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From minf...@apache.org
Subject svn commit: r1001639 - in /httpd/httpd/trunk: CHANGES docs/manual/env.xml docs/manual/mod/mod_cache.xml include/ap_mmn.h modules/cache/cache_util.h modules/cache/mod_cache.c modules/cache/mod_cache.h
Date Mon, 27 Sep 2010 09:20:41 GMT
Author: minfrin
Date: Mon Sep 27 09:20:40 2010
New Revision: 1001639

URL: http://svn.apache.org/viewvc?rev=1001639&view=rev
Log:
mod_cache: Add the cache_status hook to register the final cache
decision hit/miss/revalidate. Add optional support for an X-Cache
and/or an X-Cache-Detail header to add the cache status to the
response. PR48241

Modified:
    httpd/httpd/trunk/CHANGES
    httpd/httpd/trunk/docs/manual/env.xml
    httpd/httpd/trunk/docs/manual/mod/mod_cache.xml
    httpd/httpd/trunk/include/ap_mmn.h
    httpd/httpd/trunk/modules/cache/cache_util.h
    httpd/httpd/trunk/modules/cache/mod_cache.c
    httpd/httpd/trunk/modules/cache/mod_cache.h

Modified: httpd/httpd/trunk/CHANGES
URL: http://svn.apache.org/viewvc/httpd/httpd/trunk/CHANGES?rev=1001639&r1=1001638&r2=1001639&view=diff
==============================================================================
--- httpd/httpd/trunk/CHANGES [utf-8] (original)
+++ httpd/httpd/trunk/CHANGES [utf-8] Mon Sep 27 09:20:40 2010
@@ -2,6 +2,11 @@
 
 Changes with Apache 2.3.9
 
+  *) mod_cache: Add the cache_status hook to register the final cache
+     decision hit/miss/revalidate. Add optional support for an X-Cache
+     and/or an X-Cache-Detail header to add the cache status to the
+     response. PR48241 [Graham Leggett]
+
   *) mod_authz_host: Add 'local' provider that matches connections originating
      on the local host. PR 19938. [Stefan Fritsch]
 

Modified: httpd/httpd/trunk/docs/manual/env.xml
URL: http://svn.apache.org/viewvc/httpd/httpd/trunk/docs/manual/env.xml?rev=1001639&r1=1001638&r2=1001639&view=diff
==============================================================================
--- httpd/httpd/trunk/docs/manual/env.xml (original)
+++ httpd/httpd/trunk/docs/manual/env.xml Mon Sep 27 09:20:40 2010
@@ -59,6 +59,7 @@
     <title>Setting Environment Variables</title>
     <related>
       <modulelist>
+        <module>mod_cache</module>
         <module>mod_env</module>
         <module>mod_rewrite</module>
         <module>mod_setenvif</module>

Modified: httpd/httpd/trunk/docs/manual/mod/mod_cache.xml
URL: http://svn.apache.org/viewvc/httpd/httpd/trunk/docs/manual/mod/mod_cache.xml?rev=1001639&r1=1001638&r2=1001639&view=diff
==============================================================================
--- httpd/httpd/trunk/docs/manual/mod/mod_cache.xml (original)
+++ httpd/httpd/trunk/docs/manual/mod/mod_cache.xml Mon Sep 27 09:20:40 2010
@@ -226,6 +226,39 @@
 
