support for write operations against expiring keys, by master-controlled expiring in replication and AOF synthesizing DEL operations

src/db.c

@@ -45,7 +45,7 @@ robj *lookupKeyRead(redisDb *db, robj *key) {
 }
 
 robj *lookupKeyWrite(redisDb *db, robj *key) {
-    deleteIfVolatile(db,key);
+    expireIfNeeded(db,key);
     return lookupKey(db,key);
 }
 
@@ -321,7 +321,6 @@ void renameGenericCommand(redisClient *c, int nx) {
         return;
 
     incrRefCount(o);
-    deleteIfVolatile(c->db,c->argv[2]);
     if (dbAdd(c->db,c->argv[2],o) == REDIS_ERR) {
         if (nx) {
             decrRefCount(o);
@@ -375,7 +374,6 @@ void moveCommand(redisClient *c) {
     }
 
     /* Try to add the element to the target DB */
-    deleteIfVolatile(dst,c->argv[1]);
     if (dbAdd(dst,c->argv[1],o) == REDIS_ERR) {
         addReply(c,shared.czero);
         return;
@@ -430,8 +428,45 @@ time_t getExpire(redisDb *db, robj *key) {
     return (time_t) dictGetEntryVal(de);
 }
 
+/* Propagate expires into slaves and the AOF file.
+ * When a key expires in the master, a DEL operation for this key is sent
+ * to all the slaves and the AOF file if enabled.
+ *
+ * This way the key expiry is centralized in one place, and since both
+ * AOF and the master->slave link guarantee operation ordering, everything
+ * will be consistent even if we allow write operations against expiring
+ * keys. */
+void propagateExpire(redisDb *db, robj *key) {
+    struct redisCommand *cmd;
+    robj *argv[2];
+
+    cmd = lookupCommand("del");
+    argv[0] = createStringObject("DEL",3);
+    argv[1] = key;
+    incrRefCount(key);
+
+    if (server.appendonly)
+        feedAppendOnlyFile(cmd,db->id,argv,2);
+    if (listLength(server.slaves))
+        replicationFeedSlaves(server.slaves,db->id,argv,2);
+
+    decrRefCount(key);
+}
+
 int expireIfNeeded(redisDb *db, robj *key) {
     time_t when = getExpire(db,key);
+
+    /* If we are running in the context of a slave, return ASAP:
+     * the slave key expiration is controlled by the master that will
+     * send us synthesized DEL operations for expired keys.
+     *
+     * Still we try to return the right information to the caller, 
+     * that is, 0 if we think the key should be still valid, 1 if
+     * we think the key is expired at this time. */
+    if (server.masterhost != NULL) {
+        return time(NULL) > when;
+    }
+
     if (when < 0) return 0;
 
     /* Return when this key has not expired */
@@ -440,15 +475,7 @@ int expireIfNeeded(redisDb *db, robj *key) {
     /* Delete the key */
     server.stat_expiredkeys++;
     server.dirty++;
-    return dbDelete(db,key);
+    propagateExpire(db,key);
-}
-
-int deleteIfVolatile(redisDb *db, robj *key) {
-    if (getExpire(db,key) < 0) return 0;
-
-    /* Delete the key */
-    server.stat_expiredkeys++;
-    server.dirty++;
     return dbDelete(db,key);
 }
 

