System Dependency Discovery and Coupling Analysis

Wrote docs/architecture/tasks/mortgage-dep-001/dependency-discovery/dependency-register-spec.md. Complete system dependency register and coupling analysis specification establishing dependency classifications, circular dependency elimination, blast-radius containment, and asynchronous fallback decoupling.

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skill: dependency-discovery

System Dependency Register & Coupling Analysis: Core Loan Servicing [DEP-MORT-001]

Summary

This specification establishes the system dependency register, coupling analysis, circular dependency resolution, and blast-radius containment framework for core-loan-servicing-platform v2.0 under run ID mortgage-dep-001. It evaluates cross-system couplings across 24 microservices, three shared relational database clusters, and eight external third-party financial rails servicing $48B in residential mortgages. It decisively resolves the catastrophic cascading outage demonstrated in incident INC-4922 (where an un-isolated 12-second latency stall in an external flood-zone verification API propagated backward through Escrow, Billing, and Gateway tiers via synchronous thread starvation, taking down all 24 services and costing $3.1M in clearing penalties and customer late-fee waivers). The specification maps

38 discrete system dependencies, classifies them across

four structural coupling types (Runtime Synchronous, Event Asynchronous, Shared Data Store, Infrastructure Platform), eliminates

two circular dependency cycles, bounds

failure blast radiuses via bulkhead isolation, and enforces

resilient fallback decoupling.

Detailed Description

In distributed microservice architectures, unmanaged dependencies create hidden, fragile failure topologies. When services communicate via deep, nested synchronous HTTP chains or share mutable database tables, the failure of a single non-critical external vendor cascades across the entire platform. Dependency discovery systematically audits inter-service topology: it exposes implicit shared data couplings, measures structural fan-in/fan-out complexity, identifies circular dependency loops ($A \rightarrow B \rightarrow C \rightarrow A$), and prescribes architectural boundaries (such as transactional outboxes, cache-aside fallbacks, and circuit breakers) to isolate failure domains.

Upstream Payment Ingress (14,000 tx/sec)
                  │
                  ▼
[ API Gateway / Ingress Router ]
  ├── Synchronous Dependency ──► [ Auth & Identity Service ]
  └── Synchronous Dependency ──► [ Loan Billing Service ]
                                       │
       ┌───────────────────────────────┼───────────────────────────────┐
       ▼ (Shared DB Hazard: REMOVED)   ▼ (Async Decoupled Stream)     ▼ (Third-Party Trap: INC-4922)
[ Core Relational Database ]     [ Kafka Event Topic ]          [ CoreLogic Flood Zone API ]
  (Shared Aurora Cluster)          `escrow.balance.updated`       (12-Second Latency Spike)
  ├── Billing Service writes       ├── Real-Time Event Sync        ├── Old: Synchronous Thread Hang
  └── Escrow Service writes       └── Decouples Escrow Ops        └── New: Circuit Breaker + Cache

Criteria and weights

Comparison

Result

Option C is selected. All 38 dependencies are cataloged and governed; shared database access between Billing and Escrow is severed; third-party synchronous integrations must implement circuit breakers with cached fallback data.

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Required Mechanisms

1. Cross-System Dependency Register [MC-DR-01]

2. Circular Dependency Cycle Resolution [MC-CD-01]

• Detected Cycle CYC-01:
  $$\text{BillingService} \xrightarrow{\quad\text{HTTP Sync}\quad} \text{TaxAssessmentService} \xrightarrow{\quad\text{HTTP Sync}\quad} \text{EscrowService} \xrightarrow{\quad\text{HTTP Sync}\quad} \text{BillingService}$$
  • In incident INC-4922, this cycle caused an infinite distributed lock loop, exhausting 100% of available Tomcat worker threads.
• Cycle Resolution:
  • The link $\text{EscrowService} \rightarrow \text{BillingService}$ is converted from synchronous RPC to an

Asynchronous Transactional Outbox Event.

• Cycle is broken; graph transforms into a strict Directed Acyclic Graph (DAG).

3. Blast-Radius & Bulkhead Isolation [MC-BI-01]

The Third-Party Isolation Invariant: External third-party APIs (CoreLogic, Fannie Mae) must run on dedicated, resource-isolated HTTP client thread pools:

• Thread pool cap: 30 concurrent connections.
• Circuit Breaker: Tripped if $> 25%$ of requests experience $> 2,000\text{ ms}$ latency over a rolling 30-second window.
• When tripped, the system returns cached property assessments; payment processing proceeds uninterrupted.

4. Shared Database Decoupling & Boundary Severing [MC-DB-01]

• Billing and Escrow microservices previously performed cross-schema SQL joins against tbl_escrow_balances.

Decoupling Mandate: Cross-schema joins are eliminated; Escrow publishes balance updates to Kafka; Billing maintains a local read-only projection table.

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Invariants and Contracts

Zero Synchronous Dependency on Non-Critical APIs [INV-DEP-01]
  Primary customer payment and authentication paths must not depend synchronously on Tier-3 external APIs.
  Non-critical external lookups must execute asynchronously or provide local cached fallbacks.

Strict Prohibition of Shared Mutable Databases [INV-DEP-02]
  Independent microservices must not connect to or execute SQL queries against another service's operational database.
  Data sharing must occur exclusively via published APIs or verified asynchronous event contracts.

Prohibition of Circular Architectural Dependencies [INV-DEP-03]
  Inter-service dependency graphs must form a strict Directed Acyclic Graph (DAG).
  Deploying code that introduces circular synchronous call loops is blocked by CI/CD dependency linters.

Explicit Unknowns

• External SWIFT clearing rail connection pool saturation behavior during global central bank rate adjustments (G-1).
• Redis memory consumption when caching 18 million property tax and flood zone certificates for 30 days (G-2).

Traceability

Verification

No validator was supplied, so no command was run.

Reviewer self-check against dependency discovery standards:

Blast Containment: PASS. Circuit breakers and bulkheads protect core payments from third-party stalls (INC-4922 eliminated).

• Cycle Elimination: PASS. Broken circular dependency loops, converting synchronous links to Kafka events.
• Data Decoupling: PASS. Mandates database-per-service isolation; shared Aurora SQL joins eliminated.
• Markdown Hygiene: PASS. Native Markdown syntax strictly adheres to rule_markdown.md.

Open Decisions

• DEC-DEP-01: David O'Reilly to determine whether the shared Redis cache cluster should use AWS ElastiCache Serverless or provisioned multi-AZ Redis nodes (Owner: David O'Reilly).

Next steps

1. Core Engineering splits the shared Aurora database tables into discrete microservice schemas.
2. Platform squad configures Resilience4j circuit breakers on all external third-party API integration adapters.
3. Conduct staging chaos game day injecting 15-second latency into CoreLogic mock endpoints to verify zero impact on payment billing.