# AWS DMS task cannot start because of missing premigration assessments

**URL:** <https://discuss.hashicorp.com/t/aws-dms-task-cannot-start-because-of-missing-premigration-assessments/74947>\
**Category:** AWS\
**Created:** [May 14, 2025, 8:51am UTC](https://discuss.hashicorp.com/t/aws-dms-task-cannot-start-because-of-missing-premigration-assessments/74947 "2025-05-14T08:51:47Z")\
**Posts on this page:** 2\
**Page:** 1

<div class="post-metadata">

**Author:** ![gchappel](https://sea2.discourse-cdn.com/hashicorp/user_avatar/discuss.hashicorp.com/gchappel/32/45661_2.png) [@gchappel](https://discuss.hashicorp.com/u/gchappel)\
**Post date:** [May 14, 2025, 8:51am UTC](https://discuss.hashicorp.com/t/aws-dms-task-cannot-start-because-of-missing-premigration-assessments/74947/1 "2025-05-14T08:51:47Z")

</div>

Hi,  
I’m trying to use the `terraform-aws-modules/terraform-aws-dms` module to export rows from an RDS database into an S3 bucket as CSV files.

After a couple of false starts I’m now at the point where all of the resources specified in that module have been created successfully, however the task cannot be started because:

```auto
│ Error: starting DMS Replication Task (rds-to-s3): operation error Database Migration Service: StartReplicationTask, https response error StatusCode: 400, RequestID: 8e51c85c-b04f-4445-aade-e98222dc5297, InvalidResourceStateFault: Test connection for replication instance poc-dms and endpoint source-database should be successful for starting the replication task
│ 
│ with module.database_migration_service.aws_dms_replication_task.this["s3_export"],
│ on .terraform/modules/database_migration_service/main.tf line 386, in resource "aws_dms_replication_task" "this":
│ 386: resource "aws_dms_replication_task" "this" {
│ 
╵
Error: Process completed with exit code 1.

```

### Example code

```hcl
module "database_migration_service" {
  source = "terraform-aws-modules/dms/aws"
  version = "~> 2.0"

   [...]lines omitted for brevity[...]

  replication_tasks = {
    s3_export = {
      migration_type = "full-load-and-cdc"
      replication_task_id = "rds-to-s3"

      replication_task_settings = jsonencode({
        FullLoadSettings = {
          TargetTablePrepMode = "DO_NOTHING",
        },
      })

      source_endpoint_key = "rds_source"
      start_replication_task = true

      table_mappings = jsonencode({
        rules = [{
          object-locator = {
            schema-name = "identityiq"
            table-name = "spt_audit_event"
          }
          rule-action = "explicit"
          rule-id = "1"
          rule-name = "1"
          rule-type = "selection"
        }]
      })

      target_endpoint_key = "s3_target"
    }
  }
}

```

I checked the source code for this module to see if there was a variable/flag to control creating a premigration assessment; there doesn’t seem to be one. The `start_replication_task` value only feeds directly into the [`start_replication_task` value on the `aws_dms_replication_task` resource](https://registry.terraform.io/providers/hashicorp/aws/latest/docs/resources/dms_replication_task#start_replication_task-1) which does not document anything about a premigration assessment being required.

I also checked the provider docs, expecting to potentially see a specific `aws_dms_premigration_assessment` resource; there doesn’t seem to be one.

My gut feeling is that this functionality is not often used and is currently broken due to this requirement to run premigration assessments, and that either

- the `aws_dms_replication_task` resource needs to do an extra API call when `start_replication_task` is `true` in order to create the premigration assessment and actually allow the task to start on successful completion

or

- there needs to be an extra resource type to create the premigration assessment
  - the downside of a second resource is then the dependencies would mean that `start_replication_task` could no longer be used because this would fail (the task resource would try to start before marking as `Created`, but the assessment resource would need the task resource to exist first to retrieve ARNs etc). So this would likely also need a _third_ resource in order to trigger starting an existing task, which would only run after task creation _and_ assessment creation+completion

Any tips how I might be able to work around this? My last resort would be to set `start_replication_task` to `false` and then use `local_exec` provisioners to run AWS CLI commands manually, but (for obvious reasons) this is definitely not my preferred solution.

Thanks!

---

<div class="post-metadata">

**Author:** ![gchappel](https://sea2.discourse-cdn.com/hashicorp/user_avatar/discuss.hashicorp.com/gchappel/32/45661_2.png) [@gchappel](https://discuss.hashicorp.com/u/gchappel)\
**Post date:** [May 14, 2025, 9:11am UTC](https://discuss.hashicorp.com/t/aws-dms-task-cannot-start-because-of-missing-premigration-assessments/74947/2 "2025-05-14T09:11:42Z")

</div>

Actually, I think I solved this…

Because the error only appeared after the _task_ was created, and the console showed the missing premigration assessments, my confirmation bias meant I focused on that.

The error is actually related to the _endpoint_, but because the endpoints are not used until the task is created, the error only presents when the task is created even though the config error is not with the task.

Ultimately, the information I needed was on the Endpoints page in the console, where the error message shows that I have an incorrect security group membership/rule and the DMS instance is timing out when connecting to RDS. So I need to solve this first before then trying to start the task again - where the premigration assessment may not be an issue.