SaaS Engineering

Modernising Your Tech Stack Without a Full Rewrite: Incremental Migration, Go and Next.js

By Mostafa EL MouaddinePublished 12 October 202610 min read

Modernising your tech stack means gradually replacing whatever is holding back delivery, security or growth, without taking the service down or rewriting everything in one go. The safest approach is incremental migration: put a façade in front of the existing system, then move one module at a time. For a SaaS architecture, Go for the services, Next.js and TypeScript for the interface and Python for AI and data make a solid combination — provided the team knows how to run it, which is not always the case.

What are the signs that a tech stack needs modernising?

An old technology is not a problem in itself. A stack becomes a problem when it costs more than it returns: every change takes longer than planned, and the team spends more time working around the system than moving it forward. The signals that matter are ones you can observe:

• Slow, risky releases: every deployment is done by hand, often in the evening, with the fear of breaking something else.
• End-of-life dependencies: a framework, language or operating system that no longer receives security patches.
• Knowledge in too few heads: only one or two people understand the core of the system, and hiring for that technology is getting harder.
• Expensive scaling: the only answer to more users is a bigger server, and traffic peaks turn into incidents.
• Product roadblocks: you cannot open an API to a partner, add a language, host a second client on the same platform or cleanly plug in an AI model.

A single signal can often be dealt with locally. When several pile up, the question is no longer whether to modernise but in what order. Before choosing technologies, set down your business objectives and constraints: that is what a good software requirements specification is for.

Application rewrite: why avoid starting from scratch?

It is tempting to throw everything away and start again. In practice, a "big bang" rewrite freezes the product for months: the old system keeps evolving for customers, the new one chases a moving target, and the final cutover piles all the risk onto a single weekend. Implicit business rules — never documented but very much present in the code — are also the first casualties.

The proven alternative is incremental migration, often called the strangler fig pattern (after the strangler fig tree, an image popularised by Martin Fowler): the new system grows around the old one and replaces it feature by feature.

1. Put a façade (a reverse proxy or API gateway) in front of the existing application, with no change for users.
2. Pick a first scope that is limited, useful and low-risk: search, notifications or a reporting module, for example.
3. Rebuild it on the new stack, then route that traffic to it through the façade.
4. Measure, fix, then repeat with the next scope.
5. Delete the old code as you go, or the team will end up maintaining two systems indefinitely.

Every step delivers value and can be rolled back: if the new service misbehaves, the façade sends traffic back to the old one while you fix it. The tricky part lies elsewhere: during the transition, old and new code often share the same data. Decide from the outset which system is the source of truth for each piece of data, and shield the new code from the quirks of the legacy model with a translation layer (an "anti-corruption layer"); otherwise the technical debt migrates along with the code.

Why Go and Next.js for a SaaS architecture?

Next.js, Go, Python and TypeScript make up Codevista Innovations' core stack. The reasons are operational, and they apply to any team evaluating this combination.

Go for backend services

• Simple concurrency: goroutines and channels let you handle requests, message queues and external calls in parallel, without managing threads by hand.
• A single binary: a Go service compiles to a self-contained executable, which can be fully static. The container stays lightweight and there is no runtime to install on the server.
• Predictable performance: a compiled language, fast start-up, a generally modest memory footprint. Infrastructure is easier to size.
• Easy to operate: a rich standard library (HTTP, JSON, cryptography), built-in tooling for testing, formatting and profiling, and cross-compilation to other systems.
• Code the whole team can read: the language is deliberately small, which makes code reviews and onboarding new developers easier.

Go is particularly well suited to APIs, integration services (payments, ERP, messaging), asynchronous processing and components that have to handle heavy load.

Next.js and TypeScript for the frontend

Next.js adds server-side rendering and static generation to React. Public pages reach the browser already built, which matters for SEO, for load times on mobile and for sharing on social media. The React ecosystem, one of the most widespread in frontend development, supplies UI components and libraries, and makes hiring easier.

TypeScript brings typed contracts: the data exchanged with the backend is described once, for instance from an OpenAPI schema generated by the Go services. A field renamed in the API then breaks the build, not production. Finally, Next.js lends itself to incremental migration: its rewrite rules serve the new pages and pass the rest through to the legacy application, page by page.

Python where AI and data live

Python remains the go-to language for data and artificial intelligence: data processing, machine learning, SDKs from large language model providers. Rather than forcing that work into Go, it is better to isolate it in dedicated Python services, called through an internal API or a message queue. This split keeps each language where it fits best and lets you apply stricter security rules to the part that handles sensitive data — for example when orchestrating AI agents. The trade-off: one more language to maintain, test and monitor. If your AI boils down to a few API calls to a model provider, a Go or TypeScript service is often enough.

