Cache CLUSTER SLOTS response for improving throughput and reduced lat…

…ency. (#53)

This commit adds a logic to cache `CLUSTER SLOTS` response for reduced
latency and also updates the cache when a change in the cluster is
detected.

Historically, `CLUSTER SLOTS` command was deprecated, however all the
server clients have been using `CLUSTER SLOTS` and have not migrated to
`CLUSTER SHARDS`. In future this logic can be added to any other
commands to improve the performance of the engine.

---------

Signed-off-by: Roshan Khatri <[email protected]>

src/cluster.c

@@ -1312,24 +1312,6 @@ int clusterRedirectBlockedClientIfNeeded(client *c) {
     return 0;
 }
 
-/* Returns an indication if the replica node is fully available
- * and should be listed in CLUSTER SLOTS response.
- * Returns 1 for available nodes, 0 for nodes that have
- * not finished their initial sync, in failed state, or are
- * otherwise considered not available to serve read commands. */
-static int isReplicaAvailable(clusterNode *node) {
-    if (clusterNodeIsFailing(node)) {
-        return 0;
-    }
-    long long repl_offset = clusterNodeReplOffset(node);
-    if (clusterNodeIsMyself(node)) {
-        /* Nodes do not update their own information
-         * in the cluster node list. */
-        repl_offset = replicationGetSlaveOffset();
-    }
-    return (repl_offset != 0);
-}
-
 void addNodeToNodeReply(client *c, clusterNode *node) {
     char* hostname = clusterNodeHostname(node);
     addReplyArrayLen(c, 4);
@@ -1381,10 +1363,28 @@ void addNodeToNodeReply(client *c, clusterNode *node) {
     serverAssert(length == 0);
 }
 
+/* Returns an indication if the node is fully available
+ * and should be listed in CLUSTER SLOTS response.
+ * Returns 1 for available nodes, 0 for nodes that have
+ * not finished their initial sync, in failed state, or are
+ * otherwise considered not available to serve read commands. */
+int isNodeAvailable(clusterNode *node) {
+    if (clusterNodeIsFailing(node)) {
+        return 0;
+    }
+    long long repl_offset = clusterNodeReplOffset(node);
+    if (clusterNodeIsMyself(node)) {
+        /* Nodes do not update their own information
+         * in the cluster node list. */
+        repl_offset = getNodeReplicationOffset(node);
+    }
+    return (repl_offset != 0);
+}
+
 void addNodeReplyForClusterSlot(client *c, clusterNode *node, int start_slot, int end_slot) {
     int i, nested_elements = 3; /* slots (2) + master addr (1) */
     for (i = 0; i < clusterNodeNumSlaves(node); i++) {
-        if (!isReplicaAvailable(clusterNodeGetSlave(node, i))) continue;
+        if (!isNodeAvailable(clusterNodeGetSlave(node, i))) continue;
         nested_elements++;
     }
     addReplyArrayLen(c, nested_elements);
@@ -1396,27 +1396,27 @@ void addNodeReplyForClusterSlot(client *c, clusterNode *node, int start_slot, in
     for (i = 0; i < clusterNodeNumSlaves(node); i++) {
         /* This loop is copy/pasted from clusterGenNodeDescription()
          * with modifications for per-slot node aggregation. */
-        if (!isReplicaAvailable(clusterNodeGetSlave(node, i))) continue;
+        if (!isNodeAvailable(clusterNodeGetSlave(node, i))) continue;
         addNodeToNodeReply(c, clusterNodeGetSlave(node, i));
         nested_elements--;
     }
     serverAssert(nested_elements == 3); /* Original 3 elements */
 }
 
