feat: implement polling fallback for event communication

[takaokouji]

Summary

Server-side implementation of polling-based event notification fallback for Mesh v2. Adds recordEventsByNode mutation and getEventsSince query to support environments where WebSocket connections are blocked.

Implementation Details

GraphQL Schema Changes

New Types and Fields

• RecordEventsPayload: Response type for recordEventsByNode mutation
  • nextSince: Cursor for next getEventsSince call
• Event.cursor: Sort Key (SK) for cursor-based pagination
• Group.useWebSocket: Protocol selection flag
• Group.pollingIntervalSeconds: Polling interval (when useWebSocket=false)
• Node.useWebSocket: Inherited from group
• Node.pollingIntervalSeconds: Inherited from group

New Mutations

• createGroup: Added useWebSocket parameter
• recordEventsByNode: Records events to DynamoDB (polling only)

New Queries

• getEventsSince: Retrieves events since last fetch (cursor-based)

Server-side Timestamp Management

IMPORTANT: recordEventsByNode uses server-side timestamps, not client-side event.firedAt.

Reason: 1+ second delay exists from device event firing to recordEventsByNode call.

Implementation:

const serverTimestamp = util.time.nowISO8601();
const items = events.map(event => ({
  pk: `GROUP#${groupId}@${domain}`,
  sk: `EVENT#${serverTimestamp}#${util.autoId()}`,
  timestamp: serverTimestamp,  // Server-side timestamp
  ttl: util.time.nowEpochSeconds() + parseInt(ctx.env.MESH_EVENT_TTL_SECONDS)
}));

Benefits:

• Time consistency (no client clock skew)
• Order guarantee (request reception order)
• Consistency with since parameter in getEventsSince

Cursor-based Pagination

getEventsSince uses Sort Key (SK) as cursor:

// Request
const sk = since.startsWith('EVENT#') ? since : `EVENT#${since}`;
return {
  operation: 'Query',
  query: {
    expression: 'pk = :pk AND sk > :sk',
    expressionValues: {
      ':pk': `GROUP#${groupId}@${domain}`,
      ':sk': sk
    }
  },
  limit: 100
};

// Response
return ctx.result.items.map(item => ({
  // ...
  cursor: item.sk  // Return SK as cursor
}));

Environment Variables

• MESH_EVENT_TTL_SECONDS: 10 (events auto-delete after 10 seconds)
• MESH_POLLING_INTERVAL_SECONDS: 2 (poll every 2 seconds)

CDK Stack Changes

• Added environment variables to AppSync resolvers
• Added recordEventsByNode and getEventsSince resolvers
• Updated Lambda function to handle useWebSocket flag

Testing

Unit Tests

• recordEventsByNode saves events with server-side timestamps
• getEventsSince retrieves events with cursor pagination
• createGroup accepts useWebSocket parameter

Integration Tests

• Event recording and retrieval flow
• Cursor-based pagination
• TTL auto-deletion (10 seconds)

Cost Optimization

• WebSocket Priority: Uses WebSocket when available (no DynamoDB writes)
• Short TTL: Events auto-delete after 10 seconds to minimize storage cost
• Efficient Query: Cursor-based pagination with 100-item limit

Related

• Parent PR: feat: implement polling fallback for event communication smalruby3-develop#15
• Related Issue: feat: WebSocket 接続失敗時のポーリングによるイベント通知 smalruby3-develop#14

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