---
type: architecture
title: Pipeline Runner
description: How tenbackward.main turns one cron tick into a startup log line, a retry-wrapped pipeline pass, and a single run_complete summary line.
tags: [pipeline, runner, retry]
timestamp: 2026-08-04T17:51:00Z
---

# Purpose

`tenbackward.main` is the entry point executed by cron. Job 1083
rewrote it to: (a) install JSON logging, (b) execute a retry-wrapped
pipeline pass that is **silent when there are no candidates**, and
(c) emit a single structured `run_complete` summary on success.

# Call Flow

```
main.main()
  ├── configure_json_logging()
  ├── load_config()  ── raises ConfigError → exit 2
  ├── config.data_dir.mkdir(parents=True, exist_ok=True)
  ├── log_startup(version, site_url, run_at, tz, hashtags, throwback_prefix)
  ├── result = _run_with_retry(config)
  │       ├── for attempt in 1 .. max_retries+1:
  │       │     try: return _run_once(config)
  │       │     except Exception as exc:
  │       │           log_error("pipeline_error", exc=exc, attempt=attempt, max_attempts=attempts)
  │       ├── log_error("pipeline_failed", exc=last_exc, attempts=attempts)
  │       └── return None
  ├── if result is None: return 1
  └── log_run_summary(scanned, matched, posted, skipped, posted_ids)
```

# Extension Point: `_iter_candidates`

```python
def _iter_candidates(config: Config) -> Iterable[str]: ...
```

Job 1083 ships `_iter_candidates` as an **empty iterator** so the
silent-on-no-matches contract is the default behaviour. The future
blog-clone + matching pipeline plugs into this function without
touching `_run_once` or `_run_with_retry`.

# `_run_once` — Summary Counters

`_run_once` returns a 5-tuple:

```python
(scanned, matched, posted, skipped, posted_ids)
```

| Counter     | Meaning                                                              |
|-------------|----------------------------------------------------------------------|
| `scanned`   | Items yielded by `_iter_candidates`.                                 |
| `matched`   | Items that survived the dedupe-vs-`already_posted` check.            |
| `posted`    | Items newly recorded in `posted.json` during this run.               |
| `skipped`   | Items that matched but were already in `posted.json` (deduped).      |
| `posted_ids`| The list of post identifiers written to state.                       |

`state.save_posted` is called **only** when `posted_ids` is non-empty,
so a no-op run does not touch `posted.json` on disk.

# Retry Behaviour (`_run_with_retry`)

* `attempts = max(1, config.max_retries + 1)` — at least one attempt
  even if `MAX_RETRIES=0`.
* Any exception inside `_run_once` is caught (`noqa: BLE001` —
  intentional, the whole pass is opaque to the runner).
* Each failed attempt is recorded via
  `log_error("pipeline_error", exc=exc, attempt=attempt, max_attempts=attempts)`.
* When the budget is exhausted, the runner emits
  `log_error("pipeline_failed", exc=last_exc, attempts=attempts)` and
  returns `None` so `main` can translate it to `exit 1`.
* `time.sleep(0)` between attempts is the placeholder seam where the
  next job can introduce real backoff.

# Silent-on-No-Matches Contract

A run with zero candidates produces exactly one line:

```json
{"ts":"...","level":"INFO","message":"startup","event":"startup",...}
```

No `run_complete` line is emitted. Tests assert this by counting log
records (`tests/test_run_logging.py::test_run_summary_silent_when_no_candidates`).

# Example Sequence (Healthy Run)

1.  `startup` line at INFO.
2.  `_run_with_retry` returns `(0, 0, 0, 0, [])` (no candidates yet
    because `_iter_candidates` is empty) → `main` exits `0`
    **without** a `run_complete` line.
3.  Next-day tick repeats the cycle.

# Example Sequence (Future Blog Step in Place)

1.  `startup` line at INFO.
2.  `_iter_candidates` yields `[id-1, id-2]`. `id-1` is already in
    `posted.json`, so `skipped += 1`. `id-2` is new and appended to
    `posted_ids` and persisted.
3.  `run_complete` line at INFO with
    `scanned=2, matched=2, posted=1, skipped=1, posted_ids=["id-2"]`.

# Related

* [Logging & Run Summary](/architecture/logging.md)
* [Config Schema](/architecture/config-schema.md)
* [Cron Lifecycle](/operations/cron-lifecycle.md)