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name state-change-slice
description builds a state-change slice from an event model

Overview

State Change Slices are slices that change the system by processing a command. Each slice implements the Command-Event pattern using event sourcing.

Implementation Steps

When creating a state-change slice, you MUST create the following files in src/slices/{SliceName}/:

  1. {SliceName}Command.ts - Command handler implementation
  2. {SliceName}.test.ts - Test specifications
  3. routes.ts - API endpoint (unless explicitly told not to)
  4. **src/slices/{SliceName}/ui-prompt.md - prompt to build the UI

Command Handler Structure

Every Command handler file follows this pattern:

1. Imports

import type {Command} from '@event-driven-io/emmett'
import {CommandHandler} from '@event-driven-io/emmett';
import {ContextEvents} from "../../events/ContextEvents";
import {findEventstore} from "../../common/loadPostgresEventstore";

2. Command Type Definition

Define the command with:

  • Command name (matches slice name)
  • Data fields from specification
  • Standard metadata fields
export type {CommandName}Command = Command<'{CommandName}', {
    // data fields from spec
}>;

3. State Type

Define state needed for business logic validation:

export type {CommandName}State = {
    // fields needed for validation
    // often empty {} if no validation needed
}

export const {CommandName}InitialState = (): {CommandName}State => ({
    // initial state
});

4. Evolve Function

Updates state based on events (event sourcing projection):

export const evolve = (
    state: {CommandName}State,
    event: ContextEvents,
): {CommandName}State => {
    const {type, data} = event;

    switch (type) {
        case "{EventName}":
            // update state based on event
            return { ...state, /* updates */ };
        default:
            return state;
    }
};

5. Decide Function

Business logic that validates command and returns events:

export const decide = (
    command: {CommandName}Command,
    state: {CommandName}State,
): ContextEvents[] => {
    // validation logic
    // throw errors if validation fails

    return [{
        type: '{EventName}',
        data: {
            // event data fields
        },
    }];
};

6. Command Handler & Export

const {CommandName}CommandHandler = CommandHandler<{CommandName}State, ContextEvents>({
    evolve,
    initialState: {CommandName}InitialState
});

export const handle{CommandName} = async (id: string, command: {CommandName}Command) => {
    const eventStore = await findEventstore()
    const result = await {CommandName}CommandHandler(eventStore, id, (state: {CommandName}State) => decide(command, state))
    return {
        nextExpectedStreamVersion: result.nextExpectedStreamVersion,
        lastEventGlobalPosition: result.lastEventGlobalPosition
    }
}

Key Patterns

Destructuring

Two patterns observed:

  1. Direct access: command.data.fieldName
  2. Destructure: const {field1, field2} = command.data;

Both are acceptable - choose based on readability.

State Validation

  • Only include fields in state that are needed for validation
  • Many simple commands have empty state {}
  • Complex validations (like AddLocation) track sets or maps in state

Testing

Every command MUST have tests using DeciderSpecification:

import {DeciderSpecification} from '@event-driven-io/emmett';
import {{CommandName}Command, {CommandName}State, decide, evolve} from "./{CommandName}Command";
import {describe, it} from "node:test";

describe('{CommandName} Specification', () => {
    const given = DeciderSpecification.for({
        decide,
        evolve,
        initialState: () => ({})
    });

    it('spec: {test description}', () => {
        const command: {CommandName}Command = {
            type: '{CommandName}',
            data: {
                // test data
            },
        }

        given([/* precondition events */])
            .when(command)
            .then([{
                type: '{EventName}',
                data: {
                    // expected event data
                },
            }])
    });
});

Routes (API Endpoint)

Unless explicitly told not to, create a routes.ts file:

import {Router, Request, Response} from 'express';
import {{CommandName}Command, handle{CommandName}} from './{CommandName}Command';
import {requireUser} from "../../supabase/requireUser";
import {WebApiSetup} from "@event-driven-io/emmett-expressjs";
import {assertNotEmpty} from "../../common/assertions";

export type {CommandName}RequestPayload = {
    // fields matching command data (all optional with ?)
}

export type {CommandName}Request = Request<
    Partial<{ id: string }>,
    unknown,
Partial<{CommandName}RequestPayload>
>;

export const api =
    (
        // external dependencies
    ): WebApiSetup =>
        (router: Router): void => {
            router.post('/api/{commandname}/:id', requireRestaurantAccess, async (req: {CommandName}Request, res: Response) => {
                const principal = await requireUser(req, res, false);
                if (principal.error) {
                    return res.status(401).json(principal);
                }

                try {
                    const command: {CommandName}Command = {
                        data: {
                            // map from req.body with assertNotEmpty
                        },
                        type: "{CommandName}"
                    }

                    if (!req.params.id) throw "no id provided"

                    const result = await handle{CommandName}(assertNotEmpty(req.params.id), command);

                    return res.status(200).json({
                        ok: true,
                        next_expected_stream_version: result.nextExpectedStreamVersion?.toString(),
                        last_event_global_position: result.lastEventGlobalPosition?.toString()
                    });
                } catch (err) {
                    console.error(err);
                    return res.status(500).json({ok: false, error: 'Server error'});
                }
            });
        };

Make sure to annotate it with OpenAPI annotations in the comments, so it´s picked up by the open-api ui.

Business Logic

In *Command.ts Evolve-Function provides the state we can use to validate:

export const evolve = (
    state: PlanVacationState,
    event: ContextEvents,
): PlanVacationState => {
    const {type, data} = event;

    switch (type) {
        // case "..Event":
        case 'VacationPlanned':
            state.plannedVacations.push({id: event.data.vacation_id, from: event.data.from, to: event.data.to})
            return state;
        case 'VacationCancelled':
            state.plannedVacations = state.plannedVacations.filter(it => it.id !== event.data.vacation_id)
            return state;
        default:
            return state;
    }
};

The Decide-Function then makes the decision ( success or error )

export const decide = (
    command: PlanVacationCommand,
    state: PlanVacationState,
): ContextEvents[] => {

    state.plannedVacations.forEach(vacation => {
        if (
            command.data.from <= vacation.to &&
            command.data.to >= vacation.from
        ) {
            throw {error: "conflicting_vacations"}
        }
    })

    return [{
        type: "VacationPlanned",
        data: {
          ...
        },
    }]
};

Error Handling

In routes.ts, define an error mapper:

const errorMapping = (error:string): string => {

    switch(error) {
        case "conflicting_vacations" : return "Achtung, Betriebsurlaub überschneidet sich."
        default: return "Leider ist ein Fehler aufgetreten"
    }
}

use this directly in the route.

  router.post('/api/planvacation/:id', requireRestaurantAccess, async (req: PlanVacationRequest, res: Response) => {
                ...
                } catch (err:any) {
                    console.error(err);
                    return res.status(500).json({ok: false, error:  errorMapping(err.error)});
                }
            });
        };

slice json

in each slice folder, generate a file .slice.json

{
    "id" : "<slice id>",
    "slice": "<slice title>",
    "context": "<contextx>",
    "link": "https://miro.com/app/board/<board-id>=/?moveToWidget=<slice id>"
}

References

  • See templates/AddLocation for samples

UI Prompt

to build the UI - use this endpont "endpoint URL"

Payload example:
<payload example as JSON>

make sure to put endpoints into the api.ts and follow the rules:
- provide all headers