MyApp.Repo,
  # ...
]

4. Generate a Resource:

mix ash_scylla.gen User name:string, email:string

This creates lib/my_app/resources/user.ex with a starter template. Or define it manually:

# lib/my_app/resources/user.ex
defmodule MyApp.User do
  use Ash.Resource,
    data_layer: AshScylla.DataLayer,
    repo: MyApp.Repo

  attributes do
    uuid_primary_key :id
    attribute :name, :string
    attribute :email, :string
  end

  actions do
    defaults [:create, :read, :update, :destroy]
  end
end

5. Create a Domain:

# lib/my_app/domain.ex
defmodule MyApp.Domain do
  use Ash.Domain

  resources do
    resource MyApp.User
  end
end

6. Create Keyspace and Tables:

# Create keyspace (using the mix task)
mix ash_scylla.setup

# Or programmatically
MyApp.Repo.create_keyspace()

# Run migrations
AshScylla.Migrator.run!(MyApp.Repo.nodes(), [
  AshScylla.Migration.create_table_cql(MyApp.User),
  "CREATE INDEX IF NOT EXISTS idx_users_email ON users (email)"
])

7. Start Using It:

# Create
{:ok, user} = Ash.create(MyApp.User, %{name: "John", email: "john@example.com"})

# Read
users = MyApp.User
  |> Ash.Query.filter(email == "john@example.com")
  |> Ash.read!()

# Update
{:ok, updated} = user
  |> Ash.Changeset.for_update(:update, %{name: "John Doe"})
  |> Ash.update()

# Delete
:ok = Ash.destroy(user)

Or using the domain directly:

# Create via domain
{:ok, user} = MyApp.Domain.create_user(%{name: "John", email: "john@example.com"})

# Read via domain
users = MyApp.Domain.read_users!()

Features

Core Ash Features ✅

ScyllaDB-Specific Features 🚀

TTL (Time To Live)

Automatically expire data after a specified time:

defmodule MyApp.Session do
  use Ash.Resource,
    data_layer: AshScylla.DataLayer

  ash_scylla do
    ttl 3600  # Expire after 1 hour
  end
end

Consistency Levels

Configure read/write consistency per resource:

ash_scylla do
  consistency :quorum  # :any, :one, :two, :three, :quorum, :all, :local_quorum
end

Secondary Indexes

Query non-primary key columns efficiently:

ash_scylla do
  secondary_index :email          # Single column
  secondary_index [:name, :age]   # Composite index
end

Materialized Views

Create alternative query patterns with automatic view maintenance:

ash_scylla do
  materialized_view :users_by_email,
    primary_key: [:email, :id],
    include_columns: [:name, :age]
end

Batch Operations

Reduce network round-trips with BATCH statements:

# Bulk create (uses BATCH internally)
{:ok, users} = user_data_list
  |> Ash.bulk_create(MyApp.User, :create)

# Async partition-aware batching for large datasets
AshScylla.DataLayer.Batch.batch_insert_async(repo, statements, resource: MyApp.User, max_concurrency: 8)

Token-Based Pagination

Efficient pagination without OFFSET:

ash_scylla do
  pagination :token  # Use token-based pagination instead of OFFSET
end

Per-Action Consistency

Configure consistency levels per action:

ash_scylla do
  consistency :quorum              # Default consistency
  per_action_consistency read: :one, create: :quorum  # Per-action overrides
end

Data Modeling Best Practices

ScyllaDB is a wide-column store optimized for specific query patterns. Follow these principles:

1. Query-First Design 🎯

Design your tables around your queries, not the other way around:

# Good: Partition key supports your main query
defmodule MyApp.User do
  attributes do
    attribute :email, :string, primary_key?: true  # Partition key
    attribute :name, :string
  end
end

# Query by partition key (efficient)
MyApp.User
  |> Ash.Query.filter(email == "user@example.com")
  |> Ash.read_one()

2. Denormalization is Normal 📦

Duplicate data across tables to support different query patterns:

# Table for querying posts by author
defmodule MyApp.PostByAuthor do
  attributes do
    attribute :author_id, :uuid, primary_key?: true
    attribute :post_id, :uuid, primary_key?: true
    attribute :title, :string
    attribute :content, :string
  end
end

# Table for querying posts by date
defmodule MyApp.PostByDate do
  attributes do
    attribute :date, :date, primary_key?: true
    attribute :post_id, :uuid, primary_key?: true
    attribute :title, :string
    attribute :author_name, :string  # Denormalized
  end
end

3. Choose Partition Keys Wisely 🔑

• High cardinality: Distribute data evenly across nodes
• Query patterns: Support your most common queries
• Avoid hotspots: Don't use low-cardinality partition keys

