---
title: Writing HTTP service discovery
nav_title: HTTP SD
sort_rank: 7
---

Prometheus provides a generic [HTTP Service Discovery](https://prometheus.io/docs/prometheus/latest/configuration/configuration/#http_sd_config),
that enables it to discover targets over an HTTP endpoint.

The HTTP Service Discovery is complementary to the supported service
discovery mechanisms, and is an alternative to [File-based Service Discovery](https://prometheus.io/docs/guides/file-sd/#use-file-based-service-discovery-to-discover-scrape-targets).

## Comparison between File-Based SD and HTTP SD

Here is a table comparing our two generic Service Discovery implementations.

| Item | File SD | HTTP SD |
| ---- | ------- | ------- |
| Event Based | Yes, via inotify | No |
| Update frequency | Instant, thanks to inotify | Following refresh_interval |
| Format | Yaml or JSON | JSON |
| Transport | Local file | HTTP/HTTPS |
| Security | File-Based security | TLS, Basic auth, Authorization header, OAuth2 |

## Requirements of HTTP SD endpoints

If you implement an HTTP SD endpoint, here are a few requirements you should be
aware of.

The response is consumed as is, unmodified. On each refresh interval (default: 1
minute), Prometheus will perform a GET request to the HTTP SD endpoint. The GET
request contains a `X-Prometheus-Refresh-Interval-Seconds` HTTP header with the
refresh interval.

The SD endpoint must answer with an HTTP 200 response, with the HTTP Header
`Content-Type: application/json`. The answer must be UTF-8 formatted.
If no targets should be transmitted, HTTP 200 must also be emitted, with
an empty list `[]`. Target lists are unordered.

Prometheus caches target lists. If an error occurs while fetching an updated
targets list, Prometheus keeps using the current targets list. The targets list
is not saved across restart. The `prometheus_sd_http_failures_total` counter
metric tracks the number of refresh failures.

The whole list of targets must be returned on every scrape. There is no support
for incremental updates. A Prometheus instance does not send its hostname and it
is not possible for a SD endpoint to know if the SD requests is the first one
after a restart or not.

The URL to the HTTP SD is not considered secret. The authentication and any API
keys should be passed with the appropriate authentication mechanisms. Prometheus
supports TLS authentication, basic authentication, OAuth2, and authorization
headers.

## HTTP_SD format

```json
[
  {
    "targets": [ "<host>", ... ],
    "labels": {
      "<labelname>": "<labelvalue>", ...
    }
  },
  ...
]
```


Examples:

```json
[
    {
        "targets": ["10.0.10.2:9100", "10.0.10.3:9100", "10.0.10.4:9100", "10.0.10.5:9100"],
        "labels": {
            "__meta_datacenter": "london",
            "__meta_prometheus_job": "node"
        }
    },
    {
        "targets": ["10.0.40.2:9100", "10.0.40.3:9100"],
        "labels": {
            "__meta_datacenter": "london",
            "__meta_prometheus_job": "alertmanager"
        }
    },
    {
        "targets": ["10.0.40.2:9093", "10.0.40.3:9093"],
        "labels": {
            "__meta_datacenter": "newyork",
            "__meta_prometheus_job": "alertmanager"
        }
    }
]
```