 </section>
 
+<section id="status"><title>Cache Status and Logging</title>
+  <p>Once <module>mod_cache</module> has made a decision as to whether
or not
+  an entity is to be served from cache, the detailed reason for the decision
+  is written to the subprocess environment within the request under the
+  <strong>cache-status</strong> key. This reason can be logged by the
+  <directive module="mod_log_config">LogFormat</directive> directive as
+  follows:</p>
+
+  <example>
+    LogFormat "%{cache-status}e ..."
+  </example>
+
+  <p>Based on the caching decision made, the reason is also written to the
+  subprocess environment under one the following three keys, as appropriate:</p>
+
+  <dl>
+    <dt>cache-hit</dt><dd>The response was served from cache.</dd>
+    <dt>cache-revalidate</dt><dd>The response was stale and was successfully
+      revalidated, then served from cache.</dd>
+    <dt>cache-miss</dt><dd>The response was served from the upstream server.</dd>
+  </dl>
+
+  <p>This makes it possible to support conditional logging of cached requests
+  as per the following example:</p>
+
+  <example>
+    CustomLog cached-requests.log common env=cache-hit<br />
+    CustomLog uncached-requests.log common env=cache-miss<br />
+    CustomLog revalidated-requests.log common env=cache-revalidate<br />
+  </example>
+
+</section>
+    
 <directivesynopsis>
 <name>CacheEnable</name>
 <description>Enable caching of specified URLs using a specified storage
@@ -777,5 +810,87 @@ LastModified date.</description>
 
   </usage>
 </directivesynopsis>
+
+<directivesynopsis>
+<name>CacheHeader</name>
+<description>Add an X-Cache header to the response.</description>
+<syntax>CacheHeader <var>on|off</var></syntax>
+<default>CacheHeader off</default>
+<contextlist><context>server config</context>
+    <context>virtual host</context>
+    <context>directory</context>
+    <context>.htaccess</context>
+</contextlist>
+<compatibility>Available in Apache 2.3.9 and later</compatibility>
+  
+<usage>
+  <p>When the <directive module="mod_cache">CacheHeader</directive> directive
+  is switched on, an <strong>X-Cache</strong> header will be added to the response
+  with the cache status of this response. If the normal handler is used, this
+  directive may appear within a <directive module="core">&lt;Directory&gt;</directive>
+  or <directive module="core">&lt;Location&gt;</directive> directive.
If the quick
+  handler is used, this directive must appear within a server or virtual host
+  context, otherwise the setting will be ignored.</p>
+
+  <dl>
+    <dt><strong>HIT</strong></dt><dd>The entity was fresh,
and was served from
+    cache.</dd>
+    <dt><strong>REVALIDATE</strong></dt><dd>The entity was
stale, was successfully
+    revalidated and was served from cache.</dd>
+    <dt><strong>MISS</strong></dt><dd>The entity was fetched
from the upstream
+      server and was not served from cache.</dd>
+  </dl>
+
+  <example>
+    # Enable the X-Cache header<br />
+    CacheHeader on<br />
+  </example>
+
+  <example>
+    X-Cache: HIT from localhost<br />
+  </example>
+
+</usage>
+</directivesynopsis>
+
+<directivesynopsis>
+<name>CacheDetailHeader</name>
+<description>Add an X-Cache-Detail header to the response.</description>
+<syntax>CacheDetailHeader <var>on|off</var></syntax>
+<default>CacheDetailHeader off</default>
+<contextlist><context>server config</context>
+      <context>virtual host</context>
+      <context>directory</context>
+      <context>.htaccess</context>
+</contextlist>
+<compatibility>Available in Apache 2.3.9 and later</compatibility>
+    
+<usage>
+  <p>When the <directive module="mod_cache">CacheDetailHeader</directive>
directive
+  is switched on, an <strong>X-Cache-Detail</strong> header will be added to
the response
+  containing the detailed reason for a particular caching decision.</p>
+  
+  <p>It can be useful during development of cached RESTful services to have additional
+  information about the caching decision written to the response headers, so as to
+  confirm whether <code>Cache-Control</code> and other headers have been correctly
+  used by the service and client.</p>
   
+  <p>If the normal handler is used, this directive may appear within a
+  <directive module="core">&lt;Directory&gt;</directive> or
+  <directive module="core">&lt;Location&gt;</directive> directive. If
the quick handler
+  is used, this directive must appear within a server or virtual host context, otherwise
+  the setting will be ignored.</p>
+
+  <example>
+    # Enable the X-Cache-Detail header<br />
+    CacheDetailHeader on<br />
+  </example>
+
+  <example>
+    X-Cache-Detail: "conditional cache hit: entity refreshed" from localhost<br />
+  </example>
+
+</usage>
+</directivesynopsis>
+
 </modulesynopsis>

