NSO is capable of starting user-provided Erlang applications embedded in the same Erlang VM as NSO.

The Erlang code is packaged into applications which are automatically started and stopped by NSO if they are located at the proper place. NSO will search all packages for top-level directories called erlang-lib. The structure of such a directory is the same as a standard lib directory in Erlang. The directory may contain multiple Erlang applications. Each one must have a valid .app file. See the Erlang documentation of application and app for more info.

An Erlang package skeleton can be created by making use of the ncs-make-package command:

ncs-make-package --erlang-skeleton --erlang-application-name <appname> <package-name>

Multiple applications can be generated by using the option --erlang-application-name NAME multiple times with different names.

All application code should use the prefix ec_ for module names, application names, registered processes (if any), and named ets tables (if any), to avoid conflict with existing or future names used by NSO itself.

Erlang API

The Erlang API to NSO is implemented as an Erlang/OTP application called econfd. This application comes in two flavors. One is built into NSO to support applications running in the same Erlang VM as NSO. The other is a separate library which is included in source form in the NSO release, in the $NCS_DIR/erlang directory. Building econfd as described in the $NCS_DIR/erlang/econfd/README file will compile the Erlang code and generate the documentation.

This API can be used by applications written in Erlang in much the same way as the C and Java APIs are used, i.e. code running in an Erlang VM can use the econfd API functions to make socket connections to NSO for the data provider, MAAPI, CDB, etc. access. However, the API is also available internally in NSO, which makes it possible to run Erlang application code inside the NSO daemon, without the overhead imposed by the socket communication.

When the application is started, one of its processes should make initial connections to the NSO subsystems, register callbacks, etc. This is typically done in the init/1 function of a gen_server or similar. While the internal connections are made using the exact same API functions (e.g. econfd_maapi:connect/2) as for an application running in an external Erlang VM, any Address and Port arguments are ignored, and instead, standard Erlang inter-process communication is used.

There is little or no support for testing and debugging Erlang code executing internally in NSO since NSO provides a very limited runtime environment for Erlang to minimize disk and memory footprints. Thus the recommended method is to develop Erlang code targeted for this by using econfd in a separate Erlang VM, where an interactive Erlang shell and all the other development support included in the standard Erlang/OTP releases are available. When development and testing are completed, the code can be deployed to run internally in NSO without changes.

For information about the Erlang programming language and development tools, refer to www.erlang.org and the available books about Erlang (some are referenced on the website).

The --printlog option to ncs, which prints the contents of the NSO error log, is normally only useful for Cisco support and developers, but it may also be relevant for debugging problems with application code running inside NSO. The error log collects the events sent to the OTP error_logger, e.g. crash reports as well as info generated by calls to functions in the error_logger(3) module. Another possibility for primitive debugging is to run ncs with the --foreground option, where calls to io:format/2 etc will print to standard output. Printouts may also be directed to the developer log by using econfd:log/3.

While Erlang application code running in an external Erlang VM can use basically any version of Erlang/OTP, this is not the case for code running inside NSO, since the Erlang VM is evolving and provides limited backward/forward compatibility. To avoid incompatibility issues when loading the beam files, the Erlang compiler erlc should be of the same version as was used to build the NSO distribution.

NSO provides the VM, erlc and the kernel, stdlib, and crypto OTP applications.

Application Configuration

Applications may have dependencies to other applications. These dependencies affect the start order. If the dependent application resides in another package, this should be expressed by using the required package in the package-meta-data.xml file. Application dependencies within the same package should be expressed in the .app. See below.

The following config settings in the .app file are explicitly treated by NSO:

Example

The examples.ncs/getting-started/developing-with-ncs/18-simple-service-erlang example in the bundled collection shows how to create a service written in Erlang and execute it internally in NSO. This Erlang example is a subset of the Java example examples.ncs/getting-started/developing-with-ncs/4-rfs-service.

NSO is capable of starting one or several Python VMs where Python code in user-provided packages can run.

An NSO package containing a python directory will be considered to be a Python Package. By default, a Python VM will be started for each Python package that has a python-class-name defined in its package-meta-data.xml file. In this Python VM, the PYTHONPATH environment variable will be pointing to the python directory in the package.

If any required package that is listed in the package-meta-data.xml contains a python directory, the path to that directory will be added to the PYTHONPATH of the started Python VM and thus its accompanying Python code will be accessible.

Several Python packages can be started in the same Python VM if their corresponding package-meta-data.xml files contain the same python-package/vm-name.

