myronmeier
commented
3 years ago Thanks for raising this, @techdragon, literally days before we ran into the same issue. Upvoted! I would even go so far as to propose it be an error and not just a warning.
Here is a simple demonstration.
terraform {
required_version = "0.15.4"
}
locals {
local_dbs = {
db1 = { a = "1a" }
db1 = { a = "2a" } // notice the key here is the same as the previous (e.g. a mistake)
db3 = { a = "3a" }
}
}
output locals_map_out {
value = local.local_dbs
}The above produces these outputs
Apply complete! Resources: 0 added, 0 changed, 0 destroyed.
Outputs:
locals_map_out = {
"db1" = {
"a" = "2a"
}
"db3" = {
"a" = "3a"
}
}Notice the loss of the 1st db1 map entry. There was no error and no warning. It just silently replaced the 1st map entry with the 2nd.
The same can be demonstrated with input variables as well
variable "var_dbs" {
type = map(object({
a = string
}))
}
output variable_map_out {
value = var.var_dbs
}With this terraform.tfvars
var_dbs = {
db1 = { a = "1a" }
db1 = { a = "2a" }
db3 = { a = "3a" }
}Produces
Outputs:
variable_map_out = tomap({
"db1" = {
"a" = "2a"
}
"db3" = {
"a" = "3a"
}
})Adding more outputs to the locals example above to see if the 1st map entry is just somehow hidden indicates that it was likely lost while constructing the map itself. This similar older issue #18573 (circa TF 11?) seems to confirm that, but it also makes a prediction about duplicate map keys being an error in future versions (TF 12?), which appears to have not come true.
output locals_map_length {
value = length(local.local_dbs)
}
output locals_map_keys {
value = keys(local.local_dbs)
}
output locals_map_values {
value = values(local.local_dbs)
}
output locals_map_json {
value = jsonencode(local.local_dbs)
}Outputs:
locals_map_json = "{\"db1\":{\"a\":\"2a\"},\"db3\":{\"a\":\"3a\"}}"
locals_map_keys = [
"db1",
"db3",
]
locals_map_length = 2
locals_map_out = {
"db1" = {
"a" = "2a"
}
"db3" = {
"a" = "3a"
}
}
locals_map_values = [
{
"a" = "2a"
},
{
"a" = "3a"
},
]Even reading from JSON silently drops the duplicate key without error or warning
locals {
local_json = jsondecode( <<-EOT
{
"db1": { "a": "1a" },
"db1": { "a": "2a" },
"db3": { "a": "3a" }
}
EOT
)
}
output locals_json_out {
value = local.local_json
}Outputs:
locals_json_out = {
"db1" = {
"a" = "2a"
}
"db3" = {
"a" = "3a"
}
}
techdragon
nwsparks
philip-harvey
fernando-villalba
ams1
naitmare01
scottpiper-wiz
apparentlymart
stefansedich
nhooey
rpuserh
Shocktrooper
yermulnik
ericrichtert
lra
Current Terraform Version
Use-cases
Avoiding duplicate keys in maps.
Attempted Solutions
Linter scripts and third party tools.
Proposal
Duplicate keys in maps should raise some kind of warning. This would help prevent mistakes when working with larger and more complex maps. In particular dynamic maps and merged maps where the keys may not be visible in the terraform configuration as written, only being populated when terraform is run.
References
N.A.