# Terraform Dag > Understand Terraform's execution engine as a concurrent scheduler and distinguish between logic errors in Provider code (Go) and race conditions caused by the Directed Acyclic Graph (DAG) during TestAcc runs. Use when this capability is needed. - Skill: `tomevault-io/terraform-dag` (Agent Skill, multi-file: 2 files) - Install (CLI): `npx skillmds@latest add tomevault-io/terraform-dag` - Raw SKILL.md: https://api.skillmd.com/api/skills/tomevault-io/terraform-dag/raw - Safety review: pending (external: skill-scanner PASS, skillspector PASS) - Works with: Claude Code, Claude.ai, OpenAI Codex - Category: DevOps & Infra - Author: tomevault-io (https://skillmd.com/u/tomevault-io) - Updated: 2026-09-17 - Page: https://skillmd.com/skills/tomevault-io/terraform-dag --- # Terraform Provider Concurrency & DAG Logic The agent understands Terraform's execution engine as a concurrent scheduler. It can distinguish between logic errors in Provider code (Go) and race conditions caused by the Directed Acyclic Graph (DAG) during `TestAcc` runs. ## Core Concepts for Provider Debugging ### 1. The Parallel Execution Enabler (The DAG) * **The Scheduler:** Terraform walks the DAG and identifies "independent branches." If a test config defines five `aws_instance` resources without dependencies, the Provider's `Create` function will be called 5 times concurrently (up to the default `-parallelism=10`). * **Isolation Guarantee:** In a single walk, resource $A$ cannot pass data to resource $B$ of the same type unless an edge (dependency) exists. * **Provider gRPC Boundary:** The agent must recognize that while the Provider is a single process, the `terraform-plugin-framework` (or SDKv2) handles these CRUD calls in separate goroutines. ### 2. Debugging Heuristics (Provider vs. HCL) When analyzing test failures (e.g., `TestAccExample_basic`), the agent must apply these rules: | Symptom | Likely Cause | Debugging Action | | :--- | :--- | :--- | | `ResourceNotFound` during parallel Create | **API Race Condition.** The backend API hasn't propagated the first resource before the second one tries to reference it. | Suggest adding `time.Sleep` in the provider or a `depends_on` in the test HCL to verify. | | Interleaved Logs in `TF_LOG=TRACE` | **Parallel Execution.** Logs from `Create` for Resource A and Resource B are mixing. | Identify the `[root]` or `[id]` in logs to untangle which thread is failing. | | Consistent failure on `-parallelism=1` but pass on default | **Logic Error.** The code depends on a side effect of concurrency. | (Rare) Check for global variables/shared state in the provider struct. | | Random failure on default but pass on `-parallelism=1` | **Concurrency Bug.** The provider is not thread-safe or the API is rate-limiting concurrent writes. | Check mutexes in the `client` or `provider` struct. | ### 3. Acceptance Test Isolation * `helper/resource.ParallelTest` runs entire test cases in parallel with other tests. * A single `resource.Test` with multiple resources in one `Step` runs those resources in parallel based on the DAG. ## Execution Constraints * **No Implicit Ordering:** Never assume `Resource_A` will finish before `Resource_B` if they are the same type in the same test step. * **State Consistency:** Assume that during a parallel walk, the `State` for one resource is "locked" to that CRUD operation and cannot be modified by a neighboring resource thread. ### VCR Cassette Interpretation * **Interleaved Streams:** Recognize that a single `.yaml` cassette contains multiple interleaved HTTP streams. * **Deterministic Matching:** When analyzing Go code using `go-vcr`, prioritize looking at the `vcr.Config.Matcher`. If the Provider sends a `User-Agent` with a version string or a `Request-ID`, the agent must suggest ignoring these headers in the matcher to ensure replay stability. * **State vs. Cassette:** If the `ID` in the Terraform State doesn't match the `id` in the VCR Cassette, the "Parallel Execution Enabler" will fail because the DAG will attempt to pass an ID to a dependency that "doesn't exist" in the recorded world. --- > Source: [jackofallops/terraform-provider-developer-skills](https://github.com/jackofallops/terraform-provider-developer-skills) — distributed by [TomeVault](https://tomevault.io).