Before OpenACS 3.3, the OpenACS startup process was extremely simple: after AOLserver performed its internal initialization (reading the configuration file, loading shared libraries and module code, etc.) it scanned through the Tcl library directory (generally /var/lib/aolserver/yourservername/tcl), sourcing each file in sequence.

While this overall structure for initialization is still intact, package management has thrown a wrench into the works - there are a few extra things to do during initialization, most notably:

This document examines in detail each of the steps involved in AOLserver/OpenACS startup.

As soon as the nsd daemon is executed by the init process (or otherwise), AOLserver reads its configuration file and chroots itself if necessary. It then loads shared libraries indicated in the .ini file (e.g., the Oracle driver and nssock), and sources Tcl module files (generally in /home/aol30/modules/tcl). This step is, and has always been, the same for all AOLservers, regardless of whether they are running OpenACS.

Next AOLserver sources, in lexicographical order, each file in the /tcl directory. The first such file is 0-acs-init.tcl, which doesn't do much directly except to determine the OpenACS path root (e.g., /var/lib/aolserver/yourservername) by trimming the final component from the path to the Tcl library directory (/var/lib/aolserver/yourservername/tcl). But 0-acs-init.tcl's has an important function, namely sourcing /packages/acs-core/bootstrap.tcl, which does the following:

At this point, bootstrap.tcl is done executing. AOLserver proceeds to source the remaining files in the /tcl directory (i.e., unpackaged libraries) and begins listening for connections.

The OpenACS 5.2.3rc1 Permissions system allows developers and administrators to set access control policies at the object level, that is, any application or system object represented by a row in the acs_objects table can be access-controlled via a PL/SQL or Tcl interface. The permissions system manages a data model that then allows scripts to check permissions using another API call.

Although object level permissions seems appropriate, no developer or administrator wants to explicitly set access control rights for every user and every object on a site. Therefore, OpenACS has two auxiliary mechanisms for making this easier:

The rest of this document discusses each of these parts, and how they fit together with the permissions system.

OpenACS 5.2.3rc1 has an abstraction called a party. Parties have a recursive definition. We can illustrate how it works with the following simplified data model. First, we define the parties table, where each party has an email address and a URL for contact information.

create table parties (
    party_id  integer not null references acs_objects(object_id),
    email varchar(100),
    url varchar(100)
)

Now we define two subtypes of party, one for persons, and one for groups:

create table groups (
    group_id  not null references parties(party_id),
    group_name varchar(100) not null
)

create table persons (
    person_id not null references parties(party_id),
    first_names varchar(100) not null,
    last_name varchar(100) not null
)

The users table is also defined in this data model as a subtype of person.

Finally, we define two relations, one for group membership and one for group composition.

The composition relation expresses that every member of group A should also be a member of group B. This relation allows us to define a hierarchy of groups.

The membership relation maps groups to parties. Each member of a group is a party rather than just a user. That is, groups consist of members that are either a person or an entire group. This allows us to say that group A should be a member of another group B.

The groups data model is recursive. Modelling parties as either a person or a group provides a way to model complex hierarchical groupings of persons and groups.

The full details of the groups data model is beyond the scope of this tutorial. See Parties in OpenACS or OpenACS 4 Groups Design for more details.

NOTE: Much more detailed information about the permissions system and how to use it is available in the OpenACS Permissions Tediously Explained document.

The permissions data model is a mapping between privileges, parties and objects. Parties and objects have already been discussed. Now we focus on privileges.

In OpenACS, a privilege describes the right to perform some operation on some object. Privileges are the basic units out of which we build access control policies. For example in the Unix filesystem, access is controlled by granting users some combination of read, write, or execute privileges on files and directories. In OpenACS 5.2.3rc1, the table of privileges is organized hierarchically so that developers can define privileges that aggregate some set of privileges together. For example, if we have read, write, create and delete privileges, it might be convenient to combine them into a new privilege called "admin". Then, when a user is granted "admin" privilege, she is automatically granted all the child privileges that the privilege contains. The OpenACS 5.2.3rc1 kernel data model defines these privileges:

#
begin
 acs_privilege.create_privilege('read');
 acs_privilege.create_privilege('write');
 acs_privilege.create_privilege('create');
 acs_privilege.create_privilege('delete');
 acs_privilege.create_privilege('admin');

 acs_privilege.add_child('admin', 'read');
 acs_privilege.add_child('admin', 'write');
 acs_privilege.add_child('admin', 'create');
 acs_privilege.add_child('admin', 'delete');

 commit;
end;

Note that a user does not gain admin privileges when granted read, write, create and delete privileges, because some operations explicitly require admin privileges. No substitutions.

