Berkeley DB Java Edition
version 6.2.7

Package com.sleepycat.persist.evolve

Utilities for managing class evolution of persistent objects.

See: Description

Package com.sleepycat.persist.evolve Description

Utilities for managing class evolution of persistent objects.

Class Evolution

For persistent data that is not short lived, changes to persistent classes are almost inevitable. Some changes are compatible with existing types, and data conversion for these changes is performed automatically and transparently. Other changes are not compatible with existing types. Mutations can be used to explicitly manage many types of incompatible changes.

Not all incompatible class changes can be handled via mutations. For example, complex refactoring may require a transformation that manipulates multiple entity instances at once. Such changes are not possible with mutations but can be made by performing a store conversion.

The different categories of type changes are described below.

Key Field Changes

Unlike entity data, key data is not versioned. Therefore, the physical key format for an index is fixed once the index has been opened, and the changes allowed for key fields are very limited. The only changes allowed for key fields are:

Any other changes to a key field are incompatible and may be made only by performing a store conversion.

Key ordering, including the behavior of a custom Comparable, is also fixed, since keys are stored in order in the index. The specifications for key ordering may not be changed, and the developer is responsible for not changing the behavior of a Comparable key class. WARNING:: Changing the behavior of a Comparable key class is likely to make the index unusable.

Compatible Type Changes

Entity data, unlike key data, is versioned. Therefore, some changes can be made compatibly and other changes can be handled via mutations. Compatible changes are defined below. To make a compatible class change, a mutation is not required; however, the class version must be assigned a new (greater) integer value.

Changes to a class hierarchy are compatible in some cases. A new class may be inserted in the hierarchy. A class may be deleted from the hierarchy as long as one of the following is true: 1) it contains no persistent fields, 2) any persistent fields are deleted with field Deleter mutations, or 3) the class is deleted with a class Deleter mutation. Classes in an existing hierarchy may not be reordered compatibly, and fields may not moved from one class to another compatibly; for such changes a class Converter mutation is required.

Changes to field types in entity class definitions are compatible when they conform to the Java Language Specification definitions for Widening Primitive Conversions and Widening Reference Conversions. For example, a smaller integer type may be changed to a larger integer type, and a reference type may be changed to one of its supertypes. Automatic widening conversions are performed as described in the Java Language Specification.

Primitive types may also be compatibly changed to their corresponding primitive wrapper types, or to the wrapper type for a widened primitive type. However, changing from a primitive wrapper type to a primitive type is not a compatible change since existing null values could not be represented.

Integer primitive types (byte, short, char, int, long) and their primitive wrapper types may be compatibly changed to the BigInteger type.

Enum values may be added compatibly, but may not be deleted or renamed. As long as new values are declared after existing values, the default sort order for enum key fields will match the declaration order, i.e, the default sort order will match the enum ordinal order. If a new value is inserted (declared before an existing value), it will be sorted after all existing values but before newly added values. However, these ordering rules are only guaranteed for enums containing up to 631 values and only if existing values are not reordered. If more than 631 values are declared or the declarations of existing values are reordered, then the default sort order will be arbitrary and will not match the declaration (ordinal) order.

In addition, adding fields to a class is a compatible change. When a persistent instance of a class is read that does not contain the new field, the new field is initialized by the default constructor.

All other changes to instance fields are considered incompatible. Incompatible changes may be handled via mutations, as described next.

Note that whenever a class is changed, either compatibly or incompatibly, a new (higher) class version number must be assigned. See Entity.version() and Persistent.version() for information on assigning class version numbers.

Mutations

There are three types of mutations: Renamer, Deleter and Converter.

A class or field can be renamed using a Renamer. Renaming is not expensive, since it does not involve conversion of instance data.

A class or field can be deleted using a Deleter.

Other incompatible changes are handled by creating a Converter mutation and implementing a Conversion.convert method that manipulates the raw objects and/or simple values directly. The convert method is passed an object of the old incompatible type and it returns an object of a current type.

