Legacy System Integration

The legacy system integration is the COBOL-era database, the old CRM, and the bridge to the AI (L376) — the constraints (the legacy's: the old APIs L376, the batch L282, the no-API L376, L376), the patterns (the adapter L375, the sync L376, the file L375, L376), and the strangler (the incremental replacement L376: the new around the old, L376) — the bridge (L376) to the AI (L173) — the enterprise's (L380) legacy (L376), bridged (L376).

The one-sentence interview answer: "The legacy integration is the bridge to the old systems (L376). The constraints (L376): the legacy's (L376) — the old APIs (L376): the SOAP (L376) and the mainframe (L376); the batch (L282): the nightly (L376) exports (L376); and the no-API (L376): the files (L375) and the screens (L376). The patterns (L376): the adapter (L375) — the legacy's (L376) API wrapped (L375); the sync (L376) — the data (L313) copied (L376) to the modern (L375); and the file (L375) — the batch (L282) exchange (L376). The strangler (L376): the incremental (L376) — the new (L376) system (L375) around the old (L376), the pieces (L376) replaced (L376) one by one (L376) — the rewrite (L376) avoided (L376). The bridge (L376): the AI (L173) to the legacy (L376) — the data (L313) synced (L376) for the grounding (L280), the actions (L315) via the adapter (L375). The AI shape (L173): the enterprise (L380) — the legacy (L376) and the AI (L173) bridged (L376): the constraints (L376), the patterns (L376), and the strangler (L376) — the bridge (L376), built (L376)."

Think of the legacy integration as the old town and the new district. The old town (the legacy, L376): the cobbled streets (the old APIs, L376), the night market (the batch, L282), and the shuttered doors (the no-API, L376). The new district (the AI, L173): the modern grid (the cloud, L366). The bridge (the patterns, L376): the toll roads (the adapters, L375) — the old town's (L376) gates (L376); the ferries (the sync, L376) — the goods (the data, L313) carried (L376); and the freight (the files, L375). The expansion (the strangler, L376): the new buildings (L376) rising around the old (L376) — the old (L376) demolished (L376) piece by piece (L376). The city works because the bridge connects, and the expansion is incremental (L376).

   the old town (the legacy, L376)
   ┌────────────────────────────────────────────────────────┐
   │ the old streets (the APIs, L376) · the night market     │
   │ (the batch, L282) · the shutters (the no-API, L376)     │
   │ the bridge (the patterns, L376) — the tolls (L375), the │
   │ ferries (L376)                                          │
   │ the expansion (the strangler, L376) — the incremental   │
   └────────────────────────────────────────────────────────┘

The mental model is the old town: the streets, the bridge, and the expansion (L376).

   the AI (L173)
        │
        ▼
   ┌────────────────────── THE ADAPTER (L375) ──────────────────────────┐
   │  the legacy's (L376) API wrapped (L375) — the SOAP (L376)         │
   │  or the file (L375) — the contract (L254)                        │
   └──────────────────────────┬───────────────────────────────────────┘
                              ▼
   ┌────────────────────── THE SYNC (L376) ─────────────────────────────┐
   │  the data (L313) copied (L376) — the nightly (L376) batch (L282)  │
   │  → the modern store (L268) for the grounding (L280)               │
   └──────────────────────────┬───────────────────────────────────────┘
                              ▼
   ┌────────────────────── THE STRANGLER (L376) ────────────────────────┐
   │  the new (L376) around the old (L376) — the pieces (L376)         │
   │  replaced (L376) one by one (L376)                                │
   └──────────────────────────────────────────────────────────────────┘

The flow is the bridge: adapter → sync → strangler (L376).

• The constraints (L376). The legacy's (L376): the old APIs (L376), the batch (L282), the no-API (L376).
• The patterns (L376). The adapter (L375), the sync (L376), the file (L375).
• The strangler (L376). The incremental replacement (L376): the new (L376) around the old (L376).
• The bridge (L376). The AI (L173) to the legacy (L376): the data (L313) synced (L376), the actions (L315) via the adapter (L375).

