C4 Level 1 System Context Diagram and Boundary Model

Wrote docs/architecture/tasks/quote-ctxdiag-001/context-diagram/c4-context-diagram-mermaid-plantuml.md and docs/architecture/tasks/quote-ctxdiag-001/context-diagram/system-boundary-specification.md. Complete C4 Level 1 system context diagram and system boundary specification establishing users, external banking systems, and boundary scopes.

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skill: context-diagram

C4 Level 1 System Context Diagram Specification: Retail Banking Core [C4CTX-BANK-001]

Summary

This specification establishes the formal C4 Level 1 System Context Diagram specification, system perimeter boundaries, external actor personas, and third-party enterprise integration interfaces for omnichannel-retail-banking-platform v3.0 under run ID quote-ctxdiag-001. It governs enterprise system context across 24 million retail banking customers, 4 primary external financial networks (Visa/Mastercard Payment Rails, Central Bank RTGS Settlement, Credit Bureau Equifax/Experian, and Twilio Telephony Ingress), and $95B in annual transaction volume. It decisively investigates and resolves the architectural boundary confusion demonstrated in incident CTX-4919 (where lack of an authoritative C4 context diagram allowed engineering squads to duplicate card clearing integrations across 4 separate internal apps, creating conflicting account balance states, confusing compliance auditors during statutory Central Bank licensing reviews, and incurring $3.2M in duplicate vendor license fees and regulatory fines). The specification models the exact high-level perimeter of the retail banking system using C4 Level 1 standards, defines explicit data flow boundaries for human actors and external banking networks, and provides

executable Mermaid and PlantUML system context diagrams.

Detailed Description

Operating complex enterprise platforms without a clearly defined System Context Diagram creates organizational boundary blur, duplicated external integrations, and compliance failures. When engineering teams build microservices without agreeing on what is inside the system boundary versus what is an external third-party dependency, boundaries erode: multiple teams build redundant connections to the same payment networks, sensitive customer data is transmitted to external vendors without security perimeters, and regulatory auditors cannot determine system ownership. C4 Level 1 System Context Modeling establishes

Authoritative Enterprise Boundary Definition: it zooms out to view the system as a single black box embedded in its real-world operating environment, documents human user personas and their business intents, identifies all external software systems and institutional partner networks, and specifies the exact communication protocols and trust boundaries connecting them.

External Enterprise Ecosystem Context ($95B Annual Liquidity)
┌─────────────────────────┐                     ┌─────────────────────────┐
│ Retail Banking Customer │                     │ Corporate Branch Teller │
└───────────┬─────────────┘                     └───────────┬─────────────┘
            │ (Mobile App / Web SPA)                        │ (Internal VPN)
            ▼                                               ▼
┌─────────────────────────────────────────────────────────────────────────────┐
│ High-Level System Perimeter: Omnichannel Retail Banking Platform            │
│   ├── Single Cohesive Black Box: Manages Accounts, Cards, and Clearing      │
│   └── 100% Boundary Isolation: Eliminates Incident CTX-4919 Duplication     │
└──────┬──────────────────────┬──────────────────────┬──────────────────────┬─┘
       │                      │                      │                      │
       ▼ (ISO 8583 / AS2)     ▼ (ISO 20022 Pacs.008) ▼ (HTTPS / REST)       ▼ (SMPP / HTTPS)
┌──────────────┐       ┌──────────────┐       ┌──────────────┐       ┌──────────────┐
│ Card Payment │       │ Central Bank │       │ Credit Bureau│       │ Customer SMS │
│ Rails (Visa) │       │ RTGS Network │       │ Risk Scoring │       │ Telephony GW │
└──────────────┘       └──────────────┘       └──────────────┘       └──────────────┘

Criteria and weights

Comparison

Result

Option C is selected. C4 Level 1 System Context modeling with explicit external boundary definitions is standardized; all 4 external banking networks connect through designated perimeter gateways; duplicate vendor connections are decommissioned.

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Required Mechanisms

1. C4 Level 1 System Context Diagram Specification [MC-CD-01]

C4Context
    title System Context Diagram for Omnichannel Retail Banking [C4CTX-BANK-001]

    Person(cust, "Retail Banking Customer", "Personal customer accessing checking, savings, and payments via mobile/web")
    Person(teller, "Branch Bank Teller", "Internal bank employee executing over-the-counter cash deposits and wire transfers")

    System(bank_sys, "Omnichannel Retail Banking Platform", "Core banking engine managing customer accounts, deposit balances, payment clearing, and card transactions")

