# Startup of the Clixon backend

## Background

This document describes the configuration startup mechanism of the Clixon backend. This document describes the mechanism of Clixon version 3.10 which supports the following features:
* Loading of a "startup" XML or JSON configuration
* Loading of "extra" XML.
* Detection of in-compatible XML and Yang models in the startup configuration.
* An upgrade callback when in-compatible XML is encountered
* A "failsafe" mode allowing a user to repair the startup on errors or failed validation.

## Operation

### Modes

When the Clixon backend starts, it can start in one of four modes:
* _startup_: The configuration is loaded from a persistent `startup` database. This database is loaded, validated and committed into the running database.
* _running_: Similar to startup, but instead the `running` database is used as persistent database.*
_none_: No databases are touched - the system starts and loads existing running database without validation or commits. This is mostly a debug option.
* _init_: Similar to none, but the running database is cleared before loading

### Startup configuration

When the backend daemon is started in `startup` mode, the system loads
the `startup` database. The `running` mode is very similar, the only
difference is that the running database is copied (overwrites) the
startup database before this phase.

When loading the startup configuration, it is checked for parse
errors, the yang model-state is detected and the XML is validated
against the backend Yang models.

If yang-models do not match, an `upgrade` callback is made.

If any errors are detected, the backend tries to enter a `failsafe` mode.


### Yang model-state

Clixon has the ability to store module-state information according to
RFC7895 in the datastores. Including yang module-state in the
datastores is enabled by the following entry in the Clixon
configuration:
```
   <CLICON_XMLDB_MODSTATE>true</CLICON_XMLDB_MODSTATE>
```

If the datastore does not contain module-state info, no detection of
incompatible XML is made, and the upgrade feature described in
this section will not occur.

A backend does not perform detection of mismatching XML/Yang if:
1. The datastore was saved in a pre-3.10 system 
2. `CLICON_XMLDB_MODSTATE` was not enabled when saving the file
3. The backend configuration does not have `CLICON_XMLDB_MODSTATE` enabled.

Note that the module-state detection is independent of the other steps
of the startup operation: syntax errors, validation checks, failsafe mode, etc,
are still made, even though module-state detection does not occur.

Note also that a 3.10 Clixon system with `CLICON_XMLDB_MODSTATE` disabled
will silently ignore the module state.

Example of a (simplified) datastore with Yang module-state:
```
<config>
   <a1 xmlns="urn:example:a">some text</a1>
   <modules-state xmlns="urn:ietf:params:xml:ns:yang:ietf-yang-library">
      <module-set-id>42</module-set-id>
      <module>
         <name>A</name>
         <revision>2019-01-01</revision>
         <namespace>urn:example:a</namespace>
      </module>
   </modules-state>
</config>
```

### Upgrade callback

If the module-state of the startup configuration does not match the
module-state of the backend daemon, an _upgrade_ callback is
made. This allows the user to automatically upgrade the XML to the
recent version. As a hint, the module-state differences is passed to
the callback.

Example upgrade callback:
```
  /*! Upgrade configuration from one version to another
   * @param[in]  h      Clicon handle
   * @param[in]  xms    Module state differences
   * @retval     0      OK
   * @retval    -1      Error
   */
  int 
  example_upgrade(clicon_handle       h,
                  cxobj              *xms)
  {
    if (xms)
	clicon_log_xml(LOG_NOTICE, xms, "%s", __FUNCTION__);
    // Perform upgrade of startup XML
    return 0;
  }

  static clixon_plugin_api api = {
    "example",                              /* name */    
    ...
    .ca_upgrade=example_upgrade,            /* upgrade configuration */
  };
```

Note that this is simply a template for upgrade. Actual upgrading may
be implememted by a user.

If no action is made, and the XML is not upgraded, the next step of
the startup is made, which is XML/Yang validation.

An out-dated XML
may still pass validation and the system will go up in normal state.

However, if the validation fails, the backend will try to enter the
failsafe mode so that the user may perform manual upgarding of the
configuration.


### Extra XML

If validation succeeds and the startup configuration has been committed to the running database, a user may add "extra" XML.

There are two ways to add extra XML to running database after start. Note that this XML is not "committed" into running.

The first way is via a file. Assume you want to add this xml:
```
<config>
   <x xmlns="urn:example:clixon">extra</x>
</config>
```
You add this via the -c option:
```
clixon_backend ... -c extra.xml
```

The second way is by programming the plugin_reset() in the backend
plugin. The example code contains an example on how to do this (see plugin_reset() in example_backend.c).

The extra-xml feature is not available if startup mode is `none`. It will also not occur in failsafe mode.

### Startup status and failsafe mode

When the startup process is completed, a startup status is set and is accessible via `clixon_startup_status_get(h)` with the following values:
```
  STARTUP_ERR        XML/JSON syntax error
  STARTUP_INVALID,   XML / Yang validation failure
  STARTUP_OK         OK
```

If the startup fails, the backend looks for a `failsafe` configuration
in `CLICON_XMLDB_DIR/failsafe_db`. If such a config is not found, the
backend terminates.

If the failsafe is found, the failsafe config is loaded and
committed into the running db. The `startup` database will contain syntax
errors or invalidated XML.

A user can repair the `startup`
configuration and either restart the backend or copy the startup
configuration to candidate and the commit.

Note that the if the startup configuration contains syntactic errors
(eg `STARTUP_ERR`) you cannot access the startup via Restconf or
Netconf operations since the XML may be broken.

If the startup is not valid (no syntax errors), you can edit the XML
and then copy/commit it via CLI, Netconf or Restconf.

## Flowcharts

This section contains non-formal "flowcharts" showing the dynamics of
the configuration databases in the startup phase.

The flowchart starts in one of the the modes (non, init, startup, running):

Starting in init mode:
```
                 reset     
running   |--------+------------> GOTO EXTRA XML
```

Start in running mode:
```
running   ----+
               \ copy 
startup         +------------> GOTO STARTUP

```
Starting in startup mode:
```
                              reset     
running                         |--------+------------> GOTO EXTRA XML
                parse validate OK       / commit 
startup -------+--+-------+------------+          
```

If validation of startup fails:
```
failsafe      ----------------------+
                            reset    \ commit
running                       |-------+---------------> GOTO EXTRA XML
              parse validate fail 
startup      ---+-------------------------------------> INVALID XML
```

Load EXTRA XML:
```
running -----------------+----+------> GOTO SYSTEM UP
           reset  loadfile   / merge
tmp     |-------+-----+-----+
```

SYSTEM UP:
```
running ----+-----------------------> RUNNING
             \ copy
candidate     +---------------------> CANDIDATE
```
	     
## Thanks
Thanks matt smith and dave cornejo for input

## References

[RFC7895](https://tools.ietf.org/html/rfc7895)