Does a Joomla site write anything (session etc.) towards the Db upon browsing (no logins simply browsing)? I must make my DB read-just for maintenance reasons.

Yes, an out-of-this area Joomla installation stores session information within the DB. However, for those who have XCache or Memcached support placed on your server, you are able to alter the handler to make use of another thing. Within the after sales, visit Site > Global Configuration, then click on the System link. At the end from the right hands column, there is a setting where one can alter the session handler from 'Database' to another thing.

Yea , Joomla keeps periods for guest customers too. So each time a user visits the website , one record will be included to the consumer table. So it's better to accept site offline rather than making the database read only.