corda/docs/source/api-persistence.rst

API: Persistence

Corda offers developers the option to expose all or some part of a contract state to an Object Relational Mapping (ORM) tool to be persisted in a RDBMS. The purpose of this is to assist vault development by effectively indexing persisted contract states held in the vault for the purpose of running queries over them and to allow relational joins between Corda data and private data local to the organisation owning a node.

The ORM mapping is specified using the Java Persistence API (JPA) as annotations and is converted to database table rows by the node automatically every time a state is recorded in the node's local vault as part of a transaction.

Note

Presently the node includes an instance of the H2 database but any database that supports JDBC is a candidate and the node will in the future support a range of database implementations via their JDBC drivers. Much of the node internal state is also persisted there. You can access the internal H2 database via JDBC, please see the info in "node-administration" for details.

Schemas

Every ContractState can implement the QueryableState interface if it wishes to be inserted into the node's local database and accessible using SQL.

../../core/src/main/kotlin/net/corda/core/schemas/PersistentTypes.kt

The QueryableState interface requires the state to enumerate the different relational schemas it supports, for instance in cases where the schema has evolved, with each one being represented by a MappedSchema object return by the supportedSchemas() method. Once a schema is selected it must generate that representation when requested via the generateMappedObject() method which is then passed to the ORM.

Nodes have an internal SchemaService which decides what to persist and what not by selecting the MappedSchema to use.

../../node/src/main/kotlin/net/corda/node/services/api/SchemaService.kt

../../core/src/main/kotlin/net/corda/core/schemas/PersistentTypes.kt

The SchemaService can be configured by a node administrator to select the schemas used by each app. In this way the relational view of ledger states can evolve in a controlled fashion in lock-step with internal systems or other integration points and not necessarily with every upgrade to the contract code. It can select from the MappedSchema offered by a QueryableState, automatically upgrade to a later version of a schema or even provide a MappedSchema not originally offered by the QueryableState.

It is expected that multiple different contract state implementations might provide mappings within a single schema. For example an Interest Rate Swap contract and an Equity OTC Option contract might both provide a mapping to a Derivative contract within the same schema. The schemas should typically not be part of the contract itself and should exist independently to encourage re-use of a common set within a particular business area or Cordapp.

Note

It's advisable to avoid cross-references between different schemas as this may cause issues when evolving MappedSchema or migrating its data. At startup, nodes log such violations as warnings stating that there's a cross-reference between MappedSchema's. The detailed messages incorporate information about what schemas, entities and fields are involved.

MappedSchema offer a family name that is disambiguated using Java package style name-spacing derived from the class name of a schema family class that is constant across versions, allowing the SchemaService to select a preferred version of a schema.

The SchemaService is also responsible for the SchemaOptions that can be configured for a particular MappedSchema which allow the configuration of a database schema or table name prefixes to avoid any clash with other MappedSchema.

Note

It is intended that there should be plugin support for the SchemaService to offer the version upgrading and additional schemas as part of Cordapps, and that the active schemas be configurable. However the present implementation offers none of this and simply results in all versions of all schemas supported by a QueryableState being persisted. This will change in due course. Similarly, it does not currently support configuring SchemaOptions but will do so in the future.

Custom schema registration

Custom contract schemas are automatically registered at startup time for CorDapps. The node bootstrap process will scan for schemas (any class that extends the MappedSchema interface) in the plugins configuration directory in your CorDapp jar.

For testing purposes it is necessary to manually register the packages containing custom schemas as follows:

• Tests using MockNetwork and MockNode must explicitly register packages using the cordappPackages parameter of MockNetwork
• Tests using MockServices must explicitly register packages using the cordappPackages parameter of the MockServices makeTestDatabaseAndMockServices() helper method.

Note

Tests using the DriverDSL will automatically register your custom schemas if they are in the same project structure as the driver call.

Object relational mapping

The persisted representation of a QueryableState should be an instance of a PersistentState subclass, constructed either by the state itself or a plugin to the SchemaService. This allows the ORM layer to always associate a StateRef with a persisted representation of a ContractState and allows joining with the set of unconsumed states in the vault.

The PersistentState subclass should be marked up as a JPA 2.1 Entity with a defined table name and having properties (in Kotlin, getters/setters in Java) annotated to map to the appropriate columns and SQL types. Additional entities can be included to model these properties where they are more complex, for example collections, so the mapping does not have to be flat. The MappedSchema must provide a list of all of the JPA entity classes for that schema in order to initialise the ORM layer.

Several examples of entities and mappings are provided in the codebase, including Cash.State and CommercialPaper.State. For example, here's the first version of the cash schema.

