Feature flags concept in Spring Boot - Performance & Optimization

Performance: Feature flags concept

MEDIUM IMPACT

Feature flags affect application startup time and runtime decision-making speed, impacting how fast features are enabled or disabled without redeploying.

Toggle features dynamically in a Spring Boot app without slowing requests

Spring Boot

private final Map<String, Boolean> featureCache = new ConcurrentHashMap<>();

public boolean isFeatureEnabled(String feature) {
    return featureCache.computeIfAbsent(feature, this::loadFeatureFlag);
}

private boolean loadFeatureFlag(String feature) {
    return jdbcTemplate.queryForObject("SELECT enabled FROM feature_flags WHERE name = ?", Boolean.class, feature);
}

Caches feature flags in memory to avoid repeated database calls, reducing latency and CPU load.

📈 Performance GainSingle database call per feature, sub-millisecond flag checks after cache, reduces CPU and latency significantly

Toggle features dynamically in a Spring Boot app without slowing requests

Spring Boot

public boolean isFeatureEnabled(String feature) {
    // Reads from database on every call
    return jdbcTemplate.queryForObject("SELECT enabled FROM feature_flags WHERE name = ?", Boolean.class, feature);
}

Querying the database on every feature check causes high latency and blocks request threads.

📉 Performance CostBlocks request threads, adds 10-50ms latency per call, increases CPU usage under load

Performance Comparison

Pattern	DOM Operations	Reflows	Paint Cost	Verdict
Database call per flag check	0	0	0	[X] Bad
In-memory cached flag check	0	0	0	[OK] Good

Rendering Pipeline

Feature flag checks occur during request processing, affecting backend response time but not browser rendering directly.

→Backend Processing

→Request Handling

⚠️ BottleneckDatabase or external service calls for flag evaluation

Optimization Tips

1Avoid querying external sources for feature flags on every request.

2Cache feature flags in memory and refresh asynchronously.

3Monitor backend latency to detect feature flag performance issues.

Performance Quiz - 3 Questions

What is the main performance risk of checking feature flags by querying the database on every request?

A It increases CSS paint cost.

B It causes layout shifts in the browser.

C It increases request latency and CPU usage due to repeated blocking calls.

D It reduces bundle size.

DevTools: Spring Boot Actuator and Application Performance Monitoring (APM) tools

How to check: Use APM to measure request latency and database query counts; check logs for repeated flag queries.

What to look for: High request latency spikes and frequent database queries indicate poor feature flag performance.