A Python package skeleton can be created by making use of the ncs-make-package command:

ncs-make-package --service-skeleton python <package-name>

YANG Model

The tailf-ncs-python-vm.yang defines the python-vm container which, along with ncs.conf, is the entry point for controlling the NSO Python VM functionality. Study the content of the YANG model in the example below (The Python VM YANG Model). For a full explanation of all the configuration data, look at the YANG file and man ncs.conf. Here will follow a description of the most important configuration parameters.

Note that some of the nodes beneath python-vm are by default invisible due to a hidden attribute. To make everything under python-vm visible in the CLI, two steps are required:

1. First, the following XML snippet must be added to ncs.conf:\

   Copy

   <hide-group>
      <name>debug</name>
   </hide-group>

2. Next, the unhide command may be used in the CLI session:

   Copy

   admin@ncs(config)# unhide debug
   admin@ncs(config)#

The sanity-checks/self-assign-warning controls the self-assignment warnings for Python services with off, log, and alarm (default) modes. An example of a self-assignment:

class ServiceCallbacks(Service):
    @Service.create
    def cb_create(self, tctx, root, service, proplist):
        self.counter = 42

As several service invocations may run in parallel, self-assignment will likely cause difficult-to-debug issues. An alarm or a log entry will contain a warning and a keypath to the service instance that caused the warning. Example log entry:

<WARNING> ... Assigning to self is not thread safe: /mysrvc:mysrvc{2}

With the logging/level, the amount of logged information can be controlled. This is a global setting applied to all started Python VMs unless explicitly set for a particular VM, see Debugging of Python packages. The levels correspond to the pre-defined Python levels in the Python logging module, ranging from level-critical to level-debug.

The logging/log-file-prefix define the prefix part of the log file path used for the Python VMs. This prefix will be appended with a Python VM-specific suffix which is based on the Python package name or the python-package/vm-name from the package-meta-data.xml file. The default prefix is logs/ncs-python-vm so e.g., if a Python package named l3vpn is started, a logfile with the name logs/ncs-python-vm-l3vpn.log will be created.

The status/start and status/current contains operational data. The status/start command will show information about what Python classes, as declared in the package-meta-data.xml file, were started and whether the outcome was successful or not. The status/current command will show which Python classes that are currently running in a separate thread. The latter assumes that the user-provided code cooperates by informing NSO about any thread(s) started by the user code, see Structure of the User-provided Code.

The start and stop actions make it possible to start and stop a particular Python VM.

> yanger -f tree tailf-ncs-python-vm.yang
          
submodule: tailf-ncs-python-vm (belongs-to tailf-ncs)
  +--rw python-vm
     +--rw sanity-checks
     |  +--rw self-assign-warning?   enumeration
     +--rw logging
     |  +--rw log-file-prefix?   string
     |  +--rw level?             py-log-level-type
     |  +--rw vm-levels* [node-id]
     |     +--rw node-id    string
     |     +--rw level      py-log-level-type
     +--rw status
     |  +--ro start* [node-id]
     |  |  +--ro node-id     string
     |  |  +--ro packages* [package-name]
     |  |     +--ro package-name    string
     |  |     +--ro components* [component-name]
     |  |        +--ro component-name    string
     |  |        +--ro class-name?       string
     |  |        +--ro status?           enumeration
     |  |        +--ro error-info?       string
     |  +--ro current* [node-id]
     |     +--ro node-id     string
     |     +--ro packages* [package-name]
     |        +--ro package-name    string
     |        +--ro components* [component-name]
     |           +--ro component-name    string
     |           +--ro class-names* [class-name]
     |              +--ro class-name    string
     |              +--ro status?       enumeration
     +---x stop
     |  +---w input
     |  |  +---w name    string
     |  +--ro output
     |     +--ro result?   string
     +---x start
        +---w input
        |  +---w name    string
        +--ro output
           +--ro result?   string

Structure of the User-provided Code

The package-meta-data.xml file must contain a component of type application with a python-class-name specified as shown in the example below.

<component>
  <name>L3VPN Service</name>
  <application>
    <python-class-name>l3vpn.service.Service</python-class-name>
  </application>
</component>
<component>
  <name>L3VPN Service model upgrade</name>
  <upgrade>
    <python-class-name>l3vpn.upgrade.Upgrade</python-class-name>
  </upgrade>
</component>

The component name (L3VPN Service in the example) is a human-readable name of this application component. It will be shown when doing show python-vm in the CLI. The python-class-name should specify the Python class that implements the application entry point. Note that it needs to be specified using Python's dot notation and should be fully qualified (given the fact that PYTHONPATH is pointing to the package python directory).

