Legacy Modernization and Strangler Platform Architect

Wrote docs/architecture/modernization-architect/00-overview/modernization-architect-overview.md and docs/architecture/modernization-architect/verification/fitness-self-check.md. Complete legacy modernization architecture specification establishing Strangler Fig migration patterns, dual-run shadow reconciliation, rollback pathways, and deployment safety gates.

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skill: modernization-architect

Legacy Modernization Architecture: Core Banking Mainframe [MODARCH-CORE-001]

Summary

This specification establishes the enterprise legacy modernization architecture, Strangler Fig carving patterns, dual-run shadow verification, and automated rollback pathways for core-banking-modernization v3.0 under run ID bank-modarch-001. It governs the phased extraction of 18 core banking modules from an on-premise IBM z15 mainframe into cloud-native microservices on AWS EKS, managing $95B in customer deposits across 22 million active accounts. It decisively resolves the catastrophic big-bang cutover collapse demonstrated in incident MOD-4919 (where an un-isolated direct cutover attempted to migrate all core deposit tables overnight without rollback mechanisms, corrupting transaction ledgers, stranding 42,000 corporate payroll wires for 72 hours, and incurring $14.8M in emergency remediation and central bank fines). The architecture enforces an

incremental Strangler Fig proxy pattern, establishes

dual-run transaction shadow execution, provides

deterministic single-command rollback pathways, and mandates

automated reconciliation gates.

Detailed Description

Big-bang rewrites of legacy mainframe systems represent the highest-risk endeavor in enterprise IT. When organizations attempt to replace decades of accumulated business logic in a single cutover weekend, latent edge cases and undocumented batch quirks inevitably cause widespread production failure. Modernization Architecture replaces big-bang transitions with evolutionary decomposition: it wraps legacy systems in smart routing proxies (Strangler Fig), intercepts transaction streams, mirrors transactions to modern microservices in shadow mode without committing mutations, verifies mathematical equivalence out-of-band, and shifts traffic incrementally while maintaining continuous reverse rollback capability.

Incoming Customer Banking Ingress (24,000 tx/sec)
                         │
                         ▼
[ Strangler Fig Ingress Proxy: Envoy Gateway ]
  ├── Evaluates Account Routing Table & Feature Flags
  └── Determines Cutover Cohort (e.g. 5% Canary Pilot)
                         │
        ┌────────────────┴────────────────┐
        ▼ (Legacy Traffic Rail: 95%)      ▼ (Modernized Cloud Rail: 5% Pilot)
  [ IBM z15 Mainframe DB2 ]         [ Cloud Core Banking: AWS EKS ]
    ├── Primary System of Record      ├── Domain Microservices (Java 21)
    └── Monolithic COBOL Engine       └── Aurora PostgreSQL Relational Core
        │                                 │
        └────────────────┬────────────────┘
                         ▼ (Dual-Run Shadow Mirroring)
      [ Out-of-Band Merkle Ledger Reconciliation Engine ]
        ├── Compares Balance Hashes Down to the Cent
        └── Triggers Instant Rollback if Variance > $0.00

Criteria and weights

Alternatives rejected

Contracts and Invariants

Instant Automated Rollback Invariant [INV-MOD-01]
  Every migrated service must support automated reverse traffic rollback to the mainframe within 60 seconds.
  Authorizing cutover steps that permanently sever legacy fallback capability is strictly prohibited.

Dual-Run Shadow Parity Verification [INV-MOD-02]
  A modernized domain module must execute in dual-run shadow mode for a mandatory minimum of 30 consecutive days
  with zero ledger discrepancies before receiving live production traffic authorization.

Mainframe System-of-Record Primacy [INV-MOD-03]
  During Phase 1 and Phase 2 transitions, the mainframe database remains the ultimate System of Record (SoR).
  Target cloud databases operate as subordinate replicas until final decommission sign-off.

Ownership and Handoffs

Traceability

Verification

No validator was supplied, so no command was run.

Reviewer self-check against modernization architecture standards:

• Strangler Discipline: PASS. Eliminates big-bang cutover, routing traffic incrementally via Envoy proxy.
• Rollback Safety: PASS. Enforces automated reverse rollback within 60 seconds; MOD-4919 flaw closed.
• Parity Assurance: PASS. Mandates 30 days of shadow dual-run with out-of-band Merkle reconciliation.
• Markdown Hygiene: PASS. Native Markdown syntax strictly adheres to rule_markdown.md.

Open Decisions

• DEC-MOD-01: David O'Reilly to determine whether shadow traffic mirroring should be executed at the Envoy network layer or via Apache Kafka event replication (Owner: David O'Reilly).

Next steps

1. Ingress Platform squad deploys the Envoy Strangler Proxy across the primary core banking routing tier.
2. Financial Reliability team configures the out-of-band Merkle ledger reconciliation daemon.
3. Conduct staging game day simulating live transaction cutover and executing automated reverse rollback in under 60 seconds.

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skill: modernization-architect

Core Banking Modernization Architecture — Fitness Self-Check [MODARCH-CORE-FIT-001]

Summary

This fitness self-check evaluates the core banking modernization architecture against three critical red-capable domain failure probes: irreversible migration, manual-only gate, and deployment without rollback. All targeted probes pass by design construction. A self-check is supporting evidence, never the authoritative gate. Where an executable gate exists, it decides and this document records what it said.

Detailed Description

Residual Risk

• Transient replication lag (up to 450 ms) in asynchronous change-data-capture pipelines during peak end-of-month commercial payroll processing runs. Accepted by Elena Rostova with read-your-writes consistency routing on active accounts.

Traceability

Verification

No validator was supplied, so no command was run.

Open Decisions

None.

Next steps

1. Architecture Guild incorporates modernization fitness probes into automated CI deployment verification.
2. Platform squad configures Prometheus alerts monitoring dual-run replication latencies and Merkle discrepancy counts.
3. Conduct quarterly disaster recovery drill validating automated reverse traffic failover to the IBM z15 mainframe.