src/redis.c

@@ -435,6 +435,48 @@ void updateDictResizePolicy(void) {
 
 /* ======================= Cron: called every 100 ms ======================== */
 
+/* Try to expire a few timed out keys. The algorithm used is adaptive and
+ * will use few CPU cycles if there are few expiring keys, otherwise
+ * it will get more aggressive to avoid that too much memory is used by
+ * keys that can be removed from the keyspace. */
+void activeExpireCycle(void) {
+    int j;
+
+    for (j = 0; j < server.dbnum; j++) {
+        int expired;
+        redisDb *db = server.db+j;
+
+        /* Continue to expire if at the end of the cycle more than 25%
+         * of the keys were expired. */
+        do {
+            long num = dictSize(db->expires);
+            time_t now = time(NULL);
+
+            expired = 0;
+            if (num > REDIS_EXPIRELOOKUPS_PER_CRON)
+                num = REDIS_EXPIRELOOKUPS_PER_CRON;
+            while (num--) {
+                dictEntry *de;
+                time_t t;
+
+                if ((de = dictGetRandomKey(db->expires)) == NULL) break;
+                t = (time_t) dictGetEntryVal(de);
+                if (now > t) {
+                    sds key = dictGetEntryKey(de);
+                    robj *keyobj = createStringObject(key,sdslen(key));
+
+                    propagateExpire(db,keyobj);
+                    dbDelete(db,keyobj);
+                    decrRefCount(keyobj);
+                    expired++;
+                    server.stat_expiredkeys++;
+                }
+            }
+        } while (expired > REDIS_EXPIRELOOKUPS_PER_CRON/4);
+    }
+}
+
+
 int serverCron(struct aeEventLoop *eventLoop, long long id, void *clientData) {
     int j, loops = server.cronloops++;
     REDIS_NOTUSED(eventLoop);
@@ -533,41 +575,10 @@ int serverCron(struct aeEventLoop *eventLoop, long long id, void *clientData) {
          }
     }
 
-    /* Try to expire a few timed out keys. The algorithm used is adaptive and
+    /* Expire a few keys per cycle, only if this is a master.
-     * will use few CPU cycles if there are few expiring keys, otherwise
+     * On slaves we wait for DEL operations synthesized by the master
-     * it will get more aggressive to avoid that too much memory is used by
+     * in order to guarantee a strict consistency. */
-     * keys that can be removed from the keyspace. */
+    if (server.masterhost == NULL) activeExpireCycle();
-    for (j = 0; j < server.dbnum; j++) {
-        int expired;
-        redisDb *db = server.db+j;
-
-        /* Continue to expire if at the end of the cycle more than 25%
-         * of the keys were expired. */
-        do {
-            long num = dictSize(db->expires);
-            time_t now = time(NULL);
-
-            expired = 0;
-            if (num > REDIS_EXPIRELOOKUPS_PER_CRON)
-                num = REDIS_EXPIRELOOKUPS_PER_CRON;
-            while (num--) {
-                dictEntry *de;
-                time_t t;
-
-                if ((de = dictGetRandomKey(db->expires)) == NULL) break;
-                t = (time_t) dictGetEntryVal(de);
-                if (now > t) {
-                    sds key = dictGetEntryKey(de);
-                    robj *keyobj = createStringObject(key,sdslen(key));
-
-                    dbDelete(db,keyobj);
-                    decrRefCount(keyobj);
-                    expired++;
-                    server.stat_expiredkeys++;
-                }
-            }
-        } while (expired > REDIS_EXPIRELOOKUPS_PER_CRON/4);
-    }
 
     /* Swap a few keys on disk if we are over the memory limit and VM
      * is enbled. Try to free objects from the free list first. */

src/redis.h

@@ -752,8 +752,8 @@ void resetServerSaveParams();
 
 /* db.c -- Keyspace access API */
 int removeExpire(redisDb *db, robj *key);
+void propagateExpire(redisDb *db, robj *key);
 int expireIfNeeded(redisDb *db, robj *key);
-int deleteIfVolatile(redisDb *db, robj *key);
 time_t getExpire(redisDb *db, robj *key);
 int setExpire(redisDb *db, robj *key, time_t when);
 robj *lookupKey(redisDb *db, robj *key);

src/t_string.c

@@ -17,7 +17,6 @@ void setGenericCommand(redisClient *c, int nx, robj *key, robj *val, robj *expir
         }
     }
 
-    if (nx) deleteIfVolatile(c->db,key);
     retval = dbAdd(c->db,key,val);
     if (retval == REDIS_ERR) {
         if (!nx) {