---
title: relay, seeding & crawler sources
---

## multiple relay support

`hydrant` supports connecting to multiple relays simultaneously for firehose ingestion. when `RELAY_HOSTS` is configured with multiple URLs:

- one independent firehose stream loop is spawned per relay
- each relay maintains its own firehose cursor state
- all ingestion loops share the same worker pool and database

commit events are de-duplicated according to the repo `rev`. account / identity events are de-duplicated using the `time` field.

## direct PDS connections

a firehose source can also be a direct connection to a PDS rather than a relay. prefix the URL with `pds::` to mark it as such:

```
HYDRANT_RELAY_HOSTS=wss://bsky.network,pds::wss://pds.example.com
```

only when a source is marked as a direct PDS (`is_pds: true`), hydrant enforces host authority. relays (`is_pds: false`, the default) are exempt from this check, since they forward commits from many PDSes by design.

## firehose seeding

in relay mode, `RELAY_HOSTS` defaults to empty. set `SEED_HOSTS` to one or more relay base URLs and hydrant will call `com.atproto.sync.listHosts` on each at startup and every 30 minutes thereafter, adding every returned PDS as a firehose source:

```
HYDRANT_SEED_HOSTS=https://bsky.network
```

seeding runs as a background task so the main firehose loop is not blocked. seed URLs are fetched concurrently (up to four at a time) and the full `listHosts` pagination is consumed for each. if a request fails partway through, the hosts collected so far are still added and the failure is logged.

each discovered host is added as a persistent PDS firehose source (`is_pds: true`), equivalent to calling `POST /firehose/sources`.

banned hosts (`status: "banned"`) are skipped. all other statuses are included since the firehose ingestor retries on disconnect and transiently-unavailable hosts will reconnect on their own.

seeding runs from latest cursor on restart so new PDS' added to the upstream relay since the last start are picked up automatically (if they haven't through firehose). sources that are already running are detected and skipped, so re-seeding is idempotent.

## crawler sources

the crawler is configured separately from the firehose via `CRAWLER_URLS`. each source is a `[mode::]url` entry where the mode prefix is optional and defaults to `by_collection` in filter mode or `list_repos` in full-network mode.

- `list_repos`: enumerates the relay via `com.atproto.sync.listRepos`. relay account status is reconciled for repos already known to hydrant; filtered indexes check unknown repos with `describeRepo` before enqueueing them.
- `relay_first`: an explicit coverage-over-moderation opt-in. it completes the relay's authoritative `listRepos` pass first, then calls the same relay's `listHosts` and crawls `listRepos` on each safe, non-banned PDS. relay-listed inactive repos are retained as status tombstones, so deep pages can only add DIDs the relay did not list and never update relay-authoritative status or revision state. this may discover inactive or other repositories omitted by the relay, which also means it deliberately bypasses relay-level repository moderation for those unknown DIDs. local PDS bans and filter excludes still win.
- `by_collection`: queries `com.atproto.sync.listReposByCollection` for each configured signal. more efficient for filtered indexing since it only surfaces repos that have matching records. cursors are stored per collection. note that it won't crawl anything if no signals are specified.

```
CRAWLER_URLS=by_collection::https://lightrail.microcosm.blue,list_repos::wss://bsky.network
```

for deep discovery in a full-network indexer:

```
CRAWLER_URLS=relay_first::https://relay.fire.hose.cam
```

`relay_first` processes at most four PDSes concurrently. each PDS request uses the shared per-PDS concurrency gate and rate-tier pacing, holds its permit through the bounded response read, and feeds 429/hard failures back into the normal PDS throttler. unsafe/private DNS targets, locally or upstream-banned hosts, offline hosts, and PDSes without `listRepos` support are skipped. relay-pass, `listHosts`, and per-PDS cursors are durable; a restart resumes the last committed page, and `/crawler/cursors` clears all of them for the source.

each source maintains its own cursor so restarts resume mid-pass.

sources can also be added and removed at runtime via the `/crawler/sources` API (see [crawler management](../api/crawler.md)). dynamically added sources are persisted to the database and survive restarts. `CRAWLER_URLS` sources are startup-only: they are not written to the database and will always reappear after a restart regardless of runtime changes (unless you change the config of course).