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A backend feature shared by web, mobile, or other clients is more than an implementation reused in several places: it is a contract each client depends on. The main lesson is to design that contract around domain behavior, then account for differences in payload needs, release schedules, retries, and access boundaries. The right choices depend on the clients and system; there is no universal API shape or versioning strategy.
Design the shared API around the domain
A useful API describes what callers can do and what those operations mean. It should not expose internal code structure or simply reproduce the database schema. That separation lets a team change storage or refactor internals without forcing every client to change at the same time. Microsoft recommends evolving an API when its functionality changes, rather than changing it solely because the implementation or data store has changed. Microsoft’s API design guidance discusses this distinction.
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This contract matters whether an API is public to browser or native-app clients or used between backend services. Client-facing APIs must account for compatibility with those applications, while backend APIs also need to consider network performance. REST over HTTP is common, but Microsoft notes that binary protocols can suit performance-sensitive communication; the choice should be based on the scenario and tested rather than assumed.
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Clients may share the same domain operations while differing in what makes an interaction practical. A mobile interface might need a smaller payload or a sequence of calls suited to a constrained connection; a desktop or browser client may have different needs. Start by comparing the actual differences rather than creating separate APIs simply because there are separate apps.
- Payload shape and size: Do clients need substantially different data, or can one stable representation serve them?
- Interaction patterns: Do clients combine operations or navigate workflows differently?
- Release cadence and ownership: Can the teams coordinate changes, or do clients ship independently?
- Operational cost: Would multiple interfaces reduce client complexity enough to justify their additional implementation, testing, and support?
When client needs diverge substantially, a Backend for Frontends (BFF) pattern can provide a tailored interface for a client or client group. It is an option, not a default: additional interfaces create more code and operational work to maintain. Microsoft describes client-specific needs and BFFs in its API design guidance.
Plan compatibility for independent releases
Every separately released client increases the chance that some consumers will still depend on an older contract. A change that seems small on the server can therefore become a client-breaking change. Adding a field is often compatible if clients tolerate fields they do not recognize; removing or renaming a field can break consumers that still use it. Google Cloud also notes that internal consumers can be disrupted by breaking changes when they are not versioned. Treat compatibility as a lifecycle responsibility, not a final check before deployment. Google Cloud’s discussion of API versioning covers the coordination problem.
When a breaking change is necessary, choose a versioning method that fits the way clients route, cache, and make requests. Microsoft documents four common approaches:
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|---|---|---|
| URI | In the request path | Visible and straightforward to route; URI-based approaches can be cache-friendly. |
| Query parameter | In the request query string | Explicit in the request and can be cache-friendly. |
| Custom header | In a request header | Requires examining headers and can affect cache behavior. |
| Media type | In the media type used in the request | Can keep version details out of the URI, but requires header-aware handling and can affect caching. |
These are alternatives, not a ranking. Consider routing, cache behavior, how links represent versions, client implementation burden, and the number of versions the team can realistically support. Supporting several versions also means more combinations to test and operate. Pair a version with a migration and retirement plan so clients have a supported path forward. For the approaches and trade-offs, see Microsoft’s Web API design guidance.
Make retries safe for state-changing operations
Clients and networked systems may retry a request when they do not receive a clear response. If repeating an operation creates another side effect—such as performing the same action twice—a retry can produce an unintended result. Microsoft defines idempotency this way: “An operation is idempotent if you can call it multiple times without producing more side effects after the first call.”
Where repeated requests must not repeat an effect, design the operation so retries can be handled safely. The exact mechanism depends on the operation and system; the important point is to treat retry behavior as part of the contract rather than leaving each client to guess. Microsoft discusses idempotency in its API design guidance.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Keep authorization separate from tenant isolation
Authentication establishes who is calling, and authorization determines what that caller may do. In a multi-tenant SaaS system, those checks alone do not guarantee that a request is restricted to the caller’s tenant. The application must enforce tenant isolation as well as permissions; otherwise, an authenticated and authorized caller could still reach another tenant’s data if the boundary is not applied correctly.
For systems serving separate tenants, consider whether permissions are complex, whether enforcement must be consistent across endpoints, and what audit requirements and isolation risks apply. AWS’s guidance treats tenant isolation as distinct from authorization and discusses centralized policy patterns for multi-tenant API access. See AWS Prescriptive Guidance on multi-tenant SaaS authorization and API access control. This concern applies when a system is multi-tenant; serving several client applications does not, by itself, imply that it is.
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Use a gateway only when the system calls for one
An API gateway can provide a place to handle concerns such as monitoring, routing, protocol translation, authentication, authorization, rate limiting, and resilience controls. That does not make a gateway mandatory. Its value depends on the system, the number of consumers, and API complexity; introducing one also adds an architectural component to operate. The Software Engineering Institute at Carnegie Mellon University examines these roles and trade-offs in its report on the design, development, and testing of modern APIs.
What to carry into the next feature
Before a shared backend feature is committed to a contract, identify its consumers and the constraints that distinguish them. Decide which behavior belongs in the domain API, where client-specific adaptation is justified, how breaking changes will be introduced and retired, and whether retries or tenant boundaries affect correctness. Those choices connect backend design to the realities of clients that may evolve on different schedules.
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