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When one independently deployed frontend changes, what must it promise so the rest of the application keeps working? Before choosing a bundler or runtime tool, teams need agreements about ownership, routes, lifecycle, communication, dependencies, compatibility, and failures. A bundler can package or expose code; it cannot decide those boundaries for you.
What the microfrontend contract covers
Microfrontends are frontend modules that teams can manage, build, and deploy separately. That independence moves work to the boundaries: each module must integrate predictably with the shell and neighboring modules. The single-spa documentation describes a microfrontend as “a microservice that exists within a browser.” The analogy is useful because it highlights independent responsibility, but browser modules still need explicit rules for how they fit together.
Think of the contract as the externally visible promises each frontend makes to the rest of the application. AWS identifies routing, state, communication, and dependency management as architectural decisions; in practice, teams also need agreements on lifecycle, compatibility, quality, and operations.
Ownership
Name the team responsible for each user journey, route, and major UI region. Clarify who handles defects, accessibility issues, and operational support. Without clear ownership, a module can be independently deployed yet nobody is accountable for its behavior at the boundary.
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Composition and lifecycle
Decide who determines when a module is relevant, loaded, mounted, unmounted, or replaced. The shell or an orchestrator may make those decisions, but the module must have defined lifecycle expectations. For example, specify what setup it performs on mount and what resources or listeners it must release on unmount.
In single-spa, activity functions help determine when applications should be mounted and unmounted. Its module types distinguish route-controlling applications, parcels that do not own routes, and utility modules that export shared logic. These are different responsibilities, not interchangeable labels.
Public interfaces and compatibility
List the functions, events, properties, or links that cross a module boundary. Keep the interface narrow, document which side owns each value, and define how changes are introduced: for example, whether a breaking change requires a coordinated release or a deprecation period. These are architectural agreements teams should establish to address the communication and coordination challenges of multi-team composition.
Navigation
Agree who owns route definitions, URL state, deep links, browser history, and transitions between modules. A route should have an accountable owner, and a user should be able to navigate directly to a supported deep link without relying on an undocumented sequence of clicks. Routing is a first-order architecture decision, not a detail a bundler settles.
State and identity
Keep ordinary interface state close to the component or microfrontend that owns it. For cross-module needs such as identity or shared capabilities, define a small, stable interface rather than exposing every module to an all-purpose global store. single-spa recommends local component state or a store per microfrontend and favors importing from another microfrontend as a communication approach.
Dependencies
Set a policy for which dependencies are duplicated and which are shared. Specify acceptable version compatibility, who maintains shared versions, and how upgrades are coordinated. Sharing can reduce repeated dependency delivery, but it ties consumers to compatibility and release decisions; duplication can preserve independent upgrade timing while adding repeated code.
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Failure behavior and quality
Define what users see if a module is slow, unavailable, or fails while loading, and how the application recovers. Also align on accessibility, design-system use, security expectations, telemetry, and integration testing. AWS notes that multi-component composition brings specialized integration and end-to-end testing needs: independent deployment does not remove the need to test the assembled experience.
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Choose composition and dependency strategies separately
There is no universally correct integration choice. First decide where composition happens and who coordinates routes and lifecycles; then choose how dependencies are resolved. These choices are related, but they answer different questions: single-spa is an orchestrator for application lifecycles, while import maps and Module Federation are dependency-management options.
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| Approach | Where it fits | Trade-off to plan for |
|---|---|---|
| Build-time composition | Modules are assembled during a build. | The composition is known at build time; teams should assess whether that fits their deployment and release boundaries. |
| Server-side or render-time composition | Composition occurs on the server or during rendering. | Teams must decide how the server or rendering layer coordinates modules and their interfaces. |
| Browser runtime orchestration | Modules are resolved or activated in the browser; single-spa can coordinate application lifecycles. | Routing, activation, loading, and cross-module behavior need explicit ownership and integration tests. |
| Share nothing | Each module carries its own dependencies. | Duplication can increase delivered code, while allowing modules to upgrade independently. |
| Import maps | Web-standard runtime dependency resolution. | Shared versions require compatibility and upgrade coordination among consumers. |
| Module Federation | Runtime sharing and loading of modules or dependencies. | Sharing dependencies still creates version and release coordination; the mechanism does not define team ownership or public contracts. |
AWS describes share-nothing, import maps, and Module Federation as dependency-management strategies. They are not alternatives to lifecycle orchestration in every case: a system can use an orchestrator and separately choose a dependency strategy. Evaluate each option against your existing framework, deployment model, team boundaries, and testing capacity rather than treating a tool choice as the architecture.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Make dependency sharing a deliberate exception
Sharing every dependency can make consumers dependent on the same upgrade schedule. single-spa recommends choosing import maps or Module Federation for shared third-party dependencies, but cautions against sharing everything. It notes that a small library such as a router may be reasonable to duplicate when doing so lets teams upgrade separately.
Use a shared dependency when the reuse is valuable enough to justify a compatibility policy and an owner for upgrades. Keep it local when independent change matters more than avoiding duplication. The sources establish these qualitative trade-offs; they do not provide a measured bundle-size cost for either choice.
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- Map ownership. Assign teams to journeys, routes, UI regions, and support responsibilities.
- Define boundaries. Write down lifecycle expectations, public interfaces, navigation rules, and the owner of cross-module state.
- Set compatibility rules. Decide how interface and dependency changes are versioned, introduced, deprecated, and upgraded.
- Choose composition. Select build-time, render-time, or browser-runtime composition based on deployment and framework needs; decide who coordinates lifecycle and routing.
- Select dependency handling. Choose between duplication and runtime sharing for each dependency, with a named upgrade owner for anything shared.
- Test the assembled experience. Cover direct routes, transitions, loading and failure states, accessibility, and cross-module integration—not only each module in isolation.
Only after these agreements are clear can a bundler or runtime integration tool be compared meaningfully: it should implement chosen boundaries, not substitute for deciding them.
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