Case Study

Payday Chat

A real-time, members-only network for online business founders.

The Problem

Founders needed a private, high-signal place to network, build teams, and scale revenue — not another noisy public feed. Payday Chat is invite-only and built around real-time collaboration.

My Role

Built solo with AI-assisted development — Flutter client, NestJS API, data model, real-time layer, and deployment.

Visit Platform ↗

Highlights

Real-time messaging with presence and live events over WebSockets
One NestJS API powering both a Flutter mobile app and a web client
PostgreSQL domain model with Redis for caching and pub/sub fan-out
NGINX gateway, Stripe payments, and OAuth authentication

Stack

FlutterDartNestJSPostgreSQLRedisNGINXStripe

Constraints

Invite-only product — auth and access control had to be correct from day one.
One backend, two clients (Flutter + web) — schema and contracts had to be shared, not duplicated.
Realtime is a feature, not a nice-to-have — presence and live events must survive reconnects.

System Architecture

Clients

Flutter Mobile App

Web App

Gateway

NGINX

API

NestJS REST

WebSocket Gateway

Data

PostgreSQL

Redis (cache + pub/sub)

External

Stripe

OAuth

Key Trade-offs

The decisions worth defending — what I chose, what I turned down, and why.

Realtime transport

Chose

WebSockets with Redis pub/sub fan-out

Rejected

Long-polling or third-party realtime SaaS

Predictable latency, no per-message vendor cost, and Redis already in the stack for caching — one fewer moving part.

Mobile client framework

Chose

Flutter (single codebase for iOS + Android)

Rejected

Native Swift + Kotlin clients

Solo build — two native clients would have doubled the surface area and slowed iteration on the API.

API style

Chose

REST + dedicated WebSocket gateway

Rejected

GraphQL subscriptions

Simpler operational story, easier to cache at the NGINX layer, and the realtime channel stays an explicit, observable component.

What I'd Do Differently

An honest retrospective — the stuff I'd change with more time, more users, or a second pass.

1Introduce contract tests between the NestJS API and the Flutter client earlier — a few breakages were caught only at runtime.
2Move long-lived sockets to a dedicated process so API deploys don't drop client connections.
3Add structured event versioning from day one instead of retrofitting it once the schema started moving.

Technical Deep-Dive

Architecture, specifications, and implementation details.

New Module Checklist — Adding a NestJS Module

Follow this every time you add a new feature module to Payday.

#1. Generate Files

# Run inside the api container
docker compose exec api npx nest g module feature-name
docker compose exec api npx nest g controller feature-name
docker compose exec api npx nest g service feature-name

This creates:

src/feature-name/
├── feature-name.module.ts
├── feature-name.controller.ts
└── feature-name.service.ts

#2. Add Entity

Create src/feature-name/entities/feature-name.entity.ts:

import { Entity, Column, PrimaryGeneratedColumn, CreateDateColumn, UpdateDateColumn, ManyToOne, JoinColumn } from 'typeorm';
import { UserEntity } from '../../members/entities/user.entity';

@Entity('feature_name')    // snake_case table name
export class FeatureNameEntity {
  @PrimaryGeneratedColumn('uuid')
  id: string;

  @Column({ name: 'user_id' })
  userId: string;

  @ManyToOne(() => UserEntity, { onDelete: 'CASCADE' })
  @JoinColumn({ name: 'user_id' })
  user: UserEntity;

  @Column({ length: 255 })
  title: string;

  @CreateDateColumn({ name: 'created_at' })
  createdAt: Date;

  @UpdateDateColumn({ name: 'updated_at' })
  updatedAt: Date;
}

#3. Add DTOs

src/feature-name/dto/
├── create-feature-name.dto.ts
├── update-feature-name.dto.ts
└── query-feature-name.dto.ts

// create-feature-name.dto.ts
import { IsString, IsNotEmpty, IsOptional, MaxLength } from 'class-validator';
import { ApiProperty, ApiPropertyOptional } from '@nestjs/swagger';

export class CreateFeatureNameDto {
  @ApiProperty({ example: 'My title' })
  @IsString()
  @IsNotEmpty()
  @MaxLength(255)
  title: string;

