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README.md

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+# restic-alarm: send alerts for inactive restic backups stored in S3 buckets
+
+## Design
+
+`restic-alarm` watches over S3(-compatible) buckets containing `restic` backups,
+and sends alert emails for buckets that have been inactive (= without new
+snapshots) for too long. Alerts and inactivity duration settings as well as
+contact email are configured separately for each bucket.
+
+The buckets to watch and their respective configuration are stored in a separate
+bucket `restic-alarm-state` accessed by `restic-alarm`. Each bucket to watch
+corresponds to a file `restic-alarm-state/watch/<bucket uid>` containing the
+bucket's alert settings.
+
+The S3 key id, secret, region and endpoint URL needed by `restic-alarm` to
+access `restic-alarm-state` and the buckets to watch are passed to the process
+through (standard) environment variables.
+
+## Installation (on NixOS)
+
+1. Create a new S3 key for `restic-alarm`.
+2. Create a new S3 bucket named `restic-alarm-state` and owned by this new key.
+3. Install `restic-alarm`: import the nixos module provided by the flake as
+   `nixosModules.default`, then add:
+```
+custom.restic-alarm = {
+  enable = true;
+  env_file = "/path/to/secret/env";
+};
+```
+where `/path/to/secret/env` points to a file containing the environment
+variables for S3 access (region, endpoint, access key id, secret key). **The
+`env` file should only be readable by `root`**.
+
+A template for the `env` file would be:
+```
+AWS_DEFAULT_REGION=xxxx
+AWS_ENDPOINT_URL="https://xxxxx"
+AWS_ACCESS_KEY_ID=xxxxx
+AWS_SECRET_ACCESS_KEY=xxxxx
+```
+
+## Adding a bucket to watched by `restic-alarm`
+
+1. Give read-only access to `restic-alarm`'s S3 key for your bucket.
+2. Add a new file in the `restic-alarm-state` bucket at path `watch/<bucket
+   uid>`, containing the following information:
+```
+# Email where to send inactivity alerts
+email = "your@email"
+# Inactivity after which to send an alert (in days).
+# Here, send an alert if there has not been a new snapshot in more than 3 days. 
+inactivity = 3
+# Optional: short name for your bucket (can be anything)
+name = "my_backups"
+# Optional: interval between successive alerts (in days; default = 1 day)
+alert_interval = 1
+# Optional: duration after which we stop sending alerts (in days; default = 7 days)
+alert_duration = 7
+```
+(the file is parsed as a `toml` file)
+
+Since some of the fields are optional, a minimal configuration file can be as short as:
+```
+email = "your@email"
+inactivity = 3
+```