Added Memcached Session Replicas by changes below

1. Allow for automatic removal of failed server to session handler (MEMCACHED_BEHAVIOR_REMOVE_FAILED_SERVERS)
2. Allow X number of replicas (MEMCACHED_BEHAVIOR_NUMBER_OF_REPLICAS)
3. Added option for consistent hasing (MEMCACHED_BEHAVIOR_KETAMA)
4. Added error msg when session handler couldn't obtain a lock

memcached.ini

@@ -17,7 +17,24 @@ memcached.sess_lock_wait = 150000
 ; the default value is "memc.sess.key."
 memcached.sess_prefix = "memc.sess.key."
 
+
+; Allow consistent hasing for the session
+memcached.sess_consistent_hashing = 1
+
+; Allow failed memcached server to automatically be removed 
+memcached.sess_remove_failed = 1
+
+; Write data to a number of additional memcached servers
+; This is "poor man's HA" as libmemcached calls it.
+; If this value is positive and sess_remove_failed is enabled
+; when a memcached server fails the session will continue to be available
+; from a replica. However, if the failed memcache server
+; becomes available again it will read the session from there
+; which could have old data or no data at all
+memcached.sess_num_replicas = 0;
+
 ; memcached session binary mode
+; libmemcached replicas only work if binary mode is enabled
 memcached.sess_binary = Off
 
 ; Set the compression type

php_memcached.c

@@ -288,6 +288,9 @@ PHP_INI_BEGIN()
 	STD_PHP_INI_ENTRY("memcached.sess_binary",		"0",		PHP_INI_ALL, OnUpdateBool,		sess_binary_enabled,	zend_php_memcached_globals,	php_memcached_globals)
 	STD_PHP_INI_ENTRY("memcached.sess_lock_wait",		"150000",	PHP_INI_ALL, OnUpdateLongGEZero,sess_lock_wait,			zend_php_memcached_globals,	php_memcached_globals)
 	STD_PHP_INI_ENTRY("memcached.sess_prefix",		"memc.sess.key.",	PHP_INI_ALL, OnUpdateString, sess_prefix,		zend_php_memcached_globals,	php_memcached_globals)
+	STD_PHP_INI_ENTRY("memcached.sess_consistent_hashing",	"0",		PHP_INI_ALL, OnUpdateBool,              sess_consistent_hashing_enabled,	zend_php_memcached_globals,     php_memcached_globals)
+	STD_PHP_INI_ENTRY("memcached.sess_num_replicas",	"0",		PHP_INI_ALL, OnUpdateLong, 		sess_num_replicas,	zend_php_memcached_globals,     php_memcached_globals)
+	STD_PHP_INI_ENTRY("memcached.sess_remove_failed",	"0",		PHP_INI_ALL, OnUpdateBool,              sess_remove_failed_enabled,	zend_php_memcached_globals,     php_memcached_globals)
 #endif
 	STD_PHP_INI_ENTRY("memcached.compression_type",		"fastlz",	PHP_INI_ALL, OnUpdateCompressionType, compression_type,		zend_php_memcached_globals,	php_memcached_globals)
 	STD_PHP_INI_ENTRY("memcached.compression_factor",	"1.3",		PHP_INI_ALL, OnUpdateReal, compression_factor,		zend_php_memcached_globals,	php_memcached_globals)
@@ -3034,6 +3037,9 @@ static void php_memc_init_globals(zend_php_memcached_globals *php_memcached_glob
 #ifdef HAVE_MEMCACHED_SESSION
 	MEMC_G(sess_locking_enabled) = 1;
 	MEMC_G(sess_binary_enabled) = 1;
+	MEMC_G(sess_consistent_hashing_enabled) = 0;
+	MEMC_G(sess_num_replicas) = 0;
+	MEMC_G(sess_remove_failed_enabled) = 0;
 	MEMC_G(sess_prefix) = NULL;
 	MEMC_G(sess_lock_wait) = 0;
 	MEMC_G(sess_locked) = 0;

php_memcached.h

@@ -66,6 +66,9 @@ ZEND_BEGIN_MODULE_GLOBALS(php_memcached)
 	zend_bool sess_locked;
 	char* sess_lock_key;
 	int   sess_lock_key_len;
+	long  sess_num_replicas;
+	zend_bool sess_remove_failed_enabled;
+	zend_bool sess_consistent_hashing_enabled;
 #endif
 	char *serializer_name;
 	enum memcached_serializer serializer;

php_memcached_session.c

@@ -48,6 +48,7 @@ static int php_memc_sess_lock(memcached_st *memc, const char *key TSRMLS_DC)
 	char *lock_key = NULL;
 	int lock_key_len = 0;
 	unsigned long attempts;
+	long write_retry_attempts = 0;
 	long lock_maxwait;
 	long lock_wait = MEMC_G(sess_lock_wait);
 	time_t expiration;
@@ -64,6 +65,11 @@ static int php_memc_sess_lock(memcached_st *memc, const char *key TSRMLS_DC)
 	expiration  = time(NULL) + lock_maxwait + 1;
 	attempts = (unsigned long)((1000000.0 / lock_wait) * lock_maxwait);
 