-void clusterCommandSlots(client * c) {
-    /* Format: 1) 1) start slot
-     *            2) end slot
-     *            3) 1) master IP
-     *               2) master port
-     *               3) node ID
-     *            4) 1) replica IP
-     *               2) replica port
-     *               3) node ID
-     *           ... continued until done
-     */
+void clearCachedClusterSlotsResponse(void) {
+    for (connTypeForCaching conn_type = CACHE_CONN_TCP; conn_type < CACHE_CONN_TYPE_MAX; conn_type++) {
+        if (server.cached_cluster_slot_info[conn_type]) {
+            sdsfree(server.cached_cluster_slot_info[conn_type]);
+            server.cached_cluster_slot_info[conn_type] = NULL;
+        }
+    }
+}
+
+sds generateClusterSlotResponse(void) {
+    client *recording_client = createCachedResponseClient();
     clusterNode *n = NULL;
     int num_masters = 0, start = -1;
-    void *slot_replylen = addReplyDeferredLen(c);
+    void *slot_replylen = addReplyDeferredLen(recording_client);
 
     for (int i = 0; i <= CLUSTER_SLOTS; i++) {
         /* Find start node and slot id. */
@@ -1430,14 +1430,52 @@ void clusterCommandSlots(client * c) {
         /* Add cluster slots info when occur different node with start
          * or end of slot. */
         if (i == CLUSTER_SLOTS || n != getNodeBySlot(i)) {
-            addNodeReplyForClusterSlot(c, n, start, i-1);
+            addNodeReplyForClusterSlot(recording_client, n, start, i-1);
             num_masters++;
             if (i == CLUSTER_SLOTS) break;
             n = getNodeBySlot(i);
             start = i;
         }
     }
-    setDeferredArrayLen(c, slot_replylen, num_masters);
+    setDeferredArrayLen(recording_client, slot_replylen, num_masters);
+    sds cluster_slot_response = aggregateClientOutputBuffer(recording_client);
+    deleteCachedResponseClient(recording_client);
+    return cluster_slot_response;
+}
+
+int verifyCachedClusterSlotsResponse(sds cached_response) {
+    sds generated_response = generateClusterSlotResponse();
+    int is_equal = !sdscmp(generated_response, cached_response);
+    /* Here, we use LL_WARNING so this gets printed when debug assertions are enabled and the system is about to crash. */
+    if (!is_equal) serverLog(LL_WARNING,"\ngenerated_response:\n%s\n\ncached_response:\n%s", generated_response, cached_response);
+    sdsfree(generated_response);
+    return is_equal;
+}
+
+void clusterCommandSlots(client *c) {
+    /* Format: 1) 1) start slot
+     *            2) end slot
+     *            3) 1) master IP
+     *               2) master port
+     *               3) node ID
+     *            4) 1) replica IP
+     *               2) replica port
+     *               3) node ID
+     *           ... continued until done
+     */
+    connTypeForCaching conn_type = connIsTLS(c->conn);
+
+    if (detectAndUpdateCachedNodeHealth()) clearCachedClusterSlotsResponse();
+
+    sds cached_reply = server.cached_cluster_slot_info[conn_type];
+    if (!cached_reply) {
+        cached_reply = generateClusterSlotResponse();
+        server.cached_cluster_slot_info[conn_type] = cached_reply;
+    } else {
+        debugServerAssertWithInfo(c, NULL, verifyCachedClusterSlotsResponse(cached_reply) == 1);
+    }
+
+    addReplyProto(c, cached_reply, sdslen(cached_reply));
 }
 
 /* -----------------------------------------------------------------------------

src/cluster.h

@@ -100,6 +100,10 @@ const char *clusterNodePreferredEndpoint(clusterNode *n);
 long long clusterNodeReplOffset(clusterNode *node);
 clusterNode *clusterLookupNode(const char *name, int length);
 void clusterReplicateOpenSlots(void);
+int detectAndUpdateCachedNodeHealth(void);
+client *createCachedResponseClient(void);
+void deleteCachedResponseClient(client *recording_client);
+void clearCachedClusterSlotsResponse(void);
 
