clixon/doc/FAQ.md

# Clixon FAQ

## What is Clixon?

Clixon is a configuration management tool including a generated CLI ,
Yang parser, netconf and restconf interface and an embedded databases.

## Why should I use Clixon?

If you want an easy-to-use configuration frontend based on yang with an
open-source license.  Typically for embedded devices requiring a
config interface such as routers and switches. 

## What license is available?
CLIXON is dual license. Either Apache License, Version 2.0 or GNU
General Public License Version 2. 

## Is Clixon extendible?
Yes. All application semantics is defined in plugins with well-defined
APIs. There are currently plugins for: CLI, Netconf,  Restconf, the datastore and the backend.

## Which programming language is used?
Clixon is written in C. The plugins are written in C. The CLI
specification uses cligen (http://cligen.se)

It is possible ro write plugins in Python. It is reasonable
simple to spawn an external script from a backend (but needs to be done).

## How to best understand Clixon?
Run the ietf yang routing example, in the example directory.

## How do you build and install Clixon (and the example)?
Clixon: 
```
	./configure; 
	make; 
	sudo make install; 
	sudo make install-include
```
The example: 
```
	 cd example; 
	 make; 
	 sudo make install
```

## Do I need to setup anything?

The config demon requires a valid group to create a server UNIX socket.
Define a valid CLICON_SOCK_GROUP in the config file or via the -g option
or create the group and add the user to it. The default group is 'clicon'.
On linux:
  sudo groupadd clicon
  sudo usermod -a -G clicon user

## What about reference documentation?
Clixon uses Doxygen for reference documentation.
Build using 'make doc' and aim your browser at doc/html/index.html or
use the web resource: http://clicon.org/ref/index.html

## How do you run the example?
- Start a backend server: 'clixon_backend -Ff /usr/local/etc/routing.xml'
- Start a cli session: clixon_cli -f /usr/local/etc/routing.xml
- Start a netconf session: clixon_netconf -f /usr/local/etc/routing.xml

## How is configuration data stored?
Configuration data is stored in an XML datastore. The default is a
text-based datastore, but there also exists a key-value datastore
using qdbm. In the example the datastore are regular files found in
/usr/local/var/routing/.

## What is validate and commit?
Clixon follows netconf in its validate and commit semantics.
In short, you edit a 'candidate' configuration, which is first
'validated' for consistency and then 'committed' to the 'running' 
configuration.

A clixon developer writes commit functions to incrementaly upgrade a
system state based on configuration changes. Writing commit callbacks
is the core functionality of a clixon system.

## What is a Clixon configuration file?
Clixon options are stored in a configuration file you must specify
when you start a backend or client using -f. The example configuration
file is installed at /usr/local/etc/routing.xml.

## Can I run Clixon as docker containers?
Yes, the example works as docker containers as well. backend and cli needs a 
common file-system so they need to run as a composed pair.
```
	cd example/docker
	make docker # Prepares /data as shared file-system mount
	run.sh      # Starts an example backend and a cli
```
The containers are by default downloaded from dockerhib, but you may
build the containers locally: 
```
	cd docker
	make docker
```
You may also push the containers with 'make push' but you may then consider changing the image name in the makefile.

## How do I use netconf?

As an alternative to cli configuration, you can use netconf. Easiest is to just pipe netconf commands to the clixon_netconf application.
Example:
	echo "<rpc><get-config><source><candidate/></source><configuration/></get-config></rpc>]]>]]>" | clixon_netconf -f /usr/local/etc/routing.xml

However, more useful is to run clixon_netconf as an SSH
subsystem. Register the subsystem in /etc/sshd_config:
```
	Subsystem netconf /usr/local/bin/clixon_netconf -f /usr/local/etc/routing.xml
```
and then invoke it from a client using
```
	ssh -s <host> netconf
```

## How do I use restconf?

You can access clixon via REST API using restconf, such as using
curl. GET, PUT, POST are supported.

You need a web-server, such as nginx, and start a restconf fcgi
daemon, clixon_restconf. Read more in the restconf docs.

