MXS-1310: Add deterministic conflict resolution

The schemarouter can now resolve database mapping conflicts in a
deterministic manner. This will fix the problem of central databases which
are replicated shards being assigned in a non-deterministic manner.

Documentation/Routers/SchemaRouter.md

@@ -51,6 +51,17 @@ List of databases to ignore when checking for duplicate databases.
 
 Regular expression that is matched against database names when checking for duplicate databases.
 
+### `preferred_server`
+
+The name of a server in MaxScale which will be used as the preferred server when
+a database is found on more than one server. If a database exists on two
+servers, of which neither is the server referred by this parameter, the server
+that replies first will be assigned as the location of the database.
+
+This parameter allows deterministic conflict resolution when a sharded cluster
+has a central database server and one or more sharded databases spread across
+multiple servers which replicate from the central database server.
+
 **Note:** As of version 2.1 of MaxScale, all of the router options can also be
 defined as parameters. The values defined in _router_options_ will have priority
 over the parameters.

server/modules/routing/schemarouter/schemarouter.c

@@ -357,6 +357,17 @@ showdb_response_t parse_showdb_response(ROUTER_CLIENT_SES* rses, backend_ref_t*
                               rses->rses_client_dcb->user,
                               rses->rses_client_dcb->remote);
                 }
+                else if (rses->router->preferred_server &&
+                         strcmp(target, rses->router->preferred_server->unique_name) == 0)
+                {
+                    /** In conflict situations, use the preferred server */
+                    MXS_INFO("Forcing location of '%s' from '%s' to ''%s",
+                             data, (char*)hashtable_fetch(rses->shardmap->hash,
+                                                          data), target);
+
+                    hashtable_delete(rses->shardmap->hash, data);
+                    hashtable_add(rses->shardmap->hash, data, target);
+                }
             }
             MXS_FREE(data);
         }
@@ -651,6 +662,7 @@ MXS_MODULE* MXS_CREATE_MODULE()
             {"refresh_databases", MXS_MODULE_PARAM_BOOL, "true"},
             {"refresh_interval", MXS_MODULE_PARAM_COUNT, DEFAULT_REFRESH_INTERVAL},
             {"debug", MXS_MODULE_PARAM_BOOL, "false"},
+            {"preferred_server", MXS_MODULE_PARAM_SERVER},
             {MXS_END_MODULE_PARAMS}
         }
     };
@@ -719,6 +731,7 @@ static MXS_ROUTER* createInstance(SERVICE *service, char **options)
     router->schemarouter_config.max_sescmd_hist = config_get_integer(conf, "max_sescmd_history");
     router->schemarouter_config.disable_sescmd_hist = config_get_bool(conf, "disable_sescmd_history");
     router->schemarouter_config.debug = config_get_bool(conf, "debug");
+    router->preferred_server = config_get_server(conf, "preferred_server");
 
     if ((config_get_param(conf, "auth_all_servers")) == NULL)
     {

server/modules/routing/schemarouter/schemarouter.h

@@ -365,14 +365,15 @@ typedef struct router_instance
     unsigned int            bitvalue;    /*< Required value of server->status   */
     ROUTER_STATS            stats;       /*< Statistics for this router         */
     struct router_instance* next;        /*< Next router on the list            */
-    bool            available_slaves; /*< The router has some slaves available */
+    bool                    available_slaves; /*< The router has some slaves available */
     HASHTABLE*              ignored_dbs; /*< List of databases to ignore when the
                                           * database mapping finds multiple servers
                                           * with the same database */
-    pcre2_code*                   ignore_regex; /*< Databases matching this regex will
+    pcre2_code*             ignore_regex; /*< Databases matching this regex will
                                            * not cause the session to be terminated
                                            * if they are found on more than one server. */
-    pcre2_match_data*             ignore_match_data;
+    pcre2_match_data*       ignore_match_data;
+    SERVER*                 preferred_server; /**< Server to prefer in conflict situations */
 
 } ROUTER_INSTANCE;