Clarify purpose of Pulsar consumer subscription related properties

[vpavic]

Some while ago I opened these two issues against the Spring Pulsar project:

• Setting subscription name using spring.pulsar.consumer.subscription.name configuration property doesn't work spring-pulsar#480
• Setting subscription name using spring.pulsar.consumer.subscription.type configuration property doesn't work spring-pulsar#488

After setting up another project recently that uses Spring Pulsar for consuming messages, I've again tripped up on the same thing attempting to set Pulsar consumer subscription related settings using Spring Boot provided configuration properties.

Though Spring Pulsar reference manual now contains the following two tips:

The spring.pulsar.consumer.subscription.name property is ignored and is instead generated when not specified on the annotation.

The spring.pulsar.consumer.subscription-type property is ignored and is instead taken from the value on the annotation. However, you can set the subscriptionType = {} on the annotation to instead use the property value as the default.

IMO it would be good to revisit these properties as their purpose is (at least to me) unclear at the moment.

/cc @onobc

[onobc]

Thanks for the report @vpavic ,

subscription-name

The spring.pulsar.consumer.subscription-name is only used as a default subscription name when using the consumer factory directly. However, I expect about 99% of users will not use the consumer factory directly.
Another couple of downsides to setting a default sub name:

• As a static property it is really not valuable since it will likely vary by each consumer. More appropriate would be a subscription-name-expression that could resolve to something sensical.
• Subscription name is really important in Pulsar and leaving this to a default property could lead to problems if not used properly.

Based on the above downsides I would be in favor of deprecating (for removal) this property in 3.4.0. @philwebb wdyt?

subscription-type

The spring.pulsar.consumer.subscription-type is used as a default subscription name when using the consumer factory directly OR when using PulsarListener and setting thesubscriptionType = {}.

Note

But this is an opt-in behavior as IFF the user does @PulsarListener(subscriptionType = {}) does the spring.pulsar.consumer.subscription-type property get respected as the default.

It may be better suited if we set the default on PulsarListener(subscriptionType = {}) and then set the default spring.pulsar.consumer.subscription-type=Exclusive. This would still keep the default of Exclusive but would now let the SB config prop drive the system default.

Thoughts? @wilkinsona @vpavic ?

[vpavic]

The spring.pulsar.consumer.subscription-name is only used as a default subscription name when using the consumer factory directly.

The spring.pulsar.consumer.subscription-type is used as a default subscription name when using the consumer factory directly

I wasn't aware of this. Are there any examples of how spring.pulsar.consumer.subscription.name and .subscription.type come into play with consumer factory driven configuration?

To clarify my use case - I'm integrating a 3rd party system that exposes a Pulsar topic as a means of subscribing to events published by that system. So nothing Pulsar related is in my control and key settings that impact the listener (topic and subscription names) are changing between different environments. So I'd like to be able to control those solely using configuration properties. Current setup/workaround involve using expressions in @PulsarListener attributes but I'd like to avoid that if possible.

It may be better suited if we set the default on PulsarListener(subscriptionType = {}) and then set the default spring.pulsar.consumer.subscription-type=Exclusive. This would still keep the default of Exclusive but would now let the SB config prop drive the system default.

That sounds good, but I assume that configuration facilities behind @EnablePulsar would also have to take care that Exclusive stays as default if not specified on the annotation (for scenarios that don't involve Spring Boot and its auto-configuration).

[philwebb]

I don't think any of us the Boot team have enough experience of Pulsar to give deep advice, but on the surface both changes seem sensible. We can certainly do them in 3.4.

@onobc Do you want to raise issues/PRs for them?

[onobc]

I wasn't aware of this. Are there any examples of how spring.pulsar.consumer.subscription.name and .subscription.type come into play with consumer factory driven configuration?

There are no examples, but rather the other higher-level abstractions (the containers used by @PulsarListener, and @PulsarReader, @ReactivePulsarListener`) use the consumer factories. It is expected to be an edge case to use them directly.