Modified: httpd/httpd/trunk/include/ap_mmn.h
URL: http://svn.apache.org/viewvc/httpd/httpd/trunk/include/ap_mmn.h?rev=1001639&r1=1001638&r2=1001639&view=diff
==============================================================================
--- httpd/httpd/trunk/include/ap_mmn.h (original)
+++ httpd/httpd/trunk/include/ap_mmn.h Mon Sep 27 09:20:40 2010
@@ -269,6 +269,7 @@
  *                         ap_cache_try_lock, ap_cache_check_freshness,
  *                         cache_server_conf, cache_enable, cache_disable,
  *                         cache_request_rec and cache_provider_list private.
+ * 20100923.1 (2.3.9-dev)  Add cache_status hook.
  */
 
 #define MODULE_MAGIC_COOKIE 0x41503234UL /* "AP24" */
@@ -276,7 +277,7 @@
 #ifndef MODULE_MAGIC_NUMBER_MAJOR
 #define MODULE_MAGIC_NUMBER_MAJOR 20100923
 #endif
-#define MODULE_MAGIC_NUMBER_MINOR 0                     /* 0...n */
+#define MODULE_MAGIC_NUMBER_MINOR 1                     /* 0...n */
 
 /**
  * Determine if the server's current MODULE_MAGIC_NUMBER is at least a

Modified: httpd/httpd/trunk/modules/cache/cache_util.h
URL: http://svn.apache.org/viewvc/httpd/httpd/trunk/modules/cache/cache_util.h?rev=1001639&r1=1001638&r2=1001639&view=diff
==============================================================================
--- httpd/httpd/trunk/modules/cache/cache_util.h (original)
+++ httpd/httpd/trunk/modules/cache/cache_util.h Mon Sep 27 09:20:40 2010
@@ -92,11 +92,12 @@ extern "C" {
 #define DEFAULT_CACHE_EXPIRE    MSEC_ONE_HR
 #define DEFAULT_CACHE_LMFACTOR  (0.1)
 #define DEFAULT_CACHE_MAXAGE    5
+#define DEFAULT_X_CACHE         0
+#define DEFAULT_X_CACHE_DETAIL  0
 #define DEFAULT_CACHE_LOCKPATH "/mod_cache-lock"
 #define CACHE_LOCKNAME_KEY "mod_cache-lockname"
 #define CACHE_LOCKFILE_KEY "mod_cache-lockfile"
 
-
 /**
  * cache_util.c
  */
@@ -168,8 +169,19 @@ typedef struct {
     /** run within the quick handler */
     int quick;
     int quick_set;
+    int x_cache;
+    int x_cache_set;
+    int x_cache_detail;
+    int x_cache_detail_set;
 } cache_server_conf;
 
+typedef struct {
+    int x_cache;
+    int x_cache_set;
+    int x_cache_detail;
+    int x_cache_detail_set;
+} cache_dir_conf;
+
 /* A linked-list of authn providers. */
 typedef struct cache_provider_list cache_provider_list;
 

Modified: httpd/httpd/trunk/modules/cache/mod_cache.c
URL: http://svn.apache.org/viewvc/httpd/httpd/trunk/modules/cache/mod_cache.c?rev=1001639&r1=1001638&r2=1001639&view=diff
==============================================================================
--- httpd/httpd/trunk/modules/cache/mod_cache.c (original)
+++ httpd/httpd/trunk/modules/cache/mod_cache.c Mon Sep 27 09:20:40 2010
@@ -201,6 +201,9 @@ static int cache_quick_handler(request_r
         return DECLINED;
     }
 