# Good: User ID has high cardinality
attribute :user_id, :uuid, primary_key?: true

# Avoid: Status has low cardinality (creates hotspots)
attribute :status, :string, primary_key?: true  # Don't do this

Configuration

Resource Configuration

defmodule MyApp.User do
  use Ash.Resource,
    data_layer: AshScylla.DataLayer

  ash_scylla do
    table "users"                    # Override table name
    keyspace "custom_keyspace"        # Override keyspace
    consistency :quorum               # Consistency level
    ttl 3600                          # Default TTL (seconds)

    # Secondary indexes
    secondary_index :email
    secondary_index [:name, :age]

    # Materialized views
    materialized_view :users_by_email,
      primary_key: [:email, :id],
      include_columns: [:name, :age]
  end
end

Repo Configuration

config :my_app, MyApp.Repo,
  nodes: ["scylla-1:9042", "scylla-2:9042"],  # Cluster nodes
  keyspace: "my_app_prod",
  pool_size: 50,                                # Connections per node
  request_timeout: 300_000,                     # Query timeout (ms)
  connect_timeout: 10_000

Pool Size Guidelines:

• Development: 5-10
• Production: 25-100 (based on concurrent queries)

ScyllaDB works best with a connections-per-shard approach: pool_size = num_nodes * num_cores_per_node

Limitations

Since ScyllaDB/Cassandra is a NoSQL wide-column store, some features are not supported:

Observability

Telemetry

AshScylla emits standard :telemetry events for all query and batch operations, enabling integration with LiveDashboard, Datadog, OpenTelemetry, and other observability tools.

Query events:

• [:ash_scylla, :query, :start] - Query begins execution
• [:ash_scylla, :query, :stop] - Query finishes successfully
• [:ash_scylla, :query, :exception] - Query raises an error

Batch events:

• [:ash_scylla, :batch, :start] - Batch operation begins
• [:ash_scylla, :batch, :stop] - Batch operation finishes

Attaching a handler:

:telemetry.attach(
  "ash_scylla-logger",
  [:ash_scylla, :query, :stop],
  &MyApp.Telemetry.handle_event/4,
  nil
)

Prepared Statement Caching

For high-throughput workloads, enable the prepared statement cache to eliminate repeated query parsing overhead on ScyllaDB:

# In your supervision tree
children = [
  AshScylla.PreparedStatementCache,
  # ... other children
]

Documentation

For detailed documentation, see:

• Usage Guide - Comprehensive guide with examples
• Development Guide - Dev container setup and development workflow
• Production Guide - Multi-node cluster deployment and operations
• Implementation Summary - Technical details
• Error Handling - Error types and handling strategies
• API Documentation - Module documentation (when published)

Quick Links

• Secondary Indexes
• Materialized Views
• Batch Operations
• Consistency Levels
• TTL Support
• Performance Optimization

Testing

Run the test suite:

# All tests (unit + integration; requires Podman/Docker for testcontainers)
mix test

# Unit tests only (no ScyllaDB required)
mix test --exclude integration

# Integration tests only (requires Podman/Docker)
mix test test/scylla_integration_test.exs --only integration

# CI pipeline (unit tests + credo)
mix test.ci

Test Structure

Integration tests use testcontainer_ex to spin up a ScyllaDB instance automatically via Podman or Docker.

Contributing

Contributions are welcome! Here's how to get started:

1. Fork the repository
2. Clone your fork: git clone https://github.com/your-username/ash_scylla.git
3. Create a feature branch: git checkout -b feature/my-feature
4. Make your changes
5. Run tests: mix test
6. Commit your changes: git commit -am 'Add some feature'
7. Push to the branch: git push origin feature/my-feature
8. Create a Pull Request

Development Setup

# Install dependencies
mix deps.get

# Start ScyllaDB via Podman Compose (includes health checks)
podman-compose up -d

# Or via Docker Compose
docker compose up -d

# Or start ScyllaDB manually with Podman
podman run -p 9042:9042 scylladb/scylla:latest

# Or with Docker
docker run -p 9042:9042 scylladb/scylla:latest

# Run tests
mix test

Dev Container

A .devcontainer/devcontainer.json is provided for VS Code Dev Containers. It brings up both Elixir and ScyllaDB together via Podman Compose or Docker Compose.

License

This project is licensed under the Apache License 2.0 - see the LICENSE file for details.

Acknowledgments

• Ash Framework - The Elixir framework this data layer integrates with
• Xandra - Native Elixir CQL driver for ScyllaDB/Cassandra
• ScyllaDB - High-performance NoSQL database

Made with ❤️ for the Elixir and Ash communities