How do you optimize Drupal performance for high-traffic sites?

Long Answer

Drupal, as a robust CMS, can scale to power high-traffic websites if performance is carefully engineered across multiple layers: caching, content delivery, database tuning, and code optimization. An effective performance strategy must address both server-side execution and client delivery.

1) Core Drupal caching and object caching
Drupal provides built-in caching mechanisms—page caching, render caching, and dynamic page cache. Enabling them ensures that anonymous users see pre-built pages without repeated database queries. For object caching, integrate Redis or Memcached. Redis can store session and cache data outside the database, reducing MySQL/Postgres load.

2) Varnish as a reverse proxy
Varnish is widely used in front of Drupal. It caches entire HTTP responses and serves them directly, bypassing PHP and Drupal for anonymous traffic. The setup involves configuring Drupal headers (Cache-Control, Surrogate-Control, max-age) so Varnish knows when to invalidate. Coupling this with cache tags in Drupal ensures that content invalidates correctly when edited.

3) CDN integration
For static assets—CSS, JS, images, fonts—a Content Delivery Network (CDN) like Cloudflare, Fastly, or Akamai reduces latency globally. CDNs offload requests from origin servers and serve files from edge nodes near users. When combined with Varnish, this creates two strong layers: edge CDN cache and mid-tier Varnish. Drupal’s core and contributed modules like CDN module can automate asset URLs pointing to CDN domains.

4) Database tuning
Database performance is often the bottleneck for large Drupal sites. Best practices include:

• Adding proper indexes on frequently queried fields.
  
• Enabling query cache and analyzing slow queries with tools like EXPLAIN.
  
• Partitioning or sharding large tables (e.g., log tables) when scaling.
  
• Offloading search to external systems like Elasticsearch or Solr instead of relying on LIKE queries.
  
• Using read replicas for scaling read-heavy workloads.
  

5) PHP and application tuning
PHP runtime performance affects Drupal. Enable OPcache with sufficient memory, fine-tune PHP-FPM pools for concurrency, and configure limits (max children, workers) according to traffic peaks. Drupal 9+ benefits from Symfony improvements, but it still requires tuning of PHP runtime for maximum throughput.

6) Frontend optimization
Aggregate and minify CSS/JS using Drupal’s built-in aggregation system. Leverage lazy-loading for images and responsive images module to avoid over-delivery. Prefetch fonts and critical assets. Inline critical CSS for faster first paint.

7) Contributed modules and custom code audits
One hidden performance killer is poorly written contributed modules. Regularly audit modules and custom code for query-heavy operations. Disable unused modules and use profiling tools like XHProf or Tideways to spot bottlenecks.

8) Infrastructure considerations
High-traffic Drupal deployments usually run on clusters: multiple webheads behind load balancers, Varnish/NGINX as edge, Redis for cache, and a managed database cluster. Containerized setups (Kubernetes + Helm charts for Drupal) allow autoscaling of pods during peak traffic.

9) Monitoring and profiling
Continuous performance monitoring is essential. Use New Relic or Blackfire to profile application-level bottlenecks. Monitor cache hit ratios in Redis/Varnish and query performance metrics from the database. Track Core Web Vitals and real user monitoring (RUM) metrics to catch frontend regressions.

When layered together—Drupal caching, Varnish proxy, CDN, database tuning, and frontend optimization—Drupal websites can scale to millions of pageviews per day without degrading UX.

‍

Table

‍

Common Mistakes

Typical pitfalls include relying only on Drupal’s default caching without adding Varnish, leading to heavy server load. Storing sessions in the database instead of Redis slows down response times. Using CDNs only for media but ignoring CSS/JS caching misses major optimization. Another mistake is neglecting database indexes—large log tables quickly degrade performance. Developers often forget cache invalidation strategies, leaving users with stale content. Overloading sites with too many contributed modules without audits also creates inefficiencies. Finally, failing to monitor cache hit ratios or slow queries results in blind spots that surface only under load.

