The Tool Desk
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Enable Apache’s HTTP cache with mod_cache, add mod_cache_disk for persistence, point CacheRoot to a writable directory, and cache only responses that are public and safely repeatable. Validate the first request as a MISS and later requests as HIT or REVALIDATE; a disk cache without a size-management plan can eventually consume the filesystem.
What Apache content caching does
Apache’s HTTP-aware cache is supplied by mod_cache. It evaluates request and response headers, freshness, validators, and variants before deciding whether a stored response can be served. mod_cache_disk is the persistent disk backend: it stores cached metadata and response content under the directory named by CacheRoot.
This is different from a browser cache or a CDN. The cache runs inside the Apache server handling the request, so it can reduce repeated work between Apache and an origin application, database, or upstream proxy. It does not make every response cacheable. HTTP policy still controls whether a response may be stored and reused.
How the disk layout works
The disk backend hashes URLs and distributes entries through a directory fan-out. The common layout uses separate .header and .data files for metadata and response content. With CacheDirLevels 2 and CacheDirLength 1, each level can contain up to 64 subdirectories, producing as many as 4,096 leaf directories. The fan-out prevents a single directory from accumulating every cached object.
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Decide what is safe to cache first
Begin with public, repeatable responses. Versioned JavaScript, CSS, images, downloadable files, and generated pages that are identical for every visitor are the easiest candidates. Treat every response containing identity, permissions, account data, session state, or request-specific output as non-cacheable unless you have designed and tested a complete variation policy.
- Good candidates: public assets with versioned URLs and public pages whose output does not depend on a logged-in user.
- High-risk candidates: account pages, checkout or payment flows, administration screens, private API responses, and anything selected by an authorization header.
- Cookie caution: a response that sets or depends on user cookies can accidentally expose one user’s content to another if the cache policy is incomplete.
- Shared namespace caution: do not place private application data in a cache namespace shared by unrelated virtual hosts or applications.
Use the origin response headers to state the policy. Apache honors standard freshness and validation metadata rather than relying on a universal performance rule.
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| Header or directive | What it controls | Typical use |
|---|---|---|
Cache-Control: public |
Declares that shared caches may store the response. | Public assets or pages that are safe for all visitors. |
Cache-Control: max-age=<seconds> |
Sets the freshness lifetime. | Use a long lifetime for immutable, versioned assets; choose a shorter value for frequently changing pages. |
Cache-Control: private |
Restricts storage to a private client cache. | Personalized responses that should not be reused by Apache’s shared cache. |
Cache-Control: no-store |
Prohibits storing the response. | Secrets, payment data, credentials, and other content that must not be retained. |
Cache-Control: no-cache |
Requires validation before reuse; it does not mean “never store.” | Responses that may be stored but must be checked with the origin before serving. |
Expires |
Provides an HTTP expiration time. | Legacy-compatible freshness metadata when used consistently with Cache-Control. |
ETag |
Identifies a particular representation for validation. | Lets Apache revalidate an older entry without downloading an unchanged body. |
Last-Modified |
Supplies a modification timestamp for validation. | Allows conditional requests based on the resource’s last change. |
Vary |
Separates cached variants according to request headers. | Use when output legitimately changes with headers such as Accept-Encoding or language preferences. |
Do not add a broad cache rule merely because a URL looks public. The response’s authentication, cookies, headers, and body all matter.
Prepare the Linux host and Apache modules
Confirm module availability
Your distribution may package or load Apache modules differently. Enable both the HTTP cache module and the disk backend using the distribution’s normal module manager or Apache configuration. Then inspect the loaded module list:
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apachectl -M | grep -E 'cache_module|cache_disk_module'The output should include the cache module and the disk-cache module. If the command is named httpd on your distribution, use that executable instead. Do not assume that a configuration file exists simply because Apache is installed; a missing module must be installed or enabled before the directives will parse.
Create a dedicated cache directory
Choose a path outside all web document roots, such as /var/cache/apache/. Create it with ownership and permissions that allow the Apache service account to read and write entries while preventing unrelated users from modifying them. Keep enough free space for the workload and monitor the filesystem that contains the directory.
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- Do not place
CacheRootinside a directory served directly by a virtual host. - Do not make the directory world-writable as a shortcut for permission errors.
- Ensure the directory survives reboots and is available before Apache starts.
- Use the service account and group names supplied by your Linux distribution rather than copying a name from another system.
