Global vs. Per‑Route Serialization in Fastify: Which Approach Yields Better Performance for High‑Throughput APIs?

Goal

We want to deliver a high‑throughput, read‑only API where the JSON response shape is stable across many requests.

Constraints

• Minimize per‑request CPU spent on serialization.
• Keep startup time acceptable for rapid deployment cycles.
• Maintain flexibility to change response schemas without a full rebuild.
• Ensure that error responses and non‑JSON payloads remain unaffected.

Uncertainty

Fastify allows a global setReplySerializer that compiles a serializer from a JSON schema, or the default JSON.stringify can be used per route. The interaction between a global serializer and route‑specific schemas is not fully documented, and the impact on error handling and plugin boundaries is unclear.

Questions

1. Will a global reply serializer alter the serialization of error responses or non‑JSON content types?
2. What is the comparative startup cost of compiling a schema‑based serializer versus using the default runtime serializer?
3. How does registering setReplySerializer inside a plugin affect its scope and the behavior of routes defined in other plugins?