Medusa Development

Medusa Development

Medusa is a headless commerce framework built around modules, data models, and workflows; almost every piece of business logic — from an API route to a scheduled job — should be expressed as a workflow made of discrete, composable steps.

Workflow for Building a Medusa Feature

1. Define or extend the data model — Use the model utility from @medusajs/framework/utils to declare the module's data model(s) under src/modules/<module>/models/.
2. Write the module service — Create a service in src/modules/<module>/service.ts that extends MedusaService when the module has data models, exposing async methods for domain operations.
3. Register the module — Add the module to medusa-config.ts so Medusa's container can resolve it.
4. Build steps — Define each unit of work as a step with createStep from @medusajs/framework/workflows-sdk, including a compensation function for anything that needs to be undone on failure.
5. Compose the workflow — Wire steps together with createWorkflow, using transform for data shaping and when for conditional branches.
6. Expose the workflow — Call the workflow from an API route, a scheduled job, or a subscriber — never put business logic directly in the route/job/subscriber handler.
7. Read data with Query — Use Medusa's Query (req.scope.resolve("query") or the workflow-level useQueryGraphStep) to fetch data instead of calling module services directly for reads.

General Rules

• Don't use type aliases when importing files — import types and values directly from their source module rather than re-exporting through a local alias.
• When throwing errors, always throw MedusaError (from @medusajs/framework/utils) instead of a plain Error, so the API layer can map it to the correct HTTP status and error code.
• Always use Query to retrieve data rather than calling a module's service methods directly for reads — Query understands module links and can join data across modules in one call.

import { MedusaError } from "@medusajs/framework/utils"

if (!product) {
  throw new MedusaError(
    MedusaError.Types.NOT_FOUND,
    `Product with id "${productId}" was not found`
  )
}

Data Model Rules

• Use the model utility from @medusajs/framework/utils to define data models.
• Data model variables should be camelCase; the name passed to model.define should be snake_case.
• When adding an id field to a data model, always make it a primary key with .primaryKey().
• A data model can have only one id field — any other identifier should be a text field instead.
• Data model fields should be snake_case.

// src/modules/loyalty/models/loyalty-account.ts
import { model } from "@medusajs/framework/utils"

const LoyaltyAccount = model.define("loyalty_account", {
  id: model.id().primaryKey(),
  customer_id: model.text(),
  points_balance: model.number().default(0),
  tier: model.enum(["bronze", "silver", "gold"]).default("bronze"),
})

export default LoyaltyAccount

Service Rules

• When creating a service, always make its methods async.
• If a module has data models, make the service extend MedusaService so it inherits generated CRUD methods for each model.

// src/modules/loyalty/service.ts
import { MedusaService } from "@medusajs/framework/utils"
import LoyaltyAccount from "./models/loyalty-account"

class LoyaltyModuleService extends MedusaService({
  LoyaltyAccount,
}) {
  async addPoints(accountId: string, points: number) {
    const account = await this.retrieveLoyaltyAccount(accountId)
    return await this.updateLoyaltyAccounts({
      id: account.id,
      points_balance: account.points_balance + points,
    })
  }
}

export default LoyaltyModuleService

Workflow Rules

• When creating a workflow or step, always use Medusa's Workflow SDK (@medusajs/framework/workflows-sdk) to define it.
• When creating a feature in an API route, scheduled job, or subscriber, always create a workflow for it rather than inlining the logic in the handler.
• When creating a workflow, always create a step for each discrete unit of work in it.
• In workflows, use transform for any data transformation between steps — don't manipulate step output directly in the workflow function body.
• In workflows, use when to define conditional branches instead of a plain if around step calls.
• Don't use await when calling steps inside a workflow — step invocation returns a special reference the workflow engine resolves, and await-ing it breaks the workflow's ability to orchestrate compensation and retries.
• In workflows, don't make the workflow function itself async — the function body only describes the step graph, it doesn't execute imperatively.
• Don't add typing to a compensation function's input — the compensation function receives whatever the step's invoke function returned, and Medusa infers this automatically.
• Only use steps in a workflow — don't call services, Query, or other side-effecting code directly inside the workflow function; put that logic in a step.