Apply a baseline disk-cache configuration
Put the following directives in the global Apache configuration or in the configuration scope supported by your installation:
CacheRoot /var/cache/apache/
CacheEnable disk /
CacheDirLevels 2
CacheDirLength 1
# Temporary diagnostics while validating behavior
CacheHeader on
# Uncomment when normal authorization and request phases must run
# CacheQuickHandler offWhat each directive means
CacheRootidentifies the writable storage root. Omitting it causes configuration processing to fail for the disk backend.CacheEnable disk /enables the disk cache for URLs below the root path. Narrow this path if only one location should be cached.CacheDirLevels 2andCacheDirLength 1establish the two-level, one-character fan-out described above.CacheHeader onadds diagnostic cache headers. Keep it enabled only while troubleshooting if you do not want to expose cache state to clients.CacheQuickHandleris left at its default unless you explicitly turn it off. The security implications are significant, so decide this setting based on the request-processing requirements described below.
If your application already sets precise cache headers, let those headers determine freshness. CacheMinExpire can impose a minimum retention only when no valid expiration was supplied; it should not override a deliberate application policy. CacheStaleOnError on can serve stale data when the backend returns a 5xx response, but use it only when stale content is preferable to an error for that resource.
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Understand the quick-handler security trade-off
CacheQuickHandler defaults to on. A fresh cache hit can then be served before most normal per-request processing. That is fast, but it can bypass authorization and access-control checks. Apache’s documentation warns that, with the quick handler enabled, Allow and Deny directives can be circumvented.
Set the directive to off when a request must pass authentication, authorization, access controls, personalization logic, mod_include, or other normal processing phases before a response is returned:
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Configure Apache step by step
- Inventory the responses. List the URLs or locations you intend to cache. Mark whether each response is public, whether it varies by request headers, and whether it uses authentication or cookies.
- Enable both modules. Use your Linux distribution’s Apache module tooling, then verify the loaded modules with
apachectl -Mor the equivalenthttpdcommand. - Create and secure the cache root. Make
/var/cache/apache/or another dedicated path writable by the Apache service account and inaccessible to ordinary users. - Add the disk directives. Set
CacheRoot, a suitably scopedCacheEnable diskrule, and the directory fan-out settings. - Choose the request path. Leave the quick handler enabled only for content that does not require authorization or other normal request phases. Add
CacheQuickHandler offwhere those phases are mandatory. - Set origin headers. Add accurate
Cache-Control,Expires, validators, andVaryheaders in the application or virtual-host configuration. Explicitly mark private and sensitive responses withprivateorno-store. - Test the configuration. Run the distribution’s standard Apache configuration test before reloading. Fix every syntax, module, path, and permission error first.
- Reload Apache. Use the normal service command for your distribution, such as the service name used by your installation, and confirm that the daemon returns to an active state.
- Exercise one known-public URL. Capture the first response and repeat it under the same request conditions before drawing conclusions about hit rate.
Verify misses, hits, and revalidation
Capture response headers
Use either of these requests while diagnostics are enabled:
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curl -I https://example.test/public-resource
curl -sD - -o /dev/null https://example.test/public-resourceReplace the example host with a URL you control. The first cacheable request commonly reports:
X-Cache: MISSRepeat the request without changing cookies, authorization, or negotiating headers. A fresh stored response should report X-Cache: HIT. If the entry is stale but its validators permit a successful check with the origin, expect X-Cache: REVALIDATE. Inspect the complete response, including Cache-Control, Expires, ETag, Last-Modified, Vary, and any Set-Cookie header.
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- Record the first response headers and body.
- Repeat the same request and compare the cache status.
- Change the origin representation or expire it deliberately.
- Repeat the request and confirm that the result follows the freshness and validator headers.
- Send a request with a different header named in
Varyand verify that the correct variant is selected. - Test an authenticated or cookie-bearing request separately; it should not receive another user’s representation.
A MISS does not by itself prove that configuration is broken. The response may be explicitly private, marked no-store, expired immediately, varied by request headers, bypassed by authorization, or affected by cookies. Review Apache logs and the full header set before changing directives.
