@castore/core

Castore

Making Event Sourcing easy 😎

Event Sourcing is a data storage paradigm that saves changes in your application state rather than the state itself.

It is powerful as it enables rewinding to a previous state and exploring audit trails for debugging or business/legal purposes. It also integrates very well with event-driven architectures.

However, it is tricky to implement 😅

After years of using it at Kumo, we have grown to love it, but also experienced first-hand the lack of consensus and tooling around it. That's where Castore comes from!

---

Castore is a TypeScript library that makes Event Sourcing easy 😎

---

With Castore, you'll be able to:

• Define your event stores
• Fetch and push new events seamlessly
• Implement and test your commands
• ...and much more!

All that with first-class developer experience and minimal boilerplate ✨

🫀 Core Design

Some important decisions that we've made early on:

💭 Abstractions first

Castore has been designed with flexibility in mind. It gives you abstractions that are meant to be used anywhere: React apps, containers, Lambdas... you name it!

For instance, EventStore classes are stack agnostic: They need an EventStorageAdapter class to interact with actual data. You can code your own EventStorageAdapter (simply implement the interface), but it's much simpler to use an off-the-shelf adapter like DynamoDBEventStorageAdapter.

🙅‍♂️ We do NOT deploy resources

While some packages like DynamoDBEventStorageAdapter require compatible infrastructure, Castore is not responsible for deploying it.

Though that is not something we exclude in the future, we are a small team and decided to focus on DevX first.

⛑ Full type safety

Speaking of DevX, we absolutely love TypeScript! If you do too, you're in the right place: We push type-safety to the limit in everything we do!

If you don't, that's fine 👍 Castore is still available in Node/JS. And you can still profit from some nice JSDocs!

📖 Best practices

The Event Sourcing journey has many hidden pitfalls. We ran into them for you!

Castore is opiniated. It comes with a collection of best practices and documented anti-patterns that we hope will help you out!

Table of content

• 🎬 Getting Started
  • Installation
  • Packages structure
• 🚀 The Basics
  • Events
  • Event Types
  • Aggregates
  • Reducers
  • Event Store
  • Event Storage Adapter
  • Command
  • Event Groups
• 💪 Advanced Usage
  • Event-driven architecture
  • Message queues
  • Message queue adapters
  • Message buses
  • Message bus adapters
  • Connected Event Store
  • Snapshotting
  • Read Models
• 📖 Resources
  • Test Tools
  • React Visualizer
  • Dam
  • Packages List
  • Common Patterns

🎬 Getting Started

- Installation

# npm
npm install @castore/core

# yarn
yarn add @castore/core

- Packages structure

Castore is not a single package, but a collection of packages revolving around a core package. This is made so every line of code added to your project is opt-in, wether you use tree-shaking or not.

Castore packages are released together. Though different versions may be compatible, you are guaranteed to have working code as long as you use matching versions.

Here is an example of working package.json:

{
  ...
  "dependencies": {
    "@castore/core": "1.3.1",
    "@castore/dynamodb-event-storage-adapter": "1.3.1"
    ...
  },
  "devDependencies": {
    "@castore/test-tools": "1.3.1"
    ...
  }
}

🚀 The Basics

- Events

Event Sourcing is all about saving changes in your application state. Such changes are represented by events, and needless to say, they are quite important 🙃

Events that concern the same entity (like a Pokemon) are aggregated through a common id called aggregateId (and vice versa, events that have the same aggregateId represent changes of the same business entity). The index of an event in such a serie of events is called its version.

In Castore, stored events (also called event details) always have exactly the following properties:

• aggregateId (string)
• version (integer ≥ 1)
• timestamp (string): A date in ISO 8601 format
• type (string): A string identifying the business meaning of the event
• payload (?any = never): A payload of any type
• metadata (?any = never): Some metadata of any type

import type { EventDetail } from '@castore/core';

type PokemonAppearedEventDetail = EventDetail<
  'POKEMON_APPEARED',
  { name: string; level: number },
  { trigger?: 'random' | 'scripted' }
>;

// 👇 Equivalent to:
type PokemonAppearedEventDetail = {
  aggregateId: string;
  version: number;
  timestamp: string;
  type: 'POKEMON_APPEARED';
  payload: { name: string; level: number };
  metadata: { trigger?: 'random' | 'scripted' };
};

- EventType

Events are generally classified in events types (not to confuse with TS types). Castore lets you declare them via the EventType class:

import { EventType } from '@castore/core';

const pokemonAppearedEventType = new EventType<
  'POKEMON_APPEARED',
  { name: string; level: number },
  { trigger?: 'random' | 'scripted' }
>({ type: 'POKEMON_APPEARED' });

Note that we only provided TS types for payload and metadata properties. That is because, as stated in the core design, Castore is meant to be as flexible as possible, and that includes the validation library you want to use (if any): The EventType class can be used directly if no validation is required, or implemented by other classes which will add run-time validation methods to it 👍

See the following packages for examples:

• JSON Schema Event Type
• Zod Event Type

🔧 Technical description

Constructor:

• type (string): The event type

import { EventType } from '@castore/core';

const pokemonAppearedEventType = new EventType({ type: 'POKEMON_APPEARED' });

Properties:

• type (string): The event type

const eventType = pokemonAppearedEventType.type;
// => 'POKEMON_APPEARED'

Type Helpers:

