throw not found exception if pubsub topic is not available - spring

Question

I am using spring boot to interact with pubsub topic.
My config class for this connection look like this:
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
import org.springframework.cloud.gcp.pubsub.core.PubSubTemplate;
import org.springframework.cloud.gcp.pubsub.core.publisher.PubSubPublisherTemplate;
import org.springframework.cloud.gcp.pubsub.support.PublisherFactory;
import org.springframework.cloud.gcp.pubsub.support.converter.SimplePubSubMessageConverter;
import org.springframework.util.Assert;
import org.springframework.util.concurrent.ListenableFuture;
import org.springframework.util.concurrent.SettableListenableFuture;
import com.google.api.core.ApiFuture;
import com.google.api.core.ApiFutureCallback;
import com.google.api.core.ApiFutures;
import com.google.pubsub.v1.PubsubMessage;
public abstract class PubSubPublisher {
private static final Logger LOGGER = LoggerFactory.getLogger(PubSubPublisher.class);
private final PubSubTemplate pubSubTemplate;
protected PubSubPublisher(PubSubTemplate pubSubTemplate) {
this.pubSubTemplate = pubSubTemplate;
}
protected abstract String topic(String topicName);
public ListenableFuture<String> publish(String topicName, String message) {
LOGGER.info("Publishing to topic [{}]. Message: [{}]", topicName, message);
return pubSubTemplate.publish(topicName, message);
}
}
And I am calling this at my service, like this:
publisher.publish(topic-name, payload);
This publish method is async one, which always pass on did not wait for acknowldgrment. I make add get after publish for wait until it get the response from pubsub.
But I wanted to know if in case my topic is not already present and i try to push some message, it should throw some error like resource not found, considering using default async method only.
Might be implementing the callback would help but i am unable to do that in my code. And the current override publish method which use callback is just throwing the WARN not exception i wanted that to be exception. that is the reason i wanted to implement the callback.

Answer 1

You can check if Topic already present
from google.cloud import pubsub_v1
project_id = "projectname"
topic_name = "unknowTopic"
publisher = pubsub_v1.PublisherClient()
topic_path = publisher.topic_path(project_id, topic_name)
try:
response = publisher.get_topic(topic_path)
except Exception as e:
print(e)
This returns the error as
404 Resource not found (resource=unknowTopic).