network_cli code flow

networl_cli.txt

There are multiple moving parts in this code flow.
To start with network_cli connection plugin is applicable for network platforms that support persistent 
connection with the remote host across task.

The method called on connection object (connection.edit_config) is serialised in json string as per jsonrpc 2.0 spec [0] 
and is send over to socket [1]. The receiving end of the socket is a daemon process called "ansible-connection" 
which is spawned from task_executor [2] at the time of connection creation to the remote host.

"ansible-connection" receives the json string and calls handle_request method [3] on the object of JsonRpcServer class 
In the initialization steps, "ansible-connection" loads the connection plugin ("network-cli" plugin in this case) and 
register it with object JsonRpcServer class [4]. handle_request() in turn decodes the json string 
to fetch the method name, arguments and invokes it on the object of Connection class defined in network_cli [5]

network_cli _connect() method which is called at the time of connection initialization [6] & [7] handles the 
connection with the remote host and also loads the cliconf and terminal plugins within the Connection object 
based on the value of "network_os" [8] (nxos in this case)

So the call connection.edit_config ends up in Connection class defined in network_cli as and "edit_config" method 
is not directly defined in this class it searches for the method with the same name in nxos cliconf and terminal plugin. 
Since edit_config is defined in nxos cliconf plugin [9] it gets invoked.

nxos_vlan --> load_config --> connection.edit_config --> ansible-connection -> network_cli --> nxos cliconf
|------ module side code ---------------------------|    |--------------- controller code -----------------|

[0] http://www.jsonrpc.org/specification
[1] https://github.com/ansible/ansible/blob/devel/lib/ansible/module_utils/connection.py#L158
[2] https://github.com/ansible/ansible/blob/devel/lib/ansible/executor/task_executor.py#L917
[3] https://github.com/ansible/ansible/blob/devel/bin/ansible-connection#L123
[4] https://github.com/ansible/ansible/blob/devel/bin/ansible-connection#L89
[5] https://github.com/ansible/ansible/blob/devel/lib/ansible/utils/jsonrpc.py#L25
[6] https://github.com/ansible/ansible/blob/devel/bin/ansible-connection#L87
[7] https://github.com/ansible/ansible/blob/devel/lib/ansible/plugins/connection/network_cli.py#L287
[8] https://github.com/ansible/ansible/blob/devel/lib/ansible/plugins/connection/network_cli.py#L318
[9] https://github.com/ansible/ansible/blob/devel/lib/ansible/plugins/connection/network_cli.py#L225

You can refer the httpapi proposal here https://github.com/ansible/proposals/issues/103 which deals with the 
similar requirement. 
FYI httpapi connection support is present in devel branch and will be available with Ansible 2.6 version onwards