Exandra

Adapter module for Apache Cassandra and ScyllaDB.

Uses Xandra for communication with the underlying database.

Uses Ecto for database interfacing, running schema migrations, and querying operations.

Configuration

To configure an Ecto.Repo that uses Exandra as its adapter, you can use the application configuration or pass the options when starting the repo.

You can use the following options:

• Any of the options supported by Ecto.Repo itself, which you can see in the Ecto.Repo documentation.

• Any of the options supported by Xandra.Cluster.start_link/1.

#{Exandra.Connection.start_opts_docs()}

To configure your Ecto repository to use this adapter, you can use the :adapter option. For example, when defining the repo:

defmodule MyApp.Repo do
  use Ecto.Repo, otp_app: :my_app, adapter: Exandra
end

You can configure your database connection in config/dev.exs. Here's an example dev configuration:

# Configure your database
config :my_app, MyApp.Repo,
  migration_primary_key: [name: :id, type: :uuid], # Overrides the default type `bigserial` used for version attribute in schema migration
  nodes: ["127.0.0.1"],  # List of database connection endpoints
  keyspace: "my_app_dev", # Name of your keyspace
  sync_connect: 5000,             # Waiting time in milliseconds for the database connection
  log: :info,
  stacktrace: true,
  show_sensitive_data_on_connection_error: true,
  pool_size: 10

Note: The bigserial data type is specific to PostgreSQL databases and is not present in Scylla/Cassandra.

Schemas

You can regularly use Ecto.Schema with Exandra. For example:

defmodule User do
  use Ecto.Schema

  @primary_key {:id, Ecto.UUID, autogenerate: true}
  schema "users" do
    field :email, :string
    field :meta, Exandra.Map, key: :string, value: :string
  end
end

You can use all the usual types (:string, Ecto.UUID, and so on).

Maps

The :map type gets stored in Cassandra/Scylla as a blob of text with the map encoded as JSON. For example, if you have a schema with

field :features, :map

you can pass the field as an Elixir map when setting it, and Exandra will convert it to a map on the way from the database. Because Exandra uses JSON for this, you'll have to pay attention to things such as atom keys (which can be used when writing, but will be strings when reading) and such.

User-Defined Types (UDTs)

If one of your fields is a UDT, you can use the Exandra.UDT type for it. For example, if you have a phone_number UDT, you can declare fields with that type as:

field :home_phone, Exandra.UDT, type: :phone_number
field :office_phone, Exandra.UDT, type: :phone_number

String Keys {: .warning}

There is no validation with Exandra.UDT and the keys must be strings.

Alternatively, you can use the Exandra.EmbeddedType for Ecto.Schema-backed UDTs. For example, if you have a phone_number UDT, you can use:

field :home_phone, Exandra.EmbeddedType, using: MyApp.PhoneSchema

Finally, if you have a column of frozen UDTs list<frozen<phone_number>>, you can still use the Exandra.EmbeddedType just as before with cardinality: :many like so:

field :home_phone, Exandra.EmbeddedType, cardinality: :many, using: MyApp.PhoneSchema

Inets

Cassandra/Scylla has a native inet type which represents either an ipv4 or an ipv6 address. Exandra provides the Exandra.Inet type for these fields.

field :last_ip, Exandra.Inet

Tuples

Tuples can be declared using the Exandra.Tuple type.

field :version, Exandra.Tuple, types: [:integer, :integer, :integer]

Native Collections (Lists, Maps, Sets)

Access to native Cassandra/Scylla collections is available using the appropriate data types in the field definition.

@primary_key false
schema "hotels" do
  field :checkins, {:array, :utc_datetime}                            # list<timestamp>
  field :room_to_customer, Exandra.Map, key: :integer, value: :string # map<int, string>
  field :available_rooms, Exandra.Set, type: :integer                 # set<int>
end

Composite Collections {: .tip}

It's possible to create composite collections using the following syntax:

field :complex_type, {:array, Exandra.Map}, key: :string, value: Exandra.Set, type: :integer

This creates a list<map<tuple<integer>, set<string>>>.

However, please note that due to the limited expressivity of this representation, each collection at the same level will have the same typing information:

field :valid, Exandra.Set, type: Exandra.Set, type: :integer                                     # set<set<int>>
field :invalid, Exandra.Map, key: Exandra.Set, type: :integer, value: Exandra.Set, type: :string # map<set<int>, set<int>> not map<set<int>, set<string>>

Counter Tables

You can use the Exandra.Counter type to create counter fields (in counter tables). For example:

@primary_key false
schema "page_views" do
  field :route, :string, primary_key: true
  field :total, Exandra.Counter
end

You can only update counter fields. You'll have to use c:Ecto.Repo.update_all/2 to insert or update counters. For example, in the table above, you'd update the :total counter field with:

query =
  from page_view in "page_views",
    where: page_view.route == "/browse",
    update: [set: [total: 1]]

MyApp.Repo.update_all(query)

Batch Queries

You can run batch queries through Exandra. Batch queries are supported by Cassandra/Scylla, and allow you to run multiple queries in a single request. See Exandra.Batch for more information and examples.

Multiple keyspaces using prefixes

You can use query prefixes to query different keyspaces using the same schemas. As pointed out in the Ecto docs, migrations must be run for each prefix in this case.

Batch Queries

You can run batch queries through Exandra. Batch queries are supported by Cassandra/Scylla, and allow you to run multiple queries in a single request. See Exandra.Batch for more information and examples.

Multiple keyspaces using prefixes

You can use query prefixes to query different keyspaces using the same schemas. As pointed out in the Ecto docs, migrations must be run for each prefix in this case.

Migrations

You can use Exandra to run migrations as well, as it supports most of the DDL-related commands from Ecto.Migration. For example:

defmodule AddUsers do
  use Ecto.Migration

  def change do
    create table("users", primary_key: false) do
      add :email, :string, primary_key: true
      add :age, :int
    end
  end
end

Cassandra and Scylla Types {: .info}

When writing migrations, remember that you must use the actual types from Cassandra or Scylla, which you must pass in as an atom.

For example, to add a column with the type of a map of integer keys to boolean values, you need to declare its type as :"map<int, boolean>".

This is a non-comprehensive list of types you can use:

• :"map<key_type, value_type>" - maps (such as :"map<int, boolean>").
• :"list<type>" - lists (such as :"list<uuid>").
• :string - gets translated to the text type.
• :map - maps get stored as text, and Exandra dumps and loads them automatically.
• <udt> - User-Defined Types (UDTs) should be specified as their name, expressed as an atom. For example, a UDT called full_name would be specified as the type :full_name.
• :naive_datetime, :naive_datetime_usec, :utc_datetime, :utc_datetime_usec - these are all represented as the timestamp type.

User-Defined Types (UDTs)

Ecto.Migration doesn't support creating, altering, or dropping Cassandra/Scylla UDTs. To do those operations in a migration, use Ecto.Migration.execute/1 or Ecto.Migration.execute/2. For example, in your migration module:

def change do
  execute(
    _up_query = "CREATE TYPE full_name (first_name text, last_name text))",
    _down_query = "DROP TYPE full_name"
  )
end