Conversions can be specified in two ways: for specific fields or for all instances of a class. A different Converter constructor is used in each case. Field-specific conversions are used instead of class conversions when both are applicable.

Note that a class conversion may be not specified for an enum class. A field conversion, or a class conversion for the class declaring the field, may be used.

Note that each mutation is applied to a specific class version number. The class version must be explicitly specified in a mutation for two reasons:

  1. This provides safety in the face of multiple unconverted versions of a given type. Without a version, a single conversion method would have to handle multiple input types, and would have to distinguish between them by examining the data or type information.
  2. This allows arbitrary changes to be made. For example, a series of name changes may reuse a given name for more than one version. To identify the specific type being converted or renamed, a version number is needed.

See Entity.version() and Persistent.version() for information on assigning class version numbers.

Mutations are therefore responsible for converting each existing incompatible class version to the current version as defined by a current class definition. For example, consider that class-version A-1 is initially changed to A-2 and a mutation is added for converting A-1 to A-2. If later changes in version A-3 occur before converting all A-1 instances to version A-2, the converter for A-1 will have to be changed. Instead of converting from A-1 to A-2 it will need to convert from A-1 to A-3. In addition, a mutation converting A-2 to A-3 will be needed.

When a Converter mutation applies to a given object, other mutations that may apply to that object are not automatically performed. It is the responsibility of the Converter to return an object that conforms to the current class definition, including renaming fields and classes. If the input object has nested objects or superclasses that also need conversion, the converter must perform these nested conversions before returning the final converted object. This rule avoids the complexity and potential errors that could result if a converter mutation were automatically combined with other mutations in an arbitrary manner.

The EntityStore.evolve method may optionally be used to ensure that all instances of an old class version are converted to the current version.

Other Metadata Changes

When a class that happens to be an entity class is renamed, it remains an entity class. When a field that happens to be a primary or secondary key field is renamed, its metadata remains intact as well.

When the SecondaryKey annotation is added to an existing field, a new index is created automatically. The new index will be populated by reading the entire primary index when the primary index is opened.

When the SecondaryKey annotation is included with a new field, a new index is created automatically. The new field is required to be a reference type (not a primitive) and must be initialized to null (the default behavior) in the default constructor. Entities will be indexed by the field when they are stored with a non-null key value.

When a field with the SecondaryKey annotation is deleted, or when the SecondaryKey annotation is removed from a field without deleting it, the secondary index is removed (dropped). Removal occurs when the store is opened.

The SecondaryKey.relate property may NOT be changed. All other properties of a SecondaryKey may be changed, although avoiding changes that cause foreign key integrity errors is the responsibility of the application developer. For example, if the SecondaryKey.relatedEntity() property is added but not all existing secondary keys reference existing primary keys for the related entity, foreign key integrity errors may occur.

The PrimaryKey annotation may NOT be removed from a field in an entity class.

The PrimaryKey.sequence() property may be added, removed, or changed to a different name.

The Persistent.proxyFor() property may NOT be added, removed, or changed to a different class.

Warnings on Testing and Backups

The application developer is responsible for verifying that class evolution works properly before deploying with a changed set of persistent classes. The DPL will report errors when old class definitions cannot be evolved, for example, when a mutation is missing. To test that no such errors will occur, application test cases must include instances of all persistent classes.

Converter mutations require special testing. Since the application conversion method is allowed to return instances of any type, the DPL cannot check that the proper type is returned until the data is accessed. To avoid data access errors, application test cases must cover converter mutations for all potential input and output types.

When secondary keys are dropped or entity classes are deleted, the underlying databases are deleted and cannot be recovered from the store. This takes place when the store is opened. It is strongly recommended that a backup of the entire store is made before opening the store and causing class evolution to proceed.

