A gravitational slingshot makes use of the gravity and relative movements of celestial bodies to accelerate a spacecraft and change its trajectory.

Slingshot: edge record cache#

Applications in ATProtocol store data in users' own PDS (Personal Data Server), which are distributed across thousands of independently-run servers all over the world. Trying to access this data poses challenges for client applications:

• A PDS might be far away with long network latency
• or may be on an unreliable connection
• or overloaded when you need it, or offline, or…

Large projects like Bluesky control their performance and reliability by syncing all app-relevant data from PDSs into first-party databases. But for new apps, building out this additional data infrastructure adds significant effort and complexity up front.

Slingshot is a fast, eager, production-grade cache of data in the ATmosphere, offering performance and reliability without custom infrastructure.

Current status#

The core APIs will not change, since they are standard third-party com.atproto query APIs from ATProtocol.

Eager caching#

In many cases, Slingshot can cache the data you need before first request!

Slingshot subscribes to the global Firehose of data updates. It keeps a short-term rolling indexed window of all data, and automatically promotes content likely to be requested to its longer-term main cache. (automatic promotion is still a work in progress)

When there is a cache miss, Slingshot can often still accelerate record fetching, since it keeps a large cache of resolved identities: it can usually request from the correct PDS without extra lookups.

Precise invalidation#

The fireshose includes update and delete events, which Slingshot uses to ensure stale and deleted data is removed within a very short window. Additonally, identity and account-level events can trigger rapid cleanup of data for deactivated and deleted accounts. (some of this is still a work in progress)

Low-trust#

The "AT" in ATProtocol stands for Authenticated Transfer: all data is cryptographically signed, which makes it possible to broadcast data through third parties and trust that it's real without having to directly contact the originating server.

Two core standard query APIs are supported to balance convenience and trust. They both fetch records:

com.atproto.repo.getRecord#

• convenient JSON response format
• cannot be proven authentic

com.atproto.sync.getRecord#

• DAG-CBOR-encoded response requires extra libraries to decode, but
• includes a cryptographic proof of authenticity!

(work on this endpoint is in progress)

Service proxying#

Clients can proxy atproto queries through their own PDS with Service Proxying, and this is supported by Slingshot. The Slingshot instance must be started the --domain argument specified.

Service-proxied requests can specify a Slingshot instance via the atproto-proxy header:

GET /xrpc/com.bad-example.identity.resolveMiniDoc?identifier=bad-example.com
Host: <your pds>
atproto-proxy: did:web:<slingshot domain>#slingshot

Where <your pds> is the user's own PDS host, and <slingshot domain> is the domain that the slingshot instance is deployed at (eg. slingshot.microcosm.blue). See the Service Proxying docs for more.

Ergonomic APIs#

• Slingshot also offers variants of the getRecord endpoints that accept a full at-uri as a parameter, to save clients from needing to parse and validate all parts of a record location.

• Bi-directionally verifying identity endpoints, so you can directly exchange atproto handles for DIDs without extra steps, plus a convenient Mini-Doc verified identity summary.

Part of microcosm#

Microcosm is a collection of services and independent community-run infrastructure for ATProtocol.

Slingshot excels when combined with shallow indexing services, which offer fast queries of global data relationships but with only references to the data records. Microcosm has a few!

• 🌌 Constellation, a global backlink index (all social interactions in atproto are links!)
• 🎇 Spacedust, a firehose of all social interactions