Terraform HCL for_each vs count Resource Replacement Discipline | AgentRules.org

Rules Stacks Clients Auditor Pruner Self-Heal RFCs Benchmark CLI CI/CD Authors Submit

This specification is maintained by the AgentRules Working Group. Have enhancements or want to claim maintainership?Submit Specification →

devopshigh PriorityID: terraform-dynamic-blocks-splat

Terraform HCL for_each vs count Resource Replacement Discipline

Production Infrastructure as Code standard for Terraform and OpenTofu. Mandates using for_each with stable map keys instead of count for managing collections of cloud resources, preventing accidental cascading destructions when array elements are removed.

Prohibited Anti-Pattern: AI uses count = length(var.subnets) to provision cloud resources, causing Terraform to destroy and recreate all subsequent resources whenever an index changes.

Export Ready-to-Drop Configuration

Select format below to copy code or download file directly.

---
description: Terraform HCL for_each vs count Resource Replacement Discipline - Production Infrastructure as Code standard for Terraform and OpenTofu. Mandates using for_each with stable map keys instead of count for managing collections of cloud resources, preventing accidental cascading destructions when array elements are removed.
globs: **/*.tf, **/*.tofu
alwaysApply: false
verified_by: "agentrules.org"
license: "CC-BY-4.0"
source: "https://agentrules.org/rules/terraform-dynamic-blocks-splat"
---

# Terraform HCL for_each vs count Resource Replacement Discipline

> **Severity**: HIGH | **Category**: devops
> **Anti-Pattern Warning**: AI uses count = length(var.subnets) to provision cloud resources, causing Terraform to destroy and recreate all subsequent resources whenever an index changes.

## System Constraints
You are a Principal Terraform and Cloud Infrastructure Architect. When managing multiple instances of a resource (subnets, IAM users, security group rules), NEVER use count based on a list. Removing an item from the middle of a list causes Terraform to shift index numbers and destroy/recreate all subsequent resources. Always use for_each with stable unique keys.

## Strict Guidelines
- MUST use for_each = toset(var.items) or for_each = var.item_map for multi-instance resources.
- MUST NOT use count = length(var.items) where item order is subject to modification.
- MUST use dynamic blocks (dynamic 'ingress' { ... }) for nested block repetition.
- MUST specify lifecycle { create_before_destroy = true } on zero-downtime resource updates.
- SHOULD use splat expressions (aws_instance.web[*].id) only for referencing homogeneous outputs.

## Bad vs Good Implementations
### Safe Resource Management with for_each Map Keys in Terraform

#### Bad Antipattern (AI Common Hallucination)
> **Why it fails**: If the second CIDR is removed, the third subnet shifts from index [2] to [1]. Terraform interprets this as destroying subnet 2 and updating/recreating subnet 3, causing a production network outage.
```hcl
# FATAL: count index shifting destroys and recreates production subnets!
variable "subnet_cidrs" {
  default = ["10.0.1.0/24", "10.0.2.0/24", "10.0.3.0/24"]
}

resource "aws_subnet" "broken" {
  count      = length(var.subnet_cidrs)
  cidr_block = var.subnet_cidrs[count.index] # Removing 10.0.2.0 destroys subnet 2 AND recreates subnet 3!
}
```

#### Production Standard (Correct)
> **Standard**: Using for_each associates each subnet with a unique permanent string key ('public-a'). Removing an item deletes only that specific subnet without altering other resources.
```hcl
# Safe: Unique stable map keys guarantee zero unintended destructions
variable "subnets" {
  type = map(object({
    cidr = string
    az   = string
  }))
  default = {
    "public-a"  = { cidr = "10.0.1.0/24", az = "us-east-1a" }
    "public-b"  = { cidr = "10.0.2.0/24", az = "us-east-1b" }
    "private-a" = { cidr = "10.0.3.0/24", az = "us-east-1a" }
  }
}

resource "aws_subnet" "safe" {
  for_each          = var.subnets
  cidr_block        = each.value.cidr
  availability_zone = each.value.az
  tags = {
    Name = each.key
  }
}
```

Recommended MCP Servers (mcp-serverlist.com)

Equip agent with system tools to verify code for this stack.

