Data Migration Strategies
Learn how to move data safely when modernizing legacy systems using dual writes, backfills, validation, and zero-downtime cutover techniques.
Data Migration Strategies is a free SaaS Architecture & Startup Engineering lesson on CoddyKit — lesson 4 of 4. You can read the complete lesson below for free — then practise it hands-on in the browser with a built-in code editor and a 24/7 AI tutor. It is part of the SaaS Architecture & Startup Engineering learning path, one of 4 lessons in the course, and your progress syncs across the web and the CoddyKit app.
Why Data Migration Is Hard
Modernizing a legacy system often means moving data to a new schema or database. Unlike code, data is stateful and irreplaceable — a botched migration can corrupt or lose customer data permanently.
This lesson covers strategies to migrate safely with minimal downtime.
Big Bang vs Incremental
Two broad approaches:
- Big bang — stop the system, migrate everything, switch over (risky, requires downtime)
- Incremental — migrate gradually while both systems run
For SaaS, incremental zero-downtime migration is almost always preferred.
Schema Mapping
Before moving data, define a mapping from the old schema to the new one: which fields move where, what transforms apply, and how to handle missing or malformed values.
Document every edge case; legacy data is always messier than expected.
The Backfill
A backfill copies all existing historical data into the new store, usually in batches to avoid overwhelming the systems.
let cursor = 0;
while (true) {
const batch = oldDb.fetch(cursor, 1000);
if (batch.length === 0) break;
newDb.insertMany(batch.map(transform));
cursor = batch[batch.length - 1].id;
}Dual Writes
To keep both stores in sync during migration, the app performs dual writes: every change is written to both the old and the new system.
This ensures the new store stays current while the backfill catches up the history.
function save(record) {
oldDb.write(record);
newDb.write(transform(record));
}Validation and Reconciliation
Before trusting the new store, reconcile it against the old one: compare row counts, checksums, and spot-check records.
Investigate every discrepancy; silent data loss is the worst outcome.
Shadow Reads
Before switching, run shadow reads: serve from the old system but also read from the new one and compare results in the background.
This catches mapping bugs under real traffic without affecting users.
The Cutover
The cutover flips reads to the new system. With dual writes and validation in place, this can be done gradually using a feature flag, often per tenant.
Start with internal accounts, then a small percentage, then everyone.
Rollback Planning
Always have a rollback plan. Because dual writes keep the old store current, you can flip reads back instantly if the new system misbehaves.
Never decommission the old store until the new one has proven stable.
Decommissioning Safely
Once the new system is fully trusted, stop the dual writes, archive the old data, and finally retire the legacy store.
Keep a final backup. Premature deletion has ended careers.
Migrating in Multi-Tenant SaaS
In SaaS you can migrate tenant by tenant, limiting blast radius. If one tenant's migration fails, only they are affected, and you learn before touching others.
This natural batching is a major advantage of multi-tenancy.
Quick Check
Test your migration knowledge.
Recap
You learned safe data migration:
- Incremental over big bang for zero-downtime
- Schema mapping, backfill, and dual writes
- Validation, shadow reads, gradual cutover, and rollback
- Migrate tenant by tenant to limit blast radius
Frequently asked questions
Is the “Data Migration Strategies” lesson free?
Yes — the full text of “Data Migration Strategies” is free to read here on the web, and the SaaS Architecture & Startup Engineering course includes 4 lessons in total. To practise it interactively (a built-in code editor and a 24/7 AI tutor) and unlock the rest of the SaaS Architecture & Startup Engineering course, upgrade to CoddyKit PRO.
What will I learn in “Data Migration Strategies”?
Learn how to move data safely when modernizing legacy systems using dual writes, backfills, validation, and zero-downtime cutover techniques. You practise SaaS Architecture & Startup Engineering with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.
Do I need any experience to start SaaS Architecture & Startup Engineering?
No prior experience is required. SaaS Architecture & Startup Engineering on CoddyKit is structured for beginners through advanced learners; this is — lesson 4 of 4, so you can start here or from the beginning and move at your own pace.
How long does the “Data Migration Strategies” lesson take?
Most CoddyKit lessons take about 5–10 minutes. Each one is bite-sized and interactive, so you make steady progress and pick up exactly where you left off across the web and the app.
Can I write and run code in this SaaS Architecture & Startup Engineering lesson?
Yes. Every SaaS Architecture & Startup Engineering lesson includes a built-in code editor, so you write and run real code right in your browser and get instant AI feedback — no local setup required.
All lessons in this course
- Strangler Fig Pattern
- Replatforming vs. Refactoring
- Gradual Rollouts & Testing
- Data Migration Strategies