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Tutorial

Building an application using the Tasklist API and NestJS​

The Tasklist API provides a simple way for you to build apps powered by BPMN that require human interaction.

With this example, we'll use NestJS (one of the most popular Node.js backend frameworks) to a build a loan request review application.

Getting started​

For this tutorial we'll need:

Before moving forward​

If you have all the prerequisites from the getting started section above, you should have cloned a repo with the complete demo application we're going to build over this tutorial. The default branch in this repo has the complete application, so we need to checkout to the branch 0-getting-started before proceeding.

Inside the repo folder, you'll find some files and two folders, one of these folders is called demo-data/ and the other frontend/. As it might be evident inside each of these folders, there are two different projects.

The former will be responsible by deploying the demo process into Zeebe and generating instances for that process. The latter is a front-end application that will consume our API; this project is bootstrapped with Vite, bulma for styling, and react-query

Creating a new NestJS application​

Now let's bootstrap our NestJS app. Take the following steps:

  1. Open your terminal and go to the cloned repository folder.
  2. Run nest new api.
  3. Pick yarn as a package manager.

This will create the NestJS project for us inside the api/ folder. We can clean up the project a bit and remove the files api/app.controller.spec.ts, api/app.controller.ts, and api/app.service.ts.

We can also remove the references from the deleted files in api/app.module.ts. The file should look like this:

import { Module } from "@nestjs/common";

@Module({
  imports: [],
  controllers: [],
  providers: [],
})
export class AppModule {}

To check if everything is working as expected, run yarn workspace api run start:dev from the root folder on your terminal. You should see a message similar to the one below:

[00:00:00 AM] Starting compilation in watch mode...
[00:00:00 AM] Found 0 errors. Watching for file changes.
[Nest] 46621  - 00/00/0000, 00:00:00 AM     LOG [NestFactory] Starting Nest application...
[Nest] 46621  - 00/00/0000, 00:00:00 AM     LOG [InstanceLoader] AppModule dependencies initialized +12ms
[Nest] 46621  - 00/00/0000, 00:00:00 AM     LOG [NestApplication] Nest application successfully started +3ms

Generating the Tasklist service​

Inside the api/ folder we'll need to generate a service that will be responsible for accessing the Tasklist API. Take the following steps:

  1. Run nest g service.
  2. You'll be prompted to pick a name for the service, let's pick tasklist.
  3. Run yarn add @nestjs/axios.

A folder called tasklist/ will be created with the service definition and test; you can delete the tests if you wish. We also installed the package @nestjs/axios, so we can make requests to the Tasklist API. To make HTTP requests we need to inject the module into the service, like below:

import { Injectable } from "@nestjs/common";
import { HttpService } from "@nestjs/axios";

@Injectable()
export class TasklistService {
  constructor(private readonly http: HttpService) {}
}

Now, we're ready to make requests to the API. First, let's define a Data Transfer Object (DTO) with the shape of the tasks we're going to request. For that, we can create a file in tasklist/dto/task.dto.ts. There, we can define the DTO as follows:

type Variable = {
  name: string;
  value: string;
};

export class TaskDto {
  id: string;
  name: string;
  processName: string;
  creationTime: string;
  completionTime: string | null;
  assignee: string | null;
  variables: Variable[];
  taskState: "CREATED" | "COMPLETED" | "CANCELED";
  sortValues: [string, string];
  isFirst: boolean | null;
  formKey: string | null;
  processDefinitionId: string;
  taskDefinitionId: string;
}

We can implement the requests. For this, we need to define the Tasklist API query and define the methods on the service:

import { HttpService } from "@nestjs/axios";
import { Injectable } from "@nestjs/common";
import { firstValueFrom } from "rxjs";
import { TaskDto } from "./dto/task.dto";

const getTasksQuery = `
  query GetTasks($state: TaskState $pageSize: Int $searchAfter: [String!] $searchBefore: [String!] $taskDefinitionId: String!) {
    tasks(query: { state: $state pageSize: $pageSize searchAfter: $searchAfter searchBefore: $searchBefore taskDefinitionId: $taskDefinitionId }) {
      id
      creationTime
      variables {
        value
        name
      }
      taskState
      isFirst
      sortValues
    }
  }
`;

type QueryVariables = {
  pageSize?: number;
  searchAfter?: [string, string];
  searchBefore?: [string, string];
  state?: "CREATED" | "COMPLETED";
  taskDefinitionId?: string;
};

@Injectable()
export class TasklistService {
  constructor(private readonly http: HttpService) {}

  async getTasks(variables: QueryVariables): Promise<TaskDto[]> {
    const { http } = this;
    const { errors, data } = (
      await firstValueFrom(
        http.post("/", {
          /*
           for simplicity we just used Axios here, but since the Tasklist API is a GraphQL API
           a package like `graphql-request` might be better suited for this
          */
          query: getTasksQuery,
          variables,
        })
      )
    ).data;

    if (errors) {
      // handle error
    }

    return data.tasks;
  }
}

To keep things concise, we have one query and one method here. To see the complete implementation, review this file.

Handling the Tasklist API authentication​

We have the implementation of our service, but we still can't make requests to the Tasklist API because we're not providing any credentials to the API.