‍

Sample Answers (Junior / Mid / Senior)

Junior:
“I enable Drupal’s page and entity cache, aggregate CSS/JS, and configure CDN for images. I also use Redis for sessions instead of the database.”

Mid:
“I combine Drupal caching with Varnish reverse proxy and Redis for object caching. I integrate a CDN for static assets, optimize database queries with indexes, and profile modules for performance. I also tune PHP-FPM and OPcache for concurrency.”

Senior:
“I design a multi-layer stack: Drupal render cache + Redis, Varnish reverse proxy with cache tag invalidation, global CDN for static delivery, and tuned database clusters with replicas. I implement lazy-loading, critical CSS, and code audits to eliminate heavy queries. Infrastructure includes autoscaling with Kubernetes, load balancing, and continuous monitoring with New Relic and Blackfire. This ensures high-traffic Drupal sites maintain low latency under scale.”

‍

Evaluation Criteria

Interviewers evaluate if the candidate:

• Understands layered caching (Drupal core, Redis/Memcached, Varnish, CDN).
  
• Knows how to configure cache invalidation to prevent stale content.
  
• Demonstrates ability to tune databases with indexes, replicas, and query optimization.
  
• Mentions PHP/OPcache tuning for runtime performance.
  
• Covers frontend optimizations like aggregation, lazy loading, and responsive images.
  
• Integrates monitoring/profiling tools for continuous feedback.
  
• Shows awareness of infrastructure scaling (load balancing, Kubernetes, clustering).
  Strong answers tie strategies into a cohesive system. Weak answers only mention “enable cache” without explaining how or where.
  
  

Preparation Tips

Practice setting up Drupal with Varnish and Redis locally. Review Drupal’s cache APIs (Cache::invalidateTags(), render caching). Learn to configure CDN integration and tune settings.php for cache backends. Study MySQL/Postgres query optimization with EXPLAIN and practice creating indexes. Use New Relic or Blackfire on a test Drupal site to profile performance bottlenecks. Rehearse explaining layered architecture in a 60–90s pitch: Drupal cache → Redis → Varnish → CDN → database tuning. Finally, prepare real-world examples of how you scaled a Drupal site under traffic spikes (e.g., product launches, media events).

‍

Real-world Context

A media company running Drupal saw traffic spikes during breaking news events. By placing Varnish in front of Drupal and configuring cache tags, they served millions of anonymous requests directly from memory, reducing load by 80%. Redis replaced database session storage, cutting DB queries significantly. Adding Cloudflare CDN accelerated global delivery and absorbed DDoS attempts. Slow log queries were fixed with new indexes, and PHP-FPM pools were tuned for concurrency. As a result, page load time dropped from 3.5s to 900ms under peak traffic, ensuring both scalability and resilience.

‍

Key Takeaways

• Combine Drupal caching with Redis for object/session storage.
  
• Add Varnish as a reverse proxy with cache tags for invalidation.
  
• Integrate a CDN for global asset delivery and DDoS absorption.
  
• Optimize database queries with indexes and replicas.
  
• Tune PHP runtime (OPcache, PHP-FPM).
  
• Audit contributed modules and monitor performance continuously.
  
  

Practice Exercise

Scenario: You are asked to prepare a high-traffic Drupal site for a global campaign launch.

Tasks:

1. Enable Drupal caching layers (page, entity, render).
   
2. Configure Redis for cache/session storage.
   
3. Set up Varnish as a reverse proxy with cache tags.
   
4. Integrate Cloudflare as CDN for static assets.
   
5. Optimize the database with indexes on slow queries.
   
6. Aggregate/minify CSS/JS, enable lazy loading for images.
   
7. Tune PHP-FPM pools and OPcache memory.
   
8. Run New Relic profiling to verify cache hit ratios and DB response times.
   
9. Document an incident response plan if traffic exceeds expected load.
   

Deliverable: Prepare a 2-minute pitch explaining your multi-layer architecture and how it ensures Drupal remains fast and resilient under millions of requests.