Study the excerpt of the directory listing from a package named l3vpn below.

packages/
+-- l3vpn/
    +-- package-meta-data.xml
    +-- python/
    |   +-- l3vpn/
    |       +-- __init__.py
    |       +-- service.py
    |       +-- upgrade.py
    |       +-- _namespaces/
    |           +-- __init__.py
    |           +-- l3vpn_ns.py
    +-- src
        +-- Makefile
        +-- yang/
            +-- l3vpn.yang

Look closely at the python directory above. Note that directly under this directory is another directory named the package (l3vpn) that contains the user code. This is an important structural choice that eliminates the chance of code clashes between dependent packages (only if all dependent packages use this pattern of course).

As you can see, the service.py is located according to the description above. There is also a __init__.py (which is empty) there to make the l3vpn directory considered a module from Python's perspective.

Note the _namespaces/l3vpn_ns.py file. It is generated from the l3vpn.yang model using the ncsc --emit-python command and contains constants representing the namespace and the various components of the YANG model, which the User code can import and make use of.

The service.py file should include a class definition named Service which acts as the component's entry point. See The Application Component for details.

Notice that there is also a file named upgrade.py present which holds the implementation of the upgrade component specified in the package-meta-data.xml excerpt above. See The Upgrade Component for details regarding upgrade components.

The application Component

The Python class specified in the package-meta-data.xml file will be started in a Python thread which we call a component thread. This Python class should inherit ncs.application.Application and should implement the methods setup() and teardown().

NSO supports two different modes for executing the implementations of the registered callpoints, threading and multiprocessing.

The default threading mode will use a single thread pool for executing the callbacks for all callpoints.

The multiprocessing mode will start a subprocess for each callpoint. Depending on the user code, this can greatly improve the performance on systems with a lot of parallel requests, as a separate worker process will be created for each Service, Nano Service, and Action.

The behavior is controlled by three factors:

• callpoint-model setting in the package-meta-data.xml file.

• Number of registered callpoints in the Application.

• Operating System support for killing child processes when the parent exits.

If the callpoint-model is set to multiprocessing, more than one callpoint is registered in the Application and the Operating System supports killing child processes when the parent exits, NSO will enable multiprocessing mode.

import ncs

class Service(ncs.application.Application):
    def setup(self):
        # The application class sets up logging for us. It is accessible
        # through 'self.log' and is a ncs.log.Log instance.
        self.log.info('Service RUNNING')

        # Service callbacks require a registration for a 'service point',
        # as specified in the corresponding data model.
        #
        self.register_service('l3vpn-servicepoint', ServiceCallbacks)

        # If we registered any callback(s) above, the Application class
        # took care of creating a daemon (related to the service/action point).

        # When this setup method is finished, all registrations are
        # considered done and the application is 'started'.

    def teardown(self):
        # When the application is finished (which would happen if NCS went
        # down, packages were reloaded or some error occurred) this teardown
        # method will be called.

        self.log.info('Service FINISHED')

The Service class will be instantiated by NSO when started or whenever packages are reloaded. Custom initialization, such as registering service and action callbacks should be done in the setup() method. If any cleanup is needed when NSO finishes or when packages are reloaded it should be placed in the teardown() method.

The existing log functions are named after the standard Python log levels, thus in the example above the self.log object contains the functions debug,info,warning,error,critical. Where to log and with what level can be controlled from NSO?

The upgrade Component

The Python class specified in the upgrade section of package-meta-data.xml will be run by NSO in a separately started Python VM. The class must be instantiable using the empty constructor and it must have a method called upgrade as in the example below. It should inherit ncs.upgrade.Upgrade.

import ncs
import _ncs


class Upgrade(ncs.upgrade.Upgrade):
    """An upgrade 'class' that will be instantiated by NSO.

    This class can be named anything as long as NSO can find it using the
    information specified in <python-class-name> for the <upgrade>
    component in package-meta-data.xml.

    Is should inherit ncs.upgrade.Upgrade.

    NSO will instantiate this class using the empty contructor.
    The class MUST have a method named 'upgrade' (as in the example below)
    which will be called by NSO.
    """

    def upgrade(self, cdbsock, trans):
        """The upgrade 'method' that will be called by NSO.

        Arguments:
        cdbsock -- a connected CDB data socket for reading current (old) data.
        trans -- a ncs.maapi.Transaction instance connected to the init
                 transaction for writing (new) data.

        There is no need to connect a CDB data socket to NSO - that part is
        already taken care of and the socket is passed in the first argument
        'cdbsock'. A session against the DB needs to be started though. The
        session doesn't need to be ended and the socket doesn't need to be
        closed - NSO will do that automatically.

        The second argument 'trans' is already attached to the init transaction
        and ready to be used for writing the changes. It can be used to create a
        maagic object if that is preferred. There's no need to detach or finish
        the transaction, and, remember to NOT apply() the transaction when work
        is finished.

