# Parallel Verification > Build a Parallel Verification (PV) harness — an out-of-band, last-mile integration check that proves a system actually does what it claims by driving its real public interface and then confirming the effects against the real backend through an INDEPENDENT path the code author cannot fake. Use this whenever you are building or reviewing something whose correctness ultimately lands in an external system of record: a REST API that writes to a database, a Terraform provider, a Vault secrets engine, an Okta integration, an IBM z/OS RACF resource, a message queue, an object store, an IAM policy, etc. Trigger on phrases like "verify this end-to-end", "prove it really works", "last-mile verification", "out-of-band check", "did it actually write to the database / state / backend", "black-box integration test", "build a verification harness", or any time unit tests and acceptance tests pass but you still don't trust that the real-world effect happened. Reach for PV precisely when you want a check that does not trust th - Skill: `adamtylerlynch/parallel-verification` (Agent Skill, multi-file: 3 files) - Install (CLI): `npx skillmds@latest add adamtylerlynch/parallel-verification` - Raw SKILL.md: https://api.skillmd.com/api/skills/adamtylerlynch/parallel-verification/raw - Safety review: pending - Works with: Claude Code, Claude.ai, OpenAI Codex - Category: DevOps & Infra - Author: AdamTylerLynch (https://skillmd.com/u/adamtylerlynch) - Updated: 2026-09-17 - Page: https://skillmd.com/skills/adamtylerlynch/parallel-verification --- # Parallel Verification (PV) ## What this is, and why it matters Unit tests and acceptance tests run *inside* the system's own worldview. They trust the code's mocks, its in-memory state, and its own assertions about what happened. That trust is exactly the problem: a provider can return a state file saying a certificate exists, an API can return `201 Created`, and the code can pass every test — while nothing real ever landed in the backend. **Parallel Verification is the practice of confirming an effect from the outside.** You operate the system through its real public interface as a black box, capture what the system *claims* it did, and then go to the actual system of record through a **separate, independent path** and check whether the effect is really there. The verification path must not share code, libraries, or trust with the thing under test — that independence is the entire point. The code author (human or agent) can make the code lie; they cannot make `psql`, `RACDCERT`, the AWS CLI, or the Vault CLI lie about what is actually stored. It's called **Parallel** for two reasons: 1. It runs *alongside and outside* the normal test pyramid — a parallel track to unit/acceptance tests, doing full integration verification that those can't. 2. The deliverable is a **standalone harness** that executes independently of the context that wrote the code. A human or CI job can re-run it cold. Because it talks to real systems through real CLIs, it cannot be talked out of the truth. The harness *is* the firewall against self-deception. The two-observer principle is the heart of it: the system's **self-report** (its API response, its state file, its return value) and the backend's **ground truth** (what an independent CLI sees in the real system of record) must agree. PV is the diff between those two observers. Any gap is a finding. ## What you deliver Two artifacts, always: 1. **A reusable harness** — executable scripts (typically bash) that generate inputs, drive the system, perform last-mile verification, and exit non-zero on any mismatch. It lives in its own self-contained directory (see "Where the harness lives" below) so anyone can re-run it in isolation. It is not a throwaway; it is the proof, kept. 2. **A findings report** (`FINDINGS.md`) — the human-readable verdict: what was verified, the evidence, and any discoveries that go beyond pass/fail (severity, root cause, remediation). See the structure below. ## Where the harness lives — ask first, isolate always Before writing anything, decide *where* the harness goes — and don't assume. - **Ask the user where to create the PV harness**, defaulting to `./scratchpads/`. Repos differ — some have a `verify/` dir, a `test/e2e/` tree, or a dedicated scratch area — and a wrong guess scatters artifacts where they don't belong. Propose `./scratchpads/-pv/` and let them confirm or redirect before you create files. - **Each PV test is atomic and self-contained.** Give every test its own directory under the chosen location, holding everything it needs: its own input files (`main.tf`, request payloads), its own runner script, its own log, its own state. A test must be runnable in isolation, in any order, without depending on another test's setup or leftovers — and its cleanup trap must remove only what it created. This keeps failures localized and lets you re-run one scenario without disturbing the others. - **Don't assume scaffolding that isn't there.** Do not presume an existing harness to model from, a particular SDK or worktree layout, or a sibling repo path. Discover the real structure first — search the repo, read what's actually present — and when it isn't evident, ask rather than inventing a path. A confident wrong assumption about where code or a backing service lives quietly invalidates the entire run; verifying against the wrong thing is the one failure mode PV exists to prevent. ## The core PV loop Apply this loop regardless of backend. The backend only changes step 5's mechanics; the shape is universal. 1. **Identify the contract.** What does the system claim to do, and where does the truth ultimately live? Name the real system of record (the Postgres row, the RACF profile, the Vault path, the Okta user, the S3 object). If the effect doesn't land *somewhere external and inspectable*, PV may not apply — say so. 2. **Generate realistic inputs.** Produce payloads/configs that exercise the real surface area — not just the happy path, but the fields and combinations that are easy to get wrong (types, owners, defaults, optional attributes). In the z/OSMF examples this was a `main.tf`; for a REST API it's a set of request bodies. 3. **Operate the system as a black box.** Drive it through its real public interface — `terraform apply`, `curl`, the CLI, the SDK's public API — never by reaching into its internals. If you're testing a Terraform provider, point at the locally built provider via dev overrides and run the actual `terraform` binary. Black-box discipline is what makes the result credible. 