Modernizing Monoliths
Without Downtime
- The Challenge: Legacy monoliths are often too fragile to change but too mission-critical to turn off, creating a technical stalemate.
- The Strategy: Eschew the "Big Bang" migration. Instead, use the Strangler Fig Pattern—wrapping the legacy system in a proxy layer and migrating individual services incrementally.
- The SRE Advantage: Leveraging Site Reliability Engineering (SRE) principles like Change Data Capture (CDC) and Dark Launches ensures the transition is invisible to the end user.
For many enterprises, the legacy monolith is a double-edged sword: it powers the entire business, but it's too fragile to change and too important to turn off.
The "Big Bang" migration—where you flip a switch and pray—is a relic of the past. Today, modernizing a monolith is an exercise in Site Reliability Engineering (SRE). It requires precision, observability, and a strategy that ensures your customers never even notice the ground is moving beneath them.
CASE STUDY: THE FINTECH TRANSITION
The Challenge: A UK-based FinTech platform was running on a 10-year-old Ruby on Rails monolith. Deployment took 4 hours, and the database was at 90% capacity during peak hours.
The Constraint: Zero downtime allowed. A 5-minute outage represented £50,000 in lost transactions.
Phase 1: The "Strangler Fig" Strategy
At Esseal, we don't start by rewriting code. We start by wrapping the monolith in a Proxy Layer. By placing a modern gateway (like Nginx or Kong) in front of the legacy app, we gain the ability to route traffic based on specific paths.
We then identify the most problematic "leaf" of the monolith—usually a high-traffic service like Payments or User Authentication—and build a modern microservice for just that piece.
Phase 2: Database Synchronisation
The hardest part of zero-downtime migration is the data. If the old monolith and the new microservice aren't looking at the same truth, the system fails.
Our SRE team utilises Change Data Capture (CDC). We stream database updates from the legacy SQL server to the new cloud-native database in real-time. This allows us to run both systems in parallel, verifying that the new service produces the exact same results as the old one before we ever "cut over" the users.
Phase 3: The Dark Launch
Before we trust the new system, we perform a Dark Launch. We route a copy of live production traffic to the new microservice, but we discard its response.
This allows us to stress-test the new infrastructure under real load and monitor its performance through our SRE dashboards—without affecting a single real user.
The Result: Long-Term Reliability
In our FinTech case study, the transition didn't just fix a technical debt; it secured a long-term reliable partner in Esseal to maintain the high-availability infrastructure.
- Downtime: 0 minutes throughout the 6-month migration.
- Performance: Average API response time dropped from 450ms to 80ms.
- Agility: Deployment frequency increased from monthly to ten times daily.
How Esseal Safely Migrates Your Legacy
Migration is a high-stakes game. Our embedded SRE teams take the burden off your developers, managing the complex plumbing of proxies, data syncs, and monitoring. We provide the safety net that allows your team to continue shipping features while we modernise the engine mid-flight.
Is your monolith holding you back?
Don't risk a "Big Bang" failure. As your reliable long-term partner, we'll build a structured, SRE-led migration plan that protects your uptime and your sanity.
Plan Your Zero-Downtime Migration