        The method should return True (or None, which means that a return
        statement is not needed) if everything was OK.
        If something went wrong the method should return False or throw an
        error. The northbound client initiating the upgrade will be alerted
        with an error message.

        Anything written to stdout/stderr will end up in the general log file
        for spurious output from Python VMs. If not configured the file will
        be named ncs-python-vm.log.
        """

        # start a session against running
        _ncs.cdb.start_session2(cdbsock, ncs.cdb.RUNNING,
                                ncs.cdb.LOCK_SESSION | ncs.cdb.LOCK_WAIT)

        # loop over a list and do some work
        num = _ncs.cdb.num_instances(cdbsock, '/path/to/list')
        for i in range(0, num):
            # read the key (which in this example is 'name') as a ncs.Value
            value = _ncs.cdb.get(cdbsock, '/path/to/list[{0}]/name'.format(i))
            # create a mandatory leaf 'level' (enum - low, normal, high)
            key = str(value)
            trans.set_elem('normal', '/path/to/list{{{0}}}/level'.format(key))

        # not really needed
        return True

        # Error return example:
        #
        # This indicates a failure and the string written to stdout below will
        # written to the general log file for spurious output from Python VMs.
        #
        # print('Error: not implemented yet')
        # return False

The NSO client timeouts

The section /ncs-config/api in ncs.conf contains a number of very important timeouts. See $NCS_DIR/src/ncs/ncs_config/tailf-ncs-config.yang and ncs.conf(5) for details.

• new-session-timeout controls how long NSO will wait for the NSO Python VM to respond to a new session.

• query-timeout controls how long NSO will wait for the NSO Python VM to respond to a request to get data.

• connect-timeout controls how long NSO will wait for the NSO Python VM to initialize a Dp connection after the initial socket connect.

• action-timeout controls how long NSO will wait for the NSO Python VM to respond to an action request callback.

For new-session-timeout, query-timeout and connect-timeout, whenever any of these timeouts trigger, NSO will close the sockets from NSO to the NSO Python VM. The NSO Python VM will detect the closed socket and exit.

For action-timeout, whenever this timeout triggers, NSO will only close the sockets from the NSO Python VM to the clients without exiting the Python VM.

Debugging of Python Packages

Python code packages are not running with an attached console and the standard out from the Python VMs are collected and put into the common log file ncs-python-vm.log. Possible Python compilation errors will also end up in this file.

Normally the logging objects provided by the Python APIs are used. They are based on the standard Python logging module. This gives the possibility to control the logging if needed, e.g., getting a module local logger to increase logging granularity.

The default logging level is set to info. For debugging purposes, it is very useful to increase the logging level:

    $ ncs_cli -u admin
    admin@ncs> config
    admin@ncs% set python-vm logging level level-debug
    admin@ncs% commit

This sets the global logging level and will affect all started Python VMs. It is also possible to set the logging level for a single package (or multiple packages running in the same VM), which will take precedence over the global setting:

    $ ncs_cli -u admin
    admin@ncs> config
    admin@ncs% set python-vm logging vm-levels pkg_name level level-debug
    admin@ncs% commit

The debugging output is printed to separate files for each package and the log file naming is ncs-python-vm-pkg_name.log

Log file output example for package l3vpn:

    $ tail -f logs/ncs-python-vm-l3vpn.log
    2016-04-13 11:24:07 - l3vpn - DEBUG - Waiting for Json msgs
    2016-04-13 11:26:09 - l3vpn - INFO - action name: double
    2016-04-13 11:26:09 - l3vpn - INFO - action input.number: 21

Using Non-standard Python

There are occasions where the standard Python installation is incompatible or maybe not preferred to be used together with NSO. In such cases, there are several options to tell NSO to use another Python installation for starting a Python VM.

By default NSO will use the file $NCS_DIR/bin/ncs-start-python-vm when starting a new Python VM. The last few lines in that file read:

        if [ -x "$(which python3)" ]; then
            echo "Starting python3 -u $main $*"
            exec python3 -u "$main" "$@"
        fi
        echo "Starting python -u $main $*"
        exec python -u "$main" "$@"

As seen above NSO first looks for python3 and if found it will be used to start the VM. If python3 is not found NSO will try to use the command python instead. Here we describe a couple of options for deciding which Python NSO should start.

Configure NSO to Use a Custom Start Command (recommended)

NSO can be configured to use a custom start command for starting a Python VM. This can be done by first copying the file $NCS_DIR/bin/ncs-start-python-vm to a new file and then changing the last lines of that file to start the desired version of Python. After that, edit ncs.conf and configure the new file as the start command for a new Python VM. When the file ncs.conf has been changed reload its content by executing the command ncs --reload.