When should you not choose Go and Next.js?

No stack is right everywhere. Here are the cases where other options — or leaving things as they are — make more sense:

• The team already runs a healthy stack. A well-maintained Laravel, Django, Java or .NET application that is tested and deployed regularly does not justify switching languages. Modernise your practices before your technologies.
• A simple internal back office. For a management tool with no load concerns, a batteries-included framework with a generated admin interface will often ship faster than Go services.
• An application entirely behind a login. With no public pages, the SEO argument for Next.js disappears; a standard React application may be enough and is simpler to host.
• A business domain heavy with rules. Go favours simplicity at the cost of some repetition; some teams prefer a more expressive language for modelling a complex domain.
• No operational capacity. Several services mean monitoring, centralised logging and automated deployment. Without that foundation, a well-structured monolith is safer than poorly monitored microservices.

Also factor in the pace of Next.js: its major versions move quickly, and keeping up with them takes time that needs to be planned.

Multi-tenant architecture: preparing for international growth

A multi-tenant SaaS serves many client organisations — the tenants — on a single platform. Targeting several countries multiplies both the tenants and the isolation requirements. The isolation model has to be chosen early, because it is hard to change later:

• Shared database with a tenant ID: the most economical model, but every query must filter correctly; PostgreSQL row-level security adds a useful safeguard.
• One schema per tenant: better separation, but migrations are heavier to orchestrate.
• One database per tenant: strong isolation and the option to keep data in a given country, at the cost of more complex operations.

Beyond the database, going international forces concrete choices: several languages, including Arabic written right to left, time zones, currencies, payment methods (the CMI, Morocco's interbank card-payment centre, for card payments in Morocco, and other providers such as Stripe depending on where the invoicing entity is established) and compliance. In Morocco, Law 09-08 (loi 09-08) governs the processing of personal data, including transfers abroad; the CNDP, the national data protection authority, is the regulator to consult. Each target country may add its own rules: check them before deciding where to host the data. Our guide to custom SaaS development in Morocco covers these trade-offs in detail.

How do you sequence an application modernisation?

The order of the steps matters as much as the technologies you choose. A sequence that keeps risk under control:

1. Audit and mapping: an inventory of components, dependencies, data flows and pain points, ranked by business value and by risk.
2. Delivery foundation: continuous integration and deployment, automated tests on critical user journeys, monitoring. Without this safety net, every migration is a gamble.
3. Façade and first scope: set up the proxy, then migrate a high-value, low-risk module to validate the approach.
4. Gradual Next.js frontend: public pages first, where server-side rendering has an impact on SEO and load times, then the application screens.
5. Data last: the database is the riskiest part; migrate it domain by domain, with verification scripts and a rollback plan.
6. Decommissioning and handover: each replaced module disappears from the old system, and the in-house team builds its skills on the new one throughout the project, not at the end.

Run the project in short iterations using a method such as Scrum: each sprint delivers a verifiable scope to a staging environment before it goes to production. If you are outsourcing all or part of the work, our criteria for choosing a software development company in Casablanca will help you ask the right questions, particularly about data security.

Codevista Innovations supports this process, from assessing your existing stack to designing multi-tenant architectures in Go and Next.js, working from Casablanca for clients in Morocco and abroad. Our projects begin with a discovery and design phase, then move forward in two-week sprints delivered to staging for your approval. To take stock of where you stand, tell us about your project.

Frequently asked questions

How long does it take to modernise an existing application?

It depends on the size of the system, the quality of the existing code and how much time the team has available: no generic figure is reliable. The whole point of incremental migration is that you do not depend on a single cutover date. A first scope can go into production well before the project is finished, then the following modules come one after another according to their value. An initial audit lets you establish a realistic sequence and estimate.

Can we keep our current database when moving to Go and Next.js?

Yes, and it is often the recommended approach at the start. Go has mature drivers for the main relational databases, including PostgreSQL, MySQL and SQL Server. New services can therefore read from and write to the existing database during the transition. The schema is then migrated domain by domain once the services are stable, with consistency-check scripts and a rollback plan.

Go or Node.js for a SaaS backend?

Both are defensible choices. Node.js lets you share TypeScript between frontend and backend, which makes life easier for a small team. Go offers a single binary and native concurrency that uses every processor core without any special configuration — useful for high-load APIs and integration services. The deciding factor is often the team's skills and operational constraints rather than raw performance.

Is Next.js worth it for a business application with no SEO requirements?

Less so. Next.js server-side rendering mainly benefits public pages: SEO, load times, social media sharing. For an internal tool that sits entirely behind a login, a standard React application may be enough. Next.js is still relevant if the same product combines a public website and a customer area, or if you want a single framework for routing, internationalisation and server calls.