Constellation

Constellation is a Microcosm service that indexes backlinks across AT Protocol records. It walks records from the firehose, extracts links, and lets clients query which records point at a DID, AT-URI, or web URL.

This is useful for relationships that are public record links but are not always exposed by an AppView in the shape an app wants.

A block record links to the blocked DID.
A follow record links to the followed DID.
A list item links to both the member DID and the list URI.

Link Sources

Constellation queries usually have two important inputs:

subject: the DID, AT-URI, or URL being linked to.
source: the record collection plus the field path that contains the link.

The source format is:

<collection>:<path>

Examples:

Relationship	Subject	Source
Accounts that block a DID	blocked account DID	`app.bsky.graph.block:subject`
Accounts that follow a DID	followed account DID	`app.bsky.graph.follow:subject`
Lists containing a DID	member DID	`app.bsky.graph.listitem:subject`

Older /links/* endpoints split this into collection and path. The XRPC endpoints use the combined source value.

XRPC Endpoints

The current API is under /xrpc/blue.microcosm.links.*.

GET /xrpc/blue.microcosm.links.getBacklinksCount

Returns a total count for records whose source points at subject.

GET /xrpc/blue.microcosm.links.getBacklinkDids

Returns distinct DIDs that created matching records. This is the endpoint to use when the client only needs identities, not individual link records.

GET /xrpc/blue.microcosm.links.getBacklinks

Returns matching link records. Each record identifies the DID, collection, and rkey of the source record. A client still has to hydrate anything it wants to display.

GET /xrpc/blue.microcosm.links.getManyToMany

Returns records that link to both the primary subject and another subject. The pathToOther query parameter names the second link field.

For app.bsky.graph.listitem, the primary subject can be the member DID and pathToOther=list returns the list URI.

Use Cases

Constellation is useful when a client needs relationship data that follows from public records:

Counting records that point at a DID or AT-URI.
Finding distinct accounts that created matching records.
Hydrating backlink records through an AppView or PDS after Constellation identifies their source DID, collection, and rkey.
Finding join records, such as list memberships, where one record points at both an account and another resource.

Implementation Notes

Use XRPC endpoints for new code. The public docs still show deprecated /links/* routes, but new callers should prefer /xrpc/blue.microcosm.links.*.

Send an Accept: application/json header. The public docs mention it, and it protects callers if the service changes browser defaults.

Use a descriptive User-Agent. The public instance asks callers to include a project name and contact when possible.

Counts are indexer counts, not AppView counts. They can differ from Bluesky AppView state because they depend on firehose ingestion, deletes, account state, and whatever backfill the instance has completed.

getBacklinks returns source records, not fully hydrated users or lists. A client should hydrate returned records through an AppView or PDS and handle suspended, missing, or blocked resources.

Example

curl -s \
  -H 'Accept: application/json' \
  -H 'User-Agent: example-client contact@example.com' \
  'https://constellation.microcosm.blue/xrpc/blue.microcosm.links.getBacklinksCount?subject=did%3Aplc%3Aexample&source=app.bsky.graph.block%3Asubject'