# Applications in NSO Services provide the foundation for managing the configuration of a network. But this is not the only aspect of network automation. A holistic solution must also consider various verification procedures, one-time actions, monitoring, and so on. This is quite different from managing configuration. NSO helps you implement such automation use cases through a generic application framework. This section explores the concept of services as more general NSO applications. It gives an overview of the mechanisms for orchestrating network automation tasks that require more than just configuration provisioning. ## NSO Architecture You have seen two different ways in which you can make a configuration change on a network device. With the first, you make changes directly on the NSO copy of the device configuration. The Device Manager picks up the changes and propagates them to the affected devices. The purpose of the Device Manager is to manage different devices uniformly. The Device Manager uses the Network Element Drivers (NEDs) to abstract away the different protocols and APIs towards the devices. The NED contains a YANG data model for a supported device. So, each device type requires an appropriate NED package that allows the Device Manager to handle all devices in the same, YANG-model-based way. The second way to make configuration changes is through services. Here, the Service Manager adds a layer on top of the Device Manager to process the service request and enlists the help of service-aware applications to generate the device changes. The following figure illustrates the difference between the two approaches.

Device and Service Manager

The Device Manager and the Service Manager are tightly integrated into one transactional engine, using the CDB to store data. Another thing the two managers have in common is packages. Like Device Manager uses NED packages to support specific devices, Service Manager relies on service packages to provide an application-specific mapping for each service type. However, a network application can consist of more than just a configuration recipe. For example, an integrated service test action can verify the initial provisioning and simplify troubleshooting if issues arise. A simple test might run the `ping` command to verify connectivity. Or an application could only monitor the network and not produce any configuration at all. That is why NSO actually uses an approach where an application chooses what custom code to execute for specific NSO events. ## Callbacks as an Extension Mechanism NSO allows augmenting the base functionality of the system by delegating certain functions to applications. As the communication must happen on demand, NSO implements a system of callbacks. Usually, the application code registers the required callbacks on start-up, and then NSO can invoke each callback as needed. A prime example is a Python service, which registers the `cb_create()` function as a service callback that NSO uses to construct the actual configuration.

Service Callback

In a Python service skeleton, callback registration happens inside a class `Main`, found in `main.py`: ```python class Main(ncs.application.Application): def setup(self): # Service callbacks require a registration for a 'service point', # as specified in the corresponding data model. # self.register_service('my-svc-servicepoint', ServiceCallbacks) ``` In this code, the `register_service()` method registers the `ServiceCallbacks` class to receive callbacks for a service. The first argument defines which service that is. In theory, a single class could even handle service callbacks for multiple services but that is not a common practice. On the other hand, it is also possible that no code registered a callback for a given service. This is quite often a result of a misspelling or a bug in the code that causes the application code to crash. In these situations, NSO presents an error if you try to use the service: ``` Error: no registration found for callpoint my-svc-servicepoint/service_create of type=external ``` This error refers to the concept of a service point. Service points are declared in the service YANG model and allow NSO to distinguish ordinary data from services. They instruct NSO to invoke FASTMAP and the service callbacks when a service instance is being provisioned. That means the service skeleton YANG file also contains a service point definition, such as the following: ```yang list my-svc { description "This is an RFS skeleton service"; uses ncs:service-data; ncs:servicepoint my-svc-servicepoint; } ``` Service point therefore links the definition in the model with custom code. Some methods in the code will have names starting with `cb_`, for instance, the `cb_create()` method, letting you know quickly that they are an implementation of a callback. NSO implements additional callbacks for each service point, that may be required in some specific circumstances. Most of these callbacks perform work outside of the automatic change tracking, so you need to consider that before using them. The section [Service Callbacks](https://nso-docs.cisco.com/guides/development/introduction-to-automation/pages/Bw8TviXCSEsM9XjXBC1d#ch_svcref.cbs) offers more details. As well as services, other extensibility options in NSO also rely on callbacks and `callpoints`, a generalized version of a service point. Two notable examples are validation callbacks, to implement additional validation logic to that supported by YANG, and custom actions. The section [Overview of Extension Points](#overview-of-extension-points) provides a comprehensive list and an overview of when to use each. In summary, you implement custom behavior in NSO by providing the following three parts: * A YANG model directing NSO to use callbacks, such as a service point for services. * Registration of callbacks, telling NSO to call into your code at a given point. * The implementation of each callback with your custom logic. This way, an application in NSO can implement all the required functionality for a given use case (configuration management and otherwise) by registering the right callbacks. ## Overview of Extension Points NSO supports a number of extension points for custom callbacks:
TypeSupported InYANG ExtensionDescription
ServicePython, Java, Erlangncs:servicepointTransforms a list or container into a model for service instances. When the configuration of a service instance changes, NSO invokes Service Manager and FASTMAP, which may call service create and similar callbacks. See Developing a Simple Service for an introduction.
