Smart Proxy

The smart proxy adding mTLS, retries, cache in front of a third-party service.

Problem

A partner EDI exposes a capricious API — 100 req/s limit, no cache, no observability. How to use it reliably from a modern system?

Forces

• You cannot modify the partner — it is imposed.
• The functional contract must be respected.
• Cross-cutting features (retries, cache, traces) must not pollute business code.
• The proxy must be transparent — client calls as if nothing.

Solution

Deploy a Smart Proxy (typically Envoy, NGINX+, Kong, AWS API Gateway) in front of the partner service. The proxy adds: circuit breaker, client-side rate limiting, caching with TTL, mTLS upgrade, OpenTelemetry traces, retries with backoff. Client calls the proxy as if calling the partner — transparent.

EDI implementation

In EDI, Smart Proxy is crucial for: (1) AS2 partners with short-lived certificates — proxy handles rollover, (2) customs APIs (NCTS, ICS 2) — aggressive caching on static codes, (3) PEPPOL discovery (SMP lookups) — 1h caching. Implementation: Envoy sidecar with custom filters, or Kong with Lua plugins.

Anti-patterns

• Proxy altering business semantics — must remain transparent.
• Cache too aggressive — stale data sent downstream.
• No proxy monitoring — becomes a black hole.

Related patterns

• Circuit Breaker — proxy feature.
• Wire Tap — observation enabled by proxy.

Sources

• Hohpe G., Woolf B. — EIP, Smart Proxy (p. 558). www.enterpriseintegrationpatterns.com/patterns/messaging/SmartProxy.html
• Envoy Proxy — Documentation. www.envoyproxy.io/docs