To give a user permission to perform a particular operation on a particular object you call acs_permission.grant_permission like this:

# sql code
    acs_permission.grant_permission (
      object_id => some_object_id,
      grantee_id => some_party_id,
      privilege => 'some_privilege_name'
      );

Using just these mechanisms is enough for developers and administrators to effectively define access control for every object in a system.

Explicitly defining permissions to every object individually would become very tedious. OpenACS provides a object contexts as a means for controlling permissions of a large group of objects at the same time.

In OpenACS 5.2.3rc1, object context is a scoping mechanism. "Scoping" and "scope" are terms best explained by example: consider some hypothetical rows in the address_book table:

The first row represents an entry in User 123's personal address book, the second row represents an entry in User Group 456's shared address book, and the third row represents an entry in the site's public address book. In this way, the scoping columns identify the security context in which a given object belongs, where each context is either a person or a group of people or the general public (itself a group of people).

Every object lives in a single context. A context is just an another object that represents the security domain to which the object belongs. By convention, if an object A does not have any permissions explicitly attached to it, then the system will look at the context_id column in acs_objects and check the context object there for permissions. Two things control the scope of this search:

If security_inherit_p flag is set to 't', then the automatic search through the context happens, otherwise it does not. You might set this field to 'f' if you want to override the default permissions in a subtree of some context.

For an example of how to use context hierarchy, consider the forums application. With only row-level permissions it is not obvious how to reasonably initialize the access control list when creating a message. At best, we have to explicitly grant various read and write privileges whenever we create a message, which is tedious. A reasonable thing to do is to create an object representing a forum, and point the context_id field of a new message at the forum. Then, suppose we grant every user in the system read-access to this forum. By default, they will automatically have read-access to the new message we just inserted, since the system automatically checks permissions on the message's context. To allow the creator of the message to change the message after it has been posted we grant the user write-access on the message, and we are done.

This mechanism allows developers and administrators to define a hierarchy that matches the structure they need for access control in their application. The following picture shows a typical context hierarchy for a hypothetical site:

The top two contexts in the diagram are called "magic" numbers, because in some sense, they are created by default by OpenACS for a specific purpose. The object default_context represents the root of the context hierarchy for the entire site. All permission searches walk up the tree to this point and then stop. If you grant permissions on this object, then by default those permissions will hold for every object in the system, regardless of which subsite they happen to live in. The object security_context_root has a slightly different role. If some object has no permissions attached to it, and its value for security_inherit_p is 'f', or context_id is null, this context is used by default.

See the package developer tutorials for examples on how to use permissions code.

OpenACS 5.2.3rc1 defines three separate mechanisms for specifying access control in applications.

A PL/SQL or Tcl API is then used to check permissions in application pages.

One of OpenACS's great strengths is that code written for it is very close to the database. It is very easy to interact with the database from anywhere within OpenACS, and we have a coherent API for database access which makes this even easier.

More detailed information about the DB api is available at Database Access API.

The OpenACS database API is meant to save developers from making common mistakes and to provide a more structured syntax for specifying database operations, including transactions. Here's an example of the API.

set count 0
set tcl_var "foo"
set sql {
      	SELECT foo, bar, baz
       FROM some_table, some_other_table
       WHERE some_table.id = some_other_table.id
         and some_table.condition_p = :tcl_var
}

db_transaction {
    db_foreach my_example_query_name $sql {
        lappend rows [list $foo $bar $baz]
        incr count
    }
    foreach row $rows {
        call_some_proc $foo $bar $baz
    }
}

There are several things to note here:

Bind variables are placeholders for literal values in an SQL query being sent to the server. In the old way, data was generally passed to directly to the DB backend, via Tcl string interpolation. In the example above, the query would look like:

select foo, bar, baz
from some_table, some_other_table
where some_table.id=some_other_table.id
and some_table.condition_p = '$foo'

There are a few problems with this:

What the DB API (in conjuntion with the database drivers implemented for aolserver) do is send the SQL statement to the server for parsing, then bind values to the variables and sends those values along seperately as a second step. This seperate binding step is where the term bind variable comes from.

This split has several advantages. First, type checking happens on the literal. If the column we are comparing against holds numbers, and we send a string, we get a nice error. Second, since string literals are no longer in the query, no extra quoting is required. Third, substitution of bind variables cannot change the actual text of the query, only the literal values in the placeholders. The database API makes bind variables easy to use by hooking them smoothly into the Tcl runtime so you simply provide :tclvar and the value of $tclvar is sent to the backend to actually execute the query.

The database API parses the query and pulls out all the bind variable specifications and replaces them with generic placeholders. It then automatically pulls the values of the named Tcl vars out of the runtime environment of the script, and passes them to the database.