Troubleshoot a cache that never produces hits
| Symptom | Likely explanation | What to check or change |
|---|---|---|
| Apache refuses to reload after adding directives. | A module is not loaded, CacheRoot is missing, or the directive is in an unsupported scope. |
Run the configuration test, verify both modules, confirm the cache-root path, and read the exact error before reloading. |
Every request remains MISS. |
The response is not cacheable, has expired, varies by request, or is bypassed by authorization or cookies. | Inspect all cache headers, Vary, Set-Cookie, authentication state, and Apache logs. |
| The cache directory stays empty. | The path is not writable by Apache, the URL is outside the enabled scope, or the response forbids storage. | Check ownership and permissions, confirm the URL matches CacheEnable, and review the response policy. |
| A protected URL is served without an expected access check. | The quick handler returned a cached hit before normal authorization processing. | Set CacheQuickHandler off for that location and remove private responses already stored there. |
| Different language or encoding requests receive the wrong representation. | The response does not identify its negotiated dimensions correctly. | Set an accurate Vary header and retest each variant with matching request headers. |
| Content remains old after deployment. | The entry is still fresh under its configured lifetime. | Use versioned asset URLs, correct validators, or an explicit purge process rather than ignoring the cache globally. |
| Requests continue but cache writes fail. | The cache filesystem is full, inaccessible, or experiencing disk errors. | Check free space, inode usage, permissions, mount health, and Apache logs. Apache can stand down the cache and continue serving requests as if no cache were present. |
Limit disk usage and clear stale entries
mod_cache_disk does not enforce a disk-size or inode limit by itself. A busy site can therefore fill the filesystem even when individual entries are small. Monitor both bytes and inode consumption for the filesystem containing CacheRoot.
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Apache provides htcacheclean for inspecting cached URLs, removing entries, and maintaining size or inode limits. Run it according to the version installed on your Linux host, using the same cache-root path and an operational limit appropriate for the filesystem. Integrate its cleanup mode with your service manager or scheduled maintenance, and alert before the filesystem reaches exhaustion.
For an emergency full purge, use the cleanup tool’s removal functions. If your operational procedure requires deleting the cache tree manually, stop Apache first, remove only the contents of the dedicated cache root, recreate the directory with the correct ownership and permissions, and start Apache again. Never remove a path that also contains application files.
Plan invalidation around deployments
Cache invalidation is part of release engineering. When an asset or generated page changes, an old entry can remain valid until its freshness lifetime ends. The safest long-term pattern is to publish immutable assets under versioned URLs, such as filenames containing a build hash, and give those URLs a long cache lifetime. A new filename naturally creates a new cache key.
For HTML or API responses that must change at a stable URL, use conservative freshness and reliable ETag or Last-Modified validators. Purge or rotate entries when a deployment makes old content incorrect or unsafe. Do not solve a single bad entry by disabling caching for every response.
Quick Recap
Compare the main design choices
| Decision | Option A | Option B | Reader-facing trade-off |
|---|---|---|---|
| Storage backend | Disk cache with mod_cache_disk |
Another mod_cache backend, such as a shared-object cache where supported |
Disk offers persistent files and straightforward capacity monitoring; a shared-object backend may have different memory, eviction, and restart behavior. Choose according to workload and operational limits. |
| Request processing | CacheQuickHandler on |
CacheQuickHandler off |
The quick path is faster for public content but can bypass authorization and other phases. The normal path is safer when request processing must run. |
| Cache scope | Broad rule such as CacheEnable disk / |
Narrow rule for selected locations | A broad rule is simple but increases the chance of storing a private response. Narrow scopes reduce exposure and make testing easier. |
| Freshness | Explicit application headers | Fallback defaults or minimum expiration | Explicit headers reflect real content lifetimes. Defaults can help legacy responses but should not override a deliberate private or no-store policy. |
| Invalidation | Versioned URLs and validators | Manual purge or cache rotation | Versioning avoids ambiguity for immutable assets; purge tooling is necessary when a stable URL must change immediately. |
| Capacity control | Filesystem monitoring plus htcacheclean |
No cleanup plan | The managed approach limits bytes and inodes. Without it, the cache can consume the filesystem and create unrelated service failures. |
Production checklist
- Both
mod_cacheandmod_cache_diskare loaded. CacheRootis present, writable by Apache, outside document roots, and protected from unrelated users.CacheEnablecovers only the intended URL space.- Personalized, authenticated, session-bearing, and sensitive responses are private or uncacheable.
Varyaccurately describes negotiated representations.CacheQuickHandler offis set wherever authorization or normal request phases must run.- Configuration tests pass before every reload.
- Header tests show the expected sequence of
MISS,HIT, orREVALIDATE. - Diagnostics are disabled after troubleshooting if exposing cache state is undesirable.
htcacheclean, filesystem byte and inode monitoring, and alerting are in place.- Deployments use versioned assets or a tested purge and revalidation procedure.
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