Frequently Asked Questions - Clixon
===================================

Q: What is the best way of understanding Clixon?
A: Run the ietf yang routing example. 

Q: How do you build and install Clixon (and the example)?
A: Clixon: ./configure; make; sudo make install; sudo make install-include
Example: cd example; make; sudo make install

Q: Is there any reference documentation?
A: Clixon uses Doxygen for reference documentation.
Build using 'make doc' and aim your browser at doc/html/index.html

Q: How do you run the example?
A: Start a backend server: 'clicon_backend -Ff /usr/local/etc/routing.conf'
Start a cli session: clicon_cli -f /usr/local/etc/routing.conf
Start a netconf session: clicon_netconf -f /usr/local/etc/routing.conf

Q: How does Clixon differ from Clicon?
A: Clixon is a fork of Clicon focussing on Yang specification and XML
database.  The yang support in clicon was grown out of a key-based
scheme that was complex since it had to translate between
formats. Clixon uses only XML. A simplified commit transaction
mechanism has been added to. But Clixon is not backward compliant with
key-based Clicon applications, such as ROST.

Q: In the example, how can you alter its semantics?
- routing.conf.local - Override default settings
- The yang specifications - Alter accepted XML, database format and the configuration part of the CLI commands that are generated.
- routing_cli.cli - Change the fixed part of the CLI commands 
- routing_cli.c - New cli commands may need to write a new C-funtion.
- routing_backend.c - What happens at commit.
- routing_netconf.c - Modify semantics of netconf commands.

Q: How do you check what is in a database?
clicon_dbctrl -d <database> -p
The name of the running or candidate databases are found in the
configuration file, eg  /usr/local/var/routing/candidate_db
Example:
  /interfaces/interface/eth0/ipv4 
  /interfaces/interface/eth0/type bgp
  /interfaces/interface/eth0 
  /interfaces/interface/eth0/name eth0
Each line corresponds to a database entry (node in an XML tree). If
the node is a leaf, the value appears as the second entry.

Q: How do you write a commit rule in the example?
A: 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 clicon_backend_transaction.h

Q: How do you check what has changed on commit?
A: 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
  # Get all added i/fs
  cxobj **vec = xpath_vec_flag(target, "//interface", &len, XML_FLAG_ADD); 
  for (i=0; i<len; i++)             # Loop over added i/fs
    clicon_xml2file(stdout, vec[i]) # Print the added interface
The flags you can check for are: XML_FLAG_ADD, _DEL and _CHANGE, and their combinations.

Q How do I access the XML tree?
A: Using XPATH, find and iteration functions defined in the XML library. Example

Q: How do you write a CLI callback function?
You add an entry in routing_cli.cli
   example("This is a comment") <var:int32>("This is a variable"), mycallback("myarg");
Then define a function in routing_cli.c
   mycallback(clicon_handle h, cvec *cvv, cg_var *arg)
where 'cvv' contains the value of the variable and 'arg' contains the
function parameter 'myarg'.

Q: What are cg_var and cvec used in CLI callbacks?
Those are 'CLIgen variables' and vector of CLOgen
variables. Documented in CLIgen documentation. Some examples on usage is found in the
routing_cli.c

Q: How do you write a validation rule?
Similar to a commit rule, but instead write the transaction_validate() function.
Check for consitencies 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.