Adapters

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evoq uses an adapter pattern to integrate with different event store backends. This guide explains how to configure and implement adapters.

Overview

evoq doesn't include any specific event store implementation. Instead, it defines adapter behaviors that must be implemented to connect to your chosen backend.

┌─────────────────┐     ┌─────────────────┐     ┌─────────────────┐
│    evoq     │────>│   Adapter       │────>│  Event Store    │
│  (Framework)    │     │ (Interface)     │     │   (Backend)     │
└─────────────────┘     └─────────────────┘     └─────────────────┘

Available Adapters

The evoq_esdb_gater_adapter connects to reckon-db via the gateway protocol.

Add to your dependencies:

{deps, [
    {evoq, "0.2.0"},
    {reckon_db_gater, "0.4.3"}
]}.

Configure in sys.config:

{evoq, [
    {event_store_adapter, evoq_esdb_gater_adapter},
    {store_id, my_store}
]}

Adapter Behaviors

evoq defines three adapter behaviors:

evoq_adapter (Event Store)

Core event operations:

-callback append(StoreId, StreamId, ExpectedVersion, Events) -> {ok, Version} | {error, Reason}.
-callback read(StoreId, StreamId, StartVersion, Count, Direction) -> {ok, Events} | {error, Reason}.
-callback read_all(StoreId, StreamId, Direction) -> {ok, Events} | {error, Reason}.
-callback version(StoreId, StreamId) -> integer().
-callback exists(StoreId, StreamId) -> boolean().
-callback list_streams(StoreId) -> {ok, [StreamId]} | {error, Reason}.
-callback delete_stream(StoreId, StreamId) -> ok | {error, Reason}.

evoq_snapshot_adapter

Snapshot operations for aggregate optimization:

-callback save(StoreId, StreamId, Version, Data, Metadata) -> ok | {error, Reason}.
-callback read(StoreId, StreamId) -> {ok, Snapshot} | {error, not_found}.
-callback read_at_version(StoreId, StreamId, Version) -> {ok, Snapshot} | {error, not_found}.
-callback delete(StoreId, StreamId) -> ok | {error, Reason}.
-callback list_versions(StoreId, StreamId) -> {ok, [Version]} | {error, Reason}.

evoq_subscription_adapter

Subscription operations for event delivery:

-callback subscribe(StoreId, Type, Selector, Name, Opts) -> {ok, SubscriptionId} | {error, Reason}.
-callback unsubscribe(StoreId, SubscriptionId) -> ok | {error, Reason}.
-callback ack(StoreId, Name, StreamId, Position) -> ok | {error, Reason}.
-callback get_checkpoint(StoreId, Name) -> {ok, Position} | {error, not_found}.
-callback list(StoreId) -> {ok, [Subscription]} | {error, Reason}.

Implementing a Custom Adapter

To implement a custom adapter:

  1. Create a module that implements evoq_adapter (required)
  2. Optionally implement evoq_snapshot_adapter and evoq_subscription_adapter
  3. Configure evoq to use your adapter

Example skeleton:

-module(my_custom_adapter).
-behaviour(evoq_adapter).
-behaviour(evoq_snapshot_adapter).
-behaviour(evoq_subscription_adapter).

%% evoq_adapter callbacks
-export([append/4, read/5, read_all/3, version/2, exists/2, list_streams/1, delete_stream/2]).

%% evoq_snapshot_adapter callbacks
-export([save/5, read/2, read_at_version/3, delete/2, list_versions/2]).

%% evoq_subscription_adapter callbacks
-export([subscribe/5, unsubscribe/2, ack/4, get_checkpoint/2, list/1]).

%% Implementation...
append(StoreId, StreamId, ExpectedVersion, Events) ->
    %% Connect to your event store and append events
    {ok, NewVersion}.

%% ... implement other callbacks

Configuration

Full adapter configuration options:

{evoq, [
    %% Required: which adapter to use
    {event_store_adapter, evoq_esdb_gater_adapter},

    %% Store identifier (passed to adapter)
    {store_id, my_store},

    %% Optional: separate adapters for snapshots/subscriptions
    {snapshot_adapter, evoq_esdb_gater_snapshot_adapter},
    {subscription_adapter, evoq_esdb_gater_subscription_adapter}
]}

Local Development

For local development, use _checkouts to link adapters:

mkdir -p _checkouts
ln -s /path/to/reckon-db-gater _checkouts/reckon_db_gater

This allows modifying the adapter code alongside your application.

Testing with Mocks

Use meck to mock adapters in tests:

my_test() ->
    meck:new(evoq_esdb_gater_adapter, [passthrough]),
    meck:expect(evoq_esdb_gater_adapter, append, fun(_, _, _, Events) ->
        {ok, length(Events)}
    end),

    %% Run your test
    ok = my_module:do_something(),

    meck:unload(evoq_esdb_gater_adapter).

See Also