Added some explanations on randomization

base/mysqlxx/PoolWithFailover.cpp

@@ -10,7 +10,7 @@ static bool startsWith(const std::string & s, const char * prefix)
     return s.size() >= strlen(prefix) && 0 == memcmp(s.data(), prefix, strlen(prefix));
 }
 
-/// This is thread-safe
+/// This reads from "/dev/urandom" and thus is thread-safe
 std::random_device rd;
 
 using namespace mysqlxx;
@@ -39,6 +39,11 @@ PoolWithFailover::PoolWithFailover(const Poco::Util::AbstractConfiguration & con
             }
         }
 
+        /// PoolWithFailover objects are stored in a cache inside PoolFactory.
+        /// This cache is reset by ExternalDictionariesLoader after every SYSTEM RELOAD DICTIONAR{Y|IES}
+        /// which triggers massive re-constructing of connection pools.
+        /// The state of PRNDGs like std::mt19937 is considered to be quite heavy
+        /// thus here we attempt to optimize its construction.
         static thread_local std::mt19937 rnd_generator(rd());
         for (auto & [_, replicas] : replicas_by_priority)
         {