• EventTypeDetail: Returns the event detail TS type of an EventType

import type { EventTypeDetail } from '@castore/core';

type PokemonAppearedEventTypeDetail = EventTypeDetail<
  typeof pokemonAppearedEventType
>;

// 👇 Equivalent to:
type PokemonCaughtEventTypeDetail = {
  aggregateId: string;
  version: number;
  timestamp: string;
  type: 'POKEMON_APPEARED';
  payload: { name: string; level: number };
  metadata: { trigger?: 'random' | 'scripted' };
};

• EventTypesDetails: Return the events details of a list of EventType

import type { EventTypesDetails } from '@castore/core';

type PokemonEventTypeDetails = EventTypesDetails<
  [typeof pokemonAppearedEventType, typeof pokemonCaughtEventType]
>;
// => EventTypeDetail<typeof pokemonAppearedEventType>
// | EventTypeDetail<typeof pokemonCaughtEventType>

- Aggregate

Eventhough entities are stored as series of events, we still want to use a simpler and stable interface to represent their states at a point in time rather than directly using events. In Castore, it is implemented by a TS type called Aggregate.

☝️ Think of aggregates as "what the data would look like in CRUD"

In Castore, aggregates necessarily contain an aggregateId and version properties (the version of the latest event). But for the rest, it's up to you 🤷‍♂️

For instance, we can include a name, level and status properties to our PokemonAggregate:

import type { Aggregate } from '@castore/core';

// Represents a Pokemon at a point in time
interface PokemonAggregate extends Aggregate {
  name: string;
  level: number;
  status: 'wild' | 'caught';
}

// 👇 Equivalent to:
interface PokemonAggregate {
  aggregateId: string;
  version: number;
  name: string;
  level: number;
  status: 'wild' | 'caught';
}

- Reducer

Aggregates are derived from their events by reducing them through a reducer function. It defines how to update the aggregate when a new event is pushed:

import type { Reducer } from '@castore/core';

const pokemonsReducer: Reducer<PokemonAggregate, PokemonEventDetails> = (
  pokemonAggregate,
  newEvent,
) => {
  const { version, aggregateId } = newEvent;

  switch (newEvent.type) {
    case 'POKEMON_APPEARED': {
      const { name, level } = newEvent.payload;

      // 👇 Return the next version of the aggregate
      return {
        aggregateId,
        version,
        name,
        level,
        status: 'wild',
      };
    }
    case 'POKEMON_CAUGHT':
      return { ...pokemonAggregate, version, status: 'caught' };
    case 'POKEMON_LEVELED_UP':
      return {
        ...pokemonAggregate,
        version,
        level: pokemonAggregate.level + 1,
      };
  }
};

const myPikachuAggregate: PokemonAggregate =
  myPikachuEvents.reduce(pokemonsReducer);

☝️ Aggregates are always computed on the fly, and NOT stored. Changing them does not require any data migration whatsoever.

- EventStore

Once you've defined your event types and how to aggregate them, you can bundle them together in an EventStore class.

Each event store in your application represents a business entity. Think of event stores as "what tables would be in CRUD", except that instead of directly updating data, you just append new events to it!

In Castore, EventStore classes are NOT responsible for actually storing data (this will come with event storage adapters). But rather to provide a boilerplate-free and type-safe interface to perform many actions such as:

• Listing aggregate ids
• Accessing events of an aggregate
• Building an aggregate with the reducer
• Pushing new events etc...

import { EventStore } from '@castore/core';

const pokemonsEventStore = new EventStore({
  eventStoreId: 'POKEMONS',
  eventStoreEvents: [
    pokemonAppearedEventType,
    pokemonCaughtEventType,
    pokemonLeveledUpEventType,
    ...
  ],
  reduce: pokemonsReducer,
});
// ...and that's it 🥳

☝️ The EventStore class is the heart of Castore, it even gave it its name!

🔧 Technical description

Constructor:

• eventStoreId (string): A string identifying the event store
• eventStoreEvents (EventType[]): The list of event types in the event store
• reduce (EventType[]): A reducer function that can be applied to the store event types
• onEventPushed (?(pushEventResponse: PushEventResponse => Promise<void>)): To run a callback after events are pushed (input is exactly the return value of the pushEvent method)
• storageAdapter (?EventStorageAdapter): See EventStorageAdapter

☝️ The return type of the reducer is used to infer the Aggregate type of the EventStore, so it is important to type it explicitely.

Properties:

• eventStoreId (string)

const pokemonsEventStoreId = pokemonsEventStore.eventStoreId;
// => 'POKEMONS'

• eventStoreEvents (EventType[])

const pokemonsEventStoreEvents = pokemonsEventStore.eventStoreEvents;
// => [pokemonAppearedEventType, pokemonCaughtEventType...]

• reduce ((Aggregate, EventType) => Aggregate)

const reducer = pokemonsEventStore.reduce;
// => pokemonsReducer

• onEventPushed (?(pushEventResponse: PushEventResponse) => Promise<void>): Callback to run after events are pushed

const onEventPushed = pokemonsEventStore.onEventPushed;
// => undefined (we did not provide one in this example)

• storageAdapter ?EventStorageAdapter: See EventStorageAdapter

const storageAdapter = pokemonsEventStore.storageAdapter;
// => undefined (we did not provide one in this example)

☝️ The storageAdapter is not read-only so you do not have to provide it right away.