Terraform Plan Evaluator MCP

Inspects terraform plan JSON for resource replacement warnings caused by count index shifts.

Recommended Agent Skills (askills.dev)

Ready-to-use executable skill functions tailored to this stack.

askills: Terraform for_each Migrator

Refactors count-based resource declarations to resilient for_each map configurations.

Architectural Specification & Invariant Rationale

Architectural Invariant Design: Terraform HCL for_each vs count Resource Replacement Discipline

Terraform HCL for_each vs count Resource Replacement Discipline defines an industrial-grade behavioral standard for devops engineering. Production Infrastructure as Code standard for Terraform and OpenTofu. Mandates using for_each with stable map keys instead of count for managing collections of cloud resources, preventing accidental cascading destructions when array elements are removed. Modern software architectures require strict adherence to deterministic patterns to guarantee high availability, maintainability, and zero security vulnerabilities.

Framework Paradigm Shift & Drift MechanicsLLMs are trained on millions of open-source repositories containing outdated code, legacy idioms, and unpatched antipatterns. When coding assistants generate implementations for this stack, they default to common statistical tokens: AI uses count = length(var.subnets) to provision cloud resources, causing Terraform to destroy and recreate all subsequent resources whenever an index changes. Without an explicit system instruction file, developers spend hours debugging silent failures.

Failure Modes & Production Blast Radius

Hazard Analysis: What Breaks Without This Rule

The following failure modes have been cataloged from hundreds of hours of production code reviews where AI assistants produced non-compliant implementations:

Failure Mode	AI Symptom (The Anti-Pattern)	Severity	Production Impact & Remedy
AI Hallucination & Deprecation Drift	AI uses count = length(var.subnets) to provision cloud resources, causing Terraform to destroy and recreate all subsequent resources whenever an index changes.	High	Violating this industrial standard introduces regression bugs, security risks, and unexpected runtime failures in production. Enforce explicit behavioral system directives across all AI coding assistants in your repository.
Silent Architecture Inconsistency	AI assistants produce mixed paradigms across files, creating fragmented codebase conventions.	Medium	Increases cognitive load during code reviews and leads to maintenance debt across engineering teams. Pin project conventions in .cursor/rules, CLAUDE.md, and AGENTS.md so all team agents follow identical guardrails.

Architectural Directives in Detail

Step-by-Step Production Directives

Each rule constraint below is injected into the AI agent's system prompt to establish deterministic boundaries:

1

MUST use for_each = toset(var.items) or for_each = var.item_map for multi-instance resources.

MUST use for_each = toset(var.items) or for_each = var.item_map for multi-instance resources.

Rationale:AI models frequently generate unconstrained code here because statistical language distributions lack project-specific architectural context. Mandating this directive ensures that generated code adheres to verified industrial production requirements.

Enforcement:Integrated into the Agent Pack rule file. When Cursor, Claude Code, or Codex processes this prompt, it applies this constraint before emitting syntax.

2

MUST NOT use count = length(var.items) where item order is subject to modification.

MUST NOT use count = length(var.items) where item order is subject to modification.

Rationale:AI models frequently generate unconstrained code here because statistical language distributions lack project-specific architectural context. Mandating this directive ensures that generated code adheres to verified industrial production requirements.

Enforcement:Integrated into the Agent Pack rule file. When Cursor, Claude Code, or Codex processes this prompt, it applies this constraint before emitting syntax.

3

MUST use dynamic blocks (dynamic 'ingress' { ... }) for nested block repetition.

MUST use dynamic blocks (dynamic 'ingress' { ... }) for nested block repetition.

Rationale:AI models frequently generate unconstrained code here because statistical language distributions lack project-specific architectural context. Mandating this directive ensures that generated code adheres to verified industrial production requirements.

Enforcement:Integrated into the Agent Pack rule file. When Cursor, Claude Code, or Codex processes this prompt, it applies this constraint before emitting syntax.