Example:
```
   curl -G http://127.0.0.1/restconf/data/interfaces/interface/name=eth9/type
   [
     {
       "type": "eth" 
     }
   ]
```

## How do I use notifications?

The example has a prebuilt notification stream called "ROUTING" that triggers every 10s.
You enable the notification either via the cli:
```
cli> notify 
cli>
```
or via netconf:
```
clixon_netconf -qf /usr/local/etc/routing.xml 
<rpc><create-subscription><stream>ROUTING</stream></create-subscription></rpc>]]>]]>
<rpc-reply><ok/></rpc-reply>]]>]]>
<notification><event>Routing notification</event></notification>]]>]]>
<notification><event>Routing notification</event></notification>]]>]]>
...
```

## I want to program. How do I extend the example?
- routing.xml - Change the configuration file
- The yang specifications - This is the central part. It changes the XML, database and the config cli.
- routing_cli.cli - Change the fixed part of the CLI commands 
- routing_cli.c - Cli C-commands are placed here.
- routing_backend.c - Commit and validate functions.
- routing_netconf.c - Modify semantics of netconf commands.

## How do I write a commit function?
You write a commit function in routing_backend.c.
Every time a commit is made, transaction_commit() is called in the
backend.  It has a 'transaction_data td' argument which is used to fetch
information on added, deleted and changed entries. You access this
information using access functions as defined in clixon_backend_transaction.h

## How do I check what has changed on commit?
You use XPATHs on the XML trees in the transaction commit callback.
Suppose you want to print all added interfaces:
```
	cxobj *target = transaction_target(td); # wanted XML tree
	vec = xpath_vec_flag(target, "//interface", &len, XML_FLAG_ADD); /* Get added i/fs */
	for (i=0; i<len; i++)             /* Loop over added i/fs */
	  clicon_xml2file(stdout, vec[i], 0, 1); /* Print the added interface */
```
You can look for added, deleted and changed entries in this way.

## How do I access the XML tree?
Using XPATH, find and iteration functions defined in the XML library. Example library functions:
```
      xml_child_each(), 
      xml_find(), 
      xml_body(), 
      xml_print(), 
      xml_apply()
```
More are found in the doxygen reference.

## How do I write a CLI callback function?

1. You add an entry in routing_cli.cli
>   example("This is a comment") <var:int32>("This is a variable"), mycallback("myarg");
2. Then define a function in routing_cli.c
>   mycallback(clicon_handle h, cvec *cvv, cvec *arv)
where 'cvv' contains the value of the variable 'var' and 'argv' contains the string "myarg".

The 'cvv' datatype is a 'CLIgen variable vector'.
They are documented in [CLIgen tutorial](https://github.com/olofhagsand/cligen/blob/master/cligen_tutorial.pdf)

## How do I write a validation function?
Similar to a commit function, but instead write the transaction_validate() function.
Check for inconsistencies in the XML trees and if they fail, make an clicon_err() call.
```
    clicon_err(OE_PLUGIN, 0, "Route %s lacks ipv4 addr", name);
    return -1;
```
The validation or commit will then be aborted.

## How do I write a state data callback function?

Netconf <get> and restconf GET also returns state data, in contrast to
config data. In YANG state data is specified with "config false;".

To return state data, you need to write a backend state data callback
with the name "plugin_statedata()" where you return an XML tree.

Please look at the example for an example on how to write a state data callback.

## How do I write an RPC function?

A YANG RPC is an application specific operation. Example:
```
   rpc fib-route {
      input {
         leaf inarg { type string; }
      }
      output {
         leaf outarg { type string; }
      }
   }
```
which defines the fib-route operation present in the example (the arguments have been changed).

Clixon automatically relays the RPC to the clixon backend. To
implement the RFC, you need to register an RPC callback in the backend plugin:
Example:
```
int
plugin_init(clicon_handle h)
{
...
   backend_rpc_cb_register(h, fib_route, NULL, "fib-route");
...
}
```
And then define the callback itself:
```
static int 
fib_route(clicon_handle h,            /* Clicon handle */
	  cxobj        *xe,           /* Request: <rpc><xn></rpc> */
	  struct client_entry *ce,    /* Client session */
	  cbuf         *cbret,        /* Reply eg <rpc-reply>... */
	  void         *arg)          /* Argument given at register */
{
    cprintf(cbret, "<rpc-reply><ok/></rpc-reply>");    
    return 0;
}
```
Here, the callback is over-simplified.