 /* functions with shared implementations */
 int clusterNodeIsMyself(clusterNode *n);
@@ -113,4 +117,7 @@ int isValidAuxString(char *s, unsigned int length);
 void migrateCommand(client *c);
 void clusterCommand(client *c);
 ConnectionType *connTypeOfCluster(void);
+int isNodeAvailable(clusterNode *node);
+long long getNodeReplicationOffset(clusterNode *node);
+sds aggregateClientOutputBuffer(client *c);
 #endif /* __CLUSTER_H */

src/cluster_legacy.c

@@ -762,6 +762,7 @@ void clusterSaveConfigOrDie(int do_fsync) {
         serverLog(LL_WARNING,"Fatal: can't update cluster config file.");
         exit(1);
     }
+    clearCachedClusterSlotsResponse();
 }
 
 /* Lock the cluster config using flock(), and retain the file descriptor used to
@@ -1039,6 +1040,9 @@ void clusterInit(void) {
 
     server.cluster->mf_end = 0;
     server.cluster->mf_slave = NULL;
+    for (connTypeForCaching conn_type = CACHE_CONN_TCP; conn_type < CACHE_CONN_TYPE_MAX; conn_type++) {
+        server.cached_cluster_slot_info[conn_type] = NULL;
+    }
     resetManualFailover();
     clusterUpdateMyselfFlags();
     clusterUpdateMyselfIp();
@@ -1363,6 +1367,7 @@ clusterNode *createClusterNode(char *nodename, int flags) {
     node->repl_offset_time = 0;
     node->repl_offset = 0;
     listSetFreeMethod(node->fail_reports,zfree);
+    node->is_node_healthy = 0;
     return node;
 }
 
@@ -5862,6 +5867,14 @@ void clusterUpdateSlots(client *c, unsigned char *slots, int del) {
     }
 }
 
+long long getNodeReplicationOffset(clusterNode *node) {
+    if (node->flags & CLUSTER_NODE_MYSELF) {
+        return nodeIsSlave(node) ? replicationGetSlaveOffset() : server.master_repl_offset;
+    } else {
+        return node->repl_offset;
+    }
+}
+
 /* Add detailed information of a node to the output buffer of the given client. */
 void addNodeDetailsToShardReply(client *c, clusterNode *node) {
     int reply_count = 0;
@@ -5896,12 +5909,7 @@ void addNodeDetailsToShardReply(client *c, clusterNode *node) {
         reply_count++;
     }
 
-    long long node_offset;
-    if (node->flags & CLUSTER_NODE_MYSELF) {
-        node_offset = nodeIsSlave(node) ? replicationGetSlaveOffset() : server.master_repl_offset;
-    } else {
-        node_offset = node->repl_offset;
-    }
+    long long node_offset = getNodeReplicationOffset(node);
 
     addReplyBulkCString(c, "role");
     addReplyBulkCString(c, nodeIsSlave(node) ? "replica" : "master");
@@ -6882,9 +6890,26 @@ void clusterPromoteSelfToMaster(void) {
     replicationUnsetMaster();
 }
 
+int detectAndUpdateCachedNodeHealth(void) {
+    dictIterator di;
+    dictInitSafeIterator(&di, server.cluster->nodes);
+    dictEntry *de;
+    clusterNode *node;
+    int overall_health_changed = 0;
+    while((de = dictNext(&di)) != NULL) {
+        node = dictGetVal(de);
+        int present_is_node_healthy = isNodeAvailable(node);
+        if (present_is_node_healthy != node->is_node_healthy) {
+            overall_health_changed = 1;
+            node->is_node_healthy = present_is_node_healthy;
+        }
+    }
+
+    return overall_health_changed;
+}
+
 /* Replicate migrating and importing slot states to all replicas */
-void clusterReplicateOpenSlots(void)
-{
+void clusterReplicateOpenSlots(void) {
     if (!server.cluster_enabled) return;
 
     int argc = 5;

src/cluster_legacy.h

@@ -308,6 +308,8 @@ struct _clusterNode {
     clusterLink *link;          /* TCP/IP link established toward this node */
     clusterLink *inbound_link;  /* TCP/IP link accepted from this node */
     list *fail_reports;         /* List of nodes signaling this as failing */
+    int is_node_healthy;        /* Boolean indicating the cached node health. 
+                                   Update with updateAndCountChangedNodeHealth(). */
 };
 
 struct clusterState {

src/config.c

@@ -2586,6 +2586,12 @@ static int updateOOMScoreAdj(const char **err) {
     return 1;
 }
 