Example:

$ cd $NCS_DIR/bin
$ pwd
/usr/local/nso/bin
$ cp ncs-start-python-vm my-start-python-vm
$ # Use your favourite editor to update the last lines of the new
$ # file to start the desired Python executable.

Add the following snippet to ncs.conf:

<python-vm>
    <start-command>/usr/local/nso/bin/my-start-python-vm</start-command>
</python-vm>

The new start-command will take effect upon the next restart or configuration reload.

Changing the Path to python3 or python

Another way of telling NSO to start a specific Python executable is to configure the environment so that executing python3 or python starts the desired Python. This may be done system-wide or can be made specific for the user running NSO.

Updating the Default Start Command (not recommended)

Changing the last line of $NCS_DIR/bin/ncs-start-python-vm is of course an option but altering any of the installation files of NSO is discouraged.

The NSO Java VM is the execution container for all Java classes supplied by deployed NSO packages.

The classes, and other resources, are structured in jar files and the specific use of these classes is described in the component tag in the respective package-meta-data.xml file. Also as a framework, it starts and controls other utilities for the use of these components. To accomplish this, a main class com.tailf.ncs.NcsMain, implementing the Runnable interface is started as a thread. This thread can be the main thread (running in a java main()) or be embedded into another Java program.

When the NcsMain thread starts it establishes a socket connection towards NSO. This is called the NSO Java VM control socket. It is the responsibility of NcsMain to respond to command requests from NSO and pass these commands as events to the underlying finite state machine (FSM). The NcsMain FSM will execute all actions as requested by NSO. This includes class loading and instantiation as well as registration and start of services, NEDs, etc.

When NSO detects the control socket connection from the NSO Java VM, it starts an initialization process:

1. First, NSO sends a INIT_JVM request to the NSO Java VM. At this point, the NSO Java VM will load schemas i.e. retrieve all known YANG module definitions. The NSO Java VM responds when all modules are loaded.

2. Then, NSO sends a LOAD_SHARED_JARS request for each deployed NSO package. This request contains the URLs for the jars situated in the shared-jar directory in the respective NSO package. The classes and resources in these jars will be globally accessible for all deployed NSO packages.

3. The next step is to send a LOAD_PACKAGE request for each deployed NSO package. This request contains the URLs for the jars situated in the private-jar directory in the respective NSO package. These classes and resources will be private to the respective NSO package. In addition, classes that are referenced in a component tag in the respective NSO package package-meta-data.xml file will be instantiated.

4. NSO will send a INSTANTIATE_COMPONENT request for each component in each deployed NSO package. At this point, the NSO Java VM will register a start method for the respective component. NSO will send these requests in a proper start phase order. This implies that the INSTANTIATE_COMPONENT requests can be sent in an order that mixes components from different NSO packages.

5. Lastly, NSO sends a DONE_LOADING request which indicates that the initialization process is finished. After this, the NSO Java VM is up and running.

See Debugging Startup for tips on customizing startup behavior and debugging problems when the Java VM fails to start

YANG Model

The file tailf-ncs-java-vm.yang defines the java-vm container which, along with ncs.conf, is the entry point for controlling the NSO Java VM functionality. Study the content of the YANG model in the example below (The Java VM YANG model). For a full explanation of all the configuration data, look at the YANG file and man ncs.conf.

Many of the nodes beneath java-vm are by default invisible due to a hidden attribute. To make everything under java-vm visible in the CLI, two steps are required:

1. First, the following XML snippet must be added to ncs.conf:\

   Copy

   <hide-group>
       <name>debug</name>
   </hide-group>

2. Next, the unhide command may be used in the CLI session:

   Copy

   admin@ncs(config)# unhide debug
   admin@ncs(config)#

        > yanger -f tree tailf-ncs-java-vm.yang
          submodule: tailf-ncs-java-vm (belongs-to tailf-ncs)
  +--rw java-vm
     +--rw stdout-capture
     |  +--rw enabled?   boolean
     |  +--rw file?      string
     |  +--rw stdout?    empty
     +--rw connect-time?                     uint32
     +--rw initialization-time?              uint32
     +--rw synchronization-timeout-action?   enumeration
     +--rw exception-error-message
     |  +--rw verbosity?   error-verbosity-type
     +--rw java-logging
     |  +--rw logger* [logger-name]
     |     +--rw logger-name    string
     |     +--rw level          log-level-type
     +--ro start-status?                     enumeration
     +--ro status?                           enumeration
     +---x stop
     |  +--ro output
     |     +--ro result?   string
     +---x start
     |  +--ro output
     |     +--ro result?   string
     +---x restart
        +--ro output
           +--ro result?   string

Java Packages and the Class Loader

Each NSO package will have a specific java classloader instance that loads its private jar classes. These package classloaders will refer to a single shared classloader instance as its parent. The shared classloader will load all shared jar classes for all deployed NSO packages.