Sync Methods:

• getStorageAdapter (() => EventStorageAdapter): Returns the event store event storage adapter if it exists. Throws an UndefinedStorageAdapterError if it doesn't.

import { UndefinedStorageAdapterError } from '@castore/core';

expect(() => pokemonsEventStore.getStorageAdapter()).toThrow(
  new UndefinedStorageAdapterError({ eventStoreId: 'POKEMONS' }),
);
// => true

• buildAggregate ((eventDetails: EventDetail[], initialAggregate?: Aggregate) => Aggregate | undefined): Applies the event store reducer to a serie of events.

const myPikachuAggregate = pokemonsEventStore.buildAggregate(myPikachuEvents);

• groupEvent ((eventDetail: EventDetail, opt?: OptionsObj = {}) => GroupedEvent): See Event Groups.

Async Methods:

The following methods interact with the data layer of your event store through its EventStorageAdapter. They will throw an UndefinedStorageAdapterError if you did not provide one.

• getEvents ((aggregateId: string, opt?: OptionsObj = {}) => Promise<ResponseObj>): Retrieves the events of an aggregate, ordered by version. Returns an empty array if no event is found for this aggregateId.

  OptionsObj contains the following properties:

  • minVersion (?number): To retrieve events above a certain version
  • maxVersion (?number): To retrieve events below a certain version
  • limit (?number): Maximum number of events to retrieve
  • reverse (?boolean = false): To retrieve events in reverse order (does not require to swap minVersion and maxVersion)

  ResponseObj contains the following properties:

  • events (EventDetail[]): The aggregate events (possibly empty)

const { events: allEvents } = await pokemonsEventStore.getEvents(myPikachuId);
// => typed as PokemonEventDetail[] 🙌

// 👇 Retrieve a range of events
const { events: rangedEvents } = await pokemonsEventStore.getEvents(
  myPikachuId,
  {
    minVersion: 2,
    maxVersion: 5,
  },
);

// 👇 Retrieve the last event of the aggregate
const { events: onlyLastEvent } = await pokemonsEventStore.getEvents(
  myPikachuId,
  {
    reverse: true,
    limit: 1,
  },
);

• getAggregate ((aggregateId: string, opt?: OptionsObj = {}) => Promise<ResponseObj>): Retrieves the events of an aggregate and build it.

  OptionsObj contains the following properties:

  • maxVersion (?number): To retrieve aggregate below a certain version

  ResponseObj contains the following properties:

  • aggregate (?Aggregate): The aggregate (possibly undefined)
  • events (EventDetail[]): The aggregate events (possibly empty)
  • lastEvent (?EventDetail): The last event (possibly undefined)

const { aggregate: myPikachu } = await pokemonsEventStore.getAggregate(
  myPikachuId,
);
// => typed as PokemonAggregate | undefined 🙌

// 👇 Retrieve an aggregate below a certain version
const { aggregate: pikachuBelowVersion5 } =
  await pokemonsEventStore.getAggregate(myPikachuId, { maxVersion: 5 });

// 👇 Returns the events if you need them
const { aggregate, events } = await pokemonsEventStore.getAggregate(
  myPikachuId,
);

• getExistingAggregate ((aggregateId: string, opt?: OptionsObj = {}) => Promise<ResponseObj>): Same as getAggregate method, but ensures that the aggregate exists. Throws an AggregateNotFoundError if no event is found for this aggregateId.

import { AggregateNotFoundError } from '@castore/core';

expect(async () =>
  pokemonsEventStore.getExistingAggregate(unexistingId),
).resolves.toThrow(
  new AggregateNotFoundError({
    eventStoreId: 'POKEMONS',
    aggregateId: unexistingId,
  }),
);
// true

const { aggregate } = await pokemonsEventStore.getAggregate(aggregateId);
// => 'aggregate' and 'lastEvent' are always defined 🙌

• pushEvent ((eventDetail: EventDetail, opt?: OptionsObj = {}) => Promise<ResponseObj>): Pushes a new event to the event store. The timestamp is optional (we keep it available as it can be useful in tests & migrations). If not provided, it is automatically set as new Date().toISOString(). Throws an EventAlreadyExistsError if an event already exists for the corresponding aggregateId and version (see section below on race conditions).

  OptionsObj contains the following properties:

  • prevAggregate (?Aggregate): The aggregate at the current version, i.e. before having pushed the event. Can be useful in some cases like when using the ConnectedEventStore class
  • force (?boolean): To force push the event even if one already exists for the corresponding aggregateId and version. Any existing event will be overridden, so use with extra care, mainly in data migrations.

  ResponseObj contains the following properties:

  • event (EventDetail): The complete event (includes the timestamp)
  • nextAggregate (?Aggregate): The aggregate at the new version, i.e. after having pushed the event. Returned only if the event is an initial event, if the prevAggregate option was provided, or when using a ConnectedEventStore class connected to a state-carrying message bus or queue

const { event: completeEvent, nextAggregate } =
  await pokemonsEventStore.pushEvent(
    {
      aggregateId: myPikachuId,
      version: lastVersion + 1,
      type: 'POKEMON_LEVELED_UP', // <= event type is correctly typed 🙌
      payload, // <= payload is typed according to the provided event type 🙌
      metadata, // <= same goes for metadata 🙌
      // timestamp is optional
    },
    // Not required - Can be useful in some cases
    { prevAggregate },
  );

• listAggregateIds ((opt?: OptionsObj = {}) => Promise<ResponseObj>): Retrieves the list of aggregateId of an event store, ordered by the timestamp of their initial event. Returns an empty array if no aggregate is found.