+int invalidateClusterSlotsResp(const char **err) {
+    UNUSED(err);
+    clearCachedClusterSlotsResponse();
+    return 1;
+}
+
 int updateRequirePass(const char **err) {
     UNUSED(err);
     /* The old "requirepass" directive just translates to setting
@@ -2649,6 +2655,7 @@ int updateClusterFlags(const char **err) {
 static int updateClusterAnnouncedPort(const char **err) {
     UNUSED(err);
     clusterUpdateMyselfAnnouncedPorts();
+    clearCachedClusterSlotsResponse();
     return 1;
 }
 
@@ -3162,7 +3169,7 @@ standardConfig static_configs[] = {
     createEnumConfig("enable-protected-configs", NULL, IMMUTABLE_CONFIG, protected_action_enum, server.enable_protected_configs, PROTECTED_ACTION_ALLOWED_NO, NULL, NULL),
     createEnumConfig("enable-debug-command", NULL, IMMUTABLE_CONFIG, protected_action_enum, server.enable_debug_cmd, PROTECTED_ACTION_ALLOWED_NO, NULL, NULL),
     createEnumConfig("enable-module-command", NULL, IMMUTABLE_CONFIG, protected_action_enum, server.enable_module_cmd, PROTECTED_ACTION_ALLOWED_NO, NULL, NULL),
-    createEnumConfig("cluster-preferred-endpoint-type", NULL, MODIFIABLE_CONFIG, cluster_preferred_endpoint_type_enum, server.cluster_preferred_endpoint_type, CLUSTER_ENDPOINT_TYPE_IP, NULL, NULL),
+    createEnumConfig("cluster-preferred-endpoint-type", NULL, MODIFIABLE_CONFIG, cluster_preferred_endpoint_type_enum, server.cluster_preferred_endpoint_type, CLUSTER_ENDPOINT_TYPE_IP, NULL, invalidateClusterSlotsResp),
     createEnumConfig("propagation-error-behavior", NULL, MODIFIABLE_CONFIG, propagation_error_behavior_enum, server.propagation_error_behavior, PROPAGATION_ERR_BEHAVIOR_IGNORE, NULL, NULL),
     createEnumConfig("shutdown-on-sigint", NULL, MODIFIABLE_CONFIG | MULTI_ARG_CONFIG, shutdown_on_sig_enum, server.shutdown_on_sigint, 0, isValidShutdownOnSigFlags, NULL),
     createEnumConfig("shutdown-on-sigterm", NULL, MODIFIABLE_CONFIG | MULTI_ARG_CONFIG, shutdown_on_sig_enum, server.shutdown_on_sigterm, 0, isValidShutdownOnSigFlags, NULL),

src/connection.h

@@ -62,6 +62,12 @@ typedef enum {
 #define CONN_TYPE_TLS               "tls"
 #define CONN_TYPE_MAX               8           /* 8 is enough to be extendable */
 
+typedef enum connTypeForCaching {
+    CACHE_CONN_TCP,
+    CACHE_CONN_TLS,
+    CACHE_CONN_TYPE_MAX
+} connTypeForCaching;
+
 typedef void (*ConnectionCallbackFunc)(struct connection *conn);
 
