# Data

### Data Structures

Shared data structures enable interoperability.

Solid - Solid's data structure is RDF. [RDF](https://www.w3.org/RDF/) uses URIs to name the relationship between things, allowing structured data to be shared across different applications. RDF could potentially [be a barrier to adoption](https://hal.inria.fr/hal-01966561/document), due to its complexity.

Matrix - Matrix transports messages using JSON, and tracks conversation history through DAGs.

Mastodon - ActivityPub uses streams of JSON-LD. Mastodon was formerly compatible with OStatus, which used RSS.

IPFS - IPFS uses a custom data structure, [IPLD](https://ipld.io/), designed to treat hash-linked data structures as subsets of a unified information space.

Ssb - Ssb uses append-only logs of signed JSON.

### Mutability

Federated applications allow users to edit and delete content, handled at the server level. Content may not be guaranteed to be deleted across the entire network in some cases - protocols should require applications to honor delete messages.

P2p applications have more variance around mutability.

Ssb & Hypercore - Messages added to the append-only logs used by ssb and hypercore are immutable. Applications can choose not to display messages indicated as deleted, but the data cannot be overwritten.

IPFS - Once added to a network, content is discoverable by its hash. If a copy remains stored on the network, it is re-discoverable by this reference.

Aether - "Stale" threads that have not been referenced for 6 months get dropped by the network. This design attempts to make posts more ephemeral in a p2p setting.