Converts an old version of an object value to conform to the current class or field definition.
The listener interface called during eager entity evolution.
A mutation for converting an old version of an object value to conform to the current class or field definition.
A mutation for deleting an entity class or field.
A subclass of Converter that allows specifying keys to be deleted.
Configuration properties for eager conversion of unevolved objects.
The event passed to the EvolveListener interface during eager entity evolution.
Statistics accumulated during eager entity evolution.
The base class for all mutations.
A collection of mutations for configuring class evolution.
A mutation for renaming a class or field without changing the instance or field value.
While reading from an index, an instance of a deleted class version was encountered.
A class has been changed incompatibly and no mutation has been configured to handle the change or a new class version number has not been assigned.
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.
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
key class. WARNING:: Changing the behavior of a
Comparable key class is likely to make the index unusable.
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
Persistent.version() for information on assigning
class version numbers.
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
Deletershould be used for an entity class in all of the following circumstances:
Convertermutations for the field or enclosing class that contain embedded instances of the deleted class. A
Deletershould be used for a non-entity class in all of the following circumstances:
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
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
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:
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.
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.
method may optionally be used to ensure that all instances of an old class
version are converted to the current version.
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.
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.
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
When a field with the
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.
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.
PrimaryKey annotation may NOT be
removed from a field in an entity class.
PrimaryKey.sequence() property may be
added, removed, or changed to a different name.
Persistent.proxyFor() property may NOT
be added, removed, or changed to a different class.
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.
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:
RawStoreand the new store is opened as an
RawStoreto allow access to entities for which no compatible class exists.
RawObjectentities 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.
EntityModel.convertRawObjectmethod 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.
To perform such a conversion, two separate stores must be open at once.
Both stores may be in the same
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.
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.
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.
CommitTokenmay be used to simplify the coordination process.
getSecondaryIndexto determine whether an index is available. An
IndexNotAvailableExceptionis 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
create and populate each new secondary index. A newly elected Master node that
was just upgraded should be prepared for a delay when
called to create and populate a new secondary index.
When a DPL entity class or secondary key field is renamed by an application
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.
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