05-implementation.md

# Implementation Aspects of the goSDN Controller


## Why we do this in go
Because it rocks, but let's see afterwards what can be written here.

## Storing Information

Section XXX (Conceptual Design of a SDN Controller as Network Supervisor)
discusses the need to store information about for element inventories and
topology inventories.

### Element Inventories

Storing information about network elements and their properties is a relative
static process, at least when one considers potential changes over time.
Typically such network elements are added to a network and they will remain in
the network for a longer time, i.e., multiple minutes or even longer.

### Topology Inventory

Every network has one given physical topology (G<sub>physical</sub> ) and on
top of this at least one logical topology (G<sub>logical1</sub>). There may be
multiple logical topologies (G<sub>n+1</sub>) on top logical topologies
(G<sub>n</sub>), i.e., a recursion. Such logical topologies (G<sub>n+1</sub>)
can again have other logical topologies as recursion or other logical topologies
in parallel.

A topology consists out of interfaces, which are attached to their respective
network elements, and links between these interfaces.

Mathematically, such a topology can be described as a directed graph, whereas
the interfaces of the network elements are the nodes and the links are
the edges.

G<sub>physical</sub> ist a superset of G<sub>logical1</sub>.

The topology inventory has to store the particular graph for any topology and
also the connections between the different levels of topologies. For instance,
the G<sub>logical1</sub> is linked to G<sub>physical</sub>. (needs to be clear
if changes in n-1 graph has impact on n graph).

For further study at this point: Which type of database and implementation of
databases should be used to store the different topology graphs and their
pontential dependencies? How should the interface between gosdn and this
database look like?

Here is an attempt to describe the above text in a graphical reprensetation (not ready YET!): 

```mermaid
graph TB

  SubGraph1 --> SubGraph1Flow
  subgraph "G_logical1"
  SubGraph1Flow(SubNode 1)
  Node1_l1[Node1_l1] <--> Node2_l1[Node2_l1] <--> Node3_l1[Node3_l1] <--> Node4_l1[Node4_l1] <--> Node5_l1[Node5_l1]
  end

  subgraph "G_physical"
  Node1[Node 1] <--> Node2[Node 2] <--> Node3[Node 3]
  Node4[Node 4] <--> Node2[Node 2] <--> Node5[Node 5] 

  Node2 --> SubGraph1[Link to G_logical1]
  
end
```

### Potential other Inventories

There may be the potential need to store information beyond pure topologies,
actually about network flows, i.e., information about a group of packets
belonging together.


## YANG to code

The base of the development of goSDN are YANG modules. The RESTful API used for RESTCONF is defined in an OpenAPI 2.0 file. This API documentation is generated from the YANG module. The YANG module description is also used to generate code stubs for the goSDN RESTCONF client.

\includegraphics{gfx/yang-schematics.pdf}

### YANG

YANG defines an abstract network interface. It is the foundation of the RESTCONF protocol. Several code generators exist to generate code stubs from a given definition.

### OpenAPI

OpenAPI - formerly known as Swagger - is a framework that defines RESTful APIs. We use OenAPI documentations to define the RESTCONF server implementation of the cocsn YANG modules.

### Toolchain

We use 3 different tools for the code generation workflow. For the RESTCONF server `yanger` is used to generate the OpenAPI documentation from the YANG file. `go-swagger` is used to generate a RESTCONF server with stubs for the REST calls.

The RESTCONF  client stubs used by goSDN are generated from YANG files using YGOT.

### Dependencies

For now we can only use the OpenAPI 2.0 standard. This is because `go-swagger` does not support OpenAPI 3.0 specifications yet.