Terraform vs. CloudFormation, Heat, etc.
Tools like CloudFormation, Heat, etc. allow the details of an infrastructure to be codified into a configuration file. The configuration files allow the infrastructure to be elastically created, modified and destroyed. Terraform is inspired by the problems they solve.
Terraform similarly uses configuration files to detail the infrastructure setup, but it goes further by being both cloud-agnostic and enabling multiple providers and services to be combined and composed. For example, Terraform can be used to orchestrate an AWS and OpenStack cluster simultaneously, while enabling 3rd-party providers like Cloudflare and DNSimple to be integrated to provide CDN and DNS services. This enables Terraform to represent and manage the entire infrastructure with its supporting services, instead of only the subset that exists within a single provider. It provides a single unified syntax, instead of requiring operators to use independent and non-interoperable tools for each platform and service.
Terraform also separates the planning phase from the execution phase,
by using the concept of an execution plan. By running
the current state is refreshed and the configuration is consulted to
generate an action plan. The plan includes all actions to be taken:
which resources will be created, destroyed or modified. It can be
inspected by operators to ensure it is exactly what is expected. Using
terraform graph, the plan can be visualized to show dependent ordering.
Once the plan is captured, the execution phase can be limited to only
the actions in the plan. Other tools combine the planning and execution
phases, meaning operators are forced to mentally reason about the effects
of a change, which quickly becomes intractable in large infrastructures.
Terraform lets operators apply changes with confidence, as they know exactly
what will happen beforehand.