Note that while this looks like a simple syntactic change, it really is very different from how interpolated text queries work. You use bind variables to replace what would otherwise be a literal value in a query, and Tcl style string interpolation does not happen. So you cannot do something like:

set table "baz"
set condition "where foo = bar"

db_foreach my_query { select :table from some_table where :condition }
    

SQL will not allow a literal to occur where we've put the bind variables, so the query is syntactically incorrect. You have to remember that while the bind variable syntax looks similar to variable interpolation in Tcl, It is not the same thing at all.

Finally, the DB API has several different styles for passing bind variable values to queries. In general, use the style presented here because it is the most convenient.

set user_id 123456
set role "administrator"

db_foreach user_group_memberships_by_role {
    select g.group_id, g.group_name
    from user_groups g, user_group_map map
    where g.group_id = map.user_id
    and map.user_id = :user_id
    and map.role = :role
} {
    # do something for each group of which user 123456 is in the role
    # of "administrator"
}

      

set bind_vars [ns_set create]
ns_set put $bind_vars user_id 123456
ns_set put $bind_vars role "administrator"

db_foreach user_group_memberships_by_role {
    select g.group_id, g.group_name
    from user_groups g, user_group_map map
    where g.group_id = map.user_id
    and map.user_id = :user_id
    and map.role = :role
} -bind $bind_vars {
    # do something for each group in which user 123456 has the role
    # of "administrator"
}

      

	
db_foreach user_group_memberships_by_role {
    select g.group_id, g.group_name
    from user_groups g, user_group_map map
    where g.group_id = map.user_id
    and map.user_id = :user_id
    and map.role = :role
} -bind [list user_id 123456 role "administrator"] {
    # do something for each group in which user 123456 has the role
    # of "administrator"
}

      

#
# Given the table:
#
#   create table foo (
#           bar        integer,
#           baz        varchar(10)
#   );
#

set bar ""
set baz ""

db_dml foo_create "insert into foo(bar, baz) values(:bar, :baz)"
#
# the values of the "bar" and "baz" columns in the new row are both
# null, because Oracle has coerced the empty string (even for the
# numeric column "bar") into null in both cases

      

set bar [db_null]
set baz [db_null]

db_dml foo_create "insert into foo(bar, baz) values(:bar, :baz)"
#
# sets the values for both the "bar" and "baz" columns to null

The database library can transparently maintain pools of sequence values, so that each request for a new sequence value (using db_nextval) does not incur a roundtrip to the server. For instance, this functionality is very useful in the security/sessions library, which very frequently allocates values from the sec_id_seq sequence. To utilize this functionality for a particular sequence, register the sequence to be pooled, either using the db_register_pooled_sequence procedure at server startup time, or by including a configuration parameter of the form

PoolSequence.sequence_name_seq=count

in any configuration section in the yourservername.ini file, e.g.,

[ns/server/yourservername/acs/security]
PoolSequence.sec_id_seq=20

    

The database library will allocate this number of sequence values at server startup. It will periodically scan pools and allocate new values for sequences which are less than half-full. (This normally occurs every 60 seconds, and is configurable via the PooledSequenceUpdateInterval parameter in the [ns/server/yourservername/acs/database] configuration section.)

The Database API has several functions that wrap familiar parts of the AOLserver database API.

Note that you never have to use ns_db anymore (including ns_db gethandle)! Just start doing stuff, and (if you want) call db_release_unused_handles when you're done as a hint to release the database handle.

The OpenACS templating system allows you to give your site a consistent look and feel. It also promotes code maintainability in the presentation layer, by allowing presentation components to be reused across multiple pages. If you need to change the layout for some reason, then you only need to make that change in one location, instead of across many files.

In this problem set you will familiarise yourself with the templating system in openacs. This will be achieved through customising an existing edit-this-page application template.

Before proceeding, it is strongly advised to read the templating documentation on your openacs installation (http://localhost:8000/doc/acs-templating). The documentation lists the special tags available for ADP files.

This problem set was originally written by Nick Carroll in August 2004 for the University of Sydney Course EBUS5002.

This material is copyright 2004 by Nick Carroll. It may be copied, reused, and modified, provided credit is given to the original author.

This document is a brief introduction to the OpenACS 5.2.3rc1 Request Processor; more details can be found in the OpenACS 4 Request Processor Design. Here we cover the high level concepts behind the system, and implications and usage for the application developer.

The 5.2.3rc1 Request Processor is a global filter and set of Tcl procs that respond to every incoming URL reaching the server. The following diagram summarizes the stages of the request processor assuming a URL request like http://someserver.com/notes/somepage.adp.

Once the flow of control reaches a dynamic page, the Request Processor has populated the environment of the request with several pieces of useful information. The RP's environment is accessible through the ad_conn interface, and the following calls should be useful to you when developing dynamic pages: