Support read only replicas attaching to active replicas (Bug Snapchat#229)

Author: JohnSully

src/networking.cpp

@@ -2260,7 +2260,7 @@ int processMultibulkBuffer(client *c) {
  * 1. The client is reset unless there are reasons to avoid doing it.
  * 2. In the case of master clients, the replication offset is updated.
  * 3. Propagate commands we got from our master to replicas down the line. */
-void commandProcessed(client *c) {
+void commandProcessed(client *c, int flags) {
     long long prev_offset = c->reploff;
     if (c->flags & CLIENT_MASTER && !(c->flags & CLIENT_MULTI)) {
         /* Update the applied replication offset of our master. */
@@ -2288,7 +2288,7 @@ void commandProcessed(client *c) {
             ae.arm(c);
         long long applied = c->reploff - prev_offset;
         if (applied) {
-            if (!g_pserver->fActiveReplica)
+            if (!g_pserver->fActiveReplica && (flags & CMD_CALL_PROPAGATE))
             {
                 replicationFeedSlavesFromMasterStream(g_pserver->slaves,
                     c->pending_querybuf, applied);
@@ -2312,7 +2312,7 @@ int processCommandAndResetClient(client *c, int flags) {
     serverAssert(GlobalLocksAcquired());
 
     if (processCommand(c, flags) == C_OK) {
-        commandProcessed(c);
+        commandProcessed(c, flags);
     }
     if (serverTL->current_client == NULL) deadclient = 1;
     serverTL->current_client = NULL;

src/replication.cpp

@@ -1166,6 +1166,7 @@ void processReplconfUuid(client *c, robj *arg)
  * full resync. */
 void replconfCommand(client *c) {
     int j;
+    bool fCapaCommand = false;
 
     if ((c->argc % 2) == 0) {
         /* Number of arguments must be odd to make sure that every
@@ -1176,6 +1177,7 @@ void replconfCommand(client *c) {
 
     /* Process every option-value pair. */
     for (j = 1; j < c->argc; j+=2) {
+        fCapaCommand = false;
         if (!strcasecmp((const char*)ptrFromObj(c->argv[j]),"listening-port")) {
             long port;
 
@@ -1200,6 +1202,8 @@ void replconfCommand(client *c) {
                 c->slave_capa |= SLAVE_CAPA_PSYNC2;
             else if (!strcasecmp((const char*)ptrFromObj(c->argv[j+1]), "activeExpire"))
                 c->slave_capa |= SLAVE_CAPA_ACTIVE_EXPIRE;
+
+            fCapaCommand = true;
         } else if (!strcasecmp((const char*)ptrFromObj(c->argv[j]),"ack")) {
             /* REPLCONF ACK is used by replica to inform the master the amount
              * of replication stream that it processed so far. It is an
@@ -1242,7 +1246,16 @@ void replconfCommand(client *c) {
             return;
         }
     }
-    addReply(c,shared.ok);
+
+    if (fCapaCommand) {
+        sds reply = sdsnew("+OK");
+        if (g_pserver->fActiveReplica)
+            reply = sdscat(reply, " active-replica");
+        reply = sdscat(reply, "\r\n");
+        addReplySds(c, reply);
+    } else {
+        addReply(c,shared.ok);
+    }
 }
 
 /* This function puts a replica in the online state, and should be called just
@@ -2557,6 +2570,30 @@ int slaveTryPartialResynchronization(redisMaster *mi, connection *conn, int read
     return PSYNC_NOT_SUPPORTED;
 }
 
