Data source block syntax in Terraform - Step-by-Step Execution

Process Flow - Data source block syntax

Start Terraform config

↓

Define data source block

↓

Specify provider and data source type

↓

Set required arguments

↓

Terraform reads data source

↓

Data available for use in resources

↓

End

Terraform reads the data source block to fetch existing info, making it available for resource configuration.

Execution Sample

Terraform

data "aws_ami" "example" {
  most_recent = true
  owners      = ["amazon"]
  filter {
    name   = "name"
    values = ["amzn2-ami-hvm-*-x86_64-gp2"]
  }
}

This data block fetches the most recent Amazon Linux 2 AMI owned by Amazon.

Process Table

Step	Action	Evaluation	Result
1	Terraform reads 'data' block type and name	data "aws_ami" "example"	Identifies data source type 'aws_ami' with local name 'example'
2	Reads attribute 'most_recent'	most_recent = true	Sets to fetch the latest AMI
3	Reads attribute 'owners'	owners = ["amazon"]	Filters AMIs owned by Amazon
4	Reads 'filter' block	filter { name = "name", values = ["amzn2-ami-hvm-*-x86_64-gp2"] }	Filters AMIs matching name pattern
5	Terraform queries AWS API with filters	API call with filters	Returns AMI data matching criteria
6	Data source 'example' populated	Data available	AMI ID and attributes ready for use
7	Terraform completes data source read	End of data block processing	Data source ready for resource references

💡 Terraform finishes reading data source block and stores fetched data for later use.

Status Tracker

Variable	Start	After Step 2	After Step 3	After Step 4	After Step 5	Final
most_recent	undefined	true	true	true	true	true
owners	undefined	undefined	["amazon"]	["amazon"]	["amazon"]	["amazon"]
filter_name	undefined	undefined	undefined	"name"	"name"	"name"
filter_values	undefined	undefined	undefined	["amzn2-ami-hvm-*-x86_64-gp2"]	["amzn2-ami-hvm-*-x86_64-gp2"]	["amzn2-ami-hvm-*-x86_64-gp2"]
data_source_result	undefined	undefined	undefined	undefined	AMI data object	AMI data object

Key Moments - 3 Insights

Why do we use 'data' blocks instead of 'resource' blocks for existing infrastructure?

Data blocks fetch existing info without creating or changing it, as shown in execution_table step 5 where Terraform queries AWS to get data but does not create anything.

What happens if the filter values do not match any existing resource?

Terraform will fail to find data and throw an error during the data source read phase (execution_table step 5), because data sources must return existing info.

Can we use variables inside data source blocks?

Yes, variables can be used to set attributes inside data blocks before Terraform reads them, similar to how 'owners' or 'filter.values' could be dynamic.

Visual Quiz - 3 Questions

Look at the execution_table, at which step does Terraform actually fetch data from the cloud provider?

A Step 3

B Step 5

C Step 2

D Step 7

💡 Hint

Refer to execution_table row with 'Terraform queries AWS API with filters'

According to variable_tracker, what is the value of 'most_recent' after step 4?

A true

B false

C undefined

D null

💡 Hint

Check the 'most_recent' row under 'After Step 4' column in variable_tracker

If we remove the 'owners' attribute, what would likely happen during execution?

A Terraform will fail to parse the data block

B Terraform will ignore the data source

C Terraform will fetch all AMIs without owner filter

D Terraform will create a new AMI

💡 Hint

Consider how filters affect data source queries as shown in execution_table steps 3 and 5

Concept Snapshot

Terraform data source block syntax:
- Use 'data' keyword with type and name
- Set required arguments inside block
- Terraform reads and queries existing resources
- Data is read-only and used in resource configs
- Filters narrow down data source results
- Data source must return existing info or error

Full Transcript

This visual execution shows how Terraform processes a data source block. It starts by reading the block type and name, then reads attributes like 'most_recent' and 'owners'. Next, it reads any filter blocks to narrow the search. Terraform then queries the cloud provider API with these filters to fetch existing resource data. The fetched data is stored and made available for use in resource definitions. Variables like 'most_recent' and 'owners' are set before the query. If filters do not match any resource, Terraform errors. Data sources do not create or modify resources; they only read existing ones.