Step 05 of 10 8-12 weeks· advanced
Step 5: Build the Coexistence Layer
Stand up the bridge between mainframe and the new system — API gateway, data sync, dual auth, distributed observability.
Recommended prompts
Use one of these to do the work in your IDE
Open the template to read it in full. Click Copy prompt to grab it (with your stack values pre-filled where they apply) — then paste into Claude Code, Cursor, or wherever you build.
Recommended skills
Drop these into Claude Code for this phase
Skills auto-trigger on the right kind of request. Install once; they apply to every prompt that fits.
Recommended MCP configs
Wire these tools into Claude Code first
MCP servers give Claude Code direct access to external systems (Jira, browsers, databases). Configure once.
When you're done
Verify these in your own work before moving on
This is a checklist for you to mentally tick off in your repo and IDE — the site doesn't track it, you do.
- API gateway operational, routing to mainframe and new system
- Data sync running and monitored (lag, error rate, drift detection)
- Auth federation working — single sign-in covers both
- Centralized observability across both systems
- "Hello world" capability deployed through the new system end-to-end
Common pitfalls
What goes wrong at this step
- Direct mainframe-to-cloud-DB queries from new app — tempting; produces tight coupling that's hard to break later
- Sync lag invisible — without monitoring, drift accumulates and surfaces as customer complaints
- Auth federation half-done — either fully integrate or accept dual logins; partial integration is the worst case