org.dellroad.stuff.schema
Interface SchemaUpdate<T>

Type Parameters:

T - database transaction type

All Known Implementing Classes:

AbstractSchemaUpdate, AbstractSpringSchemaUpdate, SpringPersistentObjectSchemaUpdate, SpringSQLSchemaUpdate, SpringXSLPersistentObjectSchemaUpdate

public interface SchemaUpdate<T>

A one-time database schema update. Instances typically perform changes to the database schema, though non-schema updates that just manipulate the data in the database are perfectly valid as well.

Each SchemaUpdate has a unique name among all updates ever applied to a single database, and zero or more required predecessors, which are other dependent updates that must be applied first.

Once an update has been applied to a database, it must not be changed; otherwise, inconsistencies can exist between databases that were updated using the old version vs. databases that were updated using the new version. If an update has been applied but had the wrong behavior, instead of changing the update, it's better to create a new update that depends on the first as a predecessor and corrects the mistake.

If you do have to change an update after it has been applied, then those databases that have already seen the previous version of the update must be manually corrected so they are in the same state that the new version of the update would have left them.

Method Summary

List<? extends DatabaseAction<T>>	getDatabaseActions() Get the action(s) that comprise this update.
String	getName() Get the unique name of this update.
Set<SchemaUpdate<T>>	getRequiredPredecessors() Get the the other updates (if any) that must be applied before this update may be applied.
boolean	isSingleAction() Determine whether, if this instance contains multiple individual actions, should they be applied in a single transaction and recorded as a single update.

Method Detail

getName

String getName()

Get the unique name of this update. This name must be unique among all updates ever applied to the database and must never change once this update has been applied to any database.

Returns:

the name of this update; must not be the empty string

getRequiredPredecessors

Set<SchemaUpdate<T>> getRequiredPredecessors()

Get the the other updates (if any) that must be applied before this update may be applied.

Returns:

set of zero or more other updates

See Also:

getName()

getDatabaseActions

List<? extends DatabaseAction<T>> getDatabaseActions()

Get the action(s) that comprise this update. Ideally, individual actions should be atomic database operations, i.e., each one should either finish completely, or else leave the database in a state where it can be tried again. In any case, each action will be applied within its own transaction when transactions are supported by the database unless isSingleAction() returns true.

Returns:

a list of zero or more actions to apply

See Also:

isSingleAction()

isSingleAction

boolean isSingleAction()

Determine whether, if this instance contains multiple individual actions, should they be applied in a single transaction and recorded as a single update. Normally this is false. If true, partially completed updates can result if one of the action fails.