+void parseMasterCapa(redisMaster *mi, sds strcapa)
+{
+    if (sdslen(strcapa) < 1 || strcapa[0] != '+')
+        return;
+
+    char *szStart = strcapa + 1;    // skip the +
+    char *pchEnd = szStart;
+
+    mi->isActive = false;
+    for (;;)
+    {
+        if (*pchEnd == ' ' || *pchEnd == '\0') {
+            // Parse the word
+            if (strncmp(szStart, "active-replica", pchEnd - szStart) == 0) {
+                mi->isActive = true;
+            }
+            szStart = pchEnd + 1;
+        }
+        if (*pchEnd == '\0')
+            break;
+        ++pchEnd;
+    }
+}
+
 /* This handler fires when the non blocking connect was able to
  * establish a connection with the master. */
 void syncWithMaster(connection *conn) {
@@ -2750,16 +2787,8 @@ void syncWithMaster(connection *conn) {
      *
      * The master will ignore capabilities it does not understand. */
     if (mi->repl_state == REPL_STATE_SEND_CAPA) {
-        if (g_pserver->fActiveReplica)
-        {
-            err = sendSynchronousCommand(mi, SYNC_CMD_WRITE,conn,"REPLCONF",
-                    "capa","eof","capa","psync2","capa","activeExpire",NULL);
-        }
-        else
-        {
-            err = sendSynchronousCommand(mi, SYNC_CMD_WRITE,conn,"REPLCONF",
-                    "capa","eof","capa","psync2",NULL);
-        }
+        err = sendSynchronousCommand(mi, SYNC_CMD_WRITE,conn,"REPLCONF",
+                "capa","eof","capa","psync2","capa","activeExpire",NULL);
         if (err) goto write_error;
         sdsfree(err);
         mi->repl_state = REPL_STATE_RECEIVE_CAPA;
@@ -2774,6 +2803,8 @@ void syncWithMaster(connection *conn) {
         if (err[0] == '-') {
             serverLog(LL_NOTICE,"(Non critical) Master does not understand "
                                   "REPLCONF capa: %s", err);
+        } else {
+            parseMasterCapa(mi, err);
         }
         sdsfree(err);
         mi->repl_state = REPL_STATE_SEND_PSYNC;

src/server.cpp

@@ -1735,14 +1735,26 @@ void clientsCron(int iel) {
     freeClientsInAsyncFreeQueue(iel);
 }
 
+bool expireOwnKeys()
+{
+    if (iAmMaster()) {
+        return true;
+    } else if (!g_pserver->fActiveReplica && (listLength(g_pserver->masters) == 1)) {
+        redisMaster *mi = (redisMaster*)listNodeValue(listFirst(g_pserver->masters));
+        if (mi->isActive)
+            return true;
+    }
+    return false;
+}
+
 /* This function handles 'background' operations we are required to do
  * incrementally in Redis databases, such as active key expiring, resizing,
  * rehashing. */
 void databasesCron(void) {
     /* Expire keys by random sampling. Not required for slaves
      * as master will synthesize DELs for us. */
     if (g_pserver->active_expire_enabled) {
-        if (iAmMaster()) {
+        if (expireOwnKeys()) {
             activeExpireCycle(ACTIVE_EXPIRE_CYCLE_SLOW);
         } else {
             expireSlaveKeys();
@@ -2461,6 +2473,7 @@ void initMasterInfo(redisMaster *master)
     master->cached_master = NULL;
     master->master_initial_offset = -1;
 
+    master->isActive = false;
 
     master->repl_state = REPL_STATE_NONE;
     master->repl_down_since = 0; /* Never connected, repl is down since EVER. */
@@ -3551,7 +3564,7 @@ void call(client *c, int flags) {
             !(flags & CMD_CALL_PROPAGATE_AOF))
                 propagate_flags &= ~PROPAGATE_AOF;
 
-        if (c->cmd->flags & CMD_SKIP_PROPOGATE)
+        if ((c->cmd->flags & CMD_SKIP_PROPOGATE) && g_pserver->fActiveReplica)
             propagate_flags &= ~PROPAGATE_REPL;
 
         /* Call propagate() only if at least one of AOF / replication

src/server.h

@@ -1371,6 +1371,7 @@ struct redisMaster {
     char master_replid[CONFIG_RUN_ID_SIZE+1];  /* Master PSYNC runid. */
     long long master_initial_offset;           /* Master PSYNC offset. */
 
+    bool isActive = false;
     int repl_state;          /* Replication status if the instance is a replica */
     off_t repl_transfer_size; /* Size of RDB to read from master during sync. */
     off_t repl_transfer_read; /* Amount of RDB read from master during sync. */