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 to some common schema. For example an Interest Rate Swap contract and an Equity OTC Option contract might both provide a mapping to a common Derivative schema. The schemas should typically not be part of the contract itself and should exist independently of it to encourage re-use of a common set within a particular business area or Cordapp.

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

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/database/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/CustomVaultQuery.kt

which is then referenced within a custom flow:

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

Database Migration

As a database migration tool, we use the open source library liquibase <http://www.liquibase.org/>.

If migration is enabled, the database state is checked (and updated) during node startup. (After deploying a new version of the code that contains database migrations, they are executed at that point). Possible database changes range from schema changes to data changes.

Liquibase will create a table called DATABASECHANGELOG, that will store useful information about each change ( like timestamp, description, user, md5 hash so it can't be changed, etc) We can also "tag" the database at each release to make rollback easier.

Database changes are maintained in several xml files per MappedSchema, so that only migrations corresponding to the node’s configured schemas are run. For example, on the node-info schema, if there are any database changes for release 12, the changes will be added to a new file called: node-info.changelog-v12.xml which has to be included in node-info.changelog-master.xml.

Example:

Let's suppose that at some point, at version 12, there is a need to add a new column: contentSize to the DBAttachment entity.

This means we have to:

• In the source code, add the contentSize property and map it to a new column.
• create the column in the node_attachments table.
• Run an update to set the size of all existing attachments, to not break the code that uses the new property

class DBAttachment(
    ...
    @Column(name = "content")
    @Lob
    var content: ByteArray,

    //newly added column
    @Column(name = "content_size")
    var contentSize: Int,
    ...
)

The DBAttachment entity is included in the NodeServicesV1 schema, so we create a file node-services.changelog-v12.xml with this changeset:

<changeSet author="developer_name" id="add content_size column">
    <addColumn tableName="node_attachments">
        <column name="content_size" type="INT"/>
    </addColumn>
    <update tableName="node_attachments">
        <column name="content_size" valueComputed="length(content)"/>
    </update>
    <rollback>
        <dropColumn tableName="node_attachments" columnName="content_size"/>
    </rollback>
</changeSet>

And include it in `node-services.changelog-master.xml`:

<databaseChangeLog>
    <!--the original schema-->
    <include file="migration/node-services.changelog-init.xml"/>

    <!--migrations from previous releases-->
    <include file="migration/node-services.changelog-v4.xml"/>
    <include file="migration/node-services.changelog-v7.xml"/>

    <!--added now-->
    <include file="migration/node-services.changelog-v12.xml"/>
</databaseChangeLog>

By adding the rollback script, we give users the option to revert to an older version of the software.

An easy way to manage the db version is to tag it on every release (or on every release that has migrations) <http://www.liquibase.org/documentation/changes/tag_database.html>

Usage:

Configurations:

• To enable migration at startup, set:

  • database.runMigration = true // true by default

Command line arguments:

• To export the migration to a file use —just-generate-database-migration outputSqlFile. This will generate the delta from the last release, and will output the resulting sql into the outputSqlFile. It will not write to the db. It will not start the node! ( default value for outputSqlFile is a .sql file with the current date )
• To run the migration without starting the node: --just-run-db-migration