Configure WordPress to read from database only, never write

How do you I go about setting WordPress to a read-only state?

I have a setup that scales slaves instances with read-only database access but I’m seeing a lot of log data where WordPress still tries to write to the database.

update_{post_type}_metadata, set_transient, wpdb… basically stopping anything attempting to write would be ideal. I can easily check to see if I’m on a MASTER instance vs. SLAVE instance so I could modify hooks pretty early.

Trying to short-circuit the update metadata function seems like possibly eliminating one vector, but the rest I fear involve modifying WP Core.

add_filter ( 'update_post_metadata', '__return_false', 99 );

$result = update_post_meta( -1, 'test-key', 'test-value' );

echo $result;

Any suggestions?

HyperDB looks promising as a more long-term solution.

WordPress doesn’t work well in read-only mode. It depends on the database for cache and other tasks.

However, there are a couple of options if you’d still like to proceed.

1. Set the database user permissions to ‘READ ONLY’
2. Use a function to intercept all db queries. We used this on our theme demo site to prevent db writes.

Function to use:

/**
 * Whitelist "SELECT" and "SHOW FULL COLUMNS" queries.
 */
function my_readonly_filter( $query ) {
  global $wpdb;

  if ( preg_match( '/^\s*select|show full columns\s+/i', $query ) ) {
    return $query;
  }

  // Return arbitrary query for everything else otherwise you get 'empty query' db errors.
  return "SELECT ID from $wpdb->posts LIMIT 1;";
}
add_filter( 'query', 'my_readonly_filter' );