ActionPython, Java, Erlangtailf:actionpointDefines callbacks when an action or RPC is invoked. See Actions for an introduction.
ValidationPython, Java, Erlangtailf:validateDefines callbacks for additional validation of data when the provided YANG functionality, such as must and unique statements are insufficient. See the respective API documentation for examples; the section ValidationPoint Handler (Python), the section Validation Callbacks (Java), and Embedded Erlang applications (Erlang).
Data ProviderJava, Python (low-level API with experimental high-level API), Erlangtailf:callpointDefines callbacks for transparently accessing external data (data not stored in the CDB) or callbacks for special processing of data nodes (transforms, set, and transaction hooks). Requires careful implementation and understanding of transaction intricacies. Rarely used in NSO.
Each extension point in the list has a corresponding YANG extension that defines to which part of the data model the callbacks apply, as well as the individual name of the call point. The name is required during callback registration and helps distinguish between multiple uses of the extension. Each extension generally specifies multiple callbacks, however, you often need to implement only the main one, e.g. create for services or action for actions. In addition, NSO supports some specific callbacks from internal systems, such as the transaction or the authorization engine, but these have very narrow use and are in general not recommended. ## Monitoring for Change Services and actions are examples of something that happens directly as a result of a user (or other northbound agent) request. That is, a user takes an active role in starting service instantiation or invoking an action. Contrast this to a change that happens in the network and requires the orchestration system to take some action. In this latter case, the system monitors the notifications that the network generates, such as losing a link, and responds to the new data. NSO provides out-of-the-box support for the automation of not only notifications but also changes to the operational and configuration data, using the concept of kickers. With kickers, you can watch for a particular change to occur in the system and invoke a custom action that handles the change. The kicker system is further described in [Kicker](/guides/development/advanced-development/kicker.md). ## Running Application Code Services, actions, and other features all rely on callback registration. In Python code, the class responsible for registration derives from the `ncs.application.Application`. This allows NSO to manage the application code as appropriate, such as starting and stopping in response to NSO events. These events include package load or unload and NSO start or stop events. While the Python package skeleton names the derived class `Main`, you can choose a different name if you also update the `package-meta-data.xml` file accordingly. This file defines a component with the name of the Python class to use: ```xml < ... output omitted ... > main dns_config.main.Main ``` When starting the package, NSO reads the class name from `package-meta-data.xml`, starts the Python interpreter, and instantiates a class instance. The base `Application` class takes care of establishing communication with the NSO process and calling the `setup` and `teardown` methods. The two methods are a good place to do application-specific initialization and cleanup, along with any callback registrations you require. The communication between the application process and NSO happens through a dedicated control socket, as described in the section called [IPC Ports](/guides/administration/advanced-topics/ipc-connection.md) in Administration. This setup prevents a faulty application from bringing down the whole system along with it and enables NSO to support different application environments. In fact, NSO can manage applications written in Java or Erlang in addition to those in Python. If you replace the `python-class-name` element of a component with `java-class-name` in the `package-meta-data.xml` file, NSO will instead try to run the specified Java class in the managed Java VM. If you wanted to, you could implement all of the same services and actions in Java, too. For example, see [Service Actions](https://nso-docs.cisco.com/guides/development/introduction-to-automation/pages/1ErWKzeM15TwAei4yXWm#ch_services.actions) to compare Python and Java code. Regardless of the programming language you use, the high-level approach to automation with NSO does not change, registering and implementing callbacks as part of your network application. Of course, the actual function calls (the API) and other specifics differ for each language. The [NSO Python VM](/guides/development/core-concepts/nso-virtual-machines/nso-python-vm.md), [NSO Java VM](/guides/development/core-concepts/nso-virtual-machines/nso-java-vm.md), and [Embedded Erlang Applications](/guides/development/core-concepts/nso-virtual-machines/embedded-erlang-applications.md) cover the details. Even so, the concepts of actions, services, and YANG modeling remain the same. As you have seen, everything in NSO is ultimately tied to the YANG model, making YANG knowledge such a valuable skill for any NSO developer. ## Application Timeouts NSO uses socket communication to coordinate