[IMPORTANT] Use TaskCreate to break ALL work into small tasks BEFORE starting — including tasks for each file read. This prevents context loss from long files. For simple tasks, AI MUST ask user whether to skip.

Evidence-Based Reasoning — Speculation is FORBIDDEN. Every claim needs file:line proof. Confidence: >95% recommend freely, 80-94% with caveats, <80% DO NOT recommend — gather more evidence. Cross-service validation required for architectural changes. MUST READ .claude/skills/shared/evidence-based-reasoning-protocol.md for full protocol and checklists.

• docs/project-reference/domain-entities-reference.md — Domain entity catalog, relationships, cross-service sync (read when task involves business entities/models) (content auto-injected by hook — check for [Injected: ...] header before reading)

Quick Summary

Goal: Design and implement cross-service communication, data sync, and service boundary patterns.

Workflow:

1. Pre-Flight — Identify source/target services, data ownership, sync vs async
2. Choose Pattern — Entity Event Bus (recommended), Direct API, never shared DB
3. Implement — Producer + Consumer with dependency waiting and race condition handling
4. Test — Verify create/update/delete flows, out-of-order messages, force sync

Key Rules:

• Never access another service's database directly
• Use LastMessageSyncDate for conflict resolution (only update if newer)
• Consumers must wait for dependencies with TryWaitUntilAsync
• Messages defined in shared project (search for: shared message definitions, bus message classes)

Be skeptical. Apply critical thinking, sequential thinking. Every claim needs traced proof, confidence percentages (Idea should be more than 80%).

MANDATORY IMPORTANT MUST Plan ToDo Task to READ the following project-specific reference doc:

• backend-patterns-reference.md — backend CQRS, entity event bus, message bus patterns

If file not found, search for: cross-service message definitions, entity event producers, message bus consumers.

Cross-Service Integration Workflow

When to Use This Skill

• Designing service-to-service communication
• Implementing data synchronization
• Analyzing service boundaries
• Troubleshooting cross-service issues

Pre-Flight Checklist

• Identify source and target services
• Determine data ownership
• Choose communication pattern (sync vs async)
• Map data transformation requirements

Service Boundaries

Note: Search for project-structure-reference.md or the project's service directories to discover the platform's service map, data ownership matrix, and shared infrastructure components.

Communication Patterns

Pattern 1: Entity Event Bus (Recommended)

Use when: Source service owns data, target services need copies.

Source Service                    Target Service
┌────────────┐                   ┌────────────┐
│  Employee  │──── Create ────▶ │ Repository │
│ Repository │                   └────────────┘
└────────────┘                          │
      │                                 │
      │ Auto-raise                      │
      ▼                                 ▼
┌────────────┐                   ┌────────────┐
│  Producer  │── MsgBus  ────▶ │  Consumer  │
└────────────┘                   └────────────┘

⚠️ MUST READ: CLAUDE.md for Entity Event Bus Producer and Message Bus Consumer implementation patterns.

Pattern 2: Direct API Call

Use when: Real-time data needed, no local copy required.

// In Service A, calling Service B API
public class ServiceBApiClient
{
    private readonly HttpClient _client;

    public async Task<UserDto?> GetUserAsync(string userId)
    {
        var response = await _client.GetAsync($"/api/User/{userId}");
        if (!response.IsSuccessStatusCode) return null;
        return await response.Content.ReadFromJsonAsync<UserDto>();
    }
}

Considerations:

• Add circuit breaker for resilience
• Cache responses when possible
• Handle service unavailability

Pattern 3: Shared Database View (Anti-Pattern!)

:x: DO NOT USE: Violates service boundaries

// WRONG - Direct cross-service database access
var accountsData = await accountsDbContext.Users.ToListAsync();

Data Ownership Matrix

Note: Search for project-structure-reference.md or the project's documentation for the entity ownership matrix. Each entity should have exactly ONE owning service; consumers receive synced copies via message bus.

