declare init api

examples/routing-plugin-c/plugin.c

@@ -11,6 +11,8 @@ void pgdog_init() {
 
 Route pgdog_route_query(Query query) {
     Route route;
+    route.shard = ANY; /* No sharding */
+
     char *lowercase = strdup(query.query);
 
     for (int i = 0; i < strlen(lowercase); i++) {

pgdog-plugin/include/plugin.h

@@ -7,9 +7,29 @@
  * implements it, the query router will use its decision
  * to route the query.
  *
+ * ## Thread safety
+ *
+ * This function is not synchronized and can be called
+ * for multiple queries at a time. If accessing global state,
+ * make sure to protect access with a mutex.
+ *
+ * ## Performance
+ *
+ * This function is called for every transaction. It's a hot path,
+ * so make sure to optimize for performance in the implementation.
+ *
 */
 Route pgdog_route_query(Query query);
 
+/*
+ * Perform initialization at plugin loading time.
+ *
+ * Executed only once and execution is synchronized,
+ * so it's safe to initialize sychroniziation primitives
+ * like mutexes in this method.
+ */
+void pgdog_init();
+
 /* Create new row.
 *
 * Implemented by pgdog_plugin library.