diff --git a/Documentation/Routers/Binlogrouter.md b/Documentation/Routers/Binlogrouter.md
index f47a740c2..a329caddb 100644
--- a/Documentation/Routers/Binlogrouter.md
+++ b/Documentation/Routers/Binlogrouter.md
@@ -427,12 +427,17 @@ encryption_key_file=/var/binlogs/enc_key.txt
 From MaxScale 2.3 onwards it is possible to specify secondary masters that
 the binlog router can use in case the connection to the default master fails.
 
-**Note:** This is _only_ supported in conjunction with a Galera cluster and
-provided the following holds:
-* `@@log_slave_updates` is enabled on all servers,
-* all nodes in the Galera cluster have the *same* `server_id`, and
-* all nodes in the Galera cluster use the *same* basename for the
-  binlog files (specified in the server config file with `log_bin=basename`).
+**Note:** This is _only_ supported in a Galera Cluster environment in which:
+
+   * _Wsrep GTID mode_ is enabled in the cluster.
+   * _All_ of the requirements for wsrep GTID mode are met by the cluster.
+
+_Wsrep GTID mode_ is also imperfect, so this secondary master functionality is
+only guaranteed to work if GTIDs have not become inconsistent within the cluster.
+
+See
+[Wsrep GTID Mode](https://mariadb.com/kb/en/library/using-mariadb-gtids-with-mariadb-galera-cluster/#wsrep-gtid-mode)
+for more information.
 
 The initial setup is performed exactly like when there is but one default master.
 ```
diff --git a/maxscale-system-test/masking/masking_mysqltest/r/masking_mxs1733.result b/maxscale-system-test/masking/masking_mysqltest/r/masking_mxs1733.result
index b4f053eaa..3cf294fcf 100644
--- a/maxscale-system-test/masking/masking_mysqltest/r/masking_mxs1733.result
+++ b/maxscale-system-test/masking/masking_mysqltest/r/masking_mxs1733.result
@@ -6,6 +6,3 @@ insert into masking_mxs1733 values ("john@example.com");
 select * from masking_mxs1733;
 email
 ****************
-select * from masking_mxs1733 UNION select * from masking_mxs1733;
-email
-****************
diff --git a/maxscale-system-test/masking/masking_mysqltest/t/masking_mxs1733.test b/maxscale-system-test/masking/masking_mysqltest/t/masking_mxs1733.test
index acb4b6d97..4b66aab57 100644
--- a/maxscale-system-test/masking/masking_mysqltest/t/masking_mxs1733.test
+++ b/maxscale-system-test/masking/masking_mysqltest/t/masking_mxs1733.test
@@ -14,4 +14,3 @@ create table masking_mxs1733 (email TEXT);
 insert into masking_mxs1733 values ("john@example.com");
 
 select * from masking_mxs1733;
-select * from masking_mxs1733 UNION select * from masking_mxs1733;
diff --git a/maxutils/maxbase/include/maxbase/atomic.hh b/maxutils/maxbase/include/maxbase/atomic.hh
index b72036e3a..e523fa4c4 100644
--- a/maxutils/maxbase/include/maxbase/atomic.hh
+++ b/maxutils/maxbase/include/maxbase/atomic.hh
@@ -70,5 +70,55 @@ void store(T* t, R v, int mode = SEQ_CST)
 {
     __atomic_store_n(t, v, mode);
 }
+
+/**
+ * Perform atomic compare and exchange operation
+ *
+ * @param ptr           Variable where the value is stored
+ * @param expected      Expected value of the variable
+ * @param desired       The desired new value of the variable
+ * @param success_model On success, this memory model is used
+ * @param fail_model    On failure, this memory model is used
+ *
+ * @return True if value was exchanged, false if exchange failed
+ */
+template<class T>
+bool compare_exchange(T* ptr, T* expected, T desired, int success_model = ACQ_REL, int fail_model = ACQUIRE)
+{
+    return __atomic_compare_exchange_n(ptr, expected, desired, true, success_model, fail_model);
+}
+
+/**
+ * Add to a value if it doesn't exceed a limit
+ *
+ * If the value of `ptr` + `value` is less than or equal to `limit`, the value is atomically added.
+ *
+ * @param ptr   Pointer to value to add to
+ * @param value Value to add
+ * @param limit Upper limit that is not exceeded
+ *
+ * @return True if value was modified, false if the addition failed.