  OptionsObj contains the following properties:

  • limit (?number): Maximum number of aggregate ids to retrieve
  • initialEventAfter (?string): To retrieve aggregate ids that appeared after a certain timestamp
  • initialEventBefore (?string): To retrieve aggregate ids that appeared before a certain timestamp
  • reverse (?boolean): To retrieve the aggregate ids in reverse order
  • pageToken (?string): To retrieve a paginated result of aggregate ids

  ResponseObj contains the following properties:

  • aggregateIds (string[]): The list of aggregate ids
  • nextPageToken (?string): A token for the next page of aggregate ids if one exists. The nextPageToken carries the previously used options, so you do not have to provide them again (though you can still do it to override them).

const accAggregateIds: string = [];
const { aggregateIds: firstPage, nextPageToken } =
  await pokemonsEventStore.listAggregateIds({ limit: 20 });

accAggregateIds.push(...firstPage);

if (nextPageToken) {
  const { aggregateIds: secondPage } =
    await pokemonsEventStore.listAggregateIds({
      // 👇 Previous limit of 20 is passed through the page token
      pageToken: nextPageToken,
    });
  accAggregateIds.push(...secondPage);
}

Type Helpers:

• EventStoreId: Returns the EventStore id

import type { EventStoreId } from '@castore/core';

type PokemonsEventStoreId = EventStoreId<typeof pokemonsEventStore>;
// => 'POKEMONS'

• EventStoreEventsTypes: Returns the EventStore list of events types

import type { EventStoreEventsTypes } from '@castore/core';

type PokemonEventTypes = EventStoreEventsTypes<typeof pokemonsEventStore>;
// => [typeof pokemonAppearedEventType, typeof pokemonCaughtEventType...]

• EventStoreEventsDetails: Returns the union of all the EventStore possible events details

import type { EventStoreEventsDetails } from '@castore/core';

type PokemonEventDetails = EventStoreEventsDetails<typeof pokemonsEventStore>;
// => EventTypeDetail<typeof pokemonAppearedEventType>
// | EventTypeDetail<typeof pokemonCaughtEventType>
// | ...

• EventStoreReducer: Returns the EventStore reducer

import type { EventStoreReducer } from '@castore/core';

type PokemonsReducer = EventStoreReducer<typeof pokemonsEventStore>;
// => Reducer<PokemonAggregate, PokemonEventDetails>

• EventStoreAggregate: Returns the EventStore aggregate

import type { EventStoreAggregate } from '@castore/core';

type SomeAggregate = EventStoreAggregate<typeof pokemonsEventStore>;
// => PokemonAggregate

- EventStorageAdapter

For the moment, we didn't provide any actual way to store our events data. This is the responsibility of the EventStorageAdapter class.

import { EventStore } from '@castore/core';

const pokemonsEventStore = new EventStore({
  eventStoreId: 'POKEMONS',
  eventTypes: pokemonEventTypes,
  reduce: pokemonsReducer,
  // 👇 Provide it in the constructor
  storageAdapter: mySuperStorageAdapter,
});

// 👇 ...or set/switch it in context later
pokemonsEventStore.storageAdapter = mySuperStorageAdapter;

You can choose to build an event storage adapter that suits your usage. However, we highly recommend using an off-the-shelf adapter:

• DynamoDB Event Storage Adapter
• Redux Event Storage Adapter
• In-Memory Event Storage Adapter

If the storage solution that you use is missing, feel free to create/upvote an issue, or contribute 🤗

- Command

Modifying the state of your application (i.e. pushing new events to your event stores) is done by executing commands. They typically consist in:

• Fetching the required aggregates (if not the initial event of a new aggregate)
• Validating that the modification is acceptable
• Pushing new events with incremented versions

import { Command, tuple } from '@castore/core';

type Input = { name: string; level: number };
type Output = { pokemonId: string };
type Context = { generateUuid: () => string };

const catchPokemonCommand = new Command({
  commandId: 'CATCH_POKEMON',
  // 👇 "tuple" is needed to keep ordering in inferred type
  requiredEventStores: tuple(pokemonsEventStore, trainersEventStore),
  // 👇 Code to execute
  handler: async (
    commandInput: Input,
    [pokemonsEventStore, trainersEventStore],
    // 👇 Additional context arguments can be provided
    { generateUuid }: Context,
  ): Promise<Output> => {
    const { name, level } = commandInput;
    const pokemonId = generateUuid();

    await pokemonsEventStore.pushEvent({
      aggregateId: pokemonId,
      version: 1,
      type: 'POKEMON_CAUGHT',
      payload: { name, level },
    });

    return { pokemonId };
  },
});

Note that we only provided TS types for Input and Output properties. That is because, as stated in the core design, Castore is meant to be as flexible as possible, and that includes the validation library you want to use (if any): The Command class can be used directly if no validation is required, or implemented by other classes which will add run-time validation methods to it 👍

See the following packages for examples:

• JSON Schema Event Type

🔧 Technical description

Constructor:

• commandId (string): A string identifying the command

• handler ((input: Input, requiredEventsStores: EventStore[]) => Promise<Output>): The code to execute

• requiredEventStores (EventStore[]): A tuple of EventStores that are required by the command for read/write purposes. In TS, you should use the tuple util to preserve tuple ordering in the handler (tuple doesn't mute its input, it simply returns them)

• eventAlreadyExistsRetries (?number = 2): Number of handler execution retries before breaking out of the retry loop (See section below on race conditions)

• onEventAlreadyExists (?(error: EventAlreadyExistsError, context: ContextObj) => Promise<void>): Optional callback to execute when an EventAlreadyExistsError is raised.

