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README.md

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+# ctlog-uptime-exporter
+
+A Prometheus exporter for [Certificate Transparency](https://certificate.transparency.dev/) log uptime data published by Google at:
+
+```
+https://www.gstatic.com/ct/compliance/endpoint_uptime_24h.csv
+```
+
+The CSV reports the 24-hour uptime percentage for each CT log URL broken down by operation type (`add-chain`, `get-entries`, `get-sth`, etc.). This exporter fetches that CSV on a configurable schedule and exposes the data as Prometheus metrics.
+
+## Metrics
+
+| Metric | Labels | Description |
+|---|---|---|
+| `ct_log_uptime_ratio` | `log_url`, `endpoint` | 24h uptime as a ratio (0–1) |
+| `ct_log_uptime_fetch_success` | — | 1 if the last fetch succeeded, 0 otherwise |
+| `ct_log_uptime_fetch_timestamp_seconds` | — | Unix timestamp of the last fetch attempt |
+
+Standard `go_*` and `process_*` metrics are also exposed.
+
+## Usage
+
+```sh
+go build -o ctlog-uptime-exporter .
+./ctlog-uptime-exporter
+```
+
+Metrics are served at `http://localhost:9781/metrics`.
+
+## Flags
+
+| Flag | Default | Description |
+|---|---|---|
+| `-listen` | `:9781` | Address to listen on |
+| `-url` | `https://www.gstatic.com/ct/compliance/endpoint_uptime_24h.csv` | URL of the uptime CSV |
+| `-interval` | `12h` | How often to fetch the CSV |
+| `-jitter` | `5m` | Maximum ±jitter applied to the fetch interval |
+
+The exporter fetches the CSV once on startup, then repeats every `-interval` ± a random value up to `-jitter`. This avoids thundering-herd effects if multiple instances run in parallel.
+
+## Example Prometheus scrape config
+
+```yaml
+scrape_configs:
+  - job_name: ctlog_uptime
+    static_configs:
+      - targets: ['localhost:9781']
+```
+
+## License
+
+Apache 2.0 — see [LICENSE](LICENSE).