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https://github.com/davidafsilva/vertx-unused-waiting-threads-reproducer

Reproducer for a potential issue found in Vert.x 4.x
https://github.com/davidafsilva/vertx-unused-waiting-threads-reproducer

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Reproducer for a potential issue found in Vert.x 4.x

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= Vertx Unused Waiting Threads Reproducer



image:https://img.shields.io/badge/vert.x-4.4.4-purple.svg[link="https://vertx.io"]



Reproducer for an issue, or at least an unexpected behavior, related to the

execution of tasks under a shared worker pool where a blocked thread seems

to be halting the execution of tasks on the remaining (waiting) threads of the

pool.



Note that reproducer is a bit more complex to better represent the use-case

where I originally noticed the behavior. The reproducer mimics exactly that,

a worker verticle which spawns two type of consumers: Event-Bus (local) and

Kafka.



[source,mermaid]

----

flowchart TB;

  subgraph User

    I(Stdin Parser)

  end



  subgraph K[Kafka]

    KB(Broker)

  end



  subgraph V[Vert.x]

    VEB[Vert.x EB]

    subgraph VWV[Vert.x Worker Verticle]

     EBC[Event-Bus Consumer]

     KC[Kafka Consumer]

    end

  end



  I -- sends op --> VEB

  VEB -- delivers message --> EBC

  EBC -- sends record --> KB

  KC -- polls records --> KB

----



== Running the Reproducer



Deploy a single node Kafka broker locally through the provided docker-compose

file:

[source]

----

docker compose up

----



Run the application through Gradle:

[source]

----

./gradlew run --console=plain

----



You should see an output similar to the one below

[source]

----

Supported operations:

 [1] Send a message that does not block the Kafka Consumer handler

 [2] Send a message that blocks the Kafka Consumer handler

 [3] Unblocks any ongoing 'blocking' operation, if any

 [0] Exit



Type the numerical value for the desired operation and press 

----



Choosing `1` will send a message through the event-bus that shall be "quickly"

processed by the Kafka consumer handler. You should see omething along the 

lines of:

[source]

----

[][]  - EventBus - message received: quick

[][]  - KafkaConsumer - record received: /quick

[][]  - KafkaConsumer - record processed: /quick

----



Choosing `2` will send a message through the event-bus as well, however it shall

block on the kafka consumer handler (through the means of a sleep) until you

`3` is entered. If you try to send `1` or `2` prior to sending `3`, you'll notice 

that both consumers (event-bus and kafka) will not process any message despite

having more threads (1) available in a wait state, only one them is blocked. 

This is the target issue being reproduced here.



Full example of a test run can be found below.



[source]

----

Supported operations:

 [1] Send a message that does not block the Kafka Consumer handler

 [2] Send a message that blocks the Kafka Consumer handler

 [3] Unblocks any ongoing 'blocking' operation, if any

 [0] Exit



Type the numerical value for the desired operation and press 

1



[kafka-verticle-worker-pool-0][io.vertx.core.impl.WorkerContext@5f88e225] 2023-06-29T11:47:23.052496 - EventBus - message received: quick

[kafka-verticle-worker-pool-0][io.vertx.core.impl.WorkerContext@5f88e225] 2023-06-29T11:47:23.108731 - KafkaConsumer - record received: 1/quick

[kafka-verticle-worker-pool-0][io.vertx.core.impl.WorkerContext@5f88e225] 2023-06-29T11:47:23.108858 - KafkaConsumer - record processed: 1/quick

2



[kafka-verticle-worker-pool-1][io.vertx.core.impl.WorkerContext@5f88e225] 2023-06-29T11:47:29.393930 - EventBus - message received: slow

[kafka-verticle-worker-pool-0][io.vertx.core.impl.WorkerContext@5f88e225] 2023-06-29T11:47:29.411352 - KafkaConsumer - record received: 2/slow

1



1



3



[kafka-verticle-worker-pool-0][io.vertx.core.impl.WorkerContext@5f88e225] 2023-06-29T11:47:41.002428 - KafkaConsumer - record processed: 2/slow

[kafka-verticle-worker-pool-0][io.vertx.core.impl.WorkerContext@5f88e225] 2023-06-29T11:47:41.007458 - EventBus - message received: quick

[kafka-verticle-worker-pool-0][io.vertx.core.impl.WorkerContext@5f88e225] 2023-06-29T11:47:41.007763 - EventBus - message received: quick

[kafka-verticle-worker-pool-1][io.vertx.core.impl.WorkerContext@5f88e225] 2023-06-29T11:47:41.021551 - KafkaConsumer - record received: 3/quick

[kafka-verticle-worker-pool-1][io.vertx.core.impl.WorkerContext@5f88e225] 2023-06-29T11:47:41.021731 - KafkaConsumer - record processed: 3/quick

[kafka-verticle-worker-pool-1][io.vertx.core.impl.WorkerContext@5f88e225] 2023-06-29T11:47:41.021790 - KafkaConsumer - record received: 4/quick

[kafka-verticle-worker-pool-1][io.vertx.core.impl.WorkerContext@5f88e225] 2023-06-29T11:47:41.021839 - KafkaConsumer - record processed: 4/quick



0

closing Vert.x..

----