Redis Object Cache: When It Helps and How to Confirm

Redis object caching is often recommended as a WordPress performance solution, but it does not help
every site equally. Understanding what object caching does—and does not do—helps you determine if
Redis will benefit your specific situation. This guide explains how WordPress object caching works,
when Redis provides meaningful improvements, and how to verify your implementation is actually
functioning correctly.

I. Understanding WordPress Object Caching

Before evaluating Redis, understand what the WordPress object cache does and how it differs from page
caching.

A. What Object Cache Stores

The WordPress object cache stores the results of expensive operations during a single request. It
prevents repeated database queries for the same data.

• Database query results: When WordPress fetches options, post data, or user information,
  results are cached to avoid repeat queries within the same request.
• Computed values: Plugins can cache expensive calculations, API call results, or processed
  data.
• Transients: Temporary data with expiration times stored via the Transients API often uses
  object cache.

B. Default vs Persistent Object Cache

• Default (non-persistent): WordPress includes a built-in object cache that only persists
  for the duration of a single request. When the request ends, the cache disappears.
• Persistent cache (Redis/Memcached): External memory stores that preserve cached data
  between requests. Multiple requests can share cached data.
• Key difference: Persistent caching benefits occur when cached data from one request helps
  subsequent requests. Default caching only helps within a single page load.

C. Object Cache vs Page Cache

These caching layers serve different purposes. Understanding the difference prevents unrealistic
expectations.

• Page cache: Stores complete HTML output. Subsequent requests get the saved HTML without
  running WordPress at all.
• Object cache: Stores data fragments used while generating pages. WordPress still runs,
  but with fewer database queries.
• Relationship: When page cache serves a request, object cache is never consulted. Object
  cache helps with cache misses and dynamic pages.

II. When Redis Actually Helps

Redis provides measurable benefits in specific scenarios. Evaluating your site against these criteria
determines if Redis is worthwhile.

A. High-Traffic Dynamic Content

• Logged-in users: Page caching typically excludes logged-in users. Redis helps these users
  by reducing database load for repeated data.
• Personalized pages: Content that varies by user, location, or history cannot be
  page-cached. Redis caches common data fragments.
• E-commerce sites: Shopping carts, pricing calculations, and inventory checks generate
  database queries that Redis can cache.

B. Admin Dashboard Performance

WordPress admin pages are never page-cached and generate many database queries. Redis often improves
admin responsiveness noticeably.

• Plugin-heavy dashboards: Plugins with dashboard widgets or complex settings pages benefit
  significantly from cached data.
• Editorial workflows: Publisher sites with multiple editors see faster post editing and
  content management.
• WooCommerce admin: Order management, product editing, and report generation become more
  responsive.

C. Scenarios With Minimal Benefit

• Well-cached static content: If most visitors see page-cached content and you have few
  logged-in users, Redis adds complexity without proportional benefit.
• Low traffic sites: Without significant concurrent load, database queries are already
  fast. Redis adds infrastructure without necessity.
• Simple sites: Minimal plugins and straightforward content mean fewer database queries to
  cache.

III. Installing and Configuring Redis

Implementing Redis requires both server-side installation and WordPress integration.

A. Server Installation

• Package installation: On Ubuntu/Debian, install with
  sudo apt install redis-server.
• Start and enable: Run
  sudo systemctl enable redis-server && sudo systemctl start redis-server.
• Verify running: Test with redis-cli ping. It should respond “PONG”.
• Memory limit: Configure max memory in /etc/redis/redis.conf. 64-256MB is
  typical for single WordPress sites.

B. PHP Redis Extension

PHP needs the Redis extension to communicate with the Redis server.

• Install extension: Run sudo apt install php8.3-redis (adjust version number
  for your PHP version).
• Restart PHP-FPM: Run sudo systemctl restart php8.3-fpm to load the
  extension.
• Verify: Run php -m | grep redis. You should see “redis” in the output.

C. WordPress Plugin Configuration

• Install Redis Object Cache plugin: Available in the WordPress plugin repository. It
  provides the drop-in file and management interface.
• Enable the cache: Go to Settings > Redis and click “Enable Object Cache”. This installs
  the object-cache.php drop-in.
• wp-config.php settings: Optionally add define('WP_REDIS_HOST', '127.0.0.1');
  and other constants for custom configurations.

IV. Verifying Redis Is Working

Many sites have Redis installed but not functioning correctly. Verification confirms actual
operation.

A. Plugin Dashboard Check

• Status indicator: The Redis Object Cache plugin shows connection status. “Connected” with
  a green indicator means basic connectivity works.
• Statistics: View hit/miss ratios, memory usage, and cached keys. Active use shows
  accumulating hits and reasonable memory consumption.
• Stale data warning: If the plugin shows warnings about stale or orphaned data, Redis may
  not be flushing correctly after updates.

B. Command Line Verification

• Redis CLI monitoring: Run redis-cli monitor to watch real-time Redis
  commands. Load WordPress pages and observe activity.
• Key inspection: Run redis-cli keys "*" to see cached keys. WordPress keys
  typically include your site URL or database prefix.
• Memory usage: Run redis-cli info memory to check current memory consumption
  versus configured limits.

C. Performance Measurement

• Query Monitor plugin: Shows database queries per page. With Redis active, repeat queries
  should be served from object cache.
• Before/after comparison: Note query counts and page generation times before enabling
  Redis, then compare after.
• Admin page timing: Time dashboard page loads before and after. Admin areas typically show
  the clearest improvement.

V. Common Redis Problems

A. Connection Failures

• Symptom: Plugin shows “Not Connected” or site throws Redis connection errors.
• Cause: Redis server not running, wrong host/port, or firewall blocking connection.
• Solution: Verify Redis is running with systemctl status redis. Check
  wp-config.php host settings match actual server.

B. Cache Not Actually Used

• Symptom: Connected but hit rate stays near zero. No performance improvement.
• Cause: Missing object-cache.php drop-in or conflicting caching plugin.
• Solution: Verify wp-content/object-cache.php exists and matches the Redis
  plugin’s version.

C. Memory Exhaustion

• Symptom: Eviction warnings, inconsistent caching behavior, or site errors under load.
• Cause: Redis maxmemory too small for site’s cache needs.
• Solution: Increase maxmemory in redis.conf and restart. Monitor usage to right-size the
  allocation.

VI. Optimization Tips

• Set appropriate TTL: Transients should expire. Very long TTLs waste memory on stale data.
• Flush after updates: Clear object cache after plugin and theme updates to prevent stale
  cached code paths.
• Monitor regularly: Check Redis stats monthly. Growing memory usage may indicate cache
  bloat from poorly-behaving plugins.
• Database prefix: Use unique database prefixes if multiple WordPress sites share one Redis
  instance.

VII. Conclusion

Redis object caching provides real performance benefits for WordPress sites with dynamic content,
logged-in users, or heavy admin usage. However, it is not a universal solution—well-cached static
content sites see minimal improvement. Proper implementation requires both server installation and
WordPress configuration, but the critical step is verification. Too many sites have Redis “enabled”
but not actually functioning. Use the verification methods described here to confirm Redis is
connected, storing data, and actually reducing database load. Only then can you accurately assess
whether Redis is worth the added infrastructure complexity for your specific site.

Has Redis improved your WordPress site’s performance? Share your experience and stats in the
comments!