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Internals

This page will try to give a high level overview of how the framework is working. While the end-user of the framework won't usually touch much more than the Node and Host classes, there's a lot more going on underneath.

There's a lot of meta-programming, some domain specific languages, and a mix of event-driven and blocking code.

Data flow

Roughly, this is the current flow from the interactive shell until the actual SSH client.

.. graphviz::

   digraph internals{
       "Node" [style=filled, fillcolor=gold1];
       "ParallelNode" [style=filled, fillcolor=gold1];
       "Host" [style=filled, fillcolor=gold1];

       "Host" [shape=box];
       "HostContainer" [shape=box];
       "HostsContainer" [shape=box];
       "Node" [shape=box];
       "ParallelNode" [shape=box];
       "Env" [shape=box];
       "HostContext" [shape=box];

       "Host" -> "HostContext";
       "Host" -> "Paramiko (SSH)";
       "HostsContainer" -> "Host";
       "HostContainer" -> "Host";
       "HostsContainer" -> "HostContext";
       "HostContainer" -> "HostContext";
       "Node" -> "HostsContainer";
       "ParallelNode" -> "HostsContainer";
       "ParallelNode" -> "HostContainer";
       "Env" -> "Node";
       "Env" -> "ParallelNode";
       "Interactive shell" -> "Env";
   }

The yellow classes -- :class:`Node <deployer.node.base.Node>`, :class:`ParallelNode <deployer.node.base.ParallelNode>` and :class:`Host <deployer.host.Host>` -- are the ones which an average end-user of this framework will use. He will inherit from there to define his deployment script.

:class:`HostContainer <deployer.host_container.HostContainer>` (singular and plural) and :class:`Env <deployer.node.base.Env>` are proxy classes. They are created by the framework, but passed to the user's code at some points.

Paramiko, at the lowest level, is responsible for the SSH connection. The Host class takes care of calling Paramiko, the end-user should not directly depend on Paramiko. In the future, we may replace it with for instance twisted.conch.

At the top level, we usually have the interactive shell. But if a deployment script is called as a library, it can have any other front-end. The built-in interactive shell also has a telnet server (remote shell) and a shell which has some multithreaded execution model (parallel deployment). These are realized through Twisted Matrix, and there's some event-driven code touching the iterative blocking code.