Task 1.3: Trace input variables, local values, and outputs to identify the source of a configuration issue

• Aone(module.db[*].endpoint)

  Correct

  The splat gives a list of zero or one endpoints, and one returns null for an empty list or the single element otherwise.

• Bmodule.db[0].endpoint

  Incorrect

  It works when the instance exists, but with count = 0 the index is out of range and the plan fails.

• Clookup(module.db, "endpoint", null)

  Incorrect

  lookup works on maps; module.db is a list of module instances here, so the call fails.

• Dtry(module.db[0].endpoint, null)

  Correct

  try catches the dynamic out-of-range error when no instance exists and falls back to null.

• Emodule.db.endpoint[0]

  Incorrect

  module.db is a list, so it has no endpoint attribute to index; the index must come before the attribute.

A module call with count is a list of instance objects. one() with a splat and try() with an index both cover the zero-instance case; a bare index or attribute access does not.

• A[] is an empty tuple, which never equals a list(string); test length(var.private_subnet_ids) == 0

  Correct

  The equality operators need both values to have the same type, so the comparison is always false and count is 0. length avoids the type mismatch.

• BThe default is applied after count is evaluated, leaving var.private_subnet_ids null at comparison

  Incorrect

  Defaults are part of the variable's value before any expression that uses it is evaluated; count sees the empty list.

• Ccount accepts only a bool variable and converts the conditional to 0 whatever the condition returns

  Incorrect

  count accepts any whole number known at plan time; a conditional that returns 1 or 0 is valid.

• DAn empty list(string) default is stored as an empty set, and sets cannot be compared with == in Terraform

  Incorrect

  The type constraint is list(string), so the default is an empty list; the problem is the tuple literal on the other side of ==.

Comparing collections with == depends on exact types. The empty brackets literal is a tuple, so a list never matches it. length() or a type conversion states the intent without the trap.

• AAll three results, one after another

  Incorrect

  When commands are piped in, console prints only the output of the final command, unless an earlier one errors.

• BNothing, as console works only interactively

  Incorrect

  console can be used in non-interactive scripts by piping newline-separated commands to it.

• CThe first result only, after which console exits

  Incorrect

  console evaluates every piped line and prints the last result.

• DOnly the result of local.tags["owner"], unless an earlier line errors

  Correct

  In scripted use, only the output of the final command is printed, unless an error occurs earlier.

To see several values from one scripted console call, pipe a single expression that combines them, for example an object with one attribute per value.

• AThe number 12

  Incorrect

  The two arms must share a type, and Terraform converts both to string.

• BThe string "12"

  Correct

  A number converts to a string automatically, so the conditional always returns a string.

• CAn error about inconsistent conditional result types

  Incorrect

  Terraform first tries to find a type both arms convert to; number and string can both become string.

• Dnull, the result of arms with different types

  Incorrect

  Mismatched types are converted or rejected; they never produce null.

Both results of a conditional need a common type. Terraform converts where it can; HashiCorp recommends explicit conversion with tostring() or tonumber() when the result type matters.

• AOne block with null attributes

  Incorrect

  A null value splats to an empty tuple, so there is nothing to iterate.

• BAn error: for_each cannot be null

  Incorrect

  for_each never sees null here; the splat turns null into an empty tuple.

• CZero blocks

  Correct

  A splat on null returns an empty tuple, so the dynamic block generates no blocks; a non-null object becomes a one-element tuple.

• DOne block; a splat always returns at least one element

  Incorrect

  Only a non-null single value becomes a one-element tuple.

The splat operator turns an optional single object into a zero-or-one collection, which suits dynamic blocks and for_each.

• Aterraform console -plan

  Correct

  With -plan, the console first creates a plan, as terraform plan would, and evaluates expressions against the planned state, so values from the configuration are available.

• Bterraform output -json

  Incorrect

  terraform output prints declared outputs, not local values, and nothing has been applied yet.

• Cterraform state show aws_s3_bucket.logs

  Incorrect

  State has no object for a bucket that has not been created yet.

• Dterraform validate -json

  Incorrect

  validate checks the configuration; it does not evaluate expressions against state or a plan.

The console normally evaluates against the current state, which says little about objects that do not exist yet. -plan takes longer because it plans first, and a configuration whose data sources take actions while planning will take them in the console too.