I would like to rename a PostgreSQL (9.6) table in a way that is recoverable for my system (A java app using JPA/Hibernate)
In my java code, a JPA entity would have the following annotations @Entity @Table(name="old_name") and the database would have an equivalent table called old_name.
I would like to rename the table to new_name in a way that I can incrementally update the database and java app, allowing for failure and rollback.
Typical steps would be
- create copy of
old_nameinnew_name - ensure read/writes available in both (i.e. data is replicated both ways)
- update java app to use new table
new_name - when confident system update complete, remove
old_name
Effectively I would like a duplicate table in the same schema with the same data, both able to accept reads and writes, that can be read from JPA entities.
I am aware of the use of triggers, and would like to avoid that. I am hoping there is a technique I'm not aware of and haven't found that would make this less painful than using triggers.
I have tried to rename the table and create a "simple view" over it, however the JPA entity complains as it can't find a table with the name of the view. (Because it is a view, not a table :) and there seems no @View/@Table JPA annotation that will handle this)
I haven't yet tried the facilities listed here: http://wiki.postgresql.org/wiki/Replication,_Clustering,_and_Connection_Pooling as the majority seem to be about pooling, sharding, and I need a simple short term table replica, but I will be investigating these also.
Thanks - I would like the simplest option of course, preferring something built in to postgres/JPA but will seriously consider 3rd party options also.
This was a very interesting question, so I decided to turn this answer into a full-blown article.
Database tables
Assuming you have the following two tables:
CREATE TABLE old_post (
id int8 NOT NULL,
title varchar(255),
version int4 NOT NULL,
PRIMARY KEY (id)
)
CREATE TABLE post (
id int8 NOT NULL,
created_on date,
title varchar(255),
version int4 NOT NULL,
PRIMARY KEY (id)
)
JPA entity
The old_post table must be replicated with the newer post. Notice that the post table has more columns now than the old table.
We only need to map the Post entity:
@Entity(name = "Post")
@Table(name = "post")
public static class Post {
@Id
private Long id;
private String title;
@Column(name = "created_on")
private LocalDate createdOn = LocalDate.now();
@Version
private int version;
//Getters and setters omitted for brevity
}
Hibernate event listeners
Now, we have to register 3 event listeners to intercept the INSERT, UPDATE, and DELETE operations for the Post entity.
We can do this via the following event listeners:
public class ReplicationInsertEventListener
implements PostInsertEventListener {
public static final ReplicationInsertEventListener INSTANCE =
new ReplicationInsertEventListener();
@Override
public void onPostInsert(
PostInsertEvent event)
throws HibernateException {
final Object entity = event.getEntity();
if(entity instanceof Post) {
Post post = (Post) entity;
event.getSession().createNativeQuery(
"INSERT INTO old_post (id, title, version) " +
"VALUES (:id, :title, :version)")
.setParameter("id", post.getId())
.setParameter("title", post.getTitle())
.setParameter("version", post.getVersion())
.setFlushMode(FlushMode.MANUAL)
.executeUpdate();
}
}
@Override
public boolean requiresPostCommitHanding(
EntityPersister persister) {
return false;
}
}
public class ReplicationUpdateEventListener
implements PostUpdateEventListener {
public static final ReplicationUpdateEventListener INSTANCE =
new ReplicationUpdateEventListener();
@Override
public void onPostUpdate(
PostUpdateEvent event) {
final Object entity = event.getEntity();
if(entity instanceof Post) {
Post post = (Post) entity;
event.getSession().createNativeQuery(
"UPDATE old_post " +
"SET title = :title, version = :version " +
"WHERE id = :id")
.setParameter("id", post.getId())
.setParameter("title", post.getTitle())
.setParameter("version", post.getVersion())
.setFlushMode(FlushMode.MANUAL)