  @ApiPropertyOptional()
  @IsOptional()
  @IsString()
  description?: string;
}

#4. Wire up the Module

// feature-name.module.ts
@Module({
  imports: [
    TypeOrmModule.forFeature([FeatureNameEntity]),
    // Import other modules if needed
  ],
  controllers: [FeatureNameController],
  providers: [FeatureNameService],
  exports: [FeatureNameService],  // Export only if other modules need this service
})
export class FeatureNameModule {}

#5. Register in AppModule

// app.module.ts — add to imports array:
import { FeatureNameModule } from './feature-name/feature-name.module';

@Module({
  imports: [
    // ... existing modules ...
    FeatureNameModule,  // Add here
  ],
})

#6. Controller Boilerplate

@ApiTags('feature-name')
@ApiBearerAuth('JWT')
@UseGuards(JwtAuthGuard)
@Controller('feature-name')
export class FeatureNameController {
  constructor(private readonly service: FeatureNameService) {}

  @Post()
  @ApiOperation({ summary: 'Create a feature' })
  @ApiResponse({ status: 201, description: 'Created successfully' })
  create(
    @CurrentUserId() userId: string,
    @Body() dto: CreateFeatureNameDto,
  ) {
    return this.service.create(userId, dto);
  }

  @Get()
  @ApiOperation({ summary: 'List features (paginated)' })
  findAll(
    @CurrentUserId() userId: string,
    @Query() query: PaginationDto,
  ) {
    return this.service.findAll(userId, query);
  }

  @Get(':id')
  findOne(@Param('id', ParseUUIDPipe) id: string) {
    return this.service.findOne(id);
  }

  @Patch(':id')
  update(
    @CurrentUserId() userId: string,
    @Param('id', ParseUUIDPipe) id: string,
    @Body() dto: UpdateFeatureNameDto,
  ) {
    return this.service.update(userId, id, dto);
  }

  @Delete(':id')
  @HttpCode(HttpStatus.NO_CONTENT)
  remove(
    @CurrentUserId() userId: string,
    @Param('id', ParseUUIDPipe) id: string,
  ) {
    return this.service.remove(userId, id);
  }
}

#7. Service Boilerplate

@Injectable()
export class FeatureNameService {
  constructor(
    @InjectRepository(FeatureNameEntity)
    private readonly repo: Repository<FeatureNameEntity>,
  ) {}

  async create(userId: string, dto: CreateFeatureNameDto) {
    const entity = this.repo.create({ ...dto, userId });
    return this.repo.save(entity);
  }

  async findAll(userId: string, { page = 1, limit = 20 }: PaginationDto) {
    const [data, total] = await this.repo.findAndCount({
      where: { userId },
      order: { createdAt: 'DESC' },
      skip: (page - 1) * limit,
      take: limit,
    });
    return paginate(data, total, page, limit);
  }

  async findOne(id: string) {
    const entity = await this.repo.findOne({ where: { id } });
    if (!entity) throw new NotFoundException('Not found');
    return entity;
  }

  async update(userId: string, id: string, dto: UpdateFeatureNameDto) {
    const entity = await this.findOne(id);
    if (entity.userId !== userId) throw new ForbiddenException('Not your resource');
    Object.assign(entity, dto);
    return this.repo.save(entity);
  }

  async remove(userId: string, id: string) {
    const entity = await this.findOne(id);
    if (entity.userId !== userId) throw new ForbiddenException('Not your resource');
    await this.repo.softDelete(id);
  }
}

#8. Checklist Before Committing

Entity has @Entity('table_name') with snake_case table name
All DTO fields have class-validator decorators + @ApiProperty
Controller has @ApiTags, @ApiBearerAuth, @UseGuards(JwtAuthGuard)
All route params use ParseUUIDPipe for UUID validation
Service never throws untyped errors — always NotFoundException, ForbiddenException, etc.
Service ownership check before update/delete (compare entity.userId !== userId)
Module imported in AppModule
Module exported service if other modules need it
Swagger docs load without errors: http://localhost/docs

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