The purpose of this is first to keep integrity between packages which should not have access to each other's classes, other than the ones that are contained in the shared jars. Secondly, this way it is possible to hot redeploy the private jars and classes of a specific package while keeping other packages in a run state.

Should this class loading scheme not be desired, it is possible to suppress it by starting the NSO Java VM with the system property TAILF_CLASSLOADER set to false.

java -DTAILF_CLASSLOADER=false ...

This will force NSO Java VM to use the standard Java system classloader. For this to work, all jar's from all deployed NSO packages need to be part of the classpath. The drawback of this is that all classes will be globally accessible and hot redeploy will have no effect.

There are four types of components that the NSO Java VM can handle:

• The ned type. The NSO Java VM will handle NEDs of sub-type cli and generic which are the ones that have a Java implementation.

• The callback type. These are any forms of callbacks that are defined by the DP API.

• The application type. These are user-defined daemons that implement a specific ApplicationComponent Java interface.

• The upgrade type. This component type is activated when deploying a new version of a NSO package and the NSO automatic CDB data upgrade is not sufficient. See Writing an Upgrade Package Component for more information.

In some situations, several NSO packages are expected to use the same code base, e.g. when third-party libraries are used or the code is structured with some common parts. Instead of duplicate jars in several NSO packages, it is possible to create a new NSO package, add these jars to the shared-jar directory, and let the package-meta-data.xml file contains no component definitions at all. The NSO Java VM will load these shared jars and these will be accessible from all other NSO packages.

Inside the NSO Java VM, each component type has a specific Component Manager. The responsibility of these Managers is to manage a set of component classes for each NSO package. The Component Manager acts as an FSM that controls when a component should be registered, started, stopped, etc.

For instance, the DpMuxManager controls all callback implementations (services, actions, data providers, etc). It can load, register, start, and stop such callback implementations.

The NED Component Type

NEDs can be of type netconf, snmp, cli, or generic. Only the cli and generic types are relevant for the NSO Java VM because these are the ones that have a Java implementation. Normally these NED components come in self-contained and prefabricated NSO packages for some equipment or class of equipment. It is however possible to tailor make NEDs for any protocol. For more information on this see Network Element Drivers (NEDs) and Writing a data model for a CLI NED in NED Development

The Callback Component Type

Callbacks are the collective name for a number of different functions that can be implemented in Java. One of the most important is the service callbacks, but also actions, transaction control, and data provision callbacks are in common use in an NSO implementation. For more on how to program callback using the DP API, see DP API.

The Application Component Type

For programs that are none of the above types but still need to access NSO as a daemon process, it is possible to use the ApplicationComponent Java interface. The ApplicationComponent interface expects the implementing classes to implement a init(), finish() and a run() method.

The NSO Java VM will start each class in a separate thread. The init() is called before the thread is started. The run() runs in a thread similar to the run() method in the standard Java Runnable interface. The finish() method is called when the NSO Java VM wants the application thread to stop. It is the responsibility of the programmer to stop the application thread i.e., stop the execution in the run() method when finish() is called. Note, that making the thread stop when finish() is called is important so that the NSO Java VM will not be hanging at a STOP_VM request.

package com.tailf.ncs;

/**
 * User defined Applications should implement this interface that
 * extends Runnable, hence also the run() method has to be implemented.
 * These applications are registered as components of type
 * "application" in a Ncs packages.
 *
 * Ncs Java VM will start this application in a separate thread.
 * The init() method is called before the thread is started.
 * The finish() method is expected to stop the thread. Hence stopping
 * the thread is user responsibility
 *
 */
public interface ApplicationComponent extends Runnable {

    /**
     * This method is called by the Ncs Java vm before the
     * thread is started.
     */
    public void init();

    /**
     * This method is called by the Ncs Java vm when the thread
     * should be stopped. Stopping the thread is the responsibility of
     * this method.
     */
    public void finish();

}

An example of an application component implementation is found in SNMP Notification Receiver.

The Resource Manager

User Implementations typically need resources like Maapi, Maapi Transaction, Cdb, Cdb Session, etc. to fulfill their tasks. These resources can be instantiated and used directly in the user code. This implies that the user code needs to handle connection and close of additional sockets used by these resources. There is however another recommended alternative, and that is to use the Resource manager. The Resource manager is capable of injecting these resources into the user code. The principle is that the programmer will annotate the field that should refer to the resource rather than instantiate it.