  The EventAlreadyExistsError class contains the following properties:

  • eventStoreId (?string): The eventStoreId of the aggregate on which the pushEvent attempt failed
  • aggregateId (string): The aggregateId of the aggregate
  • version (number): The version of the aggregate

  The ContextObj contains the following properties:

  • attemptNumber (?number): The number of handler execution attempts in the retry loop
  • retriesLeft (?number): The number of retries left before breaking out of the retry loop

import { Command, tuple } from '@castore/core';

const doSomethingCommand = new Command({
  commandId: 'DO_SOMETHING',
  requiredEventStores: tuple(eventStore1, eventStore2),
  handler: async (commandInput, [eventStore1, eventStore2]) => {
    // ...do something here
  },
});

Properties:

• commandId (string): The command id

const commandId = doSomethingCommand.commandId;
// => 'DO_SOMETHING'

• requiredEventStores (EventStore[]): The required event stores

const requiredEventStores = doSomethingCommand.requiredEventStores;
// => [eventStore1, eventStore2]

• handler ((input: Input, requiredEventsStores: EventStore[]) => Promise<Output>): Function to invoke the command

const output = await doSomethingCommand.handler(input, [
  eventStore1,
  eventStore2,
]);

A few notes on commands handlers:

• Commands handlers should NOT use read models when validating that a modification is acceptable. Read models are like cache: They are not the source of truth, and may not represent the freshest state.

• Fetching and pushing events non-simultaneously exposes your application to race conditions. To counter that, commands are designed to be retried when an EventAlreadyExistsError is triggered (which is part of the EventStorageAdapter interface).

• Finally, Command handlers should be, as much as possible, pure functions. If it depends on impure functions like functions with unpredictable outputs (like id generation), mutating effects, side effects or state dependency (like external data fetching), you should pass them through the additional context arguments rather than directly importing and using them. This will make them easier to test and to re-use in different contexts, such as in the React Visualizer.

Event Groups

Some commands can have an effect on several event stores, or on several aggregates of the same event store. For instance, the CATCH_POKEMON command could write both a CAUGHT_BY_TRAINER event on a pokemon aggregate (changing its status to 'caught') and a POKEMON_CAUGHT event on a trainer aggregate (appending the pokemonId to its pokedex).

To not have your application in a corrupt state, it's important to make sure that all those events are pushed or none. In Castore, this can be done through the event groups API:

• You can use the groupEvent method to build an array of events that are to be pushed together. It has the same input interface as pushEvent but synchronously returns a GroupedEvent class.
• The EventStore class exposes a static pushEventGroup method that can be used to effectively push this event group.

await EventStore.pushEventGroup(
  pokemonsEventStore.groupEvent({
    // 👇 Correctly typed
    aggregateId: 'pikachu1',
    type: 'CAUGHT_BY_TRAINER',
    payload: { trainerId: 'ashKetchum' },
    ...
  }),
  trainersEventStore.groupEvent({
    aggregateId: 'ashKetchum',
    type: 'POKEMON_CAUGHT',
    payload: { pokemonId: 'pikachu1' },
    ...
  }),
);

Like the pushEvent API, event groups are designed to throw an EventAlreadyExistsError if the transaction has failed, making sure that commands are retried as expected when race conditions arise.

☝️ When pushing event groups on several event stores, they must use the same type of event storage adapters.

☝️ Also, be aware of technical constraints of your event storage solution. For instance, the DynamoDBEventStorageAdapter's implementation is based on DynamoDB transactions, which means that the event stores tables must be in the same region, and that a groups cannot contain more than 100 events.

💪 Advanced usage

- Event-driven architecture

Event Sourcing integrates very well with event-driven architectures. In a traditional architecture, you would need to design your system events (or messages for clarity) separately from your data. With Event Sourcing, they can simply broadcast the business events you already designed.

In Castore, we distinguish three types of message:

• AggregateExists messages which only carry aggregate ids (mainly for maintenance purposes)
• Notification messages which also carry event details
• State-carrying messages which also carry their corresponding aggregates

In Castore, they are implemented by the AggregateExistsMessage, NotificationMessage and StateCarryingMessage TS types:

// AggregateExistsMessage
import type {
  AggregateExistsMessage,
  EventStoreAggregateExistsMessage,
} from '@castore/core';

type PokemonAggregateExistsMessage = AggregateExistsMessage<'POKEMONS'>;

// 👇 Equivalent to:
type PokemonAggregateExistsMessage = {
  eventStoreId: 'POKEMONS';
  aggregateId: string;
};

// // 👇 Also equivalent to:
type PokemonAggregateExistsMessage = EventStoreAggregateExistsMessage<
  typeof pokemonsEventStore
>;

