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One free scan finds every outdated or missing driver and matches the right update for your exact hardware.Free scan · exact hardware matchWhen one independently deployed frontend changes, what promises must it keep so the rest of the application continues to work? That is the central microfrontend design question. A bundler can package or expose code; it cannot decide who owns a route, how modules communicate, or what happens when one fails. Agree on those boundaries first, then choose tools that implement them.
What the microfrontend contract covers
A microfrontend is an independently managed part of a frontend that can be built and deployed separately. The independence can help teams work on different parts of a product, but it also creates integration work at their boundaries. The single-spa documentation describes a microfrontend as “a microservice that exists within a browser.” The analogy is useful because it highlights the need for an explicit interface; browser modules still share a user experience and must coordinate.
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The contract is the set of externally visible promises each frontend makes to the shell and neighboring modules. Document decisions in these areas before selecting an integration mechanism:
The Tool Desk
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- Composition and lifecycle: Decide who determines when a module is active, loaded, mounted, unmounted, or replaced. With single-spa, activity functions let the orchestrator determine when applications mount and unmount; this is a lifecycle mechanism, not a substitute for agreeing who owns the behavior.
- Public interfaces: List the functions, properties, events, or links that may cross module boundaries. Specify compatibility expectations, how breaking changes are announced, and how old interfaces are deprecated. This is a practical response to the communication and coordination challenges involved in multi-team composition.
- Navigation: Assign ownership of route definitions, URL state, deep links, browser history, and transitions between modules. AWS identifies routing as an explicit microfrontend architecture decision.
- State and identity: Keep ordinary UI state inside the module that uses it. For cross-module identity or shared capabilities, define a narrow, stable interface rather than relying on an all-purpose global store. single-spa recommends local component state or a store per microfrontend.
- Dependencies: Set expectations for which libraries may be duplicated, which may be shared, how versions remain compatible, and who controls upgrades.
- Failure behavior: Decide what users see if a module cannot load or crashes, and how the experience can recover. An orchestration tool can manage lifecycle, but teams still need to define what a useful degraded experience means.
- Quality and operations: Align on accessibility, design-system use, telemetry, security expectations, integration tests, and release coordination. AWS notes that multi-component architectures need specialized integration and end-to-end testing.
Choose the composition model to fit the contract
Composition describes where and when independently managed pieces become part of the application. These approaches are not all alternatives at the same layer: an orchestrator such as single-spa coordinates application lifecycles, while import maps and Module Federation can resolve dependencies at runtime. AWS discusses build-time and runtime dependency strategies, including share-nothing, import maps, and Module Federation. Evaluate the choice against route ownership, communication, deployment, and the degree of coordination your teams can sustain.
#1 Best Overall
| Approach | What it addresses | Contract implications |
|---|---|---|
| Build-time composition or dependency installation | Combines modules or dependencies as part of a build. | The build and release path must account for the modules it assembles. This can suit a deployment model centered on a single application build, but it does not itself settle ownership, interfaces, or cross-module state. |
| Server-side or render-time composition | Composes frontend pieces on the server or during rendering. | Teams need agreements about which parts are rendered where and how routing and shared behavior fit the existing server and deployment model. Composition location alone does not define lifecycle or interface compatibility. |
| Browser-runtime composition with an orchestrator | Loads and coordinates modules in the browser. single-spa provides application lifecycle orchestration and can support applications built with different frameworks through adapters. | Define activity conditions, mount and unmount behavior, navigation ownership, and what happens when a module is unavailable. single-spa distinguishes route-controlling applications, parcels without routes, and utility modules that export shared logic. |
| Runtime dependency resolution with import maps or Module Federation | Determines how a browser application resolves modules or shared dependencies at runtime. | Agree on dependency versions and upgrade ownership. These mechanisms address dependency resolution; they do not by themselves establish who owns routes, lifecycle, or product behavior. |
The right choice depends on the system and organization; there is no universally correct integration option. AWS recommends making architectural decisions for the specific microfrontend system rather than treating one composition or dependency strategy as a default.
Make dependency sharing an explicit tradeoff
Sharing can reduce repeated delivery of the same dependency, but consumers then inherit compatibility and upgrade coordination. Isolating modules can preserve teams’ ability to upgrade independently, while duplication may increase the code delivered. The sources describe these tradeoffs qualitatively; there is no single payload-cost figure that applies across applications.
- Share a dependency when the value of reuse outweighs the cost of coordinating compatible versions and upgrade timing.
- Duplicate a dependency when independent upgrades matter more than avoiding duplication. single-spa gives a small library such as a router as an example that may reasonably be duplicated so teams can upgrade separately.
- Avoid sharing everything by default. single-spa warns that sharing every dependency can force consumers into coordinated upgrades. AWS describes share-nothing, web standards such as import maps, and Module Federation as dependency-management strategies.
Before implementing a strategy, write down the shared-dependency policy: which packages qualify, who publishes compatible versions, how consumers test upgrades, and what teams may do when they cannot upgrade together. The tool can enforce or enable parts of that policy; the teams still own the policy itself.
Keep communication and state narrow
Cross-module communication should be intentional and limited to information another module genuinely needs. Prefer an explicit import or other documented interface over implicit coupling through a broad global store. single-spa identifies importing from another microfrontend as its preferred communication approach and recommends local component state or a store owned by each microfrontend.
For each cross-boundary capability, name its owner and compatibility expectations. This makes it possible to change a module without requiring unrelated teams to understand its internal state. Shared identity or platform capabilities may need a common interface; routine UI details usually do not.
Independent deployment still needs integration testing
Deploying modules independently does not make end-to-end behavior independent. A route change, incompatible dependency, or broken interface can affect the composed application even if each module builds successfully on its own. AWS highlights coordination and specialized integration and end-to-end testing as challenges in multi-component architectures.
Rank #4
Test the agreed boundaries as well as each module internally: route transitions, module mounting and unmounting, supported dependency combinations, cross-module interfaces, and the defined fallback when a module fails to load. Assign responsibility for these checks; otherwise, each team can meet its local release criteria while the combined experience breaks.
Use the tool to implement decisions, not make them
Before committing to a bundler or runtime mechanism, the architecture group should be able to answer who owns each route, how modules are activated and removed, which interfaces are supported, how state crosses boundaries, which dependencies are shared, and how failures and integration tests are handled. Then compare candidate tools against those requirements and the current framework and deployment model.
Best Value
single-spa is an orchestration option for lifecycle coordination; import maps and Module Federation are dependency-resolution options. They can be used to implement selected architectural decisions, but neither category removes the need for cross-team agreements. Check the current documentation for the versions you plan to use, since implementation details and tool support can change.
References: single-spa: Concept: Microfrontends; single-spa: The Recommended Setup; single-spa: Microfrontend Types; AWS Prescriptive Guidance: Managing dependencies for cross-cutting concerns; AWS Prescriptive Guidance: Architectural decisions in micro-frontends; AWS Prescriptive Guidance: Create a portal for micro-frontends by using AWS Amplify, Angular, and Module Federation.
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