What is Thanos and why is it needed for Prometheus?

Question

Accepted Answer

**Thanos** — extension of Prometheus for long-term storage, global view and high availability.

**Prometheus limitations:**
- Data is stored for 15 days (by default).
- Single instance — single point of failure.
- No global view for multi-cluster.

**Architecture:**
- **Sidecar** — runs alongside Prometheus, sends data to Object Storage.
- **Store Gateway** — reads data from Object Storage.
- **Query (Querier)** — aggregates data from Prometheus + Object Storage.
- **Compactor** — compacts old data.
- **Ruler** — writes Recording Rules / Alerting Rules to Object Storage.

**Sidecar config:**
```yaml
containers:
  - name: thanos-sidecar
    image: quay.io/thanos/thanos:v0.32.0
    args:
      - sidecar
      - --tsdb.path=/prometheus
      - --prometheus.url=http://localhost:9090
      - --objstore.config-file=/etc/thanos/bucket.yml
    volumeMounts:
      - name: data
        mountPath: /prometheus
      - name: bucket-config
        mountPath: /etc/thanos
```

**Object Storage config (S3):**
```yaml
type: S3
config:
  bucket: thanos-metrics
  endpoint: s3.us-east-1.amazonaws.com
  sse_config:
    type: SSE-S3
```

**Query:** `thanos query --endpoint=sidecar1:10901 --endpoint=sidecar2:10901` — global view.

**Thanos vs Mimir:** Thanos simpler, Mimir (Grafana) better for high-write scale.

DevOps: What is Thanos and why is it needed for Prometheus?