Argument to Support Server-Side Calculated Replication Factor

[canhanhan]

This PR addresses issue #409.

The problem centers around self-hosted Confluent Kafka requiring the replication_factor in the kafka_topic resource to be set to -1 for the configuration of confluent.placement.constraints (used for multi-region clusters). The kafka_topic resource requires that the "replication_factor" is at least 1 or greater.

This causes a client-side error: This most likely occurs because of a request being malformed by the client library or the message was sent to an incompatible broker. See the broker logs for more details. and a server-side InvalidRequestException: Both replicationFactor and confluent.placement.constraints are set. Both cannot be used at the same time.

In this PR, I'm introducing a new argument named managed_replication_factor to resolve this issue. This new addition is an optional boolean parameter; when given a 'true' value, it indicates server-side computation of the replication_factor. It instructs the provider to disregard any changes to the replication_factor, setting the value instead to -1 when creating a topic.

This approach enables the usage of confluent.placement.constraints config on Confluent Kafka, illustrated as follows:

resource "kafka_topic" "my_topic" {
  name                       = "my_topic"
  partitions                 = 3
  replication_factor         = 1 # ignored
  managed_replication_factor = true

  config = {
    "confluent.placement.constraints" = jsonencode({
      "version" : 1,
      "replicas" : [
        { "count" : 2, "constraints" : { "rack" : "east" } },
        { "count" : 2, "constraints" : { "rack" : "west" } }
      ],
      "observers" : []
    })
  }
}

If managed_replication_factor is set on an Apache Kafka cluster, the broker will use its default.replication.factor setting, which defaults to 1.

I've also added acceptance tests for a few scenarios like switching the 'managed_replication_factor' on and off, and modifying partition/replication_factor values.

@Mongey, is this something you would consider merging into the provider? Happy to make any changes based on your feedback.

[Mongey]

@canhanhan could the same outcome not be achieved by ignoring changes to the replication_factor via the builtin terraform lifecycle?

resource "kafka_topic" "my_topic" {
  name               = "my_topic"
  partitions         = 3
  replication_factor = 1 # ignored

  config = {
    "confluent.placement.constraints" = jsonencode({
      "version" : 1,
      "replicas" : [
        { "count" : 2, "constraints" : { "rack" : "east" } },
        { "count" : 2, "constraints" : { "rack" : "west" } }
      ],
      "observers" : []
    })
  }

  lifecycle {
    ignore_changes = [
      replication_factor,
    ]
  }
}

[canhanhan]

@canhanhan could the same outcome not be achieved by ignoring changes to the replication_factor via the builtin terraform lifecycle?

Hi @Mongey, not quite so:

1. For new topic creation: replication_factor must be -1. Any other value will cause Both replicationFactor and confluent.placement.constraints are set. Both cannot be used at the same time errors. Currently, replication_factor validation requires a positive integer (https://github.com/Mongey/terraform-provider-kafka/blob/main/kafka/resource_kafka_topic.go#L43).
2. For existing topic updates: The built-in ignore_changes feature works.

If you prefer, I can submit a PR which allows "-1" as a valid value for replication_factor instead of adding a new property.