cluster import/export of hash slots implemented in the query redirection engine

src/cluster.c

@@ -1605,12 +1605,19 @@ file_rd_err:
 /* Return the pointer to the cluster node that is able to serve the query
  * as all the keys belong to hash slots for which the node is in charge.
  *
- * If keys in query spawn multiple nodes NULL is returned. */
-clusterNode *getNodeByQuery(redisClient *c, struct redisCommand *cmd, robj **argv, int argc, int *hashslot) {
+ * If the returned node should be used only for this request, the *ask
+ * integer is set to '1', otherwise to '0'. This is used in order to
+ * let the caller know if we should reply with -MOVED or with -ASK.
+ *
+ * If the request contains more than a single key NULL is returned,
+ * however a request with more then a key argument where the key is always
+ * the same is valid, like in: RPOPLPUSH mylist mylist.*/
+clusterNode *getNodeByQuery(redisClient *c, struct redisCommand *cmd, robj **argv, int argc, int *hashslot, int *ask) {
     clusterNode *n = NULL;
+    robj *firstkey = NULL;
     multiState *ms, _ms;
     multiCmd mc;
-    int i;
+    int i, slot = 0;
 
     /* We handle all the cases as if they were EXEC commands, so we have
      * a common code path for everything */
@@ -1620,7 +1627,9 @@ clusterNode *getNodeByQuery(redisClient *c, struct redisCommand *cmd, robj **arg
         if (!(c->flags & REDIS_MULTI)) return server.cluster.myself;
         ms = &c->mstate;
     } else {
-        /* Create a fake Multi State structure, with just one command */
+        /* In order to have a single codepath create a fake Multi State
+         * structure if the client is not in MULTI/EXEC state, this way
+         * we have a single codepath below. */
         ms = &_ms;
         _ms.commands = &mc;
         _ms.count = 1;
@@ -1629,6 +1638,8 @@ clusterNode *getNodeByQuery(redisClient *c, struct redisCommand *cmd, robj **arg
         mc.cmd = cmd;
     }
 
+    /* Check that all the keys are the same key, and get the slot and
+     * node for this key. */
     for (i = 0; i < ms->count; i++) {
         struct redisCommand *mcmd;
         robj **margv;
@@ -1641,24 +1652,48 @@ clusterNode *getNodeByQuery(redisClient *c, struct redisCommand *cmd, robj **arg
         keyindex = getKeysFromCommand(mcmd,margv,margc,&numkeys,
                                       REDIS_GETKEYS_PRELOAD);
         for (j = 0; j < numkeys; j++) {
-            int slot = keyHashSlot((char*)margv[keyindex[j]]->ptr,
-                sdslen(margv[keyindex[j]]->ptr));
-            struct clusterNode *slotnode;
-
-            slotnode = server.cluster.slots[slot];
-            if (hashslot) *hashslot = slot;
-            /* Node not assigned? (Should never happen actually
-             * if we reached this function).
-             * Different node than the previous one?
-             * Return NULL, the cluster can't serve multi-node requests */
-            if (slotnode == NULL || (n && slotnode != n)) {
-                getKeysFreeResult(keyindex);
-                return NULL;
+            if (firstkey == NULL) {
+                /* This is the first key we see. Check what is the slot
+                 * and node. */
+                firstkey = margv[keyindex[j]];
+
+                slot = keyHashSlot((char*)firstkey->ptr, sdslen(firstkey->ptr));
+                n = server.cluster.slots[slot];
+                redisAssert(n != NULL);
             } else {
-                n = slotnode;
+                /* If it is not the first key, make sure it is exactly
+                 * the same key as the first we saw. */
+                if (!equalStringObjects(firstkey,margv[keyindex[j]])) {
+                    decrRefCount(firstkey);
+                    getKeysFreeResult(keyindex);
+                    return NULL;
+                }
             }
         }
         getKeysFreeResult(keyindex);
     }
-    return (n == NULL) ? server.cluster.myself : n;
+    if (ask) *ask = 0; /* This is the default. Set to 1 if needed later. */
+    /* No key at all in command? then we can serve the request
+     * without redirections. */
+    if (n == NULL) return server.cluster.myself;
+    if (hashslot) *hashslot = slot;
+    /* This request is about a slot we are migrating into another instance?
+     * Then we need to check if we have the key. If we have it we can reply.
+     * If instead is a new key, we pass the request to the node that is
+     * receiving the slot. */
+    if (n == server.cluster.myself &&
+        server.cluster.migrating_slots_to[slot] != NULL)
+    {
+        if (lookupKeyRead(&server.db[0],firstkey) == NULL) {
+            if (ask) *ask = 1;
+            return server.cluster.migrating_slots_to[slot];
+        }
+    }
+    /* Handle the case in which we are receiving this hash slot from
+     * another instance, so we'll accept the query even if in the table
+     * it is assigned to a different node. */
+    if (server.cluster.importing_slots_from[slot] != NULL)
+        return server.cluster.myself;
+    /* It's not a -ASK case. Base case: just return the right node. */
+    return n;
 }

src/redis.c

@@ -1096,13 +1096,15 @@ int processCommand(redisClient *c) {
             addReplyError(c,"The cluster is down. Check with CLUSTER INFO for more information");
             return REDIS_OK;
         } else {
-            clusterNode *n = getNodeByQuery(c,cmd,c->argv,c->argc,&hashslot);
+            int ask;
+            clusterNode *n = getNodeByQuery(c,cmd,c->argv,c->argc,&hashslot,&ask);
             if (n == NULL) {
-                addReplyError(c,"Invalid cross-node request");
+                addReplyError(c,"Multi keys request invalid in cluster");
                 return REDIS_OK;
             } else if (n != server.cluster.myself) {
                 addReplySds(c,sdscatprintf(sdsempty(),
-                    "-MOVED %d %s:%d\r\n",hashslot,n->ip,n->port));
+                    "-%s %d %s:%d\r\n", ask ? "ASK" : "MOVED",
+                    hashslot,n->ip,n->port));
                 return REDIS_OK;
             }
         }

src/redis.h

@@ -1083,7 +1083,7 @@ unsigned int keyHashSlot(char *key, int keylen);
 clusterNode *createClusterNode(char *nodename, int flags);
 int clusterAddNode(clusterNode *node);
 void clusterCron(void);
-clusterNode *getNodeByQuery(redisClient *c, struct redisCommand *cmd, robj **argv, int argc, int *hashslot);
+clusterNode *getNodeByQuery(redisClient *c, struct redisCommand *cmd, robj **argv, int argc, int *hashslot, int *ask);
 
 /* Git SHA1 */
 char *redisGitSHA1(void);