= Floodlight Controller Internals =
This is a quick recap/overview of the internal workings of !OpenFlowHub's Floodlight !OpenFlow controller. There is a good amount of official documentation at openflowhub.org; the contents of this page are side-notes compiled based on some code reading done in 2012.

As a convention, classes and interfaces are shown in `terminal type`, and methods and variables in ''italics''. 

== Loading and Initialization == 
Docs: http://www.openflowhub.org/display/floodlightcontroller/Module+loading+system

Floodlight can be broken down into two main components: A module loader and all of the modules that implement Floodlight's services and functions. Initialization is therefore characterized by four main steps:
 1. Read in configurations
 1. Load modules
 1. Initialize core controller module/service 
 1. Register modules with controller service  

=== Loading ===
The startup process of Floodlight begins with the reading in of configuration files and loading of necessary functional components (modules). 

There are two main configuration files:
 1. net.floodlightcontroller.core.module.IFloodlightModule: the list of all available modules
 1. floodlightdefault.properties: a list of service-providing modules and their configurations
 
The files are read by the module loading system to find and properly load all of the modules. Module interdependencies are found with a DFS search for dependencies of each module beginning with the service-providing ones listed in the second file. A module is anything that implements the `IFloodlightModule` interface, and this interface provides a method, ''getModuleDependencies()'', which facilitates the building of this dependency tree. the methods in the module loader that are responsible for this task are ''loadModulesFromContig()'' and ''loadModulesFromList()''. The procedure produces the smallest collection of modules to be loaded.  

A list of modules that come with the base controller can be found [http://www.openflowhub.org/display/floodlightcontroller/Modules here].

Once the list is complete each module is activated by calling its ''init()'' and ''startUp()'' methods. ''init()'' typically initializes references to services that a module depends on, and other things that do not count on a module's dependencies being fully active; ''startUp()'' takes care of them after everyone's ''init()'' are called.     

=== Initialization and module registration ===
The first module to always be activated (by the virtue of being first in floodlightdefault.properties) is the `FloodlightProvider`, which provides the key service that implements Floodlight's controller functions (managing connections from switches, keepalives, event dispatch, etc). The actual implementation at the heart of the controller service is the class `Controller`, implementing the `IFloodlightProviderService` interface. When people mention `IFloodlightProvider`, they are referring to a collection of methods that are part of this interface, and when ''floodlightProvider'' is referenced it is usually an instantiation of `Controller` (or some other implementation of `IFloodlightProviderService`).    

The module loader calls each module's ''startUp()'' method to register them with `Controller`, typically either as an `IOFMessageListener` or an `IOFSwitchListener`. The identification depends on what kind of event a service is interested in receiving from `Controller`. Services interested in new messages are the former, and those interested in the joining/leaving of switches are the latter. Services may belong to both categories. In `Controller`, the two groups are organized into two lists, ''messageListeners'' and ''switchListeners''. Each method essentially "adds itself" to either or both of these lists when their ''startUp()'' method is called, using the add/remove methods for each type of listener provided by `IFloodlightProviderService` . 

For example, taking a look at the `Hub` module:

 {{{
    public void init(FloodlightModuleContext context)                              
            throws FloodlightModuleException {
        floodlightProvider =                                                         
                context.getServiceImpl(IFloodlightProviderService.class);
    }

    public void startUp(FloodlightModuleContext context) {
        floodlightProvider.addOFMessageListener(OFType.PACKET_IN, this);           
    }
 }}}   

Several things should be mentioned here: 
 * `FloodlightModuleContext` maintains a !HashMap called ''serviceMap'' that keeps a mapping between a service and its implementing class 
 * the mapping between the service and its class is in terms of the interface that the class implements
 * `Hub` initializes a reference to an implementation of `IFloodlightProviderService` called ''floodlightProvider'' in `init()`
 * `Hub` is interested in receiving !PacketIns from switches, so it registers itself as a IOFMessageListener in `startUp()`  

''startUp()'' is also where service-specific variables in floodlightdefault.properties will be used to do per-module configurations. A module accesses the contents of the file through `FloodlightModuleContext` via the method ''getConfigParams()''.
[[BR]]  

modules are loaded and initialized, Floodlight will begin listening for incoming connections from switches and sending out LLDP messages to gather topology information. 

== Handling switches ==
`OFChannelHandler` is the inner class of `Controller` responsible for listening for incoming connections from switches. When a switch establishes a connection, a new instantiation of `OFSwitchImpl` is created for the switch. `OFSwitchImpl` is a representation of an individual switch, comprised of the channel, DPID, and information gleaned from the FEATURE_REPLY, among other bits and pieces. Changes are made to a switch through the manipulation of the `OFSwitchImpl` representing it.    

Another thing that occurs upon a switch establishing contact (or disappearing) is the dispatch of a switch event. `SwitchUpdate` is the `Controller` inner class responsible for calling a registered `IOFSwitchListener`'s ''addedSwitch()'' or ''removedSwitch()'' methods. This only occurs after the switch state is designated ''READY''.     

=== The !OpenFlow Handshake ===
The !OpenFlow handshake can be outlined as below:

 1. Exchange of HELLO messages 
 2. Controller: send FEATURES_REQUEST
 3. switch: send FEATURES_REPLY
 4. Controller: send GET_CONFIG_REQUEST
 5. Switch: send GET_CONFIG_REPLY

Stage 5 is the point where the switch is deemed ''READY'' and listeners are notified of its arrival. The components that execute the handshake are implemented in `Controller` as a collection of methods that are called from ''processOFMessage()''. ''processOFMessage()'' is also responsible for keeping up with the ECHO_REQUEST/REPLY keepalive messages.