Monorepo vs Multirepo in SAP BTP Cloud Foundry (MTA/CAP)

We as developers on SAP’s Business Technology Platform (BTP) Cloud Foundry environments often build full‑stack applications using the Multi‑Target Application (MTA) model. Typical projects include a HANA Cloud instance, one or several CAP APIs, an Application Router, and one or more UI modules (SAPUI5, React, Angular, Vue, or other HTML technology) packaged together. A key architectural decision is how to organize the source code: monorepo (one repository for all modules) or multirepo (each component in its own repo). Both have trade-offs in this context. This article compares the two approaches in depth – covering definitions, SAP‑specific pros and cons, impacts on MTA deployment (including blue‑green strategies), pipeline performance, and some mention on how it impacts monolith vs microservices patterns – to help teams choose the right strategy.

Monorepo: Pros and Cons for SAP BTP

A monorepo consolidates all application components in one place. Benefits: It greatly simplifies cross-module development and consistency. Teams see all code in one place, which “lowers barriers of entry” for new developers. Shared libraries or UI components can be managed centrally. Monorepos make refactoring and testing across projects easier, and they centralize collaboration. For example, running integration or end‑to‑end tests on a CAP OData service and its UI in one pipeline is straightforward. Dependency management is simpler too: version upgrades apply to the workspace at once. In practice, SAP’s tools can leverage this: for instance, using a package manager like pnpm or a framework like Nx lets a BTP monorepo share node modules and build steps across CAP and UI projects, avoiding duplicate installs. (SAP’s easy‑UI5 tutorial itself creates a monorepo structure by default.) By definition, a monorepo “provides easier access” to all code and can foster a culture of shared ownership.

However, monorepos also have drawbacks in the SAP Cloud Foundry context. Large repos can strain Git and CI. Monorepos can suffer performance issues: combining many sub-projects can slow down code pulls and make CI/CD pipelines complex. In SAP terms, building an MTA means using the Cloud MTA Build Tool (mbt) to package all modules. A monorepo CI job may need to run mbt build (including npm install in each module) on every commit, which can be slow for many UI or service modules. Blue-green deployments also swap entire MTAs at once, so a single misbehaving module could affect the whole pipeline.

Security and branching can also be tricky. With a monorepo, granting access to one part of the app gives access to all code, which may be undesirable in large organizations. Monorepos “make security difficult” since team boundaries blur. Finally, scaling a monorepo requires discipline: good module boundaries (e.g. using domain-driven design) and careful CI configuration. Without that, even small changes can trigger full builds across many unrelated components.

Key Monorepo trade-offs:

Pros: One codebase (single repo) for all UI5/React/Angular/Vue/HTML5 frontends and CAP services. Eases cross-project changes, shared libraries, unified versioning and docs.
Cons: Large repo clones and builds can slow CI (heavy CPU/memory usage). Every commit may run builds for all modules unless tools skip unaffected parts. Managing access control or branching per service is harder.

Multirepo: Pros and Cons for SAP BTP

In a multirepo setup, each SAP BTP component lives in its own repo. Benefits: Clear ownership and isolation. Teams can grant permissions per repo (e.g. UI team vs backend team). Independent repositories mean independent lifecycles: one service can be versioned, branched, or even written in a different language (Java vs Node.js CAP) without impacting others. Multi-repos enable “almost instantaneous” independent versioning because little coordination is needed across repos. Similarly, assigning business-domain ownership is straightforward: each repo “can satisfy a specific business domain”. From a CI/CD perspective, each pipeline is narrower: for example, deploying a single CAP microservice or UI module is faster since only that code is packaged into an MTA. SAP’s CI/CD pipelines (e.g. Project Piper for MTAs) can target one repository at a time, which simplifies resource management. Multirepo setups naturally fit distributed microservices architectures: you can deploy each microservice (and its UI) on its own schedule. It also improves security: rights can be locked down per repo.

However, multirepos incur their own challenges. Cross-cutting changes become more complex. For instance, if you update a shared library or need a common OData model change that affects multiple CAP services and UIs, you must coordinate commits and releases across repos. As a warning, refactoring across a multi-repo codebase can be difficult and prone to duplication. End-to-end testing is harder too: automated tests may need to pull multiple MTAs or stub out other services. Dependency drift is a risk — one team might upgrade a dependency (say, SAP CAP SDK) while another lags, causing incompatibilities. In SAP Cloud Foundry specifically, a multirepo approach often means multiple smaller MTAs, which might each need their own blue-green deploy or route configuration, complicating atomic updates of the entire app. Lastly, too much isolation can undermine team communication: separate repos can lead to silos unless process and documentation are strong.

Key Multirepo trade-offs:

Pros: Strong modularity and autonomy. Independent versioning, CI/CD, and permissions per component. Better fits large teams and microservices.
Cons: Harder to implement cross-service changes or shared interfaces (more code duplication). Integration testing and coordination overhead increases.

Impact on MTA Deployments and Blue-Green Updates

SAP’s MTA model packages multiple modules (UIs, CAP services, HANA models, Application Router, etc.) into one deployable archive. In a monorepo, there is typically a single mta.yaml at the root defining all modules. This means one mbt build produces an .mtar with every UI and service. Deploying with cf deploy --strategy blue-green then updates the entire app at once. For example, the Cloud Foundry deploy tool spins up the “green” (idle) version of each module (e.g. web-idle, srv-idle) and only switches the routes to live (renaming them from -idle to main apps) after a successful build. This enables zero-downtime updates of the whole MTA.