+ */
+template<class T>
+bool add_limited(T* ptr, T value, T limit)
+{
+    T expected;
+    T next_value;
+
+    do
+    {
+        expected = mxb::atomic::load(ptr, mxb::atomic::ACQUIRE);
+
+        if (limit < expected + value)
+        {
+            return false;
+        }
+
+        next_value = expected + value;
+    }
+    while (!mxb::atomic::compare_exchange(ptr, &expected, next_value));
+
+    return true;
+}
 }
 }
diff --git a/server/core/dcb.cc b/server/core/dcb.cc
index 38b0b7ae0..f3c4a8533 100644
--- a/server/core/dcb.cc
+++ b/server/core/dcb.cc
@@ -1188,12 +1188,13 @@ void dcb_final_close(DCB* dcb)
             dcb_stop_polling_and_shutdown(dcb);
         }
 
-        if (dcb->server)
+        if (dcb->server && dcb->persistentstart == 0)
         {
             // This is now a DCB::Role::BACKEND_HANDLER.
             // TODO: Make decisions according to the role and assert
             // TODO: that what the role implies is preset.
-            mxb::atomic::add(&dcb->server->stats.n_current, -1, mxb::atomic::RELAXED);
+            MXB_AT_DEBUG(int rc = ) mxb::atomic::add(&dcb->server->stats.n_current, -1, mxb::atomic::RELAXED);
+            mxb_assert(rc > 0);
         }
 
         if (dcb->fd != DCBFD_CLOSED)
@@ -1249,9 +1250,13 @@ static bool dcb_maybe_add_persistent(DCB* dcb)
         && server->persistpoolmax()
         && (server->status & SERVER_RUNNING)
         && !dcb->dcb_errhandle_called
-        && dcb_persistent_clean_count(dcb, owner->id(), false) < server->persistpoolmax()
-        && mxb::atomic::load(&server->stats.n_persistent) < server->persistpoolmax())
+        && dcb_persistent_clean_count(dcb, owner->id(), false) < server->persistpoolmax())
     {
+        if (!mxb::atomic::add_limited(&server->stats.n_persistent, 1, (int)server->persistpoolmax()))
+        {
+            return false;
+        }
+
         DCB_CALLBACK* loopcallback;
         MXS_DEBUG("Adding DCB to persistent pool, user %s.", dcb->user);
         dcb->was_persistent = false;
@@ -1277,8 +1282,8 @@ static bool dcb_maybe_add_persistent(DCB* dcb)
 
         dcb->nextpersistent = server->persistent[owner->id()];
         server->persistent[owner->id()] = dcb;
-        mxb::atomic::add(&dcb->server->stats.n_persistent, 1);
-        mxb::atomic::add(&dcb->server->stats.n_current, -1, mxb::atomic::RELAXED);
+        MXB_AT_DEBUG(int rc = ) mxb::atomic::add(&server->stats.n_current, -1, mxb::atomic::RELAXED);
+        mxb_assert(rc > 0);
         return true;
     }
 
diff --git a/server/modules/routing/avrorouter/avro_file.cc b/server/modules/routing/avrorouter/avro_file.cc
index 946b70827..81354a2be 100644
--- a/server/modules/routing/avrorouter/avro_file.cc
+++ b/server/modules/routing/avrorouter/avro_file.cc
@@ -376,7 +376,8 @@ bool notify_cb(DCB* dcb, void* data)
 
     if (dcb->service == service && dcb->role == DCB::Role::CLIENT)
     {
-        poll_fake_write_event(dcb);
+        auto session = (AvroSession*)dcb->session->router_session;
+        session->queue_client_callback();
     }
 
     return true;
diff --git a/server/modules/routing/avrorouter/avrorouter.hh b/server/modules/routing/avrorouter/avrorouter.hh
index 5feda0b4b..de3921bbe 100644
--- a/server/modules/routing/avrorouter/avrorouter.hh
+++ b/server/modules/routing/avrorouter/avrorouter.hh
@@ -173,6 +173,8 @@ public:
      */
     int routeQuery(GWBUF* buffer);
 
+    void queue_client_callback();
+
 private:
     AvroSession(Avro* instance, MXS_SESSION* session);
 
@@ -186,7 +188,6 @@ private:
     bool stream_data();
     void rotate_avro_file(std::string fullname);
     void client_callback();
-    void queue_client_callback();
 };
 
 void read_table_info(uint8_t* ptr,