+    /* we've got a cache hit! tell everyone who cares */
+    cache_run_cache_status(cache->handle, r, AP_CACHE_HIT, "cache hit");
+
     /* if we are a lookup, we are exiting soon one way or another; Restore
      * the headers. */
     if (lookup) {
@@ -453,6 +456,9 @@ static int cache_handler(request_rec *r)
         return DECLINED;
     }
 
+    /* we've got a cache hit! tell everyone who cares */
+    cache_run_cache_status(cache->handle, r, AP_CACHE_HIT, "cache hit");
+
     rv = ap_meets_conditions(r);
     if (rv != OK) {
         return rv;
@@ -816,7 +822,7 @@ static int cache_save_filter(ap_filter_t
                                   && exp < r->request_time)
     {
         /* if a Expires header is in the past, don't cache it */
-        reason = "Expires header already expired, not cacheable";
+        reason = "Expires header already expired; not cacheable";
     }
     else if (!conf->ignorequerystring && r->parsed_uri.query && exps
== NULL &&
              !ap_cache_liststr(NULL, cc_out, "max-age", NULL) &&
@@ -844,7 +850,7 @@ static int cache_save_filter(ap_filter_t
         /* Note: mod-include clears last_modified/expires/etags - this
          * is why we have an optional function for a key-gen ;-)
          */
-        reason = "No Last-Modified, Etag, Expires, Cache-Control:max-age or Cache-Control:s-maxage
headers";
+        reason = "No Last-Modified; Etag; Expires; Cache-Control:max-age or Cache-Control:s-maxage
headers";
     }
     else if (r->header_only && !cache->stale_handle) {
         /* Forbid HEAD requests unless we have it cached already */
@@ -903,6 +909,9 @@ static int cache_save_filter(ap_filter_t
                      "cache: %s not cached. Reason: %s", r->unparsed_uri,
                      reason);
 
+        /* we've got a cache miss! tell anyone who cares */
+        cache_run_cache_status(cache->handle, r, AP_CACHE_MISS, reason);
+
         /* remove this filter from the chain */
         ap_remove_output_filter(f);
 
@@ -1009,6 +1018,10 @@ static int cache_save_filter(ap_filter_t
     }
 
     if (rv != OK) {
+        /* we've got a cache miss! tell anyone who cares */
+        cache_run_cache_status(cache->handle, r, AP_CACHE_MISS,
+                "cache miss: create_entity failed");
+
         /* Caching layer declined the opportunity to cache the response */
         ap_remove_output_filter(f);
         cache_remove_lock(conf, cache, r, cache->handle ?
@@ -1211,6 +1224,17 @@ static int cache_save_filter(ap_filter_t
                      "cache: attempt to remove url from cache unsuccessful.");
             }
 
+            /* we've got a cache conditional hit! tell anyone who cares */
+            cache_run_cache_status(cache->handle, r, AP_CACHE_REVALIDATE,
+                    "conditional cache hit: entity refresh failed");
+
+        }
+        else {
+
+            /* we've got a cache conditional hit! tell anyone who cares */
+            cache_run_cache_status(cache->handle, r, AP_CACHE_REVALIDATE,
+                    "conditional cache hit: entity refreshed");
+
         }
 
         /* let someone else attempt to cache */
@@ -1224,12 +1248,20 @@ static int cache_save_filter(ap_filter_t
         ap_log_error(APLOG_MARK, APLOG_DEBUG, rv, r->server,
                      "cache: store_headers failed");
 
+        /* we've got a cache miss! tell anyone who cares */
+        cache_run_cache_status(cache->handle, r, AP_CACHE_MISS,
+                "cache miss: store_headers failed");
+
         ap_remove_output_filter(f);
         cache_remove_lock(conf, cache, r, cache->handle ?
                 (char *)cache->handle->cache_obj->key : NULL, NULL);
         return ap_pass_brigade(f->next, in);
     }
 
