replica redirect read&write to primary in standalone mode (#325)

To implement #319 

1. replica is able to redirect read and write commands to it's primary
in standalone mode
    * reply with "-REDIRECT primary-ip:port"
2. add a subcommand `CLIENT CAPA redirect`, a client can announce the
capability to handle redirection
    * if a client can handle redirection, the data access commands (read and
write) will be redirected
3. allow `readonly` and `readwrite` command in standalone mode, may be a
breaking change
    * a client with redirect capability cannot process read commands on a
replica by default
    * use READONLY command can allow read commands on a replica

---------

Signed-off-by: zhaozhao.zz <zhaozhao.zz@alibaba-inc.com>

src/cluster.c

@@ -1449,20 +1449,12 @@ void askingCommand(client *c) {
  * In this mode replica will not redirect clients as long as clients access
  * with read-only commands to keys that are served by the replica's primary. */
 void readonlyCommand(client *c) {
-    if (server.cluster_enabled == 0) {
-        addReplyError(c, "This instance has cluster support disabled");
-        return;
-    }
     c->flags |= CLIENT_READONLY;
     addReply(c, shared.ok);
 }
 
 /* The READWRITE command just clears the READONLY command state. */
 void readwriteCommand(client *c) {
-    if (server.cluster_enabled == 0) {
-        addReplyError(c, "This instance has cluster support disabled");
-        return;
-    }
     c->flags &= ~CLIENT_READONLY;
     addReply(c, shared.ok);
 }

src/commands.def

@@ -1089,6 +1089,28 @@ struct COMMAND_ARG CLIENT_CACHING_Args[] = {
 {MAKE_ARG("mode",ARG_TYPE_ONEOF,-1,NULL,NULL,NULL,CMD_ARG_NONE,2,NULL),.subargs=CLIENT_CACHING_mode_Subargs},
 };
 
+/********** CLIENT CAPA ********************/
+
+#ifndef SKIP_CMD_HISTORY_TABLE
+/* CLIENT CAPA history */
+#define CLIENT_CAPA_History NULL
+#endif
+
+#ifndef SKIP_CMD_TIPS_TABLE
+/* CLIENT CAPA tips */
+#define CLIENT_CAPA_Tips NULL
+#endif
+
+#ifndef SKIP_CMD_KEY_SPECS_TABLE
+/* CLIENT CAPA key specs */
+#define CLIENT_CAPA_Keyspecs NULL
+#endif
+
+/* CLIENT CAPA argument table */
+struct COMMAND_ARG CLIENT_CAPA_Args[] = {
+{MAKE_ARG("capability",ARG_TYPE_STRING,-1,NULL,NULL,NULL,CMD_ARG_MULTIPLE,0,NULL)},
+};
+
 /********** CLIENT GETNAME ********************/
 
 #ifndef SKIP_CMD_HISTORY_TABLE
@@ -1552,6 +1574,7 @@ struct COMMAND_ARG CLIENT_UNBLOCK_Args[] = {
 /* CLIENT command table */
 struct COMMAND_STRUCT CLIENT_Subcommands[] = {
 {MAKE_CMD("caching","Instructs the server whether to track the keys in the next request.","O(1)","6.0.0",CMD_DOC_NONE,NULL,NULL,"connection",COMMAND_GROUP_CONNECTION,CLIENT_CACHING_History,0,CLIENT_CACHING_Tips,0,clientCommand,3,CMD_NOSCRIPT|CMD_LOADING|CMD_STALE|CMD_SENTINEL,ACL_CATEGORY_CONNECTION,CLIENT_CACHING_Keyspecs,0,NULL,1),.args=CLIENT_CACHING_Args},
+{MAKE_CMD("capa","A client claims its capability.","O(1)","8.0.0",CMD_DOC_NONE,NULL,NULL,"connection",COMMAND_GROUP_CONNECTION,CLIENT_CAPA_History,0,CLIENT_CAPA_Tips,0,clientCommand,-3,CMD_NOSCRIPT|CMD_LOADING|CMD_STALE,ACL_CATEGORY_CONNECTION,CLIENT_CAPA_Keyspecs,0,NULL,1),.args=CLIENT_CAPA_Args},
 {MAKE_CMD("getname","Returns the name of the connection.","O(1)","2.6.9",CMD_DOC_NONE,NULL,NULL,"connection",COMMAND_GROUP_CONNECTION,CLIENT_GETNAME_History,0,CLIENT_GETNAME_Tips,0,clientCommand,2,CMD_NOSCRIPT|CMD_LOADING|CMD_STALE|CMD_SENTINEL,ACL_CATEGORY_CONNECTION,CLIENT_GETNAME_Keyspecs,0,NULL,0)},
 {MAKE_CMD("getredir","Returns the client ID to which the connection's tracking notifications are redirected.","O(1)","6.0.0",CMD_DOC_NONE,NULL,NULL,"connection",COMMAND_GROUP_CONNECTION,CLIENT_GETREDIR_History,0,CLIENT_GETREDIR_Tips,0,clientCommand,2,CMD_NOSCRIPT|CMD_LOADING|CMD_STALE|CMD_SENTINEL,ACL_CATEGORY_CONNECTION,CLIENT_GETREDIR_Keyspecs,0,NULL,0)},
 {MAKE_CMD("help","Returns helpful text about the different subcommands.","O(1)","5.0.0",CMD_DOC_NONE,NULL,NULL,"connection",COMMAND_GROUP_CONNECTION,CLIENT_HELP_History,0,CLIENT_HELP_Tips,0,clientCommand,2,CMD_LOADING|CMD_STALE|CMD_SENTINEL,ACL_CATEGORY_CONNECTION,CLIENT_HELP_Keyspecs,0,NULL,0)},