To achieve this, we need to rename the file .env.example to .env (the file needs to be on the root because we'll reuse it to generate the demo data), and the content of this file should look like this:

ZEEBE_ADDRESS="<cluster-id>.bru-2.zeebe.camunda.io:443"
ZEEBE_CLIENT_ID="k2FKt_PNMrRUFQO-QOR9MtCygvGsT.sm"
ZEEBE_CLIENT_SECRET="C-o5WFhvoZKv4-oQGHWg~d2MObjdr-GUv3cdqRS3~6fCoHaLleEEwnOqRToQvWda"
ZEEBE_AUTHORIZATION_SERVER_URL="https://login.cloud.camunda.io/oauth/token"
TASKLIST_API_ADDRESS="https://bru-2.tasklist.camunda.io/<cluster-id>/graphql"
ZEEBE_AUTHORIZATION_AUDIENCE="tasklist.camunda.io"

You can find all this information on the API tab of the cluster page. The client id and secret should be on the file you downloaded in the getting started section.

Now that we have our credentials, we can authenticate and inject the JWT token into every request we make into Tasklist API.

For this, we need to turn our Tasklist service into part of a module. Run nest g module and name it tasklist, the same we named the service. This will generate the module file and update app.module.ts. We need to edit the app.module.ts file to use only the module:

import { Module } from "@nestjs/common";
import { TasklistModule } from "./tasklist/tasklist.module";

@Module({
  imports: [TasklistModule],
  controllers: [],
  providers: [],
})
export class AppModule {}

We can install the package @nestjs/config and finally implement the authentication:

import { Logger, Module, OnModuleInit } from "@nestjs/common";
import { ConfigModule, ConfigService } from "@nestjs/config";
import { HttpModule, HttpService } from "@nestjs/axios";
import { TasklistService } from "./tasklist.service";
import { firstValueFrom, map } from "rxjs";

type AuthResponse = {
  access_token: string;
  scope: string;
  expires_in: number;
  token_type: string;
};

@Module({
  imports: [
    HttpModule,
    ConfigModule.forRoot({
      envFilePath: "../.env",
    }),
  ],
  providers: [TasklistService],
  exports: [TasklistService, HttpModule, ConfigModule],
})
export class TasklistModule implements OnModuleInit {
  logger = new Logger(TasklistModule.name);

  constructor(
    private readonly http: HttpService,
    private readonly config: ConfigService
  ) {}

  public async onModuleInit() {
    const {
      http: { axiosRef },
      config,
      logger,
    } = this;
    const credentials = await this.fetchCredentials();

    logger.log("Tasklist credentials fetched");

    axiosRef.defaults.baseURL = config.get("TASKLIST_API_ADDRESS");
    axiosRef.defaults.headers[
      "Authorization"
    ] = `Bearer ${credentials.access_token}`;
    axiosRef.defaults.headers["Content-Type"] = "application/json";
    setTimeout(this.onModuleInit.bind(this), credentials.expires_in * 1000); // we need convert minutes to milliseconds
  }

  private async fetchCredentials() {
    const { http, config } = this;

    return firstValueFrom(
      http
        .post<AuthResponse>(config.get("ZEEBE_AUTHORIZATION_SERVER_URL"), {
          client_id: config.get("ZEEBE_CLIENT_ID"),
          client_secret: config.get("ZEEBE_CLIENT_SECRET"),
          audience: config.get("ZEEBE_AUTHORIZATION_AUDIENCE"),
          grant_type: "client_credentials",
        })
        .pipe(map((response) => response.data))
    );
  }
}

When this module is initialized, we can read the credentials using the @nestjs/config package, authenticate into the API, and inject the JWT into Axios. We also set a timeout to request a new token when the first one expires.

Creating your application API​

We're now able to implement our actual business logic, but first we need to install some packages to create our custom GraphQL API.

Run yarn add @nestjs/graphql graphql apollo-server-express.

We'll have to generate a module, a service, and a resource. To achieve this, run the following commands:

nest g module
nest g service
nest g resource

Use the name loanRequests for all options. For the resource generation, select the option GraphQL (code first) and you don't have to generate the CRUD entry points.

We can now change our app.module.ts file to its final form:

import { Module } from "@nestjs/common";
import { GraphQLModule } from "@nestjs/graphql";
import { LoanRequestsModule } from "./loan-requests/loan-requests.module";

@Module({
  imports: [
    GraphQLModule.forRoot({
      autoSchemaFile: true,
      playground: true,
    }),
    LoanRequestsModule,
  ],
})
export class AppModule {}

And the loan-requests/loan-requests.module.ts to:

import { Module } from "@nestjs/common";
import { LoanRequestsService } from "./loan-requests.service";
import { TasklistModule } from "src/tasklist/tasklist.module";
import { LoanRequestsResolver } from "./loan-requests.resolver";

@Module({
  imports: [TasklistModule],
  providers: [LoanRequestsResolver, LoanRequestsService],
  exports: [LoanRequestsService, TasklistModule],
})
export class LoanRequestsModule {}

We just need to implement the service, which will have three methods (one to get all requests, one to get a single request, and one to make a decision.)

We will also have four resolvers for the GraphQL API (two mutations and two queries).

Find the full implementation here.

You can run yarn start:dev inside the api/ folder and the NestJS app should start without errors.

To test your API, you can access localhost:3000/graphl on your browser and should see our custom GraphQL API playground.

Demo data generation and sample frontend​

To test our app with a real frontend, we can change the port inside api/main.ts to 6000. Then, run from the root folder yarn start:demo-data to start the backend, frontend, and demo data generation, or just yarn start if you don't need any new data.