Add README

README.md

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+# prometheus-restic-backblaze
+
+**A prometheus exporter to pull restic snapshot timestamps from Backblaze**
+
+---
+
+[Restic](https://restic.net/) works very well with [Backblaze B2 cloud storage](https://www.backblaze.com/b2/cloud-storage.html).
+
+I wanted to have a way to monitor restic backups in Backblaze with prometheus. We're talking about "black box" monitoring here, so not monitoring the backup process itself but **only** whether a snapshot actually exists in the end.
+
+This prometheus exporter reads restic snapshots in backblaze and reports `restic_latest_snapshot_timestamp` metrics with the unix timestamp of the latest snapshot it can find for a tuple of `(repository, host, paths)`, which will be added as labels.
+
+It can be used as a Docker container, see below.
+
+## Configuration
+
+The config is simple YAML and contains a list of repositories to monitor:
+
+```yaml
+repositories:
+  - name: name of your repository
+    bucket: the b2 bucket
+    folder: the subfolder inside the b2 bucket
+    b2_account_id: your_b2_api_id
+    b2_account_key: your_b2_api_key
+    restic_password: your_restic_password
+```
+
+## Build and run the container
+
 ```
 $ docker build -t prometheus-restic-backblaze .
-
-$ docker run -v $(pwd)/config.yml:/etc/restic-config.yml prometheus-restic-backblaze
+$ docker run -e LISTEN_PORT=8080 -v $(pwd)/config.yml:/etc/restic-config.yml prometheus-restic-backblaze
+```
+
+## Example alerting rules
+
+Here is an example alertmanager alert:
+
+```yaml
+- alert: BackupTooOld
+  expr: time() - restic_latest_snapshot_timestamp > 14 * 24 * 60 * 60 # => 14 days
+  annotations:
+    summary: |
+      Backup for too old: {{$value|humanizeDuration}}
+
+```
+
+It's very important to also add an `absent()` rule, otherwise there would be no alert if the exporter fails:
+
+```yaml
+- alert: BackupDataMissingMycloud
+  expr: absent(restic_latest_snapshot_timestamp{repository="your_repository"})
+  annotations:
+    summary: |
+      No data for backup from repository {{$labels.repository}}
 ```