Detailed Design & Implementation

Data Design and Migration Strategy

Source of Truth and Ownership

During transition, Hub remains the authoritative source for core transactional domain data. Downstream systems maintain consumer-specific projections built from event notifications and query-driven retrieval.

Data Movement Patterns

  • Event notification communicates that a change occurred.
  • GraphQL retrieval provides consumer-specific field selection for synchronization.
  • REST commands provide controlled write-back into Hub for approved use cases.

Reference flow per domain transaction:

  1. Domain write is committed in Hub.
  2. Event is published with identifiers and correlation metadata.
  3. Consumer receives event and requests a targeted GraphQL projection.
  4. Consumer stores its projection and records event processing checkpoint.
  5. Reconciliation jobs periodically compare authoritative and projection state.

Data Integrity Controls

  • Idempotent event consumption using event identifiers and deduplication checkpoints.
  • Correlation IDs propagated across event, query, and command flows.
  • Replay/reconciliation capability for failed or delayed synchronization.
  • Validation-on-write in Hub to ensure domain invariants remain enforced.

Migration Approach

  • Baseline load: seed consumer stores from current state snapshots.
  • Incremental sync: apply event-driven updates post-baseline.
  • Reconciliation windows: scheduled consistency checks and automated drift reporting.
  • Cutover readiness: objective criteria for each domain before switching system ownership.

Suggested cutover readiness checklist per domain:

Criterion Target
Baseline load completion 100% of in-scope entities loaded
Event processing reliability >= 99.9% successful handling over agreed observation window
Data drift 0 critical drift defects and agreed non-critical threshold
Operational readiness Runbooks, alerts, and on-call ownership signed off
Rollback readiness Rollback drill completed successfully

API and Interface Specifications

Event Contracts

Event envelope should include:

  • Event ID, event type, version, and timestamp.
  • Aggregate/entity identifier.
  • Tenant/context metadata.
  • Correlation and causation identifiers.

Event payload should remain compact and stable, providing enough information for consumers to fetch enriched data as required.

Example event envelope:

{
    "eventId": "7f3e5f6b-95b8-4f90-9d30-0f54af0b2a17",
    "eventType": "Hub.CustomerUpdated.v1",
    "occurredAtUtc": "2026-04-22T09:15:00Z",
    "tenantId": "tenant-a",
    "aggregateType": "Customer",
    "aggregateId": "C12345",
    "correlationId": "c4f0b3aa-0bde-4e6b-9f0e-8936a4f63f89",
    "causationId": "9b6f5c5a-5f3a-4511-b2c9-c3c7d1c47ab8",
    "payload": {
        "changedFields": ["email", "status"],
        "version": 18
    }
}

GraphQL Query Surface

Design goals:

  • Consumer-selected fields to avoid over-fetching.
  • Query authorization by role and tenant scope.
  • Schema evolution with backward-compatible deprecations.
  • Pagination and filtering for high-volume domains.

Example consumer query:

query CustomerSyncProjection($id: ID!) {
    customer(id: $id) {
        id
        status
        email
        updatedAtUtc
        billingProfile {
            tier
            isActive
        }
    }
}

REST Command Surface

Design goals:

  • Explicit resource and command semantics for create/update/delete operations.
  • Standardized error contract and validation response model.
  • Idempotency support where retry behavior is expected.
  • Contract versioning strategy with deprecation windows.

Example command request:

PATCH /api/customers/C12345
Authorization: Bearer <token>
Idempotency-Key: 536f0ef5-b95e-4a2f-8d8b-f4f10d7aa2d6
Content-Type: application/json

{
    "status": "Active",
    "email": "new.email@example.com"
}

Standard error payload shape:

{
    "code": "ValidationFailed",
    "message": "Request contains invalid fields.",
    "correlationId": "c4f0b3aa-0bde-4e6b-9f0e-8936a4f63f89",
    "errors": [
        {
            "field": "email",
            "reason": "InvalidFormat"
        }
    ]
}

Security and Compliance Controls

Identity and Access

  • Modern authentication for all integration clients.
  • Role- and scope-based authorization for query and command endpoints.
  • Principle of least privilege for service identities.

Data Protection

  • Encryption in transit for all APIs and event transport channels.
  • Encryption at rest for persisted operational and projection data.
  • Controlled handling of sensitive data in logs and telemetry.

Audit and Traceability

  • Immutable audit trail for command operations.
  • Event lineage traceability from producer through consumers.
  • End-to-end request tracing with correlation IDs.

Compliance Readiness

  • Retention and deletion controls aligned with policy requirements.
  • Data minimization in event payloads and API responses.
  • Security review gates before production rollout milestones.

Implementation Sequence

  1. Establish cross-cutting foundations (identity, telemetry, correlation, contract governance).
  2. Introduce event publication for selected low-risk domains.
  3. Deliver GraphQL read models for consumer pull integration.
  4. Expose modernized REST command endpoints for controlled write-back.
  5. Expand domain coverage iteratively with domain-specific cutover criteria.
  6. Decommission legacy integration pathways as replacement coverage reaches target thresholds.

Delivery governance at each step should include:

  • Architecture sign-off for contract and compatibility implications.
  • Security sign-off for identity, authorization, and data handling controls.
  • Operations sign-off for monitoring, alerting, and rollback coverage.

Acceptance Criteria

The implementation is considered complete when all of the following are true:

  • Critical domain events are published with required metadata and schema validation.
  • Consumer sync projections are demonstrably current within agreed freshness SLO.
  • Command API contracts pass functional, security, and backward-compatibility tests.
  • Reconciliation jobs report no unresolved critical drift defects.
  • Production runbooks, dashboards, and alert routes are validated in drills.

Non-Functional Expectations

  • Reliability: graceful degradation and retry-safe integration patterns.
  • Performance: bounded latency for event-to-consumer synchronization.
  • Scalability: horizontal scale for query and event consumer workloads.
  • Supportability: operational dashboards and alerts aligned to SLOs.