+    /* we've got a cache miss! tell anyone who cares */
+    cache_run_cache_status(cache->handle, r, AP_CACHE_MISS,
+            "cache miss: attempting entity save");
+
     return cache_save_store(f, in, conf, cache);
 }
 
@@ -1309,9 +1341,102 @@ static int cache_filter(ap_filter_t *f, 
     return ap_pass_brigade(f->next, in);
 }
 
+/**
+ * If configured, add the status of the caching attempt to the subprocess
+ * environment, and if configured, to headers in the response.
+ *
+ * The status is saved below the broad category of the status (hit, miss,
+ * revalidate), as well as a single cache-status key. This can be used for
+ * conditional logging.
+ *
+ * The status is optionally saved to an X-Cache header, and the detail of
+ * why a particular cache entry was cached (or not cached) is optionally
+ * saved to an X-Cache-Detail header. This extra detail is useful for
+ * service developers who may need to know whether their Cache-Control headers
+ * are working correctly.
+ */
+static int cache_status(cache_handle_t *h, request_rec *r, ap_cache_status_e status,
+        const char *reason)
+{
+    cache_server_conf
+            *conf =
+                    (cache_server_conf *) ap_get_module_config(r->server->module_config,
+                            &cache_module);
+
+    cache_dir_conf *dconf = ap_get_module_config(r->per_dir_config, &cache_module);
+    int x_cache = 0, x_cache_detail = 0;
+
+    switch (status) {
+    case AP_CACHE_HIT: {
+        apr_table_setn(r->subprocess_env, AP_CACHE_HIT_ENV, reason);
+        break;
+    }
+    case AP_CACHE_REVALIDATE: {
+        apr_table_setn(r->subprocess_env, AP_CACHE_REVALIDATE_ENV, reason);
+        break;
+    }
+    case AP_CACHE_MISS: {
+        apr_table_setn(r->subprocess_env, AP_CACHE_MISS_ENV, reason);
+        break;
+    }
+    }
+
+    apr_table_setn(r->subprocess_env, AP_CACHE_STATUS_ENV, reason);
+
+    if (dconf && dconf->x_cache_set) {
+        x_cache = dconf->x_cache;
+    }
+    else {
+        x_cache = conf->x_cache;
+    }
+    if (x_cache) {
+        apr_table_setn(r->headers_out, "X-Cache", apr_psprintf(r->pool, "%s from %s",
+                status == AP_CACHE_HIT ? "HIT" : status == AP_CACHE_REVALIDATE ?
+                        "REVALIDATE" : "MISS", r->server->server_hostname));
+    }
+
+    if (dconf && dconf->x_cache_detail_set) {
+        x_cache_detail = dconf->x_cache_detail;
+    }
+    else {
+        x_cache_detail = conf->x_cache_detail;
+    }
+    if (x_cache_detail) {
+        apr_table_setn(r->headers_out, "X-Cache-Detail", apr_psprintf(r->pool,
+                "\"%s\" from %s", reason, r->server->server_hostname));
+    }
+
+    return OK;
+}
+
 /* -------------------------------------------------------------- */
 /* Setup configurable data */
 