.executeUpdate();
}
}
@Override
public boolean requiresPostCommitHanding(
EntityPersister persister) {
return false;
}
}
public class ReplicationDeleteEventListener
implements PreDeleteEventListener {
public static final ReplicationDeleteEventListener INSTANCE =
new ReplicationDeleteEventListener();
@Override
public boolean onPreDelete(
PreDeleteEvent event) {
final Object entity = event.getEntity();
if(entity instanceof Post) {
Post post = (Post) entity;
event.getSession().createNativeQuery(
"DELETE FROM old_post " +
"WHERE id = :id")
.setParameter("id", post.getId())
.setFlushMode(FlushMode.MANUAL)
.executeUpdate();
}
return false;
}
}
The 3 event listeners can be registered using a Hibernate Integrator:
public class ReplicationEventListenerIntegrator
implements Integrator {
public static final ReplicationEventListenerIntegrator INSTANCE =
new ReplicationEventListenerIntegrator();
@Override
public void integrate(
Metadata metadata,
SessionFactoryImplementor sessionFactory,
SessionFactoryServiceRegistry serviceRegistry) {
final EventListenerRegistry eventListenerRegistry =
serviceRegistry.getService(EventListenerRegistry.class);
eventListenerRegistry.appendListeners(
EventType.POST_INSERT,
ReplicationInsertEventListener.INSTANCE
);
eventListenerRegistry.appendListeners(
EventType.POST_UPDATE,
ReplicationUpdateEventListener.INSTANCE
);
eventListenerRegistry.appendListeners(
EventType.PRE_DELETE,
ReplicationDeleteEventListener.INSTANCE
);
}
@Override
public void disintegrate(
SessionFactoryImplementor sessionFactory,
SessionFactoryServiceRegistry serviceRegistry) {
}
}
And, to instruct Hibernate to use this custom Integrator, you need to set up the hibernate.integrator_provider configuration property:
<property name="hibernate.integrator_provider"
value="com.vladmihalcea.book.hpjp.hibernate.listener.ReplicationEventListenerIntegrator "/>
Testing time
Now, when persisting a Post entity:
Post post1 = new Post();
post1.setId(1L);
post1.setTitle(
"The High-Performance Java Persistence book is to be released!"
);
entityManager.persist(post1);
Hibernate will execute the following SQL INSERT statements:
Query:["INSERT INTO old_post (id, title, version) VALUES (?, ?, ?)"], Params:[(1, The High-Performance Java Persistence book is to be released!, 0)]
Query:["insert into post (created_on, title, version, id) values (?, ?, ?, ?)"], Params:[(2018-12-12, The High-Performance Java Persistence book is to be released!, 0, 1)]
When doing another transaction that updates an existing Post entity and creates a new Post entity:
Post post1 = entityManager.find(Post.class, 1L);
post1.setTitle(post1.getTitle().replace("to be ", ""));
Post post2 = new Post();
post2.setId(2L);
post2.setTitle(
"The High-Performance Java Persistence book is awesome!"
);
entityManager.persist(post2);
Hibernate replicates all actions to the old_post table as well:
Query:["select tablerepli0_.id as id1_1_0_, tablerepli0_.created_on as created_2_1_0_, tablerepli0_.title as title3_1_0_, tablerepli0_.version as version4_1_0_ from post tablerepli0_ where tablerepli0_.id=?"], Params:[(1)]
Query:["INSERT INTO old_post (id, title, version) VALUES (?, ?, ?)"], Params:[(2, The High-Performance Java Persistence book is awesome!, 0)]
Query:["insert into post (created_on, title, version, id) values (?, ?, ?, ?)"], Params:[(2018-12-12, The High-Performance Java Persistence book is awesome!, 0, 2)]
Query:["update post set created_on=?, title=?, version=? where id=? and version=?"], Params:[(2018-12-12, The High-Performance Java Persistence book is released!, 1, 1, 0)]
Query:["UPDATE old_post SET title = ?, version = ? WHERE id = ?"], Params:[(The High-Performance Java Persistence book is released!, 1, 1)]
When deleting a Post entity:
Post post1 = entityManager.getReference(Post.class, 1L);
entityManager.remove(post1);
The old_post record is deletected as well:
Query:["DELETE FROM old_post WHERE id = ?"], Params:[(1)]
Query:["delete from post where id=? and version=?"], Params:[(1, 1)]
For more details, check out this article.
Code available on GitHub.