work with applications, such as a Python or Java service. In addition to the control socket, NSO uses a number of worker sockets to process individual requests: performing service mapping or executing an action, for example. We collectively call these data provider applications, since the data provider protocol underpins all of them. The communication with data provider applications is subject to timeouts in order to manage the execution time of requests. These are defined in section `/ncs-config/api` in `ncs.conf`: * `ncs-config/api/action-timeout` * `ncs-config/api/query-timeout` * `ncs-config/api/new-session-timeout` * `ncs-config/api/connect-timeout` For executing actions invoked by the clients, NSO uses `action-timeout` to ensures the response from data provider is received within the given time. If the data provider fails to do so within the stipulated timeout, NSO will kill the worker sockets executing the actions and trigger the abort action defined in `cb_abort()` without restarting the NSO VMs. The following code shows a trivial implementation of an abort action callback: ```python class MyTestAction(Action): def cb_abort(self, uinfo): self.log.info('Action aborted: ') ``` There are some important points worth noting for action timeout: * An action callback that times out in one user instance will not affect the result of an action callback in another user instance. This is because NSO executes actions using multiple worker sockets, and an action timeout will only terminate the worker socket executing that specific action. * Implementing your own abort action callback in `cb_abort` allows you to handle actions that are timing out. If `cb_abort` is not defined, NSO cannot trigger the abort action during a timeout, preventing it from unlocking the action for a user session. Consequently, you must wait for the action callback to finish before attempting it again. {% hint style="info" %} See [examples.ncs/sdk-api/action-abort-py](https://github.com/NSO-developer/nso-examples/tree/6.7/sdk-api/action-abort-py) for an example of how to implement an abortable Python action that spawns a separate worker process using the multiprocessing library and returns the worker's outcome via a result queue or terminates the worker if the action is aborted. {% endhint %} For NSO operational data queries, NSO uses `query-timeout` to ensure the data provider return operational data within the given time. If the data provider fails to do so within the stipulated timeout, NSO will close its end of the control socket to the data provider. The NSO VMs will detect the socket close and exit. For connection initiation requests between NSO and data providers, NSO uses `connect-timeout` to ensure the data provider send the initial message after connecting the socket to NSO within the given time. If the data provider fails to do so within the stipulated timeout, NSO will close its end of the control socket to the data provider. The NSO VMs will detect the socket close and exit. For requests invoked by NSO, NSO uses `new-session-timeout` to ensure the data provider respond to the control socket request within the given time. If the data provider fails to do so within the stipulated timeout, NSO will close its end of the control socket to the data provider. The NSO VMs will detect the socket close and exit. ## Application Updates As your NSO application evolves, you will create newer versions of your application package, which will replace the existing one. If the application becomes sufficiently complex, you might even split it across multiple packages. When you replace a package, NSO must redeploy the application code and potentially replace the package-provided part of the YANG schema. For the latter, NSO can perform the data migration for you, as long as the schema is backward compatible. This process is documented in [Automatic Schema Upgrades and Downgrades](https://nso-docs.cisco.com/guides/development/introduction-to-automation/pages/FxpCNgv5QKnfWrJw4nXf#ug.cdb.upgrade) and is automatic when you request a reload of the package with `packages reload` or a similar command. If your schema changes are not backward compatible, you can implement a data migration procedure, which NSO invokes when upgrading the schema. Among other things, this allows you to reuse and migrate the data that is no longer present in the new schema. You can specify the migration procedure as part of the `package-meta-data.xml` file, using a component of the `upgrade` type. See [The Upgrade Component](https://nso-docs.cisco.com/guides/development/introduction-to-automation/pages/jwFVej1l7kjXuaM9axfN#ncs.development.pythonvm.upgrade) (Python) and [examples.ncs/service-management/upgrade-service](https://github.com/NSO-developer/nso-examples/tree/6.7/service-management/upgrade-service) example (Java) for details. Note that changing the schema in any way requires you to recompile the `.fxs` files in the package, which is typically done by running `make` in the package's `src` folder. However, if the schema does not change, you can request that only the application code and templates be redeployed by using the ` packages package`` `` `*`my-pkg`*` `` ``redeploy ` command. --- # Agent Instructions: Querying This Documentation If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question. 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