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Understanding Drupal: A Complete Guide to Caching Layers

Matt Glaman

Drupal's caching system is powerful only when you understand how its layers connect. This book walks you through cacheable metadata, render caching, HTTP headers, and reverse proxy configuration; the complete stack from page assembly to CDN edge.

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About

About

About the Book

Drupal's caching architecture spans multiple layers: render arrays in memory, cache backends in the database, HTTP headers on the response, and rules in the reverse proxy. Getting any layer wrong costs you either performance or correctness.

This book covers the full stack. You will learn how cacheable metadata flows through the render pipeline, how cache backends store and invalidate data, how HTTP response headers instruct reverse proxies, and how to write modules that participate correctly at every layer. Chapters progress from core concepts to debugging real-world cache failures and writing cacheable code from scratch.

The audience ranges from site builders who want to understand cache tags to module developers building custom cache backends. Every chapter works through real Drupal 11 APIs, grounded in code.

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Author

About the Author

Matt Glaman

Matt Glaman is a Principal Software Engineer at Acquia, specializing in Drupal and PHP. He's an active contributor to the Drupal open source ecosystem, focusing on developer tooling, caching architecture, and modern PHP practices. Matt writes at mglaman.dev and is a strong advocate of static analysis, clean APIs, and code that ships value rather than impresses.

Contents

Table of Contents

Preface

  1. Why I wrote this book
  2. Who is this book for?
  3. What to expect
  4. Do you need a Drupal site for this book?

Introduction

  1. Why does caching matter?

Cacheable metadata: tags, contexts, and max age

  1. What are render arrays?
  2. What is cache invalidation
  3. What is cacheable metadata?
  4. Cacheable dependencies
  5. Cache tags
  6. Cache contexts
  7. Cache tags vs. cache contexts
  8. Cache max-age (time-to-live)
  9. Bubbleable rendering metadata

Cache items, their backends, and stores

  1. Retrieving, storing, and deleting cache items
  2. Available cache backend stores
  3. Defining a custom cache bin
  4. Defining cache bins with chained backend stores
  5. Changing a cache bin’s backend store
  6. Disabling a cache bin for development
  7. Managing cache with Drush

Render (fragment) cache

  1. Rendering in Drupal
  2. Discovery and compilation caches
  3. Specifying render cache keys
  4. Bubbling metadata
  5. Placeholders for late rendering
  6. Caching non-markup data alongside rendered markup

Caching responses

  1. Cacheable response objects
  2. Access checks and response cacheability
  3. Dynamic page cache
  4. Internal page cache
  5. When to use Dynamic Page Cache vs Internal Page Cache
  6. Caching 4xx responses
  7. Streaming responses with BigPipe

HTTP caching headers

  1. How Drupal determines cacheability
  2. Working with cacheable responses
  3. Important limitations and gotchas
  4. The Vary header
  5. The Cache-Control header
  6. The Expires header
  7. The Last-Modified header
  8. The ETag header
  9. The Surrogate-Control header
  10. Extending HTTP cache headers with the HTTP Cache Control module

Reverse proxies and CDNs

  1. What are reverse proxies and CDNs?
  2. Making Drupal aware it is behind a reverse proxy or CDN
  3. Exposing cache tags through HTTP headers
  4. The Purge module
  5. Varnish reverse proxy
  6. Nginx as a reverse proxy
  7. Bypassing the cache for authenticated requests
  8. Content Delivery Networks (CDNs)
  9. Edge Side Includes

Debugging your cache

  1. Turning on development mode to bypass Twig and markup caching
  2. Disabling specific render and response caches during development
  3. Debug response cacheable metadata with HTTP response headers
  4. Inspecting cache headers in the browser
  5. Render debug output for cacheable metadata
  6. Reviewing the cache bins and cache IDs
  7. Inspecting caches with Drush
  8. Visualization modules

Writing code with cacheability in mind

  1. Using the “list” cache tag when listing entities
  2. Cacheable responses
  3. Working with cache contexts
  4. Caching forms
  5. Cacheable objects as dependencies
  6. Traits for implementing cacheable dependency interfaces
  7. Capturing correct metadata
  8. Capturing leaked metadata when rendering
  9. Implementing lazy builders
  10. Preloading cache tags for checksum optimization
  11. Prewarming caches to prevent stampedes
  12. Forcing uncacheable content with CacheOptionalInterface
  13. The Variation Cache API
  14. Testing cache metadata

Afterword

  1. What You’ve Learned
  2. Where to Go Next
  3. Staying Connected
  4. Thank You

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