MD-SAL Binding Query Language User Guide¶
Query language based API for work with YANG based models. MD-SAL component provides a binding query language to interact with the underlying data store. This API provides an easy and type-safe mechanism for retrieving and processing data from generated DOM based on queries. On the DOM layer the expression can be transmitted, and it gives the possibility to move the execution to the storage backend. This can reduce app/backend interchange data. This API is a part of the MD-SAL component and can be found inside the org.opendaylight.mdsal.binding.api.query package.
QueryExpression - Built sequence-based expression. QueryExpression is similar to an SQL query expression. While SQL operates on tables and rows, QueryExpression operates on a subtree. Creates by QueryFactory.
QueryExecutor - Interface to execute query expression and retrieve execution result.
QueryResult - Result execution of QueryExpression by QueryExecutor.
Query result will contain Objects which can be represented using the next methods:
stream - Returns sequential Stream of values from the query result.
parallelStream - Returns parallel Stream of values from the query result.
getValues - Returns List of generic Objects from the query result.
getItems - Returns List of Items(Object and InstanceIdentifier) from query result.
A QueryExpression is built up of three items, which specify what to search, looking for something matching a predicate. This is similar in structure to being an SQL query: FROM what SELECT something WHERE predicate.
Query execution workflow:
Query Factory -> Root Path -> Query Builder -> (Extract child node -> Matcher) -> build
Query Factory - Primary entry point for creating Query.
Root Path - Specify Subtree root path for start from the query. This corresponds to the what part of a query. Just as with SQL tables, this path has to point to at most one item.
Query Builder - Intermediate builder stage, which allows providing a specification of the query. On query completed call _build_ method finalize the creation of simple query.
Extract child node - Add a child path component to the query specification of what needs to be extracted. This constitutes an intermediate step of specifying the something part of what needs to be found.
Matcher - Specify a matching pattern for request using Leaf’s getter method and appropriate matcher. This constitutes the predicate part of the query. Every candidate has to match pattern for the request.
Child node can be specified in the next ways:
using child container class;
using an exact match in a keyed list using List and Key types;
using child case class and child class;
Leaf’s value can be retrieved passing method reference from container type.
Query engine supports Empty, string, int8, int16, int32, int64, uint8, uint16, uint32, uint64, Identity, TypeObject leaf’s value types. Appropriate MatchBuilder pattern applied according to leaf’s value type.
Create a simple executor:
QueryExecutor executor = SimpleQueryExecutor.builder(CODEC) .add(new FooBuilder() .setSystem(BindingMap.of( new SystemBuilder().setName("SystemOne").setAlarms(BindingMap.of( new AlarmsBuilder() .setId(Uint64.ZERO) .setCritical(Empty.getInstance()) .setAffectedUsers(BindingMap.of()).build(), new AlarmsBuilder() .setId(Uint64.ONE) .setAffectedUsers(BindingMap.of()).build())) .build(), new SystemBuilder().setName("SystemTwo").setAlarms(BindingMap.of( new AlarmsBuilder() .setId(Uint64.ZERO) .setCritical(Empty.getInstance()) .setAffectedUsers(BindingMap.of( )).build())).build())) .build()) .build();
Create query expression and execute it using executor above:
QueryExpression<System> query = new DefaultQueryFactory(CODEC).querySubtree(InstanceIdentifier.create(Foo.class)) .extractChild(System.class) .matching() .leaf(System::getName).contains("One") .build(); final QueryResult result = executor.executeQuery(query); List items = result.getItems();
This expression will retrieve System node with name containing “One” from DOM tree.
QueryExpression<Alarms> query = new DefaultQueryFactory(CODEC).querySubtree(InstanceIdentifier.create(Foo.class)) .extractChild(System.class) .extractChild(Alarms.class) .matching() .leaf(Alarms::getId).valueEquals(Uint64.ZERO) .build(); final QueryResult result = executor.executeQuery(query); List items = result.getItems();
The result of this query expression will be a list of two items - Alarms with Id of ZERO.