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Randomising vSphere Datastore Selection With Terraform

This week I ran into a customer who wasn’t using datastore clusters in their vSphere environment and was looking for ways to randomise the placement of workloads.

Consider the following code.

Ignoring that I’ve dropped variables, outputs and data sources all into a single file, what can you see? A selection of datastores declared as a variable that we are going to retrieve the ids for.

The next step is to find a way to randomise these. Enter the random_shuffle resource type.

The random_shuffle resource will return a list of values, based on the result_count value that you specify. This gives some nice flexibility, such as the situation where you were intending to deploy multiple machines, and wanted a randomised datastore placement for each of them.
To achieve that you could do something like this.

Now we are starting to cook with gas!

In closing, there are a few things to be aware of when using any of the random resources. They don’t regenerate every time you perform a plan/apply. You can choose to taint (terraform taint random_shuffle.this) to have the randomness be generated, or you can tie the random generation to another object such as your list of datastores as a keeper. Changes to the list of datastores would regenerate the result of the random_shuffle. Bear in mind that the downstream effect of this could be to change the placement of your existing virtual machine disks.

Happy Terraforming!