---
type: architecture
title: System Architecture
description: Component map of 10Backward — how config, logging, the pipeline runner, state persistence, and container entrypoint are wired together.
tags: [architecture, overview]
timestamp: 2026-08-04T17:51:00Z
---

# Overview

`10Backward` is a Mastodon daily-throwback bot that runs as a single
containerised cron job. Job 1083 adds a structured logging layer and a
retry-capable pipeline runner on top of the scaffold shipped by Job 1082.

The container boots, validates environment configuration, renders a
`/etc/cron.d/tenbackward` entry that fires once per day at the
configured `RUN_AT`, and then runs `python -m tenbackward` in the
foreground of the `cron` process. Each scheduled invocation calls
`tenbackward.main.main()`, which:

1. Installs the JSON logging formatter on the root logger.
2. Loads and validates the `Config`.
3. Ensures the data directory exists.
4. Emits a `startup` log line.
5. Executes one pipeline pass wrapped in a retry loop.
6. Emits a `run_complete` summary (skipped silently if there were no candidates).

# Components

| Component                | Responsibility                                                                                             |
|--------------------------|------------------------------------------------------------------------------------------------------------|
| `entrypoint.sh`          | Validates required env vars, renders the cron file from `RUN_AT`/`TZ`, then `exec cron -f`.                |
| `tenbackward.main`       | CLI entry point; orchestrates startup logging, retry-wrapped pipeline pass, and run summary.               |
| `tenbackward.config`     | Loads `.env` + process env, applies defaults, validates schema, produces a typed `Config` dataclass.       |
| `tenbackward.logging_setup` | JSON formatter (one log record per line), secret redaction, and `log_startup` / `log_error` / `log_run_summary` helpers. |
| `tenbackward.state`      | Read/write of `posted.json` (atomic temp-file replace).                                                    |
| `tenbackward.blog`       | GitPython `ensure_repo()` — clones the configured blog repo on first run, fast-forwards it via `pull --ff-only` thereafter, with exponential-backoff retries on transient network errors. |
| `/etc/cron.d/tenbackward`| Rendered cron file. One daily line that `cd /app` and runs `python -m tenbackward`.                        |

# Communication & Wiring

```
+-------------------+      env vars       +-------------------------+
|     entrypoint    | -------------------> |   config.load_config    |
|     (.sh)         |                     |   (Config dataclass)    |
+---------+---------+                     +-----------+-------------+
          |                                           |
          |  installs cron file                        |  feeds
          v                                           v
+-------------------+                     +-------------------------+
| /etc/cron.d/       | -- daily fires --> |   tenbackward.main.main |
| tenbackward        |                    |   - configure logging   |
+-------------------+                    |   - retry-wrapped pass  |
                                         +-----------+-------------+
                                                     |
                                                     v
                                         +-------------------------+
                                         | _run_with_retry(...)    |
                                         |  -> _run_once(...)      |
                                         |     -> state.{load,save}|
                                         +-------------------------+
```

* **Env → Config.** `load_config()` merges a `.env` file (when
  present) under the process environment via `dotenv_values` + `load_dotenv`,
  applies defaults, then validates the merged map before producing the
  `Config` dataclass.
* **Cron → main.** Each scheduled tick re-runs `python -m tenbackward`,
  so every run is a fresh interpreter invocation.
* **Pipeline state.** `_run_once` reads `posted.json` via
  `state.load_posted()` for dedupe, then writes it back via
  `state.save_posted()` only when at least one new post was recorded.

# Key Files

| Path                                | Responsibility                                                                  |
|-------------------------------------|---------------------------------------------------------------------------------|
| `/repo/entrypoint.sh`               | Bootstraps cron; validates required env vars; renders `/etc/cron.d/tenbackward`. |
| `/repo/src/tenbackward/main.py`     | CLI entry, retry wrapper, pipeline counters.                                     |
| `/repo/src/tenbackward/config.py`   | Env merging, defaults, validation, `Config` dataclass, `ConfigError`.            |
| `/repo/src/tenbackward/logging_setup.py` | JSON formatter, secret redaction, structured event helpers.                 |
| `/repo/src/tenbackward/state.py`    | `posted.json` read/write with atomic temp-file replace.                         |
| `/repo/src/tenbackward/blog.py`     | `ensure_repo()` — GitPython clone + fast-forward pull with `2^n` retry/backoff; raises `BlogRepoError` on local modifications or exhausted retries. |
| `/repo/.env.example`                | Canonical list of environment variables.                                        |
| `/repo/docker-compose.yml`          | Service definition; binds env vars from the host `.env`.                        |
| `/repo/Dockerfile`                  | Builds the runtime image.                                                       |
| `/repo/crontab/tenbackward.cron`    | Cron template shipped in the image.                                             |
| `/repo/crontab/install-cron.sh`     | Installs the rendered cron file with mode 0644; refuses placeholder leftovers.  |
| `/repo/tests/`                       | Pytest suite covering config, logging, runner, cron, and entrypoint validation. |

# Related

* [Config Schema](/architecture/config-schema.md)
* [Logging & Run Summary](/architecture/logging.md)
* [Pipeline Runner](/architecture/pipeline-runner.md)
* [Daily Run Guide](/guides/daily-run.md)
* [Environment Variable Setup](/operations/environment-setup.md)
* [Cron Lifecycle](/operations/cron-lifecycle.md)