@Resource(type=ResourceType.MAAPI, scope=Scope.INSTANCE)
public Maapi m;

This way the NSO Java VM and the Resource manager can keep control over used resources and also can intervene e.g. close sockets at forced shutdowns.

The Resource manager can handle two types of resources: MAAPI and CDB.

package com.tailf.ncs.annotations;

/**
 * ResourceType set by the Ncs ResourceManager
 */
public enum ResourceType {

    MAAPI(1),
    CDB(2);
}

For both the Maapi and Cdb resource types a socket connection is opened towards NSO by the Resource manager. At a stop, the Resource manager will disconnect these sockets before ending the program. User programs can also tell the resource manager when its resources are no longer needed with a call to ResourceManager.unregisterResources().

The resource annotation has three attributes:

• type defines the resource type.

• scope defines if this resource should be unique for each instance of the Java class (Scope.INSTANCE) or shared between different instances and classes (Scope.CONTEXT). For CONTEXT scope the sharing is confined to the defining NSO package, i.e., a resource cannot be shared between NSO packages.

• qualifier is an optional string to identify the resource as a unique resource. All instances that share the same context-scoped resource need to have the same qualifier. If the qualifier is not given it defaults to the value DEFAULT i.e., shared between all instances that have the DEFAULT qualifier.

package com.tailf.ncs.annotations;

/**
 * Annotation class for Action Callbacks Attributes are callPoint and callType
 */
@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.FIELD)
public @interface Resource {

    public ResourceType type();

    public Scope scope();

    public String qualifier() default "DEFAULT";

}

package com.tailf.ncs.annotations;

/**
 * Scope for resources managed by the Resource Manager
 */
public enum Scope {

    /**
     * Context scope implies that the resource is
     * shared for all fields having the same qualifier in any class.
     * The resource is shared also between components in the package.
     * However sharing scope is confined to the package i.e sharing cannot
     * be extended between packages.
     * If the qualifier is not given it becomes "DEFAULT"
     */
    CONTEXT(1),
    /**
     * Instance scope implies that all instances will
     * get new resource instances. If the instance needs
     * several resources of the same type they need to have
     * separate qualifiers.
     */
    INSTANCE(2);
}

When the NSO Java VM starts it will receive component classes to load from NSO. Note, that the component classes are the classes that are referred to in the package-meta-data.xml file. For each component class, the Resource Manager will scan for annotations and inject resources as specified.

However, the package jars can contain lots of classes in addition to the component classes. These will be loaded at runtime and will be unknown by the NSO Java VM and therefore not handled automatically by the Resource Manager. These classes can also use resource injection but need a specific call to the Resource Manager for the mechanism to take effect. Before the resources are used for the first time the resource should be used, a call of ResourceManager.registerResources(...) will force the injection of the resources. If the same class is registered several times the Resource manager will detect this and avoid multiple resource injections.

MyClass myclass = new MyClass();
try {
    ResourceManager.registerResources(myclass);
} catch (Exception e) {
    LOGGER.error("Error injecting Resources", e);
}

The Alarm Centrals

The AlarmSourceCentral and AlarmSinkCentral, which is part of the NSO Alarm API, can be used to simplify reading and writing alarms. The NSO Java VM will start these centrals at initialization. User implementations can therefore expect this to be set up without having to handle the start and stop of either the AlarmSinkCentral or the AlarmSourceCentral. For more information on the alarm API, see Alarm Manager.

Embedding the NSO Java VM

As stated above the NSO Java VM is executed in a thread implemented by the NcsMain. This implies that somewhere a java main() must be implemented that launches this thread. For NSO this is provided by the NcsJVMLauncher class. In addition to this, there is a script named ncs-start-java-vm that starts Java with the NcsJVMLauncher.main(). This is the recommended way of launching the NSO Java VM and how it is set up in a default installation. If there is a need to run the NSO Java VM as an embedded thread inside another program. This can be done simply by instantiating the class NcsMain and starting this instance in a new thread.

NcsMain ncsMain   = NcsMain.getInstance(host);
Thread  ncsThread = new Thread(ncsMain);

ncsThread.start();

However, with the embedding of the NSO Java VM comes the responsibility to manage the life cycle of the NSO Java VM thread. This thread cannot be started before NSO has started and is running or else the NSO Java VM control socket connection will fail. Also, running NSO without the NSO Java VM being launched will render runtime errors as soon as NSO needs NSO Java VM functionality.

Logging

NSO has extensive logging functionality. Log settings are typically very different for a production system compared to a development system. Furthermore, the logging of the NSO daemon and the NSO Java VM is controlled by different mechanisms. During development, we typically want to turn on the developer-log. The sample ncs.conf that comes with the NSO release has log settings suitable for development, while the ncs.conf created by a System Install are suitable for production deployment.