• An enterprise engagement (L379). The legacy CRM (L376) — the bridge (L376) to the AI (L173).
• A mainframe (L376). The COBOL (L376) — the batch (L282) exports (L376) — the file (L375).
• An old CRM (L376). The SOAP (L376) — the adapter (L375) — the sync (L376).
• A data warehouse (L375). The nightly (L376) sync (L376) — the grounding (L280).
• Anything enterprise (L380). The bridge (L376) — the constraints, the patterns, the strangler (L376).

The through-line: the bridge is the legacy's — the constraints, the patterns, and the strangler (L376).

Say it in four moves:

1. The constraints. "The old APIs (L376), the batch (L282), the no-API (L376)."
2. The patterns. "The adapter (L375), the sync (L376), the file (L375)."
3. The strangler. "The incremental — the new around the old (L376)."
4. The bridge. "The data (L313) synced, the actions (L315) via the adapter (L375)."

• The adapter is the seam (L375). The legacy's (L376) quirks (L376) wrapped (L375) — the contract (L254) — the AI (L173) sees the clean (L375) API (L375).
• The sync is the grounding's (L376). The data (L313) copied (L376) — the nightly (L376) batch (L282) — the RAG's (L349) grounding (L280) on the fresh (L335) data (L376).
• The strangler is the risk's (L376). The incremental (L376) — the reversible (L304) — the big-bang (L376) avoided (L376).
• The file is the no-API's (L375). The screens (L376) and the files (L375) — the batch (L282) exchange (L376).
• The ownership is the legacy's (L375). The old system's (L376) owner (L372) — the bridge's (L376) owner (L375).

• The rewrite (L376). The big-bang (L376) — the risk (L376) — the strangler (L376) is the path (L376).
• The no-adapter (L375). The AI (L173) calling the legacy (L376) directly (L376) — the coupling (L252) — the adapter (L375) is the seam (L375).
• The sync-blind grounding (L280). The RAG (L349) on the stale (L335) data (L376) — the nightly (L376) sync (L376) is the freshness (L335).
• The no-API ignored (L376). The screens (L376) — the file (L375) exchange (L376) — the batch (L282).
• The ownership-less (L375). The bridge (L376) un-owned (L375) — the changes (L376) unmade (L375).

• Wrap the adapter (L375) — the legacy's (L376) quirks (L376) contained (L375).
• Sync for the grounding (L376) — the nightly (L376) batch (L282) — the freshness (L335).
• Strangle the legacy (L376) — the incremental (L376), the reversible (L304).
• File the no-API (L375) — the batch (L282) exchange (L376).
• Own the bridge (L375) — the owner (L372) accountable (L375).

Q: Walk me through the legacy integration.

A: The bridge to the old systems (L376). The constraints — the old APIs (L376), the batch (L282), the no-API (L376). The patterns — the adapter (L375), the sync (L376), the file (L375). The strangler — the incremental (L376). And the bridge — the data (L313) and the actions (L315).

Q: How do you bridge the AI to the legacy?

A: The adapter (L375): the legacy's (L376) SOAP (L376) or files (L375) wrapped (L375) — the clean (L375) contract (L254); and the sync (L376): the data (L313) copied (L376) nightly (L282) to the modern store (L268) — the RAG's (L349) grounding (L280) on the fresh (L335) data (L376).

Q: What's the strangler?

A: The incremental replacement (L376): the new (L376) system (L375) built around the old (L376) — the seam (L375) at the boundary (L376) — the pieces (L376) replaced (L376) one by one (L376) — the old (L376) retired (L376). The big-bang (L376) rewrite (L376) is the risk (L376); the strangler (L376) is the reversible (L304) path (L376).

Q: What if the legacy has no API?

A: The file (L375): the batch (L282) exchange (L376) — the legacy (L376) exports (L376) the file (L375), the new (L375) imports (L375) it (L376). The screens (L376) — the UI scraping (L376) — the last resort (L376); the file (L375) is the standard (L376).

• What are the constraints (L376)?
• How do you bridge the AI to the legacy (L376)?
• What's the strangler (L376)?
• What if the legacy has no API (L376)?
• What's the adapter (L375)?

