The Model-Driven Service Adaptation Layer (MD-SAL) is message-bus inspired extensible middleware component that provides messaging and data storage functionality based on data and interface models defined by application developers (i.e. user-defined models).
Defines a common-layer, concepts, data model building blocks and messaging patterns and provides infrastructure / framework for applications and inter-application communication.
Provide common support for user-defined transport and payload formats, including payload serialization and adaptation (e.g. binary, XML or JSON).
The MD-SAL uses YANG as the modeling language for both interface and data definitions, and provides a messaging and data-centric runtime for such services based on YANG modeling.
The MD-SAL provides two different API types (flavours):
MD-SAL Binding: MD-SAL APIs which extensively uses APIs and classes generated from YANG models, which provides compile-time safety.
MD-SAL DOM: (Document Object Model) APIs which uses DOM-like representation of data, which makes them more powerful, but provides less compile-time safety.
Model-driven nature of the MD-SAL and DOM-based APIs allows for behind-the-scene API and payload type mediation and transformation to facilitate seamless communication between applications - this enables for other components and applications to provide connectors / expose different set of APIs and derive most of its functionality purely from models, which all existing code can benefit from without modification. For example RESTCONF Connector is an application built on top of MD-SAL and exposes YANG-modeled application APIs transparently via HTTP and adds support for XML and JSON payload type.
Basic concepts are building blocks which are used by applications, and from which MD-SAL uses to define messaging patterns and to provide services and behavior based on developer-supplied YANG models.
- Data Tree
All state-related data are modeled and represented as data tree, with possibility to address any element / subtree
Operational Data Tree - Reported state of the system, published by the providers using MD-SAL. Represents a feedback loop for applications to observe state of the network / system.
Configuration Data Tree - Intended state of the system or network, populated by consumers, which expresses their intention.
- Instance Identifier
Unique identifier of node / subtree in data tree, which provides unambiguous information, how to reference and retrieve node / subtree from conceptual data trees.
Asynchronous transient event which may be consumed by subscribers and they may act upon it.
asynchronous request-reply message pair, when request is triggered by consumer, send to the provider, which in future replies with reply message.
In MD-SAL terminology, the term ‘RPC’ is used to define the input and output for a procedure (function) that is to be provided by a provider, and mediated by the MD-SAL, that means it may not result in remote call.
MD-SAL provides several messaging patterns using broker derived from basic concepts, which are intended to transfer YANG modeled data between applications to provide data-centric integration between applications instead of API-centric integration.
Remote Procedure Calls - unicast between consumer and provider, where consumer sends request message to provider, which asynchronously responds with reply message.
Publish / Subscribe
Notifications - multicast transient message which is published by provider and is delivered to subscribers.
Data Change Events - multicast asynchronous event, which is sent by data broker if there is change in conceptual data tree, and is delivered to subscribers.
Transactional access to Data Tree
Transactional reads from conceptual data tree - read-only transactions with isolation from other running transactions.
Transactional modification to conceptual data tree - write transactions with isolation from other running transactions.