+	/* Set the number of write retry attempts to the number of replicas times the number of attempts to remove a server */
+	if (MEMC_G(sess_remove_failed_enabled)) {
+		write_retry_attempts = MEMC_G(sess_num_replicas) * ( memcached_behavior_get(memc, MEMCACHED_BEHAVIOR_SERVER_FAILURE_LIMIT) + 1);
+	}
+
 	lock_key_len = spprintf(&lock_key, 0, "lock.%s", key);
 	do {
 		status = memcached_add(memc, lock_key, lock_key_len, "1", sizeof("1")-1, expiration, 0);
@@ -73,6 +79,11 @@ static int php_memc_sess_lock(memcached_st *memc, const char *key TSRMLS_DC)
 			MEMC_G(sess_lock_key_len) = lock_key_len;
 			return 0;
 		} else if (status != MEMCACHED_NOTSTORED && status != MEMCACHED_DATA_EXISTS) {
+			if (write_retry_attempts > 0) {
+				write_retry_attempts--;
+				continue;
+			}
+			php_error_docref(NULL TSRMLS_CC, E_WARNING, "Write of lock failed");
 			break;
 		}
 
@@ -195,6 +206,30 @@ success:
 				}
 			}
 
+			if (MEMC_G(sess_consistent_hashing_enabled)) {
+				if (memcached_behavior_set(memc_sess->memc_sess, MEMCACHED_BEHAVIOR_KETAMA, (uint64_t) 1) == MEMCACHED_FAILURE) {
+					php_error_docref(NULL TSRMLS_CC, E_WARNING, "failed to set memcached consistent hashing");
+					return FAILURE;
+				}
+			}
+
+			/* Allow libmemcached remove failed servers */
+			if (MEMC_G(sess_remove_failed_enabled)) {
+				if (memcached_behavior_set(memc_sess->memc_sess, MEMCACHED_BEHAVIOR_REMOVE_FAILED_SERVERS, (uint64_t) 1) == MEMCACHED_FAILURE) {
+					php_error_docref(NULL TSRMLS_CC, E_WARNING, "failed to set: remove failed servers");
+					return FAILURE;
+				}
+			}
+
+			/* Allow replicas section */
+			long num_replicas = MEMC_G(sess_num_replicas);
+			if (num_replicas > 0) {
+				/* Set the number of replicas libmemcached will use */
+				if (memcached_behavior_set(memc_sess->memc_sess, MEMCACHED_BEHAVIOR_NUMBER_OF_REPLICAS, (uint64_t) num_replicas) == MEMCACHED_FAILURE) {
+					php_error_docref(NULL TSRMLS_CC, E_WARNING, "failed to set memcached client replicating");
+					return FAILURE;
+				}
+			}
 			return SUCCESS;
 		}
 	}
@@ -243,6 +278,7 @@ PS_READ_FUNC(memcached)
 
 	if (MEMC_G(sess_locking_enabled)) {
 		if (php_memc_sess_lock(memc_sess->memc_sess, key TSRMLS_CC) < 0) {
+			php_error_docref(NULL TSRMLS_CC, E_WARNING, "Unable to clear session lock record");
 			return FAILURE;
 		}
 	}
@@ -263,6 +299,7 @@ PS_WRITE_FUNC(memcached)
 {
 	int key_len = strlen(key);
 	time_t expiration = 0;
+	long write_try_attempts = 1;
 	memcached_return status;
 	memcached_sess *memc_sess = PS_GET_MOD_DATA();
 	size_t key_length;
@@ -277,13 +314,22 @@ PS_WRITE_FUNC(memcached)
 	if (PS(gc_maxlifetime) > 0) {
 		expiration = PS(gc_maxlifetime);
 	}
-	status = memcached_set(memc_sess->memc_sess, key, key_len, val, vallen, expiration, 0);
 
-	if (status == MEMCACHED_SUCCESS) {
-		return SUCCESS;
-	} else {
-		return FAILURE;
+	/* Set the number of write retry attempts to the number of replicas times the number of attempts to remove a server plus the initial write */
+	if (MEMC_G(sess_remove_failed_enabled)) {
+		write_try_attempts = 1 + MEMC_G(sess_num_replicas) * ( memcached_behavior_get(memc_sess->memc_sess, MEMCACHED_BEHAVIOR_SERVER_FAILURE_LIMIT) + 1);
 	}
+
+	do {
+		status = memcached_set(memc_sess->memc_sess, key, key_len, val, vallen, expiration, 0);
+		if (status == MEMCACHED_SUCCESS) {
+			return SUCCESS;
+		} else {
+			write_try_attempts--;
+		}
+	} while (write_try_attempts > 0);
+
+	return FAILURE;
 }
 
 PS_DESTROY_FUNC(memcached)