The senior read: the strangler is the path — the incremental, the reversible (L376).

// The legacy bridge (L376) — the adapter, the sync, the strangler (L376).
// 1 · THE ADAPTER (L375) — the legacy's SOAP wrapped (L376).
class LegacyCrmAdapter {
  constructor(soapClient) { this.soap = soapClient; }   // L376

  // THE CONTRACT (L254) — the clean shape (L375).
  async getCustomer(id) {
    const raw = await this.soap.call('GetCustomer', { id });  // L376
    return { id: raw.CustomerID, name: raw.Name, tier: raw.Tier };  // L254
  }
}

// 2 · THE SYNC (L376) — the nightly batch (L282).
async function nightlySync() {
  const customers = await legacyAdapter.exportAll();    // the file (L375)
  await modernStore.upsert(customers);                  // the store (L268)
  // the RAG (L349) grounding (L280) on the fresh (L335) data (L376)
}

// 3 · THE STRANGLER (L376) — the incremental (L376).
//   phase 1: the adapter (L375) + the sync (L376) — the read path (L376)
//   phase 2: the new CRM (L375) for the writes (L376)
//   phase 3: the legacy (L376) retired (L376) — the reversible (L304)

// 4 · THE GATE (L324) — the legacy writes approved (L376).
if (action.target === 'legacy') await approvalGate(action);   // L324

What the reader must SEE — the bridge, applied:

  LegacyCrmAdapter + clean contract → the seam (L375, L254)
  nightlySync → the store         → the freshness (L335, L376)
  phase 1/2/3                     → the strangler (L376)
  the approval on the legacy      → the gate (L324)

  The adapter, the sync, the strangler (L376).

• The adapter is the integration's latency (L375). The SOAP (L376) — the slow (L376) — the cached (L171) reads (L376).
• The sync is the freshness (L335). The nightly (L376) — the RAG (L349) on the day-old (L376) data (L313).
• The strangler is the risk's (L376). The pieces (L376) — the reversible (L304) — the business (L360) continuous (L376).
• The file is the batch's (L282). The export (L375) — the warehouse's (L375) load (L376).

• The legacy integration = the legacy's bridge (L376): the constraints, the patterns, the strangler.
• The constraints: the old APIs (L376), the batch (L282), the no-API (L376).
• The patterns: the adapter (L375), the sync (L376), the file (L375).
• The strangler: the incremental — the new around the old (L376).
• The bridge: the data (L313) synced, the actions (L315) via the adapter (L375).

LEGACY SYSTEM INTEGRATION = the bridge to the AI

THE CONSTRAINTS (L376)
  the old APIs (L376) — the SOAP (L376), the mainframe (L376)
  the batch (L282) — the nightly (L376) exports (L376)
  the no-API (L376) — the files (L375), the screens (L376)

THE PATTERNS (L376)
  the adapter (L375) — the legacy's (L376) API wrapped (L375)
  the sync (L376) — the data (L313) copied (L376) nightly (L282)
  the file (L375) — the batch (L282) exchange (L376)

THE STRANGLER (L376)
  the incremental (L376) — the new (L376) around the old (L376)
  the pieces (L376) replaced (L376) one by one (L376)
  the reversible (L304) — the rewrite (L376) avoided (L376)

THE BRIDGE (L376)
  the data (L313) synced (L376) — the grounding (L280)
  the actions (L315) via the adapter (L375) — gated (L324)

INTERVIEW, 4 MOVES
  1 constraints "the old APIs, the batch, the no-API (L376)"
  2 patterns    "the adapter, the sync, the file (L376)"
  3 strangler   "the incremental (L376)"
  4 bridge      "the data synced, the actions adapted (L376)"

You now hold the bridge: the constraints, the patterns, and the strangler — with the old town connected and the expansion incremental. The old town and the new district are linked — and the demolition is piece by piece (L376).

Next: the portability, the fallback, and when multi-cloud is the wrong answer — Multi-Cloud Concepts (L377).