Allow reads during an active replica load, optionally allow writes as an experimental feature

Former-commit-id: 655bb6e21c5e814980a7b8b3ac1481ef142c1845

keydb.conf

@@ -417,6 +417,13 @@ dir ./
 #
 replica-serve-stale-data yes
 
+# Active Replicas will allow read only data access while loading remote RDBs
+# provided they are permitted to serve stale data.  As an option you may also
+# permit them to accept write commands.  This is an EXPERIMENTAL feature and
+# may result in commands not being fully synchronized
+#
+# allow-write-during-load no
+
 # You can modify the number of masters necessary to form a replica quorum when
 # multi-master is enabled and replica-serve-stale-data is "no".  By default 
 # this is set to -1 which implies the number of known masters (e.g. those

src/config.cpp

@@ -2236,6 +2236,7 @@ standardConfig configs[] = {
     createBoolConfig("appendonly", NULL, MODIFIABLE_CONFIG, g_pserver->aof_enabled, 0, NULL, updateAppendonly),
     createBoolConfig("cluster-allow-reads-when-down", NULL, MODIFIABLE_CONFIG, g_pserver->cluster_allow_reads_when_down, 0, NULL, NULL),
     createBoolConfig("multi-master-no-forward", NULL, MODIFIABLE_CONFIG, cserver.multimaster_no_forward, 0, validateMultiMasterNoForward, NULL),
+    createBoolConfig("allow-write-during-load", NULL, MODIFIABLE_CONFIG, g_pserver->fWriteDuringActiveLoad, 0, NULL, NULL),
 
     /* String Configs */
     createStringConfig("aclfile", NULL, IMMUTABLE_CONFIG, ALLOW_EMPTY_STRING, g_pserver->acl_filename, "", NULL, NULL),

src/networking.cpp

@@ -1264,9 +1264,10 @@ static void acceptCommonHandler(connection *conn, int flags, char *ip, int iel)
 void acceptOnThread(connection *conn, int flags, char *cip)
 {
     int ielCur = ielFromEventLoop(serverTL->el);
+    bool fBootLoad = (g_pserver->loading == LOADING_BOOT);
 
     int ielTarget = 0;
-    if (g_pserver->loading)
+    if (fBootLoad)
     {
         ielTarget = IDX_EVENT_LOOP_MAIN;    // During load only the main thread is active
     }
@@ -1290,10 +1291,10 @@ void acceptOnThread(connection *conn, int flags, char *cip)
             szT = (char*)zmalloc(NET_IP_STR_LEN, MALLOC_LOCAL);
             memcpy(szT, cip, NET_IP_STR_LEN);
         }
-        int res = aePostFunction(g_pserver->rgthreadvar[ielTarget].el, [conn, flags, ielTarget, szT] {
+        int res = aePostFunction(g_pserver->rgthreadvar[ielTarget].el, [conn, flags, ielTarget, szT, fBootLoad] {
             connMarshalThread(conn);
             acceptCommonHandler(conn,flags,szT,ielTarget);
-            if (!g_fTestMode && !g_pserver->loading)
+            if (!g_fTestMode && !fBootLoad)
                 rgacceptsInFlight[ielTarget].fetch_sub(1, std::memory_order_relaxed);
             zfree(szT);
         });
@@ -1302,7 +1303,7 @@ void acceptOnThread(connection *conn, int flags, char *cip)
             return;
         // If res != AE_OK we can still try to accept on the local thread
     }
-    if (!g_fTestMode && !g_pserver->loading)
+    if (!g_fTestMode && !fBootLoad)
         rgacceptsInFlight[ielTarget].fetch_sub(1, std::memory_order_relaxed);
 
     aeAcquireLock();

src/rdb.cpp

@@ -2059,7 +2059,7 @@ robj *rdbLoadObject(int rdbtype, rio *rdb, sds key, uint64_t mvcc_tstamp) {
  * needed to provide loading stats. */
 void startLoading(size_t size, int rdbflags) {
     /* Load the DB */
-    g_pserver->loading = 1;
+    g_pserver->loading = (rdbflags & RDBFLAGS_REPLICATION) ? LOADING_REPLICATION : LOADING_BOOT;
     g_pserver->loading_start_time = time(NULL);
     g_pserver->loading_loaded_bytes = 0;
     g_pserver->loading_total_bytes = size;

src/server.cpp

@@ -3849,9 +3849,13 @@ int processCommand(client *c, int callFlags) {
     /* Loading DB? Return an error if the command has not the
      * CMD_LOADING flag. */
     if (g_pserver->loading && !(c->cmd->flags & CMD_LOADING)) {
+        /* Active Replicas can execute read only commands, and optionally write commands */
+        if (!(g_pserver->loading == LOADING_REPLICATION && g_pserver->fActiveReplica && ((c->cmd->flags & CMD_READONLY) || g_pserver->fWriteDuringActiveLoad)))
+        {
             addReply(c, shared.loadingerr);
             return C_OK;
         }
+    }
 
     /* Lua script too slow? Only allow a limited number of commands.
      * Note that we need to allow the transactions commands, otherwise clients
@@ -4490,7 +4494,7 @@ sds genRedisInfoString(const char *section) {
             "aof_last_cow_size:%zu\r\n"
             "module_fork_in_progress:%d\r\n"
             "module_fork_last_cow_size:%zu\r\n",
-            g_pserver->loading.load(std::memory_order_relaxed),
+            !!g_pserver->loading.load(std::memory_order_relaxed),   /* Note: libraries expect 1 or 0 here so coerce our enum */
             g_pserver->dirty,
             g_pserver->rdb_child_pid != -1,
             (intmax_t)g_pserver->lastsave,

src/server.h

@@ -104,6 +104,9 @@ typedef long long ustime_t; /* microsecond time type. */
 #include "endianconv.h"
 #include "crc64.h"
 
+#define LOADING_BOOT 1
+#define LOADING_REPLICATION 2
+
 extern int g_fTestMode;
 
 struct redisObject;
@@ -2002,6 +2005,7 @@ struct redisServer {
     int watchdog_period;  /* Software watchdog period in ms. 0 = off */
 
     int fActiveReplica;                          /* Can this replica also be a master? */
+    int fWriteDuringActiveLoad;                  /* Can this active-replica write during an RDB load? */
 
     // Format:
     //  Lower 20 bits: a counter incrementing for each command executed in the same millisecond