# Prometheus (and Prometheus-Compatible Backends)

Epimetheus can ingest metrics into Prometheus via two paths. Any backend that exposes the Prometheus Remote Write API (including **VictoriaMetrics**) is supported by pointing `-prometheus=` at that backend's write URL (e.g. `http://victoriametrics:8428/api/v1/write`).

## Ingestion paths (overview)

```
                    Epimetheus
                         │
         ┌───────────────┼───────────────┐
         │               │               │
         ▼               ▼               ▼
   Realtime mode    Historic/Backfill   Watch mode
   (current data)   (old data)          (CSV file mtime)
         │               │               │
         ▼               │               │
   ┌───────────┐         │               │
   │Pushgateway │         │               │
   │ (HTTP POST)│         │               │
   └─────┬─────┘         │               │
         │ Scrape         │               │
         │ (15–30s)       │               │
         ▼               ▼               ▼
   ┌─────────────────────────────────────────────┐
   │  Prometheus / VictoriaMetrics               │
   │  Remote Write API: /api/v1/write            │
   │  (realtime: via Pushgateway scrape;         │
   │   historic/watch: direct POST)              │
   └─────────────────────────────────────────────┘
```

## Ingestion Paths

### Realtime: Pushgateway

- **Used by:** realtime mode, and auto mode for samples < 5 minutes old.
- **Flow:** Epimetheus pushes to Pushgateway (HTTP POST); Prometheus scrapes Pushgateway on its schedule. Timestamps become "now" at scrape time.
- **Flags:** `-pushgateway` (default `http://localhost:9091`), `-job`, `-continuous`.

### Historic: Remote Write API

- **Used by:** historic mode, backfill mode, auto mode for samples ≥ 5 minutes old, and watch mode (when `-prometheus` is set).
- **Flow:** Epimetheus sends samples to the Remote Write endpoint (e.g. `/api/v1/write`). Timestamps from the data are preserved.
- **Flags:** `-prometheus` (default `http://localhost:9090/api/v1/write`).

The Remote Write receiver must be enabled on Prometheus for historic/watch/backfill/auto with old data. See [Setup: Prometheus](../operations/setup-prometheus.md).

## Prometheus-Compatible Backends (e.g. VictoriaMetrics)

Backends that implement the [Prometheus Remote Write](https://prometheus.io/docs/concepts/remote_write_spec/) API work with Epimetheus without any code changes. Use their write endpoint as the `-prometheus=` URL.

**Example (VictoriaMetrics):**

```bash
./epimetheus -mode=watch -file=data.csv -metric-name=myapp \
  -prometheus=http://victoriametrics:8428/api/v1/write
```

Replace host/port with your VictoriaMetrics (or other compatible) write URL. Realtime mode still uses Pushgateway (scraped by your Prometheus or VictoriaMetrics); for watch/historic/backfill/auto, only the `-prometheus=` target changes.

## Time Ranges

| Time range | Status | Method |
|------------|--------|--------|
| Current (< 5 min) | Supported | Pushgateway |
| 1 hour old | Supported | Remote Write |
| 1 day to 1 month old | Supported | Remote Write |
| 6+ months | May be rejected (retention) | Remote Write |
| Years old | Likely rejected; use `promtool tsdb create-blocks-from` | — |
| Future (> 5 min ahead) | Rejected | — |

Out-of-order samples (older than existing data for the same series) require out-of-order ingestion to be enabled on the backend, or use different labels. See [Troubleshooting](../operations/troubleshooting.md).

## Retention and Configuration

Check your backend's retention (e.g. Prometheus `retention`, VictoriaMetrics settings). For very old data you may need to increase retention or enable out-of-order ingestion. See [Setup: Prometheus](../operations/setup-prometheus.md) for Prometheus-specific options.