// NotificationMessage
import type {
  NotificationMessage,
  EventStoreNotificationMessage,
} from '@castore/core';

type PokemonEventNotificationMessage = NotificationMessage<
  'POKEMONS',
  PokemonEventDetails
>;

// 👇 Equivalent to:
type PokemonEventNotificationMessage = {
  eventStoreId: 'POKEMONS';
  event: PokemonEventDetails;
};

// 👇 Also equivalent to:
type PokemonEventNotificationMessage = EventStoreNotificationMessage<
  typeof pokemonsEventStore
>;

// StateCarryingMessage
import type {
  StateCarryingMessage,
  EventStoreStateCarryingMessage,
} from '@castore/core';

type PokemonEventStateCarryingMessage = StateCarryingMessage<
  'POKEMONS',
  PokemonEventDetails,
  PokemonAggregate
>;

// 👇 Equivalent to:
type PokemonEventStateCarryingMessage = {
  eventStoreId: 'POKEMONS';
  event: PokemonEventDetails;
  aggregate: PokemonAggregate
};

// 👇 Also equivalent to:
type PokemonEventStateCarryingMessage = EventStoreStateCarryingMessage<
  typeof pokemonsEventStore
>;

All types of message can be published through message channels, i.e. Message Queues or Message Buses.

- MessageQueue

Message Queues store the published messages until they are handled by a worker. The worker is unique and predictible. It consumes all messages indifferently of their content.

You can use the AggregateExistsMessageQueue, NotificationMessageQueue or StateCarryingMessageQueue classes to implement message queues:

import { NotificationMessageQueue } from '@castore/core';

const appMessageQueue = new NotificationMessageQueue({
  messageQueueId: 'APP_MESSAGE_QUEUE',
  sourceEventStores: [pokemonsEventStore, trainersEventStore],
});

await appMessageQueue.publishMessage({
  // 👇 Typed as NotificationMessage of one of the source event stores
  eventStoreId: 'POKEMONS',
  event: {
    type: 'POKEMON_LEVELED_UP',
    ...
  },
});

// Similar for AggregateExistsMessageQueue and StateCarryingMessageQueue

🔧 Technical description

Constructor:

• messageQueueId (string): A string identifying the message queue
• sourceEventStores (EventStore[]): List of event stores that the message queue will broadcast events from
• messageQueueAdapter (?MessageChannelAdapter): See section on MessageQueueAdapters

Properties:

• messageChannelId (string)

const appMessageQueueId = appMessageQueue.messageChannelId;
// => 'APP_MESSAGE_QUEUE'

• sourceEventStores (EventStore[])

const appMessageQueueSourceEventStores = appMessageQueue.sourceEventStores;
// => [pokemonsEventStore, trainersEventStore...]

• messageChannelAdapter ?MessageChannelAdapter: See section on MessageQueueAdapters

const appMessageQueueAdapter = appMessageQueue.messageChannelAdapter;
// => undefined (we did not provide one in this example)

☝️ The messageChannelAdapter is not read-only so you do not have to provide it right away.

Async Methods:

The following methods interact with the messaging solution of your application through a MessageQueueAdapter. They will throw an UndefinedMessageChannelAdapterError if you did not provide one.

• publishMessage ((message: Message, opt?: OptionsObj = {}) => Promise<void>): Publish a Message (of the appropriate type) to the message queue.

  OptionsObj contains the following properties:

  • replay (?boolean): Signals that the event is not happening in real-time, e.g. in maintenance or migration operations. This information can be used downstream to react appropriately (e.g. prevent sending notification emails). Check the implementation of you adapter for more details.

• publishMessages (messages: Message[], opt?: OptionsObj) => Promise<void>): Publish several Messages (of the appropriate type) to the message queue. Options are similar to the publishMessage options.

• getAggregateAndPublishMessage ((message: NotificationMessage) => Promise<void>): (StateCarryingMessageQueues only) Append the matching aggregate (with correct version) to a NotificationMessage and turn it into a StateCarryingMessage before publishing it to the message queue. Uses the message queue event stores: Make sure that they have correct adapters set up.

Type Helpers:

• MessageChannelMessage: Given a MessageQueue, returns the TS type of its messages

import type { MessageChannelMessage } from '@castore/core';

type AppMessage = MessageChannelMessage<typeof appMessageQueue>;

// 👇 Equivalent to:
type AppMessage = EventStoreNotificationMessage<
  typeof pokemonsEventStore | typeof trainersEventStore...
>;

- MessageQueueAdapter

Similarly to event stores, MessageQueue classes provide a boilerplate-free and type-safe interface to publish messages, but are NOT responsible for actually doing so. This is the responsibility of the MessageQueueAdapter, that will connect it to your actual messaging solution:

import { EventStore } from '@castore/core';

const messageQueue = new NotificationMessageQueue({
  ...
  // 👇 Provide it in the constructor
  messageQueueAdapter: mySuperMessageQueueAdapter,
});

// 👇 ...or set/switch it in context later
messageQueue.messageChannelAdapter = mySuperMessageQueueAdapter;
// Named `messageChannelAdapter` as queues inherit from the `MessageChannel` class

You can code your own MessageQueueAdapter (simply implement the MessageChannelAdapter interface), but we highly recommend using an off-the-shelf adapter:

• SQS Message Queue Adapter
• In-Memory Message Queue Adapter

If the messaging solution that you use is missing, feel free to create/upvote an issue, or contribute 🤗

The adapter packages will also expose useful generics to type the arguments of your queue worker. For instance:

import type {
  SQSMessageQueueMessage,
  SQSMessageQueueMessageBody,
} from '@castore/sqs-message-queue-adapter';

const appMessagesWorker = async ({ Records }: SQSMessageQueueMessage) => {
  Records.forEach(({ body }) => {
    // 👇 Correctly typed!
    const recordBody: SQSMessageQueueMessageBody<typeof appMessageQueue> =
      JSON.parse(body);
  });
};

- MessageBus

Message Buses are used to spread messages to multiple listeners. Contrary to message queues, they do not store the message or wait for the listeners to respond. Often, filter patterns can also be used to trigger listeners or not based on the message content.