4. **Capture the system's self-report ("expected").** Record what the system says it did: the API response body, `terraform.tfstate`, the CLI's output, the returned IDs/serials. Parse it into structured values (jq is your friend) — these become the expectations you'll check against reality. 5. **Last-mile verify against ground truth ("actual").** Go to the real system of record through an independent path and read what's actually there. - REST API → Postgres: `psql` and `SELECT` the rows the API claims to have written. - Terraform provider for AWS: the `aws` CLI to describe the real resource. - Terraform provider for IBM z/OS RACF: SSH + `tsocmd "RACDCERT ... LIST"`. - Vault engine: the `vault` CLI to read/list the secret path. - Okta engine: Okta's REST API or CLI to fetch the user/group/app. **The last-mile method is not always obvious. When it isn't, ask the user** rather than guessing — they know their environment, credentials, and what "the truth" means here. See `references/last-mile-catalog.md` for the decision framework and a backend→method catalog. 6. **Compare field by field.** Diff expected against actual for every attribute that matters. Emit `PASS` / `FAIL` / `WARN` per check (WARN for things like a format mismatch that may be benign). A single comparison is worth a hundred assertions the code makes about itself. 7. **Probe beyond create.** The interesting bugs live in the rest of the lifecycle. Exercise and verify: - **Idempotency** — re-run with no input change; there must be no drift. For Terraform, `plan -detailed-exitcode` returning `2` means drift (a bug); `0` means clean. - **Update / renewal** — change an input; confirm the backend changed to match. - **Teardown** — destroy/delete; confirm the backend is actually clean, with no orphaned or ghost entries left behind. - **Failure & rollback** — where feasible, force a failure mid-operation and confirm the system doesn't leave partial garbage. 8. **Report.** Produce the machine summary (counts, exit code) and the `FINDINGS.md` writeup. Surface not just pass/fail but anything you learned — a missing permission, a non-standard output format, a confusing-but-correct behavior. Those discoveries are often the most valuable output. ## Choosing the last-mile method This is the judgment-heavy step, so give it real thought. A good last-mile path is **independent** (doesn't reuse the system-under-test's own code to read back), **authoritative** (reads the actual system of record, not a cache or a mirror), and **inspectable at the field level** (you can extract the specific values to compare). When the path is genuinely unclear — an unusual backend, an internal system, ambiguous "truth", or missing credentials — **ask the user**. Offer the options you can see and let them pick or correct you. Guessing a last-mile method and verifying against the wrong source of truth is worse than asking: it produces a confident green result that means nothing. `references/last-mile-catalog.md` holds the framework plus a catalog of known backend → verification-path mappings. ## Harness conventions Don't reinvent the scaffolding each time. `references/harness-conventions.md` distills the reusable bash patterns — colored `PASS/FAIL/WARN` logging with counters, argument parsing, credential resolution order (flags > creds file > env), prerequisite and connectivity checks, jq-based state parsing, phased runners (`--phase N`), cross-phase fact capture, cleanup traps that always tear down, and meaningful exit codes. Read it before writing a harness so your output matches the proven shape and a reviewer recognizes it instantly. Key invariants worth stating up front: - **Exit non-zero on any FAIL.** The harness's exit code is its verdict; CI depends on it. - **Always clean up.** Use a trap so a mid-run failure still tears down created resources — orphans poison the next run and the real environment. - **Capture evidence.** Log the actual backend output you compared against, so a reader can see *why* a check passed or failed, not just that it did. ## The findings report (`FINDINGS.md`) Structure it so a reviewer gets the verdict in five seconds and the depth on demand. Model it on this shape (drawn from the z/OSMF CSR verification): ```markdown # Verification — Findings () **Result: PASS|FAIL.** ## What was verified | Check | Result | Evidence | |---|---|---| | | PASS | | ## State trajectory (if lifecycle was exercised) ## New findings discovered during testing ### FINDING N — (<SEVERITY>, <merge impact>) <what you observed, root cause, and concrete remediation / action items> ## Recommendation <ship / ship-after-fix / block, with the reasoning> ## Reproducing <exact commands + prerequisites to re-run the harness> ``` The `Evidence` column matters most: cite the real serial, row, or CLI output you saw. Evidence is what makes the report trustworthy rather than another assertion. ## Backend-specific recipes Read the one that matches the system under test — each has the concrete generate→operate→last-mile recipe and the gotchas for that backend: - `references/terraform-providers.md` — any Terraform provider: dev overrides / local provider, `apply`, parsing `terraform.tfstate`, `plan -detailed-exitcode` for idempotency, the apply/plan/renew/destroy phase pattern, and how last-mile branches by what the provider targets (AWS, IBM z/OS, Vault, Okta…). - `references/rest-api-database.md` — REST/gRPC APIs whose effects land in a database: payload generation, black-box driving with `curl`, capturing responses, and last-mile `SELECT`s via `psql`/`mysql`/`mongosh`, including the reverse check (delete via API → confirm gone in DB). - `references/vault-and-secrets.md` — Vault engines and secret stores: operating via the `vault` CLI or the Terraform Vault provider, then reading back the path. - `references/okta-and-saas-apis.md` — Okta and SaaS-API backends: driving via API/CLI and last-mile via the provider's own REST API or CLI. - `references/ibm-zos-racf.md` — IBM z/OS RACF over SSH + TSO (`tsocmd`, `RACDCERT`), including the message codes that signal real failures. If the backend isn't covered, apply the core loop, use `references/last-mile-catalog.md` to choose a path (asking the user when unsure), and follow `references/harness-conventions.md` for the scaffolding.