feat: adding kotlin sample

[pengying]

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[pengying]

• fix(samples): filter out heartbeat messages from webhook stream UI #198
• feat(samples): add TypeScript backend using Grid TypeScript SDK #197
• feat: adding kotlin sample #192 👈 (View in Graphite)
• main

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[greptile-apps]

Greptile Overview

Greptile Summary

This PR adds a comprehensive Kotlin sample application for the Grid API, demonstrating the complete payout flow with a React frontend and Ktor backend. The implementation includes a step-by-step wizard for creating customers, linking external accounts, creating quotes, and executing payments, with real-time webhook streaming via SSE.

Key Changes:

• Kotlin backend using Ktor 3.x with thin proxy pattern - translates JSON requests to Grid SDK calls
• React frontend with Vite, TypeScript, and Tailwind CSS implementing wizard flow
• Gradle integration that builds frontend and serves it from backend resources
• Webhook verification and SSE streaming for real-time event updates
• Comprehensive documentation including architecture diagrams and setup instructions
• Proper error handling, structured logging with colorized output

Architecture:
The backend acts as a credential-holding proxy while the frontend orchestrates the user flow. Webhooks are verified, unwrapped using the Grid SDK, and broadcast to frontend clients via Server-Sent Events. The design enables easy replication in other languages by reusing the shared frontend.

The implementation is production-quality for a sample app with proper security practices (webhook signature verification, credential management via environment variables).

Confidence Score: 4/5

• Safe to merge with minor ambiguity in quote source detection logic
• Well-structured sample application with proper architecture, comprehensive documentation, and secure webhook handling. The buildQuoteSource logic in Quotes.kt:91 has ambiguous detection (checks both sourceType and presence of currency field) but the frontend consistently sends sourceType: "REALTIME_FUNDING" so it works in practice. All other routes are clean with proper error handling.
• samples/kotlin/src/main/kotlin/com/grid/sample/routes/Quotes.kt - review the source type detection logic

Important Files Changed

Filename	Overview
samples/kotlin/src/main/kotlin/com/grid/sample/Application.kt	Sets up Ktor server with CORS, SSE, routing, and serves frontend static files
samples/kotlin/src/main/kotlin/com/grid/sample/routes/Quotes.kt	Implements quote creation and execution routes; has ambiguous source type detection logic
samples/kotlin/src/main/kotlin/com/grid/sample/routes/Webhooks.kt	Handles webhook reception, unwraps events using SDK, broadcasts to SSE stream
samples/kotlin/src/main/kotlin/com/grid/sample/routes/ExternalAccounts.kt	Creates external bank accounts with proper beneficiary and account info handling
samples/kotlin/src/main/kotlin/com/grid/sample/routes/Customers.kt	Creates individual customers with proper optional field handling
samples/kotlin/build.gradle.kts	Gradle build with node plugin for frontend integration, proper dependencies
samples/frontend/src/components/WebhookStream.tsx	SSE-connected webhook stream component with auto-reconnect and expandable events
samples/frontend/vite.config.ts	Vite config with API proxy to backend and build output to Kotlin resources

Sequence Diagram

sequenceDiagram
    participant Browser
    participant Vite as Vite Dev Server
    participant Ktor as Ktor Backend
    participant Grid as Grid API
    participant Webhooks as Grid Webhooks

    Browser->>Vite: Load App (http://localhost:5173)
    Vite-->>Browser: React Frontend
    Browser->>Ktor: SSE Connection (/api/sse)
    Ktor-->>Browser: Connected

    Note over Browser,Ktor: Step 1: Create Customer
    Browser->>Vite: POST /api/customers
    Vite->>Ktor: Proxy to :8080
    Ktor->>Grid: customers.create()
    Grid-->>Ktor: Customer Response
    Ktor-->>Browser: Customer JSON

    Note over Browser,Ktor: Step 2: Create External Account
    Browser->>Vite: POST /api/customers/{id}/external-accounts
    Vite->>Ktor: Proxy to :8080
    Ktor->>Grid: externalAccounts.create()
    Grid-->>Ktor: Account Response
    Ktor-->>Browser: Account JSON

    Note over Browser,Ktor: Step 3: Create Quote
    Browser->>Vite: POST /api/quotes
    Vite->>Ktor: Proxy to :8080
    Ktor->>Grid: quotes.create()
    Grid-->>Ktor: Quote Response
    Ktor-->>Browser: Quote JSON

    Note over Browser,Ktor: Step 4: Execute Quote
    Browser->>Vite: POST /api/quotes/{id}/execute
    Vite->>Ktor: Proxy to :8080
    Ktor->>Grid: quotes.execute()
    Grid-->>Ktor: Updated Quote
    Ktor-->>Browser: Quote JSON

    Note over Browser,Ktor: Step 5: Sandbox Fund
    Browser->>Vite: POST /api/sandbox/send-funds
    Vite->>Ktor: Proxy to :8080
    Ktor->>Grid: sandbox.sendFunds()
    Grid-->>Ktor: Funding Response
    Ktor-->>Browser: Confirmation JSON

    Note over Webhooks,Browser: Webhook Flow
    Webhooks->>Ktor: POST /api/webhooks
    Ktor->>Ktor: Verify signature & unwrap
    Ktor->>Ktor: Broadcast to WebhookStream
    Ktor-->>Browser: SSE event with webhook data

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