Can Fastify plugin encapsulation support phased rollout without downtime

Yes, Fastify's plugin encapsulation can support a phased rollout without downtime, provided you avoid the fastify-plugin wrapper for the components you intend to isolate. By leveraging the default encapsulation provided by Avvio, you can run multiple versions of a logic set (e.g., a legacy plugin and a canary plugin) within the same instance without their decorators or hooks leaking into each other's scopes.

Implementation Strategy

To achieve a phased rollout or canary routing without interfering with business logic, follow these architectural constraints:

1. Routing and Canary Logic

The onRequest hook is the correct point for canary routing. Because it is the first hook in the lifecycle, it allows you to redirect, mirror, or reject requests before they reach the preHandler logic or the final route handler. This ensures that the overhead of routing decisions does not pollute the actual request processing pipeline.

2. Preserving Encapsulation

To prevent scope leakage during a rolling update within a single process:

• Avoid fastify-plugin: Do not wrap your versioned plugins in fastify-plugin. The wrapper explicitly breaks encapsulation, making decorators and hooks global.
• Use Scoped Registration: Register different versions of your service as separate plugins. Fastify will create a new inheritance chain for each, ensuring v1 decorators are invisible to v2.
• Asynchronous Loading: Fastify handles asynchronous registration via Avvio; the server will not start listening until all plugins in the boot sequence have resolved, preventing race conditions where a route is hit before its scoped decorators are ready.

3. Graceful Shutdown

The .close() method is considered complete only after the server stops accepting new connections and all active requests have finished processing. Plugin teardown (via onClose hooks) occurs during this sequence, ensuring that database connections or file handles are closed only after the final in-flight request is served.

Verification Steps

To verify your isolation and shutdown behavior, use these scoped tests:

// 1. Verify Encapsulation
fastify.register(async function v1() { 
  fastify.decorate('version', '1.0'); 
});
fastify.register(async function v2() { 
  // This should NOT see '1.0'
  console.log(fastify.version); 
});

// 2. Verify Shutdown
// Trigger a 5-second request, then call fastify.close().
// The process should not exit until the request completes.

Missing Diagnostic Detail: Are you performing the phased rollout via a load balancer (external) or by routing traffic between different plugin scopes within a single Fastify instance (internal)? If external, Fastify's internal encapsulation is less critical than its .close() behavior.