src/commands/client-capa.json

@@ -0,0 +1,29 @@
+{
+    "CAPA": {
+        "summary": "A client claims its capability.",
+        "complexity": "O(1)",
+        "group": "connection",
+        "since": "8.0.0",
+        "arity": -3,
+        "container": "CLIENT",
+        "function": "clientCommand",
+        "command_flags": [
+            "NOSCRIPT",
+            "LOADING",
+            "STALE"
+        ],
+        "acl_categories": [
+            "CONNECTION"
+        ],
+        "reply_schema": {
+            "const": "OK"
+        },
+        "arguments": [
+            {
+                "multiple": "true",
+                "name": "capability",
+                "type": "string"
+            }
+        ]
+    }
+}

src/networking.c

@@ -168,6 +168,7 @@ client *createClient(connection *conn) {
     c->bulklen = -1;
     c->sentlen = 0;
     c->flags = 0;
+    c->capa = 0;
     c->slot = -1;
     c->ctime = c->last_interaction = server.unixtime;
     c->duration = 0;
@@ -3589,6 +3590,13 @@ NULL
         } else {
             addReplyErrorObject(c, shared.syntaxerr);
         }
+    } else if (!strcasecmp(c->argv[1]->ptr, "capa") && c->argc >= 3) {
+        for (int i = 2; i < c->argc; i++) {
+            if (!strcasecmp(c->argv[i]->ptr, "redirect")) {
+                c->capa |= CLIENT_CAPA_REDIRECT;
+            }
+        }
+        addReply(c, shared.ok);
     } else {
         addReplySubcommandSyntaxError(c);
     }

src/server.c

@@ -3867,6 +3867,12 @@ int processCommand(client *c) {
         }
     }
 
+    if (!server.cluster_enabled && c->capa & CLIENT_CAPA_REDIRECT && server.primary_host && !mustObeyClient(c) &&
+        (is_write_command || (is_read_command && !(c->flags & CLIENT_READONLY)))) {
+        addReplyErrorSds(c, sdscatprintf(sdsempty(), "-REDIRECT %s:%d", server.primary_host, server.primary_port));
+        return C_OK;
+    }
+
     /* Disconnect some clients if total clients memory is too high. We do this
      * before key eviction, after the last command was executed and consumed
      * some client output buffer memory. */

src/server.h

@@ -429,6 +429,9 @@ extern int configOOMScoreAdjValuesDefaults[CONFIG_OOM_COUNT];
 #define CLIENT_REPLICATION_DONE (1ULL << 51)         /* Indicate that replication has been done on the client */
 #define CLIENT_AUTHENTICATED (1ULL << 52)            /* Indicate a client has successfully authenticated */
 
+/* Client capabilities */
+#define CLIENT_CAPA_REDIRECT (1 << 0) /* Indicate that the client can handle redirection */
+
 /* Client block type (btype field in client structure)
  * if CLIENT_BLOCKED flag is set. */
 typedef enum blocking_type {
@@ -1205,6 +1208,7 @@ typedef struct client {
     uint64_t flags; /* Client flags: CLIENT_* macros. */
     connection *conn;
     int resp;                            /* RESP protocol version. Can be 2 or 3. */
+    uint32_t capa;                       /* Client capabilities: CLIENT_CAPA* macros. */
     serverDb *db;                        /* Pointer to currently SELECTed DB. */
     robj *name;                          /* As set by CLIENT SETNAME. */
     robj *lib_name;                      /* The client library name as set by CLIENT SETINFO. */

tests/integration/replica-redirect.tcl

@@ -0,0 +1,36 @@
+start_server {tags {needs:repl external:skip}} {
+    start_server {} {
+        set primary_host [srv -1 host]
+        set primary_port [srv -1 port]
+
+        r replicaof $primary_host $primary_port
+        wait_for_condition 50 100 {
+            [s 0 master_link_status] eq {up}
+        } else {
+            fail "Replicas not replicating from primary"
+        }
+
+        test {replica allow read command by default} {
+            r get foo
+        } {}
+
+        test {replica reply READONLY error for write command by default} {
+            assert_error {READONLY*} {r set foo bar}
+        }
+
+        test {replica redirect read and write command after CLIENT CAPA REDIRECT} {
+            r client capa redirect
+            assert_error "REDIRECT $primary_host:$primary_port" {r set foo bar}
+            assert_error "REDIRECT $primary_host:$primary_port" {r get foo}
+        }
+
+        test {non-data access commands are not redirected} {
+            r ping
+        } {PONG}
+
+        test {replica allow read command in READONLY mode} {
+            r readonly
+            r get foo
+        } {}
+    }
+}