The NSO Java VM uses Log4j for logging and will read its default log settings from a provided log4j2.xml file in the ncs.jar. Following that, NSO itself has java-vm log settings that are directly controllable from the NSO CLI. We can do:

admin@ncs(config)# java-vm java-logging logger com.tailf.maapi level level-trace
admin@ncs(config-logger-com.tailf.maapi)# commit
Commit complete.

This will dynamically reconfigure the log level for package com.tailf.maapi to be at the level trace. Where the Java logs end up is controlled by the log4j2.xml file. By default, the NSO Java VM writes to stdout. If the NSO Java VM is started by NSO, as controlled by the ncs.conf parameter /java-vm/auto-start, NSO will pick up the stdout of the service manager and write it to:

admin@ncs(config)# show full-configuration java-vm stdout-capture
java-vm stdout-capture file /var/log/ncs/ncs-java-vm.log

(The details pipe command also displays default values)

The NSO Java VM Timeouts

The section /ncs-config/api in ncs.conf contains a number of very important timeouts. See $NCS_DIR/src/ncs/ncs_config/tailf-ncs-config.yang and ncs.conf(5) in Manual Pages for details.

• new-session-timeout controls how long NSO will wait for the NSO Java VM to respond to a new session.

• query-timeout controls how long NSO will wait for the NSO Java VM to respond to a request to get data.

• connect-timeout controls how long NSO will wait for the NSO Java VM to initialize a DP connection after the initial socket connect.

• action-timeout controls how long NSO will wait for the NSO Java VM to respond to an action request callback.

For new-session-timeout, query-timeout, and connect-timeout, whenever any of these timeouts trigger, NSO will close the sockets from NSO to the NSO Java VM. The NSO Java VM will detect the socket close and exit.

For action-timeout, whenever this timeout triggers, NSO will close only the sockets from NSO Java VM to the clients without exiting the Java VM.

If NSO is configured to start (and restart) the NSO Java VM, the NSO Java VM will be automatically restarted. If the NSO Java VM is started by some external entity, if it runs within an application server, it is up to that entity to restart the NSO Java VM.

Debugging Startup

When using the auto-start feature (the default), NSO will start the NSO Java VM (as outlined in the start of this section), there are a number of different settings in the java-vm YANG model (see $NCS_DIR/src/ncs/yang/tailf-ncs-java-vm.yang) that controls what happens when something goes wrong during the startup.

The two timeout configurations connect-time and initialization-time are most relevant during startup. If the Java VM fails during the initial stages (during INIT_JVM, LOAD_SHARED_JARS, or LOAD_PACKAGE) either because of a timeout or because of a crash, NSO will log The NCS Java VM synchronization failed in ncs.log.

After logging, NSO will take action based on the synchronization-timeout-action setting:

• log: NSO will log the failure, and if auto-restart is set to true NSO will try to restart the Java VM

• log-stop (default): NSO will log the failure, and if the Java VM has not stopped already NSO will also try to stop it. No restart action is taken.

• exit: NSO will log the failure, and then stop NSO itself.

If you have problems with the Java VM crashing during startup, a common pitfall is running out of memory (either total memory on the machine, or heap in the JVM). If you have a lot of Java code (or a loaded system) perhaps the Java VM did not start in time. Try to determine the root cause, check ncs.log and ncs-java-vm.log, and if needed increase the timeout.

For complex problems, for example with the class loader, try logging the internals of the startup:

admin@ncs(config)# java-vm java-logging logger com.tailf.ncs level level-all
admin@ncs(config-logger-com.tailf.maapi)# commit
Commit complete.

Setting this will result in a lot more detailed information in ncs-java-vm.log during startup.

When the auto-restart setting is true (the default), it means that NSO will try to restart the Java VM when it fails (at any point in time, not just during startup). NSO will at most try three restarts within 30 seconds, i.e., if the Java VM crashes more than three times within 30 seconds NSO gives up. You can check the status of the Java VM using the java-vm YANG model. For example in the CLI:

admin@ncs# show java-vm
java-vm start-status started
java-vm status running

The start-status can have the following values:

• auto-start-not-enabled: Autostart is not enabled.

• stopped: The Java VM has been stopped or is not yet started.

• started: The Java VM has been started. See the leaf 'status' to check the status of the Java application code.

• failed: The Java VM has been terminated. If auto-restart is enabled, the Java VM restart has been disabled due to too frequent restarts.

The status can have the following values:

• not-connected: The Java application code is not connected to NSO.

• initializing: The Java application code is connected to NSO, but not yet initialized.

• running: The Java application code is connected and initialized.

• timeout: The Java application connected to NSO, but failed to initialize within the stipulated timeout 'initialization-time'.