Using a monorepo means this process naturally covers every part of the application. In contrast, a multirepo approach may involve separate MTAs or apps. You might deploy each service’s MTA individually. Coordinating a synchronized blue-green swap across several repositories is more complex. Teams may choose to blue-green each service independently or use feature toggles. In practice, monorepos allow an atomic MTA deployment; multi‑repos offer granularity but require orchestration (for example, using pipelines or “umbrella” scripts to update many repos in lockstep). Cloud Foundry’s deploy command handles one MTA at a time, so multi‑repo teams must script cross‑application routing if a global swap is needed.

CI/CD Pipeline Performance and Resource Use

Repository strategy also affects build pipelines and cloud resource consumption. Monorepo pipelines tend to be heavier. Cloning a large monorepo into a CI container is costly: Git must generate packfiles for all commits and files, a CPU/memory intensive task. For example, if your repo has dozens of frontends and services, git clone will use significant memory. Continuous builds may also spend time rebuilding unaffected parts. Fortunately, modern tools mitigate this: frameworks like Nx or Turborepo can compute “affected” projects and only run npm install or builds on changed modules. In other words, a CAP change won’t rebuild all UI modules if set up correctly. Caching can further accelerate repeated builds. But without such optimization, a monorepo CI/CD job must at least install dependencies for and build every module listed in mta.yaml every time, consuming more CPU, memory, and time.

Multirepo pipelines are smaller per component. Each CI job clones one repo and builds one service or UI. This reduces clone overhead. It also allows parallel pipelines: different teams can build their modules simultaneously. However, each repo must maintain its own pipeline definition, and coordinating cross-repo dependencies (e.g. updating shared libs) may require additional CI triggers or manual steps. Overall, multi‑repo gives finer control of resources per service at the cost of managing multiple pipelines. In either case, monitoring build times and using caching/incremental tools is crucial to keep deployments speedy on SAP BTP.

Monolithic vs Microservices Architecture

It’s important to distinguish repository strategy from service architecture. A monolith is a single deployable application containing all functionality, whereas microservices are many small, independent services. (This is independent of whether the code is in one repo or many.) You can store multiple microservices in a single monorepo or give each microservice its own repo. Conversely, a monolithic CAP application could be developed in one repo or split into parts. A common confusion is calling a monorepo a “monolith” – but they’re different concepts. As one developer explains: “A monolith can be broken up into microservices, but a monorepo can only be broken down into individual repositories.”. In practice on SAP BTP, small standalone apps (e.g. a single SAPUI5 frontend with one CAP backend) effectively form a monolith, and might live fine in one repo. A larger microservices architecture (e.g. multiple CAP services, each with its own UI or API) often aligns with a multirepo setup so each service can be deployed and scaled independently. However, some teams prefer a monorepo even for microservices to share code (common libraries or gateway logic) – at the cost of a more complex CI.

In summary, monolith vs microservices is about how the app runs, while monorepo vs multirepo is about how the code is managed. A monolithic BTP app can use either repo strategy, and a microservices-based solution can as well. The key is coordinating teams: a monorepo can help keep related services in sync, whereas multirepos encourage clear service boundaries and independent lifecycles.

Recommendations: Team Size and Application Complexity

There is no one-size-fits-all answer, but some general guidelines emerge:

Small teams or simple projects: A monorepo often simplifies life. With few developers, having one repo (with one set of CI/CD pipelines) reduces overhead. Shared code (e.g. a common CAP module or UI component) is easy to manage together. For example, a two-person team building a CAP service and a Fiori UI may prefer one repo. SAP’s own tutorials and generators (easy-UI5, CAP starter) often default to a unified project for ease of setup.
Large teams or many services: Multirepo scales better. If different groups own different business domains (e.g. one team for orders service, another for invoicing, plus separate UI teams), separating repos aligns with domain-driven design. Each team can move fast without coordinating every change. Independent versioning and deployment of each service improves agility. For large enterprises with many microservices, micro‑repositories reduce merge conflicts and CI bottlenecks. They also allow strict access controls per project, which is often required in large organizations.
Mixed complexity: Some projects use a hybrid approach. For instance, one repo per logical grouping (e.g. frontend repo, backend repo) or use Git submodules. SAP BTP’s CI/CD tooling (Project Piper, the SAP CI/CD service) can accommodate both: you can trigger a single pipeline from a monorepo, or set up multiple pipelines for multi‑repos and even integrate them via scripts or pipelines-as-code.
Blue‑Green and Updates: For any size, if zero‑downtime updates are critical, be aware that monorepos make blue-green easier to execute in one step, but usually they require more resources allocation in BTP for bigger 'idle' instances. With multirepos, planing / orchestration is required to deploy multiple apps or routes together, but usually deployments of each moule are much faster and require less resources allocation and can share the 'idle' resources when deploying sequentially.

In summary, consider team structure and app architecture. For a focused CAP/UI5 application with a small team, a monorepo can accelerate development. For a platform of many APIs and frontends developed by independent teams, multirepos improve maintainability.

Conclusion

Choosing between a monorepo and multirepo is a strategic decision for SAP BTP projects. Monorepos centralize code and simplify sharing across CAP backends and UI modules, but can tax CI and build systems as applications scale. Multirepos give teams autonomy and fine-grained control (independent versioning, security, CI), but require more effort to coordinate changes and testing across components. In practice, many SAP teams choose based on team size and project complexity: smaller apps and teams lean monorepo, larger and more distributed efforts favor multirepo. Whatever the choice, following best practices (using efficient build tools, clear module boundaries, automated testing) ensures that the chosen strategy serves development velocity and reliability in SAP BTP Cloud Foundry.

Sources: Bellow some interesting posts about this topic