Test Migrated Clojure Performance Against Budgets

Post-migration performance testing answers one narrow question: does the migrated Clojure path meet the budget that matters for this system? It should not be a generic contest between Java and Clojure. It should be a controlled comparison of the old and new behavior under representative workload.

The previous page proved equivalence. This page checks whether the equivalent behavior is fast enough, stable enough, and observable enough to ship.

Define The Budget First

Do not start by optimizing. Start by defining what acceptable performance means.

A migrated path can be slightly slower and still be acceptable. It can also be faster locally and unacceptable in production. Budgets make that distinction explicit.

Use Representative Workloads

Performance tests fail when the workload is too clean.

Keep a small set of local benchmarks for developer feedback, but use production-shaped load tests before cutover.

Benchmark The Pure Slice

For a pure Clojure function, a local benchmark can isolate transformation cost.

 1(ns migration.summary-bench
 2  (:require [criterium.core :as criterium]
 3            [migration.summary :as summary]))
 4
 5(def orders
 6  (vec (for [idx (range 10000)]
 7         {:order/id (str "O-" idx)
 8          :order/amount 42M
 9          :order/rejected? (zero? (mod idx 17))})))
10
11(criterium/quick-bench
12  (summary/summarize orders))

This does not replace system testing. It tells you whether the pure transformation is obviously expensive before you blame databases, queues, or HTTP.

Profile Before Optimizing

Use profiling evidence to choose the optimization target.

Do not add type hints, transients, or primitive arrays until the profile tells you they address the real cost.

Add Regression Gates

Performance tests should influence release decisions.

CI does not need to run every expensive benchmark on every commit. Use a bounded gate for ordinary changes and a deeper gate before cutover or large migration slices.

Interpret Performance Results

Performance validation is part of risk management. It is not a reason to micro-optimize every migrated function.

Practice

1. Define one latency or throughput budget for a migrated slice.
2. Build a representative input set with normal, boundary, and large cases.
3. Benchmark the pure Clojure function separately from the full system path.
4. Identify which regression gate should run in CI and which should run before cutover.

Key Takeaways

• Performance testing should be tied to explicit budgets.
• Representative workloads matter more than clean toy inputs.
• Benchmark pure functions separately from full system paths.
• Profile before applying optimization tactics.
• Regression gates should prevent behavior changes, load failures, and hidden allocation regressions.

Quiz: Performance Testing