The Client-Server and Peer-to-Peer Paradigms

When to Employ This Paradigm

• For traditional applications that have centralized services, such as web or mobile clients communicating with backend APIs.
• For systems exploring decentralized or "offline-first" capabilities that rely on peer-to-peer synchronization.
• To formally document trust boundaries, client-server version negotiation, and API evolution strategies.

Adoption Steps

1. Define Responsibilities: Clearly delineate which logic and data reside on the client versus the server, with the goal of minimizing duplication.
2. Document the Contracts: Formally document all APIs, data schemas, authentication flows, and any capability negotiation required for handling different client versions.
3. Plan for Version Skew: Implement a strategy to manage different client and server versions, such as using feature flags, Accept headers for content negotiation, or semantic versioning for APIs.
4. Address Connectivity Issues: If the application is not purely client-server, design for intermittent connectivity. This may involve implementing offline caching, data synchronization protocols, or peer discovery and membership services.
5. Secure All Communications: Enforce the use of TLS for all data in transit. Implement authorization policies, rate limiting, and detailed telemetry for every endpoint.

Key Deliverables

• An Architecture Decision Record (ADR) that covers the roles of clients, servers, and peers, defines the trust boundaries, and outlines deployment assumptions.
• Formal API or protocol specifications, along with a suite of compatibility tests.
• Runbooks detailing the coordination required for rollouts, such as client release waves, backward-compatibility support, or operational procedures for a peer-to-peer network.

Risks & Mitigations