Store Conversion

When mutations are not sufficient for handling class changes, a full store conversion may be performed. This is necessary for two particular types of class changes:

To perform a full store conversion, a program is written that performs the following steps to copy the data from the old store to a new converted store:

  1. The old store is opened as a RawStore and the new store is opened as an EntityStore.
  2. All entities are read from the old store. Entities are read using a RawStore to allow access to entities for which no compatible class exists.
  3. The RawObject entities are then converted to the format desired. Raw objects can be arbitrarily manipulated as needed. The updated raw objects must conform to the new evolved class definitions.
  4. The updated raw entities are converted to live objects by calling the EntityModel.convertRawObject method of the new store. This method converts raw objects obtained from a different store, as long as they conform to the new evolved class definitions.
  5. The new live objects are written to the new EntityStore using a PrimaryIndex as usual.

To perform such a conversion, two separate stores must be open at once. Both stores may be in the same Environment, if desired, by giving them different store names. But since all data is being rewritten, there are performance advantages to creating the new store in a new fresh environment: the data will be compacted as it is written, and the old store can be removed very quickly by deleting the old environment directory after the conversion is complete.

Upgrading a Replication Group

When changes to persistent classes are made in a ReplicatedEnvironment, special handling is necessary when the application is upgraded on the nodes in the replication group. Upgraded means that the application on a node is stopped, the updated application classes are installed, and the application is started again.

As usual in any sort of replication group upgrade, the Replica nodes must be upgraded first and the Master node must be upgraded last. If an upgraded node is elected Master before all of the Replica nodes have been upgraded, either because of a user error or an unexpected failover, the IncompatibleClassException will be thrown.

There are two considerations that must be taken into account during the upgrade process: new indexes that are temporarily unavailable on a Replica, and exceptions that result from renamed entity classes and secondary keys.

Note that these considerations only apply when a hot upgrade is performed, i.e., when the replication group will contain a mix of upgraded and non-upgraded nodes. If all nodes in the group are first taken down and then the nodes are upgraded and restarted, then no special considerations are necessary and this documentation is not applicable.

Defining New Indexes in a Replication Group

When a new entity class is added, which defines a new PrimaryIndex, or a new secondary key is added, which defines a new SecondaryIndex, the indexes will not be immediately available on an upgraded node. A new index will not be fully available (i.e., on every node) until all the nodes have been upgraded, the index has been created (and populated, in the case of a secondary index) on the Master node, and the index has been replicated to each Replica node via the replication stream.

When a node is first upgraded it will start out as a Replica node, and any newly defined indexes will not be available. The application has two choices for handling this condition.

  1. An application may be able to coordinate among its nodes, by its own means, to inform all nodes when an index has been created and populated on the Master. Such an application can choose to access a new index only after it knows the index is available. Such coordination is not directly supported by JE, although a transaction with a CommitToken may be used to simplify the coordination process.
  2. An application may call getPrimaryIndex or getSecondaryIndex to determine whether an index is available. An IndexNotAvailableException is thrown by these methods when the index has not yet been created or when a secondary index is currently being populated via the replication stream.

When an upgraded node is elected Master (this is typically near the end of the the upgrade process), it must call getPrimaryIndex to create each new primary index, and getSecondaryIndex to create and populate each new secondary index. A newly elected Master node that was just upgraded should be prepared for a delay when getSecondaryIndex is called to create and populate a new secondary index.

Renaming Entity Classes and Keys in a Replication Group

When a DPL entity class or secondary key field is renamed by an application using a Renamer mutation, this will result internally in the underlying database for that entity class or secondary key being renamed. The actual renaming of the database first occurs on the upgraded Master node and is then replicated to each Replica node.

When the application on a Master or Replica node first accesses the store after the database has been renamed, a DatabasePreemptedException will be thrown. When this happens, the application must close any cursors and transactions that are open for that store, and then close the store and reopen it.

Berkeley DB Java Edition
version 6.2.7

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