Monolith to Microservices: Migration Guide

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Monolith to Microservices: Migration Guide

1. The Strangler Fig Pattern: Growth over Decay

Named after a tree that grows around another tree until the old tree dies.

Intercept: Put an API Gateway (Module 185) in front of your monolith.
Extract: Build ONE new Microservice (e.g., "The Payment Service").
Route: Tell the Gateway: "Any request for /payments goes to the new service. Everything else stays in the old monolith."
Repeat: Do this for $10$ more features over the next year until the monolith is empty.

2. Dealing with the "Shared Database"

This is the hardest part. The monolith and the new microservice both need the users table.

The Phase 1: Both services talk to the SAME database. (This is okay for a few months).
The Phase 2: The Microservice gets its own database. It "Syncs" data with the monolith database using a Message Queue (Module 186).
The Final Phase: You move the "Source of Truth" to the Microservice and delete the columns from the monolith.

3. Communication: Service Mesh (Istio)

During the migration, you have 1 monolith and 10 microservices all talking to each other.

The network becomes a mess.
The Solution: Use a Service Mesh (Istio).
It handles the "Routing" and "Retries" automatically so your developers don't have to write thousands of lines of fragile "Networking Code" during the migration.

4. The "No-Go" Rule: When to Stop?

Sometimes, you should keep the monolith.

If a feature is "Stable" and never has bugs, Don't touch it.
There is no prize for "100% Microservices."
A professional architecture is often a "Modernized Monolith" with a few high-scale microservices around it. This is the most cost-effective and safe way to run a company in 2026.

Frequently Asked Questions

Is it faster after the migration? Usually, the developer Build times are $10x$ faster. But the User Latency might be 20% slower because of the network calls. You are trading a small amount of "User speed" for a massive amount of "Developer Productivity."

What if we fail? With the Strangler Fig, you can't "Fail." If a new microservice doesn't work, you just tell the Gateway to send the traffic back to the old monolith. It's a "Zero-Risk" way to innovate.

Key Takeaway

Migration is about "Risk Mitigation." By mastering the Strangler Fig pattern and the discipline of data synchronization, you gain the ability to modernize even the oldest, messiest legacy systems without crashing your business. You graduate from "Managing legacy debt" to "Architecting Modern Assets."

Part of the Software Architecture Hub — engineering the split.