Synchronization Patterns

Full Sync (Initial/Recovery)

// For initial data population or recovery
public class FullSyncJob : BackgroundJobExecutor // project background job base (see docs/project-reference/backend-patterns-reference.md)
{
    public override async Task ProcessAsync(object? param)
    {
        // Fetch all from source
        var allEmployees = await sourceApi.GetAllAsync();

        // Upsert to local
        foreach (var batch in allEmployees.Batch(100))
        {
            await localRepo.CreateOrUpdateManyAsync(
                batch.Select(MapToLocal),
                dismissSendEvent: true);
        }
    }
}

Incremental Sync (Event-Driven)

// Normal operation via message bus
internal sealed class EmployeeSyncConsumer : MessageBusConsumer<EmployeeEventBusMessage> // project message bus base (see docs/project-reference/backend-patterns-reference.md)
{
    public override async Task HandleLogicAsync(EmployeeEventBusMessage message, string routingKey)
    {
        // Check if newer than current (race condition prevention)
        if (existing?.LastMessageSyncDate > message.CreatedUtcDate)
            return;

        // Apply change
        await ApplyChange(message);
    }
}

Conflict Resolution

Use LastMessageSyncDate for ordering - only update if message is newer. See CLAUDE.md Message Bus Consumer pattern for full implementation.

Integration Checklist

Before Integration

• Define data ownership clearly
• Document which fields sync
• Plan for missing dependencies
• Define conflict resolution strategy

Implementation

• Message defined in shared project
• Producer filters appropriate events
• Consumer waits for dependencies
• Race condition handling implemented
• Soft delete handled

Testing

• Create event flows correctly
• Update event flows correctly
• Delete event flows correctly
• Out-of-order messages handled
• Missing dependency handled
• Force sync works

Troubleshooting

Message Not Arriving

# Check message broker queues (search for: queue management commands)

# Check producer is publishing
grep -r "HandleWhen" --include="*Producer.cs" -A 5

# Check consumer is registered
grep -r "AddConsumer" --include="*.cs"

Data Mismatch

# Compare source and target counts
# In source service DB
SELECT COUNT(*) FROM Employees WHERE IsActive = 1;

# In target service DB
SELECT COUNT(*) FROM SyncedEmployees;

Stuck Messages

// Check for waiting dependencies
Logger.LogWarning("Waiting for Company {CompanyId}", companyId);

// Force reprocess
await messageBus.PublishAsync(message.With(m => m.IsForceSync = true));

Anti-Patterns to AVOID

:x: Direct database access

// WRONG
await otherServiceDbContext.Table.ToListAsync();

:x: Synchronous cross-service calls in transaction

// WRONG
using var transaction = await db.BeginTransactionAsync();
await externalService.NotifyAsync();  // If fails, transaction stuck
await transaction.CommitAsync();

:x: No dependency waiting

// WRONG - FK violation if company not synced
await repo.CreateAsync(employee);  // Employee.CompanyId references Company

// CORRECT
await Util.TaskRunner.TryWaitUntilAsync(() => companyRepo.AnyAsync(...));

:x: Ignoring message order

// WRONG - older message overwrites newer
await repo.UpdateAsync(entity);

// CORRECT - check timestamp
if (existing.LastMessageSyncDate <= message.CreatedUtcDate)

Verification Checklist

• Data ownership clearly defined
• Message bus pattern used (not direct DB)
• Dependencies waited for in consumers
• Race conditions handled with timestamps
• Soft delete synchronized properly
• Force sync mechanism available
• Monitoring/alerting in place

Related

• arch-security-review
• api-design

Closing Reminders

• MUST break work into small todo tasks using TaskCreate BEFORE starting
• MUST search codebase for 3+ similar patterns before creating new code
• MUST cite file:line evidence for every claim (confidence >80% to act)
• MUST add a final review todo task to verify work quality MANDATORY IMPORTANT MUST READ the following files before starting:
• MUST READ .claude/skills/shared/evidence-based-reasoning-protocol.md before starting