feat: add group existence validation for all mutations

[takaokouji]

Summary

Implements comprehensive group existence validation for all group-related mutations to ensure that operations on dissolved groups fail gracefully with appropriate error messages.

Resolves #8

Implementation Details

Approach: Option C (Full Protection)

Implemented group existence validation for all three critical mutations:

1. joinGroup: Uses ConditionCheck in TransactWriteItems

   • Atomic validation within the transaction
   • Returns DynamoDB error: ConditionalCheckFailed
   • No additional DynamoDB requests

2. reportDataByNode: Converted to Pipeline Resolver

   • Before Function: checkGroupExists (GetItem)
   • Main Function: Original reportDataByNode logic
   • Returns custom error: GroupNotFound with message "does not exist"

3. fireEventByNode: Converted to Pipeline Resolver

   • Before Function: checkGroupExists (GetItem)
   • Main Function: Original fireEventByNode logic
   • Returns custom error: GroupNotFound with message "does not exist"

New Components

Pipeline Function: js/functions/checkGroupExists.js

• Reusable group existence validation
• Performs DynamoDB GetItem for GROUP#METADATA
• Returns custom GroupNotFound error on failure
• Used by both reportDataByNode and fireEventByNode

CDK Stack Updates: lib/mesh-v2-stack.ts

• Created CheckGroupExistsFunction (AppSync Function)
• Created ReportDataByNodeFunction (AppSync Function)
• Created FireEventByNodeFunction (AppSync Function)
• Registered Pipeline Resolvers for both mutations

Resolver Updates: js/resolvers/Mutation.joinGroup.js

• Added ConditionCheck as first item in TransactWriteItems
• Validates group existence before creating node records

Cost Optimization

Why this approach is cost-effective:

• Write operations (PutItem): $1.25 per million requests
• Read operations (GetItem): $0.25 per million requests
• Ratio: Writes are 5x more expensive than reads

Cost Savings:

• Prevents wasteful PutItem operations to deleted groups
• One GetItem ($0.25/M) can prevent one PutItem ($1.25/M)
• Net savings: $1.00 per million prevented writes
• For dissolved groups receiving high-frequency updates (15 ops/sec), savings can be substantial

Example Calculation:

• Group dissolved but client continues sending data for 1 minute
• Prevented writes: 15 ops/sec × 60 sec = 900 requests
• Saved cost: 900 × ($1.25 - $0.25) / 1,000,000 = $0.0009 per incident
• With multiple groups and clients, savings accumulate

Test Coverage

New Integration Tests: spec/requests/group_existence_validation_spec.rb

7 comprehensive test cases covering:

1. ✅ joinGroup: Non-existent group → Error
2. ✅ joinGroup: Dissolved group → Error
3. ✅ reportDataByNode: Non-existent group → Error
4. ✅ reportDataByNode: Dissolved group → Error
5. ✅ fireEventByNode: Non-existent group → Error
6. ✅ fireEventByNode: Dissolved group → Error
7. ✅ E2E: All mutations fail after dissolveGroup

Test Results:

Group Existence Validation
  joinGroup mutation
    グループが存在しない場合
      エラーを返す
    グループが削除された後
      エラーを返す
  reportDataByNode mutation
    グループが存在しない場合
      エラーを返す
    グループが削除された後
      エラーを返す
  fireEventByNode mutation
    グループが存在しない場合
      エラーを返す
    グループが削除された後
      エラーを返す
  E2E: dissolveGroup後のすべてのmutation検証
    dissolveGroup後、すべてのグループ操作がエラーを返す

Finished in 4.67 seconds
7 examples, 0 failures

Error Messages

joinGroup (ConditionCheck):

Transaction cancelled, please refer cancellation reasons for specific reasons 
[ConditionalCheckFailed...]

reportDataByNode & fireEventByNode (Pipeline Resolver):

Group {groupId}@{domain} does not exist or has been dissolved
ErrorType: GroupNotFound

Breaking Changes

None. This is backward compatible - only adds validation, doesn't change existing successful operations.

Deployment

Staging Deployment:

• Stack: MeshV2Stack-stg
• Endpoint: https://mpe3yhgk6zdxfhjbay2vqcq5qe.appsync-api.ap-northeast-1.amazonaws.com/graphql
• All tests passed on staging environment

Files Changed

• js/functions/checkGroupExists.js (new) - Reusable validation function
• js/resolvers/Mutation.joinGroup.js (modified) - Added ConditionCheck
• lib/mesh-v2-stack.ts (modified) - Pipeline Resolver configuration
• spec/requests/group_existence_validation_spec.rb (new) - Integration tests

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🤖 Generated with Claude Code

Co-Authored-By: Claude Sonnet 4.5 noreply@anthropic.com

[takaokouji]

JavaScript Client Implementation Complete

Added real-time group dissolution detection to the JavaScript client prototype (examples/javascript-client/).

Changes

Implementation (app.js):

• Added handleGroupDissolved callback function (lines 706-735)
  • Unsubscribes from all active subscriptions (data, events, dissolve)
  • Clears group state and updates UI
  • Displays error notification to user
• Integrated subscription call in handleJoinGroup (line 384-388)

Documentation (README.md):

• Updated subscription documentation to reflect WebSocket implementation
• Added group dissolution detection to features section
• Added test scenarios for dissolution detection
• Updated implementation status to show all features complete

Testing

To test with 2 browser windows:

1. Window 1 (Host): Create and join a group
2. Window 2 (Member): Join the same group
3. Window 1: Click "Dissolve Group"
4. Expected: Window 2 automatically detects dissolution and shows error message

Implementation Status

✅ Backend: Group existence validation (Phase 2-1, 2-2, 2-4)
✅ Backend: dissolveGroup with Lambda (Phase 2-4)
✅ Backend: onGroupDissolve subscription (Phase 2-4)
✅ Frontend: JavaScript client dissolution detection (Phase 2-4)

This completes the full end-to-end implementation of group dissolution detection.

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