On Fri, Nov 11, 2011 at 11:09 AM, Jason Gionta <jjgio...@ncsu.edu> wrote:
> Hi all,
> I am developing a module in which I need to create a cache for the duration
> of the server process. This cache will basically keep state information in
> which requests will query and write too. As a test, I have implemented a
> simple cache using a hash table in which I read the count of the hash table
> and write that count into the hash table, there by incrementing the count by
> one. I allocate the memory for the hash table using the server process pool
> as this memory should have the life time of the process.
> While I expect the count to increment by one after each request. It seems
> like if there is a time gap between requests (over 10 seconds), the count
> gets reset (apr_hash_count = 0). I have no idea how or why the hash table
> is seemingly being emptied as I only increment and insert with a new key and
> value. Can someone shed some light on this issue? Is there a process pool
> timeout? I have compiled apache without threads to make things easier (as
> with threads gave me even more unexpected results)
Once keepalive timeout expires the client reconnects and a different
child process handles that connection, and its hash is still in the
initial state?
> Here is the basic code for the cache.... Thanks for any help!
> static void *http_create_server_config(apr_pool_t *p,server_rec *s)
> {
> http_config *pconfig=ap_get_module_config(s->module_config,
> &http_module);
> if(pconfig!=NULL)
> return pconfig;
>
> pconfig=apr_pcalloc(p,sizeof(http_config));
> pconfig->enabled=0;
> pconfig->server = s;
> apr_status_t rv;
> apr_pool_t * pchild = s->process->pool;
> /* Derive our own pool from pchild */
> rv = apr_pool_create(&pconfig->test_pool, pchild);
> if (rv != APR_SUCCESS) {
> ap_log_perror(APLOG_MARK, APLOG_CRIT, rv, pchild,
> "Failed to create subpool for my_module");
> }
> /* Set up a thread mutex for when we need to manipulate the cache */
> #if APR_HAS_THREADS
> rv = apr_thread_mutex_create(&pconfig->test_mutex,
> APR_THREAD_MUTEX_DEFAULT, pchild);
> if (rv != APR_SUCCESS) {
> ap_log_perror(APLOG_MARK, APLOG_CRIT, rv, pchild,
> "Failed to create mutex for my_module");
> }
> #endif
> /* Finally, create the cache itself (and prime it if applicable) */
> pconfig->test_cache = apr_hash_make(pconfig->test_pool);
>
> return pconfig;
> }
>
> static apr_status_t * test_cache(request_rec * r)
> {
> apr_status_t rv;
> http_config *svr =
> ap_get_module_config(r->server->module_config, &http_module);
> const char *key;
> char * tempkey=NULL;
> int val;
> #if APR_HAS_THREADS
> /* If it isn't cached, we need to compute it and save it to
> * the cache. That's a critical section, so we need the mutex.
> */
> rv = apr_thread_mutex_lock(svr->test_mutex);
> if (rv != APR_SUCCESS) {
> ap_log_rerror(APLOG_MARK, APLOG_ERR, rv, r,
> "Failed to acquire thread mutex");
> return HTTP_SERVICE_UNAVAILABLE;
> }
> /* In case of a race condition between cache lookup and
> * obtaining the lock, perform the lookup again.
> * Not a performance problem unless this happens a lot.
> */
> val = apr_hash_count(svr->test_cache);
> key = apr_itoa(svr->test_pool, val);
> tempkey = apr_hash_get(svr->test_cache, key, APR_HASH_KEY_STRING);
> if (tempkey == NULL) {
> /* OK, we really do need to compute it */
> apr_hash_set(svr->test_cache, key, APR_HASH_KEY_STRING, key);
> }
> else
> {
> ap_log_rerror(APLOG_MARK, APLOG_ERR, rv, r,
> "TEST_CACHE: FAILED! %s", tempkey);
> return APR_ERELATIVE;
> }
> #else
> /* No threads = no risk of a race condition. Just set it. */
> val = apr_hash_count(svr->test_cache);
> key = apr_itoa(svr->test_pool, val);
> apr_hash_set(svr->test_cache, key, APR_HASH_KEY_STRING, key);
> #endif
> #if APR_HAS_THREADS
> rv = apr_thread_mutex_unlock(svr->test_mutex);
> if (rv != APR_SUCCESS) {
> /* Something is seriously wrong. We need to log it,
> * but it doesn't -– of itself -– invalidate this request.
> */
> ap_log_rerror(APLOG_MARK, APLOG_ERR, rv, r,
> "Failed to release thread mutex");
> return rv;
> }
> #endif
> ap_log_rerror(APLOG_MARK, APLOG_ERR, rv, r,
> "TEST_CACHE: COUNT=%d", val);
> return APR_SUCCESS;
> }
> static apr_status_t my_filter(ap_filter_t *f, apr_bucket_brigade *bb)
> {
> test_cache(f->r);
> }
> --
> Jason Gionta
> North Carolina State University
> jjgio...@ncsu.edu
>
--
Born in Roswell... married an alien...
