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Recently I came across a situation where am building AWS infrastructure using terraform to setup a clustered environment for some applications. Thing is when I apply terraform scripts it builds all the necessary modules and spins multiple instances altogether and then finishes. This may be meant to do like this and there is nothing to blame anyways terraform works greatly to build such infra.

When I'm trying to setup such infra to deploy an application in a clustered way, here am using a configuration management tool. While building ec2 instances CM scripts gets invoked and configured accordingly. Problem comes when there is some dependency on the modules.

Consider a scenario that 2(A & B) components are part of Autoscale group and 2(C & D) components are normal ec2-instances. Here if I wish to build A first and then C since C instance got dependency on A which has to be fully configured first or vice versa, how can I control the order in which terraform helps me to achieve this.

Please can someone helps me achieving it.

Thanks in advance

Mannam Venkatanarendra
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The other answer is correct in the literal sense, but overall this is something to avoid. Build your CM code so that it will keep re-trying to converge until it succeeds. With Chef in particular, you can use the chef-client cookbook to deploy a service which runs Chef converges automatically at a given interval (30 minutes by default but you might want to make that shorter). Running things in the "right" order sounds nice, but when dealing with byzantine failures you'll thank your past self for ensuring reliable convergence no matter the order.

coderanger
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You can use the depends_on parameter. Resources can be made explicitly dependant on other resources. Terraform will in turn only build the resource once dependent resources have provisioned successfully.

https://www.terraform.io/intro/getting-started/dependencies.html

justMiles
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The question has a broad nature and the other answers are right in their own rights. What I would like to add is that making use of modules to determine order of logical sub projects works well too.

In terraform you can force procedural order with depends_on in resource level but you cannot use it for modules. However for modules you can use the output of one module as input to the other one, which would help you manage procedural order on modules level.

So, in your case, I would put A & B in one module, C & D into another and use the output variables from one to the other to control order.

Sertac
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