Code Overview

This page gives a brief overview of the code structure that implements Congress.

1. External information

The main source of information is the Congress wiki. There are two separate codebases that implement Congress: the server and the python client bindings.

The structure of the client code is the same as that for other recent OpenStack python clients. The bulk of the Congress code is contained within the server. The remainder of this page describes the layout of the server code.

2. Server directory structure

Here are the most important components of the code, described by how they are laid out in the repository.

  • congress/ instantiates message bus and installs datasource drivers and policy engine onto the bus
  • congress/datalog: implementation of Datalog policy language
  • congress/policy_engines: entities running on the message bus that understand policy languages
  • congress/datasources: datasource drivers: thin wrappers/adapters for integrating services like Nova, Neutron
  • congress/dse: message bus that the policy engine and datasources use to communicate
  • congress/api: API data models (entry points into the system from the API)
  • contrib: code for integrating into other services, e.g. devstack, horizon, tempest

3. Datalog

First is a description of the files and folders in congress/datalog. These files implement Datalog: the language Congress uses for describing policies.

  • congress/datalog/Congress.g: Antlr3 grammar defining the syntax of Datalog. make uses Congress.g to generate and, which contain the code used to convert strings into Python datastructures.
  • congress/datalog/
    • Convert policy strings into Python datastructures that represent those strings.
    • Includes datastructures for individual policy statements.
    • Also includes additional syntax checks that are not handled by the grammar.
  • congress/datalog/ unification routines used at the heart of the policy reasoning algorithms.

Second is a brief overview of the fundamental datastructures used to represent individual policy statements.

  • congress/datalog/ represents a single rule of the form head1, ..., headn :- body1, ..., bodym. Each headi and bodyi are Literals.
  • congress/datalog/ represents a possibly negated atom of the form [not] table(arg1, ..., argn). Each argi is a term.
  • congress/datalog/ represents an argument to a Literal. Is either a Variable or an ObjectConstant.
  • congress/datalog/ special kind of Term that represents a fixed string or number.
  • congress/datalog/ special kind of Term that is a placeholder used in a rule to represent an ObjectConstant.

Third is an overview of the datastructures used to represent entire policies. There are several different kinds of policies that you can choose from when creating a new policy. Each makes different tradeoffs in terms of time/space or in terms of the kind of policy statements that are permitted. Internally these are called ‘theories’.

  • congress/datalog/ represents an arbitrary collection of rules (without recursion). No precomputation of table contents is performed. Small memory footprint, but query time can be large. (A Prolog implementation of rules.) This is the default datastructure used when creating a new policy.
  • congress/datalog/ represents a collection of rules, with indexing for faster query evaluation. Used by NonrecursiveRuleTheory.
  • congress/datalog/ represents a collection of non-negated Literals without variables, e.g. p(1, "alice"). Designed for minimal memory overhead.
  • congress/datalog/ represents an arbitrary collection of rules (even allows recursion). Contents of all tables are computed and stored each time policy changes. Large memory footprint, but query time is small when asking for the contents of any table. Not actively maintained.
  • congress/datalog/ represents a collection of non-negated Literals without variables, e.g. p(1, "alice". Similar to a FactSet but with additional overhead. Used by the MaterializedViewTheory internally. Not actively maintained.

4. Policy engines

The congress/policy_engines directory contains implementations and wrappers for policy engines. At the time of writing, there are 2 policy engines in this directory: the domain-agnostic policy engine ( and the skeleton of a policy engine specialized for VM-placement ( We detail only the domain-agnostic policy engine.

4.1 Domain-agnostic policy engine

Source code found in congress/policy_engines/

  • class Runtime is the top-level class for the policy engine. It implements the creation/deletion of (different kinds of) policies, the insertion/deletion of policy statements, and all the other functionality built on top of the Datalog implementation.
  • class DseRuntime inherits from Runtime to make it run on the DSE message bus. It handles publishing/subscribing to the tables exported by the datasources.

Below we give a list of the top-level entry points to the domain-agnostic Runtime class—the top-level class for the domain agnostic policy engine.

  • create_policy, delete_policy: implement multiple policies
  • select: ask for the answer to a standard database query (e.g. the contents of a table) for a specified policy
  • insert, delete: insert or delete a single policy statement into a specified policy
  • update: batch of inserts/deletes into multiple policies
  • simulate: apply a sequence of updates (temporarily), answer a query, and roll-back the updates.
  • TriggerRegistry: central datastructure for triggers (the mechanism used to implement manual-reactive-enforcement rules). See initialize_tables and _update_obj to see how and when triggers are executed.