You can use the AggregateExistsMessageBus, NotificationMessageBus or StateCarryingMessageBus classes to implement message buses:

import { NotificationMessageBus } from '@castore/core';

const appMessageBus = new NotificationMessageBus({
  messageBusId: 'APP_MESSAGE_BUSES',
  sourceEventStores: [pokemonsEventStore, trainersEventStore...],
});

await appMessageBus.publishMessage({
  // 👇 Typed as NotificationMessage of one of the source event stores
  eventStoreId: 'POKEMONS',
  event: {
    type: 'POKEMON_LEVELED_UP',
    ...
  }
})

// Similar for AggregateExistsMessageBus and StateCarryingMessageBus

🔧 Technical description

Constructor:

• messageBusId (string): A string identifying the message bus
• sourceEventStores (EventStore[]): List of event stores that the message bus will broadcast events from
• messageBusAdapter (?MessageChannelAdapter): See section on MessageBusAdapters

Properties:

• messageBusId (string)

const appMessageBusId = appMessageBus.messageBusId;
// => 'APP_MESSAGE_BUS'

• sourceEventStores (EventStore[])

const appMessageBusSourceEventStores = appMessageBus.sourceEventStores;
// => [pokemonsEventStore, trainersEventStore...]

• messageChannelAdapter ?MessageChannelAdapter: See section on MessageBusAdapters

const appMessageBusAdapter = appMessageBus.messageChannelAdapter;
// => undefined (we did not provide one in this example)

☝️ The messageChannelAdapter is not read-only so you do not have to provide it right away.

Async Methods:

The following methods interact with the messaging solution of your application through a MessageBusAdapter. They will throw an UndefinedMessageChannelAdapterError if you did not provide one.

• publishMessage ((message: Message, opt?: OptionsObj = {}) => Promise<void>): Publish a Message (of the appropriate type) to the message bus.

  OptionsObj contains the following properties:

  • replay (?boolean): Signals that the event is not happening in real-time, e.g. in maintenance or migration operations. This information can be used downstream to react appropriately (e.g. prevent sending notification emails). Check the implementation of you adapter for more details.

• publishMessages (messages: Message[], opt?: OptionsObj) => Promise<void>): Publish several Messages (of the appropriate type) to the message bus. Options are similar to the publishMessage options.

• getAggregateAndPublishMessage ((message: NotificationMessage) => Promise<void>): (StateCarryingMessageBuses only) Append the matching aggregate (with correct version) to a NotificationMessage and turn it into a StateCarryingMessage before publishing it to the message bus. Uses the message bus event stores: Make sure that they have correct adapters set up.

Type Helpers:

• MessageChannelMessage: Given a MessageBus, returns the TS type of its messages

import type { MessageChannelMessage } from '@castore/core';

type AppMessage = MessageChannelMessage<typeof appMessageBus>;

// 👇 Equivalent to:
type AppMessage = EventStoreNotificationMessage<
  typeof pokemonsEventStore | typeof trainersEventStore...
>;

- MessageBusAdapter

Similarly to event stores, MessageBus classes provide a boilerplate-free and type-safe interface to publish messages, but are NOT responsible for actually doing so. This is the responsibility of the MessageBusAdapter, that will connect it to your actual messaging solution:

import { EventStore } from '@castore/core';

const messageBus = new NotificationMessageBus({
  ...
  // 👇 Provide it in the constructor
  messageBusAdapter: mySuperMessageBusAdapter,
});

// 👇 ...or set/switch it in context later
messageBus.messageChannelAdapter = mySuperMessageBusAdapter;
// Named `messageChannelAdapter` as buses inherit from the `MessageChannel` class

You can code your own MessageBusAdapter (simply implement the MessageChannelAdapter interface), but we highly recommend using an off-the-shelf adapter:

• EventBridge Message Bus Adapter
• In-Memory Message Bus Adapter

If the messaging solution that you use is missing, feel free to create/upvote an issue, or contribute 🤗

The adapter packages will also expose useful generics to type the arguments of your bus listeners. For instance:

import type { EventBridgeMessageBusMessage } from '@castore/event-bridge-message-bus-adapter';

const pokemonMessagesListener = async (
  // 👇 Specify that you only listen to the pokemonsEventStore messages
  eventBridgeMessage: EventBridgeMessageBusMessage<
    typeof appMessageQueue,
    'POKEMONS'
  >,
) => {
  // 👇 Correctly typed!
  const message = eventBridgeMessage.detail;
};

- ConnectedEventStore

If your storage solution exposes data streaming capabilities (such as DynamoDB streams), you can leverage them to push your freshly written events to a message bus or queue.

You can also use the ConnectedEventStore class. Its interface matches the EventStore one, but successfully pushing a new event will automatically forward it to a message queue/bus, and pushing a event group will also automatically forward the events to their respective message queues/buses:

import { ConnectedEventStore } from '@castore/core';

const connectedPokemonsEventStore = new ConnectedEventStore(
  // 👇 Original event store
  pokemonsEventStore,
  // 👇 Type-safe (appMessageBus MUST be able to carry pokemon events)
  appMessageBus,
);

// Will push the event in the event store
// ...AND publish it to the message bus if it succeeds 🙌
await connectedPokemonsEventStore.pushEvent({
  aggregateId: pokemonId,
  version: 2,
  type: 'POKEMON_LEVELED_UP',
  ...
});

Note that setting a connected event store storageAdapter and onEventPushed properties will override those of the original event store instead.

If the message bus or queue is a state-carrying one, the pushEvent method will re-fetch the aggregate to append it to the message before publishing it. You can reduce this overhead by providing the previous aggregate as an option:

await connectedPokemonsEventStore.pushEvent(
  {
    aggregateId: pokemonId,
    version: 2,
    ...
  },
  // 👇 Aggregate at version 1
  { prevAggregate: pokemonAggregate },
  // Removes the need to re-fetch 🙌
);

Compared to data streams, connected event stores have the advantage of simplicity, performances and costs. However, they strongly decouple your storage and messaging solutions: Make sure to anticipate any issue that might arise (consistency, non-caught errors etc.).

🔧 Technical description

Constructor:

• eventStore (EventStore): The event store to connect
• messageChannel (MessageBus | MessageQueue): A message bus or queue to forward events to

Properties:

A ConnectedEventStore will implement the interface of its original EventStore, and extend it with two additional properties:

• eventStore (EventStore): The original event store

const eventStore = connectedPokemonsEventStore.eventStore;
// => pokemonsEventStore

• messageChannel (MessageBus | MessageQueue): The provided message bus or queue

const messageChannel = connectedPokemonsEventStore.messageChannel;
// => appMessageBus

Note that the storageAdapter property will act as a pointer toward the original event store storageAdapter:

originalEventStore.storageAdapter = myStorageAdapter;
connectedEventStore.storageAdapter; // => myStorageAdapter

connectedEventStore.storageAdapter = anotherStorageAdapter;
originalEventStore.storageAdapter; // => anotherStorageAdapter

- Snapshotting

As events pile up in your event stores, the performances and costs of your commands can become an issue.

One solution is to periodially persist snapshots of your aggregates (e.g. through a message bus listener), and only fetch them plus the subsequent events instead of all the events.

Snapshots are not implemented in Castore yet, but we have big plans for them, so stay tuned 🙂

- Read Models

Even with snapshots, using the event store for querying needs (like displaying data in a web page) would be slow and inefficient, if not impossible depending on the access pattern.

In Event Sourcing, it is common to use a special type of message bus listener called projections, responsible for maintaining data specifically designed for querying needs, called read models.

Read models allow for faster read operations, as well as re-indexing. Keep in mind that they are eventually consistent by design, which can be annoying in some use cases (like opening a resource page directly after its creation).

Read models are not implemented in Castore yet, but we have big plans for them, so stay tuned 🙂

📖 Resources

- Test Tools

Castore comes with a handy Test Tool package that facilitates the writing of unit tests: It allows mocking event stores, populating them with an initial state and resetting them to it in a boilerplate-free and type-safe way.

- Dam

Dam is a suite of utils that facilitates data migration and maintenance operations with Castore (for instance, dispatching all the events of an event store - ordered by their timestamps - to a message queue).

- React Visualizer

Castore also comes with a handy React Visualizer library: It exposes a React component to visualize, design and manually test Castore event stores and commands.

- Packages List

Event Types

• JSON Schema Event Type: DRY EventType definition using JSON Schemas and json-schema-to-ts
• Zod Event Type: DRY EventType definition using zod

Event Storage Adapters

• DynamoDB Event Storage Adapter: Implementation of the EventStorageAdapter interface based on DynamoDB.
• Redux Event Storage Adapter: Implementation of the EventStorageAdapter interface based on a Redux store, along with tooling to configure the store and hooks to read from it efficiently.
• In-Memory Event Storage Adapter: Implementation of the EventStorageAdapter interface using a local Node/JS object. To be used in manual or unit tests.

Commands

• JSON Schema Command: DRY Command definition using JSON Schemas and json-schema-to-ts

Message Queue Adapters

• SQS Message Queue Adapter: Implementation of the MessageQueueAdapter interface based on AWS SQS.
• In-Memory Message Queue Adapter: Implementation of the MessageQueueAdapter interface using a local Node/JS queue. To be used in manual or unit tests.

Message Buses Adapters

• EventBridge Message Bus Adapter: Implementation of the MessageBusAdapter interface based on AWS EventBridge.
• In-Memory Message Bus Adapter: Implementation of the MessageBusAdapter interface using a local Node/JS event emitter. To be used in manual or unit tests.

- Common Patterns

• Simulating a future/past aggregate state: ...coming soon
• Snapshotting: ...coming soon
• Projecting on read models: ...coming soon
• Replaying events: ...coming soon
• Migrating events: ...coming soon