// src/workflows/redeem-loyalty-points.ts
import {
  createStep,
  createWorkflow,
  StepResponse,
  transform,
  when,
  WorkflowResponse,
} from "@medusajs/framework/workflows-sdk"
import { MedusaError } from "@medusajs/framework/utils"
import { LOYALTY_MODULE } from "../modules/loyalty"
import LoyaltyModuleService from "../modules/loyalty/service"

type RedeemPointsInput = {
  accountId: string
  points: number
}

const deductPointsStep = createStep(
  "deduct-points-step",
  async (input: RedeemPointsInput, { container }) => {
    const loyaltyService: LoyaltyModuleService = container.resolve(LOYALTY_MODULE)
    const account = await loyaltyService.retrieveLoyaltyAccount(input.accountId)

    if (account.points_balance < input.points) {
      throw new MedusaError(
        MedusaError.Types.INVALID_DATA,
        "Insufficient points balance"
      )
    }

    const previousBalance = account.points_balance
    const updated = await loyaltyService.updateLoyaltyAccounts({
      id: account.id,
      points_balance: previousBalance - input.points,
    })

    return new StepResponse(updated, { accountId: account.id, previousBalance })
  },
  async (compensationInput, { container }) => {
    if (!compensationInput) return
    const loyaltyService: LoyaltyModuleService = container.resolve(LOYALTY_MODULE)
    await loyaltyService.updateLoyaltyAccounts({
      id: compensationInput.accountId,
      points_balance: compensationInput.previousBalance,
    })
  }
)

export const redeemLoyaltyPointsWorkflow = createWorkflow(
  "redeem-loyalty-points",
  (input: RedeemPointsInput) => {
    const account = deductPointsStep(input)

    const tierDowngrade = when(account, (acc) => acc.points_balance === 0)
      .then(() => {
        return transform({ account }, (data) => ({
          ...data.account,
          tier: "bronze" as const,
        }))
      })

    return new WorkflowResponse(account)
  }
)

API Routes and Reading Data

• Expose workflows through API routes under src/api/; the route handler should validate input, call the workflow, and shape the HTTP response — nothing more.
• Always use Query to retrieve data for reads (list/detail endpoints) instead of resolving a module service directly.

// src/api/store/loyalty/[id]/route.ts
import type { MedusaRequest, MedusaResponse } from "@medusajs/framework/http"

export async function GET(req: MedusaRequest, res: MedusaResponse) {
  const query = req.scope.resolve("query")

  const { data: accounts } = await query.graph({
    entity: "loyalty_account",
    fields: ["id", "points_balance", "tier"],
    filters: { id: req.params.id },
  })

  res.json({ loyalty_account: accounts[0] })
}

export async function POST(req: MedusaRequest, res: MedusaResponse) {
  const { result } = await redeemLoyaltyPointsWorkflow(req.scope).run({
    input: req.validatedBody as { accountId: string; points: number },
  })

  res.json({ loyalty_account: result })
}

Subscribers and Scheduled Jobs

• Subscribers and scheduled jobs should call a workflow, exactly like API routes — they are just a different trigger for the same business logic.

// src/subscribers/order-placed.ts
import type { SubscriberArgs, SubscriberConfig } from "@medusajs/framework"
import { redeemLoyaltyPointsWorkflow } from "../workflows/redeem-loyalty-points"

export default async function orderPlacedHandler({ event, container }: SubscriberArgs<{ id: string }>) {
  await redeemLoyaltyPointsWorkflow(container).run({
    input: { accountId: event.data.id, points: 0 },
  })
}

export const config: SubscriberConfig = { event: "order.placed" }

Admin Customization Rules

• When sending requests from admin customizations (widgets, custom pages), always use Medusa's JS SDK (@medusajs/js-sdk) rather than raw fetch.
• Use TailwindCSS for styling admin customizations, matching the conventions of Medusa Admin's own UI.

// src/admin/widgets/loyalty-widget.tsx
import { defineWidgetConfig } from "@medusajs/admin-sdk"
import { useQuery } from "@tanstack/react-query"
import { sdk } from "../lib/sdk"

const LoyaltyWidget = ({ data }: { data: { id: string } }) => {
  const { data: result } = useQuery({
    queryFn: () => sdk.admin.customer.retrieve(data.id),
    queryKey: ["customer", data.id],
  })
  return <p className="text-ui-fg-subtle">{result?.customer.email}</p>
}

export const config = defineWidgetConfig({ zone: "customer.details.after" })
export default LoyaltyWidget

Common Mistakes

• Calling a module service directly from an API route handler instead of going through a workflow — this skips retries, compensation, and the standard event/observability hooks workflows provide.
• Throwing a plain Error instead of MedusaError, which loses the mapped HTTP status code and structured error type on the API response.
• Awaiting a step call inside a workflow function, which breaks the workflow engine's ability to build the step graph.
• Reading data by resolving a module service instead of using Query, which misses cross-module joins and links that Query resolves automatically.
• Naming a data model field or model.define name in camelCase instead of snake_case, causing inconsistency with the rest of the schema.

Additional Resources

• Medusa Documentation: https://docs.medusajs.com/llms-full.txt