 typedef struct ConnectionType {

src/networking.c

@@ -319,6 +319,44 @@ int prepareClientToWrite(client *c) {
     return C_OK;
 }
 
+/* Returns everything in the client reply linked list in a SDS format.
+ * This should only be used only with a caching client. */
+sds aggregateClientOutputBuffer(client *c) {
+    sds cmd_response = sdsempty();
+    listIter li;
+    listNode *ln;
+    clientReplyBlock *val_block;
+    listRewind(c->reply,&li);
+
+    /* Here, c->buf is not used, thus we confirm c->bufpos remains 0. */
+    serverAssert(c->bufpos == 0);
+    while ((ln = listNext(&li)) != NULL) {
+        val_block = (clientReplyBlock *)listNodeValue(ln);
+        cmd_response = sdscatlen(cmd_response, val_block->buf,val_block->used);
+    }
+    return cmd_response;
+}
+
+/* This function creates and returns a fake client for recording the command response 
+ * to initiate caching of any command response.
+ *
+ * It needs be paired with `deleteCachedResponseClient` function to stop caching. */
+client *createCachedResponseClient(void) {
+    struct client *recording_client = createClient(NULL);
+    /* Allocating the `conn` allows to prepare the caching client before adding
+     * data to the clients output buffer by `prepareClientToWrite`. */
+    recording_client->conn  = zcalloc(sizeof(connection));
+    return recording_client;
+}
+
+/* This function is used to stop caching of any command response after `createCachedResponseClient` is called.
+ * It returns the command response as SDS from the recording_client's reply buffer. */
+void deleteCachedResponseClient(client *recording_client) {
+    zfree(recording_client->conn);
+    recording_client->conn = NULL;
+    freeClient(recording_client);
+}
+
 /* -----------------------------------------------------------------------------
  * Low level functions to add more data to output buffers.
  * -------------------------------------------------------------------------- */

src/server.h

@@ -2027,6 +2027,7 @@ struct valkeyServer {
     unsigned long long cluster_link_msg_queue_limit_bytes;  /* Memory usage limit on individual link msg queue */
     int cluster_drop_packet_filter; /* Debug config that allows tactically
                                    * dropping packets of a specific type */
+    sds cached_cluster_slot_info[CACHE_CONN_TYPE_MAX];
     /* Scripting */
     mstime_t busy_reply_threshold;  /* Script / module timeout in milliseconds */
     int pre_command_oom_state;         /* OOM before command (script?) was started */
@@ -2680,6 +2681,8 @@ void initThreadedIO(void);
 client *lookupClientByID(uint64_t id);
 int authRequired(client *c);
 void putClientInPendingWriteQueue(client *c);
+client *createCachedResponseClient(void);
+void deleteCachedResponseClient(client *recording_client);
 
 /* logreqres.c - logging of requests and responses */
 void reqresReset(client *c, int free_buf);

tests/cluster/tests/04-resharding.tcl

@@ -125,7 +125,7 @@ test "Cluster consistency during live resharding" {
     } else {
         fail "Resharding is not terminating after some time."
     }
-
+    wait_for_cluster_propagation
 }
 
 test "Verify $numkeys keys for consistency with logical content" {

tests/unit/cluster/announced-endpoints.tcl

@@ -9,12 +9,18 @@ start_cluster 2 2 {tags {external:skip cluster}} {
         set count [expr [llength $::servers] + 1]
         set used_port [find_available_port $baseport $count]
 
+        # We execute CLUSTER SLOTS command to trigger the `debugServerAssertWithInfo` in `clusterCommandSlots` function, ensuring
+        # that the cached response is invalidated upon updating any of cluster-announce-tls-port or cluster-announce-port.
+        R 0 CLUSTER SLOTS
+        R 1 CLUSTER SLOTS
+
         R 0 config set cluster-announce-tls-port $used_port
         R 0 config set cluster-announce-port $used_port
 
         assert_match "*:$used_port@*" [R 0 CLUSTER NODES]
+        assert_match "*$used_port*" [R 0 CLUSTER SLOTS]
         wait_for_condition 50 100 {
-            [string match "*:$used_port@*" [R 1 CLUSTER NODES]]
+            ([string match "*:$used_port@*" [R 1 CLUSTER NODES]] && [string match "*$used_port*" [R 1 CLUSTER SLOTS]])
         } else {
             fail "Cluster announced port was not propagated via gossip"
         }