../../finance/src/main/kotlin/net/corda/finance/schemas/CashSchemaV1.kt

Note

If Cordapp needs to be portable between Corda OS (running against H2) and Corda Enterprise (running against a standalone database), consider database vendors specific requirements. Ensure that table and column names are compatible with the naming convention of the database vendors for which the Cordapp will be deployed, e.g. for Oracle database, prior to version 12.2 the maximum length of table/column name is 30 bytes (the exact number of characters depends on the database encoding).

Identity mapping

Schema entity attributes defined by identity types (AbstractParty, Party, AnonymousParty) are automatically processed to ensure only the X500Name of the identity is persisted where an identity is well known, otherwise a null value is stored in the associated column. To preserve privacy, identity keys are never persisted. Developers should use the IdentityService to resolve keys from well know X500 identity names.

JDBC session

Apps may also interact directly with the underlying Node's database by using a standard JDBC connection (session) as described by the Java SQL Connection API

Use the ServiceHub jdbcSession function to obtain a JDBC connection as illustrated in the following example:

../../node/src/test/kotlin/net/corda/node/services/persistence/HibernateConfigurationTest.kt

JDBC session's can be used in Flows and Service Plugins (see "flow-state-machines")

The following example illustrates the creation of a custom corda service using a jdbcSession:

../../docs/source/example-code/src/main/kotlin/net/corda/docs/kotlin/CustomVaultQuery.kt

which is then referenced within a custom flow:

../../docs/source/example-code/src/main/kotlin/net/corda/docs/kotlin/CustomVaultQuery.kt

For examples on testing @CordaService implementations, see the oracle example here <oracles>

JPA Support

In addition to jdbcSession, ServiceHub also exposes the Java Persistence API to flows via the withEntityManager method. This method can be used to persist and query entities which inherit from MappedSchema. This is particularly useful if off-ledger data must be maintained in conjunction with on-ledger state data.

Note

Your entity must be included as a mappedType in as part of a MappedSchema for it to be added to Hibernate as a custom schema. See Samples below.

The code snippet below defines a PersistentFoo type inside FooSchemaV1. Note that PersistentFoo is added to a list of mapped types which is passed to MappedSChema. This is exactly how state schemas are defined, except that the entity in this case should not subclass PersistentState (as it is not a state object). See examples:

public class FooSchema {}

@CordaSerializable
public class FooSchemaV1 extends MappedSchema {
    FooSchemaV1() {
        super(FooSchema.class, 1, ImmutableList.of(PersistentFoo.class));
    }

    @Entity
    @Table(name = "foos")
    class PersistentFoo implements Serializable {
        @Id
        @Column(name = "foo_id")
        String fooId;

        @Column(name = "foo_data")
        String fooData;
    }
}

object FooSchema

object FooSchemaV1 : MappedSchema(schemaFamily = FooSchema.javaClass, version = 1, mappedTypes = listOf(PersistentFoo::class.java)) {
    @Entity
    @Table(name = "foos")
    class PersistentFoo(@Id @Column(name = "foo_id") var fooId: String, @Column(name = "foo_data") var fooData: String) : Serializable
}

Instances of PersistentFoo can be persisted inside a flow as follows:

PersistentFoo foo = new PersistentFoo(new UniqueIdentifier().getId().toString(), "Bar");
node.getServices().withEntityManager(entityManager -> {
    entityManager.persist(foo);
    entityManager.flush();
    return null;
});

val foo = FooSchemaV1.PersistentFoo(UniqueIdentifier().id.toString(), "Bar")
serviceHub.withEntityManager {
    persist(foo)
}

And retrieved via a query, as follows:

node.getServices().withEntityManager((EntityManager entityManager) -> {
    CriteriaQuery<PersistentFoo> query = entityManager.getCriteriaBuilder().createQuery(PersistentFoo.class);
    Root<PersistentFoo> type = query.from(PersistentFoo.class);
    query.select(type);
    return entityManager.createQuery(query).getResultList();
});

val result: MutableList<FooSchemaV1.PersistentFoo> = services.withEntityManager {
    val query = criteriaBuilder.createQuery(FooSchemaV1.PersistentFoo::class.java)
    val type = query.from(FooSchemaV1.PersistentFoo::class.java)
    query.select(type)
    createQuery(query).resultList
}

Please note that suspendable flow operations such as:

• FlowSession.send
• FlowSession.receive
• FlowLogic.receiveAll
• FlowLogic.sleep
• FlowLogic.subFlow

Cannot be used within the lambda function passed to withEntityManager.