That sounds good, but I assume that configuration facilities behind @EnablePulsar would also have to take care that Exclusive stays as default if not specified on the annotation (for scenarios that don't involve Spring Boot and its auto-configuration).

Yes, the default would still be exclusive w/ or w/o using Spring Boot. Although this would only help w/ subscriptionType and it sounds like ultimately you want to be able to specify topic and subscription name using config props.

So I'd like to be able to control those solely using configuration properties. Current setup/workaround involve using expressions in @PulsarListener attributes but I'd like to avoid that if possible.

These properties are only defaults to be used when the user does not specify them on the consumer.
Assuming you have more than 1 topic/sub in the application you would need the property to contain a SpEL that would be resolved at listener container creation time.

Do you mind providing 2 sample PulsarListener signatures you are currently using to solve this? I want to be sure I am on the same page. Thank you @vpavic

Something like:

@PulsarListener(topic = "${env}-topic-1", sub = "sub-${env}-topic1")
void listen1(String msg)

@PulsarListener(topic = "${env}-topic-2", sub = "sub-${env}-topic2")
void listen2(String msg)

[vpavic]

Assuming you have more than 1 topic/sub in the application you would need the property to contain a SpEL that would be resolved at listener container creation time.

That's the point - there's only a single topic/subscription. From my experience, that isn't an uncommon pattern. Over the years I've seen quite a few systems that expose a single subscribable interface (basically a topic) that's then used to receive different messages. Anyway, to be more specific in my case it's Tuya - you can read more details about this in their developer documentation.

What I currently have is basically this (my.other.prop is environment specific):

spring.pulsar.client.authentication.plugin-class-name=my.custom.Authentication
spring.pulsar.consumer.topics=${my.other.prop}/out/event

@PulsarListener(subscriptionName = "${my.other.prop}-sub", subscriptionType = SubscriptionType.Failover)
void receive(byte[] message) {
	// process the message
}

[onobc]

Thanks @vpavic - the single topic/sub simplifies things greatly.

If you could have these properties:

spring.pulsar.client.authentication.plugin-class-name=my.custom.Authentication
spring.pulsar.consumer.topics=${my.other.prop}/out/event
spring.pulsar.consumer.subscription.name=${my.other.prop}-sub
spring.pulsar.consumer.subscription.type=Failover

and this listener:

@PulsarListener
void receive(byte[] message) {
	// process the message
}

then you would be good. Correct?

The good news is that topic property is already respected and I am adding support for subscription.type here. The final piece will be to do something similar for subscription.name next.

[vpavic]

This sounds like what I'd ideally like to have, thanks.

So it looks like deprecation of spring.pulsar.consumer.subscription.name you mentioned in #42053 (comment) is off the table, right?

In any case, let me know if I can help with any of the work needed to deliver this, either on Spring Pulsar or Spring Boot side.

[onobc]

So it looks like deprecation of spring.pulsar.consumer.subscription.name you mentioned in #42053 (comment) is off the table, right?

Correct. After digging further and understanding your requirements ai bit more, leaving the subscription name here and giving it the same treatment we did for the subscription type makes sense. Also, the fact that the name can be dynamic w/ property substitution does make it more valuable than a static value (which would be pretty useless).

In any case, let me know if I can help with any of the work needed to deliver this, either on Spring Pulsar or Spring Boot side.
So very much appreciated @vpavic but I am already halfway through the name portion. The type portion merged yesterday. I expect no changes on the Boot side as we are leaving the properties as-is. I also plan on adding either a sample or IT on the framework side that exercises your use case.

Thank you for raising these issues!

[onobc]

The output of "Clarify purpose of Pulsar consumer subscription related properties" is to make them work as described in the config props. This will be done mostly in the Spring for Apache Pulsar project. First part is complete spring-projects/spring-pulsar#818. Remaining is in process and I will link the PR here once submitted.

There is a small change required on the Boot side to support subscription name. Submitting a PR for this soon.