Drupal 7 Module Development

Node Access
The {node_access} table contains six columns, each with a specific role in the API.
• nid is an integer and is the foreign key to the {node} table, used as the JOIN
field for SELECT statements.
• gid is an integer and represents the grant id declared by a node access
module. All gids are paired with an appropriate realm, as defined by
the module.
• realm is a string (maximum 255 characters) that indicates the name of the
access rule assigned by a module. Modules may define multiple realms, and
each realm may have multiple gids. Taken together, a realm/gid pair creates
a specific access grant for the given node id.
• grant_view is an integer that provides a Boolean value indicating that
the grant in this row allows the node to be viewed by users with the
proper permission.
• grant_update is an integer that provides a Boolean value indicating that
the grant in this row allows the node to be edited by users with the
proper permission.
• grant_delete is an integer that provides a Boolean value indicating that
the grant in this row allows the node to be deleted by users with the
proper permission.
Taken together, each row of the {node_access} table defines a rule set for a node. A
node may have more than one rule set. Note that the {node_access} table itself does
not care about the publication status of the node. When defining your modules's
realms, you should take into account how to handle access to unpublished content.
Because they can write multiple records to the {node_access}
table and add JOINs to most node queries, node access modules
can be resource intensive. In order to keep performance high,
sites that plan to use node access modules should generally allow
for 5-10% additional processing power for the database server.
Defining your module's access rules
Now that we understand how Drupal limits access to nodes, we are ready to explore
the API for module developers. Before we write any code, however, we should
write out the access rules we wish to enforce and review them for accuracy and
potential trouble.
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