Best Practice for creating GCP Pub/Sub Topics if they might already exists

Question

we have a common use case for our applications where we have long lived Topics that could span across application deploys but may also not exist.

We are trying to find the best practice for the pattern in GCP pub/sub topics using the provided Java client where we can CreateIfNotExists, or GetAndCreateIfNotExists.

The current implementation we are using is to attempt to create a Topic and basically swallow the exception if it is AlreadyExistsException.

@Override
    public void createTopicIfNotExists(TopicName topicName) {
        try {
            this.topicAdminClient.createTopic(topicName);
        } catch (AlreadyExistsException e) {
            // topic already exists
        } catch (Exception e) {
            throw e;
        }
    }

I don't think this is ideal because are basically using an exception as logic which is inherently bad practice. If we try to get a topic when it doesn't exist the client throws a DoesNot exist exception so we can't do it the other way.

I have seen things on github about the JS client being able to autoCreate on a Get command, does anyone know if that is available in the java client.

See here: https://stackoverflow.com/a/38878368/5385625 Is there an equivalent in the Java client.

score 3 · Answer 1 · answered Jul 14 '21 at 03:54

3

Unfortunately this feature is not available in the java client library. You can file a feature request since this is a potential useful feature!

What I could suggest is prior to creating a topic you can perform getTopic() to check if a topic is existing or not, to prevent having exceptions.

Sample code from getTopic() Java docs:

 try (TopicAdminClient topicAdminClient = TopicAdminClient.create()) {
   TopicName topic = TopicName.ofProjectTopicName("[PROJECT]", "[TOPIC]");
   Topic response = topicAdminClient.getTopic(topic.toString());
 }

answered Jul 14 '21 at 03:54

Ricco D

6,873
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If the topic doesn’t exist then getTopic will throw an exception, right? – Sam Orozco Jul 14 '21 at 14:45
The client will throw an exception if the topic is not found. `Caused by: io.grpc.StatusRuntimeException: NOT_FOUND: Resource not found` – Sam Orozco Jul 14 '21 at 16:12

Samarth Singal · Answer 2 · 2021-07-15T22:48:52.723

1

The recommended approach is to catch the NotFoundEception on getTopic for knowing when a topic already exists. Is there a particular concern you have with processing the exception in this way?

edited Jul 15 '21 at 22:48

answered Jul 14 '21 at 19:57

Samarth Singal

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4

Where are you getting that guidance? I guess using the exceptions this way feel wrong to me because of the old adage "don't use exceptions as control flow". But at the end of the day it isn't a big deal and I am fine with it, for now. – Sam Orozco Jul 14 '21 at 20:28
I don't think this is "using exceptions as control flow" in the ways that are so discouraged. If you were to do this logic within a loop or as some means of control in cod e run per message, then yes, it would not be good. You'll want to do this topic check/creation as early in your application as you can. If the client library were to offer autoCreate, it would do exactly what you show. This method has its own dangers, e.g., if one were to misspell the topic, it would just create the topic and your published messages would be lost because there would be no subscription tied to this topic. – Kamal Aboul-Hosn Jul 15 '21 at 14:54

Best Practice for creating GCP Pub/Sub Topics if they might already exists

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