+static void *create_dir_config(apr_pool_t *p, char *dummy)
+{
+    cache_dir_conf *dconf = apr_pcalloc(p, sizeof(cache_dir_conf));
+
+    dconf->x_cache = DEFAULT_X_CACHE;
+    dconf->x_cache_detail = DEFAULT_X_CACHE_DETAIL;
+
+    return dconf;
+}
+
+static void *merge_dir_config(apr_pool_t *p, void *basev, void *addv) {
+    cache_dir_conf *new = (cache_dir_conf *) apr_pcalloc(p, sizeof(cache_dir_conf));
+    cache_dir_conf *add = (cache_dir_conf *) addv;
+    cache_dir_conf *base = (cache_dir_conf *) basev;
+
+    new->x_cache = (add->x_cache_set == 0) ? base->x_cache : add->x_cache;
+    new->x_cache_set = add->x_cache_set || base->x_cache_set;
+    new->x_cache_detail = (add->x_cache_detail_set == 0) ? base->x_cache_detail
+            : add->x_cache_detail;
+    new->x_cache_detail_set = add->x_cache_detail_set
+            || base->x_cache_detail_set;
+
+    return new;
+}
+
 static void * create_cache_config(apr_pool_t *p, server_rec *s)
 {
     const char *tmppath;
@@ -1361,6 +1486,8 @@ static void * create_cache_config(apr_po
         ps->lockpath = apr_pstrcat(p, tmppath, DEFAULT_CACHE_LOCKPATH, NULL);
     }
     ps->lockmaxage = apr_time_from_sec(DEFAULT_CACHE_MAXAGE);
+    ps->x_cache = DEFAULT_X_CACHE;
+    ps->x_cache_detail = DEFAULT_X_CACHE_DETAIL;
     return ps;
 }
 
@@ -1435,6 +1562,14 @@ static void * merge_cache_config(apr_poo
         (overrides->quick_set == 0)
         ? base->quick
         : overrides->quick;
+    ps->x_cache =
+        (overrides->x_cache_set == 0)
+        ? base->x_cache
+        : overrides->x_cache;
+    ps->x_cache_detail =
+        (overrides->x_cache_detail_set == 0)
+        ? base->x_cache_detail
+        : overrides->x_cache_detail;
     return ps;
 }
 
@@ -1782,6 +1917,52 @@ static const char *set_cache_lock_maxage
     return NULL;
 }
 
+static const char *set_cache_x_cache(cmd_parms *parms, void *dummy, int flag)
+{
+
+    if (parms->path) {
+        cache_dir_conf *dconf = (cache_dir_conf *)dummy;
+
+        dconf->x_cache = flag;
+        dconf->x_cache_set = 1;
+
+    }
+    else {
+        cache_server_conf *conf =
+            (cache_server_conf *)ap_get_module_config(parms->server->module_config,
+                                                      &cache_module);
+
+        conf->x_cache = flag;
+        conf->x_cache_set = 1;
+
+    }
+
+    return NULL;
+}
+
+static const char *set_cache_x_cache_detail(cmd_parms *parms, void *dummy, int flag)
+{
+
+    if (parms->path) {
+        cache_dir_conf *dconf = (cache_dir_conf *)dummy;
+
+        dconf->x_cache_detail = flag;
+        dconf->x_cache_detail_set = 1;
+
+    }
+    else {
+        cache_server_conf *conf =
+            (cache_server_conf *)ap_get_module_config(parms->server->module_config,
+                                                      &cache_module);
+
+        conf->x_cache_detail = flag;
+        conf->x_cache_detail_set = 1;
+
+    }
+
+    return NULL;
+}
+
 static int cache_post_config(apr_pool_t *p, apr_pool_t *plog,
                              apr_pool_t *ptemp, server_rec *s)
 {
@@ -1859,6 +2040,11 @@ static const command_rec cache_cmds[] =
                   "temp directory."),
     AP_INIT_TAKE1("CacheLockMaxAge", set_cache_lock_maxage, NULL, RSRC_CONF,
                   "Maximum age of any thundering herd lock."),
+    AP_INIT_FLAG("CacheHeader", set_cache_x_cache, NULL, RSRC_CONF | ACCESS_CONF,
+                 "Add a X-Cache header to responses. Default is off."),
+    AP_INIT_FLAG("CacheDetailHeader", set_cache_x_cache_detail, NULL,
+                 RSRC_CONF | ACCESS_CONF,
+                 "Add a X-Cache-Detail header to responses. Default is off."),
     {NULL}
 };
 