4

MUST specify lifecycle { create_before_destroy = true } on zero-downtime resource updates.

MUST specify lifecycle { create_before_destroy = true } on zero-downtime resource updates.

Rationale:AI models frequently generate unconstrained code here because statistical language distributions lack project-specific architectural context. Mandating this directive ensures that generated code adheres to verified industrial production requirements.

Enforcement:Integrated into the Agent Pack rule file. When Cursor, Claude Code, or Codex processes this prompt, it applies this constraint before emitting syntax.

5

SHOULD use splat expressions (aws_instance.web[*].id) only for referencing homogeneous outputs.

SHOULD use splat expressions (aws_instance.web[*].id) only for referencing homogeneous outputs.

Rationale:AI models frequently generate unconstrained code here because statistical language distributions lack project-specific architectural context. Mandating this directive ensures that generated code adheres to verified industrial production requirements.

Enforcement:Integrated into the Agent Pack rule file. When Cursor, Claude Code, or Codex processes this prompt, it applies this constraint before emitting syntax.

Technical Questions & Answers

Frequently Asked Technical Questions

Common questions developers face when configuring AI coding assistants for this stack:

Q:Why is Terraform HCL for_each vs count Resource Replacement Discipline critical for AI coding assistants?

Production Infrastructure as Code standard for Terraform and OpenTofu. Mandates using for_each with stable map keys instead of count for managing collections of cloud resources, preventing accidental cascading destructions when array elements are removed. Without explicit behavioral boundaries, AI coding assistants (Cursor, Claude Code, Windsurf, Codex) rely on training data averages, which frequently produce deprecated, inefficient, or vulnerable code patterns.

Q:How does adopting this rule prevent AI hallucinations?

This rule explicitly declares what the AI must avoid (AI uses count = length(var.subnets) to provision cloud resources, causing Terraform to destroy and recreate all subsequent resources whenever an index changes.) and prescribes verified positive examples. Grounding the LLM with negative and positive pairs eliminates guesswork.

Q:Which AI coding environments are supported?

This specification is multi-platform compatible: Cursor (.cursor/rules/*.mdc & .cursorrules), Anthropic Claude Code (CLAUDE.md), Windsurf (.windsurfrules), Codex / Cline (AGENTS.md), and GitHub Copilot (.github/copilot-instructions.md).

Q:Can I combine this rule with other technology stacks?

Yes. Use the Interactive Stack Builder or download an Agent Pack to merge this rule with other frontend, backend, database, security, and DevOps rules without syntax collision.

Open Specification · CC BY 4.0·Free for Personal, Team & Commercial In-Repo Use

Citation & Open Source Attribution Guidelines

This specification is licensed under Creative Commons Attribution 4.0 International (CC BY 4.0). You are completely free to adapt and share this rule in your workspace. If citing, including in public AI benchmark datasets, or redistributing, attribution to AgentRules.org is required.

Related DEVOPS Rules

View all rules →

Kubernetes Gateway API v1 HTTPRoute Canary and ReferenceGrant Invariants

Enforces strict conformance to Kubernetes Gateway API v1 specifications during migration from legacy Ingress controllers. Mandates explicit ReferenceGrant resources for cross-namespace routing, strict relative integer weighting for HTTPRoute canary releases, and explicit backendRef port declarations.

ArgoCD ApplicationSet Matrix Generator Invariants & Cascade Deletion Guards

Enforces strict structural constraints on ArgoCD ApplicationSet Matrix generators combining cluster selectors and Git directories. Prevents catastrophic multi-cluster cascade pruning, accidental secret exposure through parameter interpolation, and runaway Application CR explosion via explicit sync policies, preservation finalizers, and parameter validation.

Karpenter Resilient Topology, Interruption Queue, and Consolidation Budget Standard

Enforces deterministic disruption budgets, multi-instance-type diversification, and event-driven SQS interruption queue handling across Karpenter NodePools and EC2NodeClasses. Eliminates cascading eviction loops, instance starvation, and availability downtime during AWS Spot rebalances and underutilized node consolidation.

1 Rule in Stack

Terraform