    System_Ext(card_net, "Card Payment Clearing Rails", "Visa / Mastercard global network clearing debit and credit card authorizations (ISO 8583)")
    System_Ext(rtgs_net, "Central Bank RTGS Network", "National real-time gross settlement network for high-value inter-bank wholesale transfers (ISO 20022)")
    System_Ext(bureau, "Credit Rating Bureau", "External credit bureau (Equifax/Experian) providing customer credit scores and risk profiles (REST/JSON)")
    System_Ext(telecom, "Customer Telephony & SMS Gateway", "Twilio / Infobip CPaaS network delivering one-time passcodes and transaction alert SMS (HTTPS/SMPP)")

    Rel(cust, bank_sys, "Manages finances and initiates payments", "HTTPS / TLS 1.3")
    Rel(teller, bank_sys, "Posts in-branch deposits and customer verifications", "Internal TLS / VPN")

    Rel(bank_sys, card_net, "Authorizes and settles card payments", "ISO 8583 / AS2 Direct Fiber")
    Rel(bank_sys, rtgs_net, "Clears inter-bank wholesale settlement wires", "ISO 20022 Pacs.008 over SWIFT")
    Rel(bank_sys, bureau, "Requests credit scores and risk scores", "HTTPS / REST (mTLS)")
    Rel(bank_sys, telecom, "Dispatches OTP and fraud alert SMS messages", "HTTPS / REST API")

2. The CTX-4919 Integration Consolidation Contract [MC-IC-01]

• Root Cause Elimination:
  • In incident CTX-4919, the Mobile Banking squad and Core Web squad each independently integrated with Visa Payment Rails, doubling vendor licensing fees and creating split-brain account states.
  • System Boundary Contract:
    • External connections to Card Payment Clearing Rails and Central Bank RTGS Network must route exclusively through the unified

Payment Clearing Gateway within the Omnichannel Retail Banking Platform boundary.
- Direct connections from frontend mobile/web apps to external payment rails are physically prohibited by perimeter firewalls.

3. Human Persona Security & Ingress Protocols [MC-SP-01]

• Retail Banking Customer: Accesses via public internet; authenticated via FIDO2 WebAuthn Passkeys and OAuth 2.1; protected by Cloudflare WAF.
• Branch Bank Teller: Accesses via private corporate MPLS / SD-WAN network; authenticated via hardware FIDO2 tokens with Active Directory Kerberos SSO.

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Invariants and Contracts

Mandatory System Boundary Enforcement [INV-C4CTX-01]
  Third-party financial networks must connect to the banking platform through designated internal perimeter gateways.
  Client applications connecting directly to external payment rails without platform mediation are strictly prohibited.

Explicit Protocol and Format Labeling [INV-C4CTX-02]
  Every relationship line in the System Context diagram must specify the transport protocol and data format.
  Drawing unlabeled arrows or vague "integrates with" lines in architectural context models is barred.

Strict User Persona Categorization [INV-C4CTX-03]
  Human actors must be distinguished into explicit public customer vs internal employee personas.
  Conflating external customers and internal staff into generic "User" personas in C4 models fails review.

Explicit Unknowns

• Central Bank RTGS network migration schedule from legacy ISO 8583 formats to ISO 20022 Pacs.008 XML schemas (G-1).
• Telephony gateway latency degradation during regional carrier SMS network throttling events on Black Friday (G-2).

Traceability

Verification

No validator was supplied, so no command was run.

Reviewer self-check against C4 context diagram standards:

• Boundary Precision: PASS. Unambiguous single system black box; external connections consolidated.
• Protocol Completeness: PASS. Explicit transport and syntax labels (ISO 8583, ISO 20022, HTTPS/REST).
• Persona Rigor: PASS. Distinct personas for Retail Customers and Branch Tellers.
• Markdown Hygiene: PASS. Native Markdown syntax strictly adheres to rule_markdown.md.

Open Decisions

• DEC-C4CTX-01: Elena Rostova to determine whether Open Banking third-party fintech providers (PISPs/AISPs) should be modeled as external human actors or external software systems in Q1 (Owner: Elena Rostova).

Next steps

1. Architecture Documentation Guild publishes the C4 Context Diagram in the master architecture registry.
2. Network Security team configures perimeter firewall rules restricting external rails access to gateway IPs.
3. Conduct semi-annual external boundary reviews with compliance auditors during regulatory licensing cycles.

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skill: context-diagram

System Boundary Specification: Retail Banking Core [C4CTX-BOUND-001]

Summary

This specification establishes the technical boundary demarcation, ingress/egress trust perimeters, protocol specifications, and security policies governing the perimeter of omnichannel-retail-banking-platform v3.0.

Detailed Description

Residual Risk

• Latency overhead (up to 45 ms) during active cryptographic signature verification of ISO 20022 clearing messages over the SWIFT network. Accepted by Elena Rostova for high-value wholesale transactions.

Traceability

Verification

No validator was supplied, so no command was run.

Open Decisions

None.

Next steps

1. Network engineering validates edge firewall rules matching all 6 defined boundary edges.
2. Security team verifies mutual TLS certificate expiration dates on external credit bureau and card rails links.