#Tailable(spring-data-reactive-mongodb) equivalent in spring-data-r2dbc - postgresql

Question

I am trying my hands on spring-data-r2dbc. I am try this on Postgresql. I have tried spring-data-mongodb-reactive before. I couldn't help but to compare both.
I see that Query Derivation is not yet supported. But I was wondering if there is an equivalent for #Tailable. This way I would be notified of the database changes in real time. Ca anyone share any code samples with respect to this.
I understand that the underlying database should support this. I believe Postgresql does support this kinda thing using Logical Decoding(Correct me if I am wrong here).
Is there a #Tailable equivalent in spring-data-r2dbc ?

Answer 1

I was on the same issue not sure if you found a solution or not but I was able to accomplish something similar by doing the following. First, I added trigger to my table
CREATE TRIGGER trigger_name
AFTER INSERT OR DELETE OR UPDATE
ON table_name
FOR EACH ROW
EXECUTE PROCEDURE trigger_function_name;
This will set a trigger on the table whenever a row, is updated, deleted, or inserted. Then it will call the trigger function I have set up which looked something like this:
CREATE FUNCTION trigger_function_name
RETURNS trigger
LANGUAGE 'plpgsql'
COST 100
VOLATILE NOT LEAKPROOF
AS
$BODY$
DECLARE
payload JSON;
BEGIN
payload = row_to_json(NEW);
PERFORM pg_notify('notification_name', payload::text);
RETURN NULL;
END;
$BODY$;
This will allow me to 'listen' to the any of these updates from my spring boot project and it will send the entire row as a payload.
Next, in my spring boot project I configured a connection to my db.
#Configuration
#EnableR2dbcRepositories("com.(point to wherever repository is)")
public class R2DBCConfig extends AbstractR2dbcConfiguration {
#Override
#Bean
public ConnectionFactory connectionFactory() {
return new PostgresqlConnectionFactory(PostgresqlConnectionConfiguration.builder()
.host("host")
.database("db")
.port(port)
.username("username")
.password("password")
.schema("schema")
.connectTimeout(Duration.ofMinutes(2))
.build());
}
}
With that I Autowire (dependency injection) it into the constructor in my service class and cast it to a r2dbc PostgressqlConnection class like so:
this.postgresqlConnection = Mono.from(connectionFactory.create()).cast(PostgresqlConnection.class).block();
Now we want to 'listen' to our table and get the notified when perform some update to our table. To do that we set up an initialization method that is performed after dependency injection by using the #PostContruct annotation
#PostConstruct
private void postConstruct() {
postgresqlConnection.createStatement("LISTEN notification_name").execute()
.flatMap(PostgresqlResult::getRowsUpdated).subscribe();
}
Notice that we listen to whatever name we put inside the pg_notify method. Also we want to set up a method to close the the connection when the bean is about to be tossed away, like so:
#PreDestroy
private void preDestroy() {
postgresqlConnection.close().subscribe();
}
Now I simply create a method that returns a Flux of whatever is currently in my table, and I also merge it with my notifications, as I said before the notifications come in as a json, so I had to deserialize it and I decided to use ObjectMapper. So, it will look something like this:
private Flux<YourClass> getUpdatedRows() {
return postgresqlConnection.getNotifications().map(notification -> {
try {
//deserialize json
return objectMapper.readValue(notification.getParameter(), YourClass.class);
} catch (IOException e) {
//handle exception
}
});
}
public Flux<YourClass> getDocuments() {
return documentRepository.findAll().share().concatWith(getUpdatedRows());
}
Hope this helps.
Cheers!