@@ -1869,6 +2055,8 @@ static void register_hooks(apr_pool_t *p
     ap_hook_quick_handler(cache_quick_handler, NULL, NULL, APR_HOOK_FIRST);
     /* cache handler */
     ap_hook_handler(cache_handler, NULL, NULL, APR_HOOK_REALLY_FIRST);
+    /* cache status */
+    cache_hook_cache_status(cache_status, NULL, NULL, APR_HOOK_MIDDLE);
     /* cache filters
      * XXX The cache filters need to run right after the handlers and before
      * any other filters. Consider creating AP_FTYPE_CACHE for this purpose.
@@ -1949,10 +2137,20 @@ static void register_hooks(apr_pool_t *p
 AP_DECLARE_MODULE(cache) =
 {
     STANDARD20_MODULE_STUFF,
-    NULL,                   /* create per-directory config structure */
-    NULL,                   /* merge per-directory config structures */
+    create_dir_config,      /* create per-directory config structure */
+    merge_dir_config,       /* merge per-directory config structures */
     create_cache_config,    /* create per-server config structure */
     merge_cache_config,     /* merge per-server config structures */
     cache_cmds,             /* command apr_table_t */
     register_hooks
 };
+
+APR_HOOK_STRUCT(
+    APR_HOOK_LINK(cache_status)
+)
+
+APR_IMPLEMENT_EXTERNAL_HOOK_RUN_ALL(cache, CACHE, int, cache_status,
+                                    (cache_handle_t *h, request_rec *r,
+                                     ap_cache_status_e status,
+                                     const char *reason), (h, r, status, reason),
+                                    OK, DECLINED)

Modified: httpd/httpd/trunk/modules/cache/mod_cache.h
URL: http://svn.apache.org/viewvc/httpd/httpd/trunk/modules/cache/mod_cache.h?rev=1001639&r1=1001638&r2=1001639&view=diff
==============================================================================
--- httpd/httpd/trunk/modules/cache/mod_cache.h (original)
+++ httpd/httpd/trunk/modules/cache/mod_cache.h Mon Sep 27 09:20:40 2010
@@ -29,6 +29,7 @@
 #include "httpd.h"
 #include "apr_date.h"
 #include "apr_optional.h"
+#include "apr_hooks.h"
 
 /* Create a set of CACHE_DECLARE(type), CACHE_DECLARE_NONSTD(type) and
  * CACHE_DECLARE_DATA with appropriate export and import tags for the platform
@@ -106,6 +107,17 @@ typedef struct {
     apr_status_t (*commit_entity)(cache_handle_t *h, request_rec *r);
 } cache_provider;
 
+typedef enum {
+    AP_CACHE_HIT,
+    AP_CACHE_REVALIDATE,
+    AP_CACHE_MISS
+} ap_cache_status_e;
+
+#define AP_CACHE_HIT_ENV "cache-hit"
+#define AP_CACHE_REVALIDATE_ENV "cache-revalidate"
+#define AP_CACHE_MISS_ENV "cache-miss"
+#define AP_CACHE_STATUS_ENV "cache-status"
+
 
 /* cache_util.c */
 /* do a HTTP/1.1 age calculation */
@@ -150,6 +162,22 @@ CACHE_DECLARE(apr_table_t *)ap_cache_cac
 
 
 /* hooks */
+
+/**
+ * Cache status hook.
+ * This hook is called as soon as the cache has made a decision as to whether
+ * an entity should be served from cache (hit), should be served from cache
+ * after a successful validation (revalidate), or served from the backend
+ * and potentially cached (miss).
+ *
+ * A basic implementation of this hook exists in mod_cache which writes this
+ * information to the subprocess environment, and optionally to request
+ * headers. Further implementations may add hooks as appropriate to perform
+ * more advanced processing, or to store statistics about the cache behaviour.
+ */
+APR_DECLARE_EXTERNAL_HOOK(cache, CACHE, int, cache_status, (cache_handle_t *h,
+        request_rec *r, ap_cache_status_e status, const char *reason));
+
 APR_DECLARE_OPTIONAL_FN(apr_status_t,
                         ap_cache_generate_key,
                         (request_rec *r, apr_pool_t*p, const char **key));



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