# libp2p-bitswap
Implementation of the bitswap protocol.
## Efficiently syncing dags of blocks
Bitswap is a very simple protocol. It was adapted and simplified for ipfs-embed. The message
format can be represented by the following enums.
```rust
pub enum BitswapRequest {
Have(Cid),
Block(Cid),
}
pub enum BitswapResponse {
Have(bool),
Block(Vec<u8>),
}
```
The mechanism for locating providers can be abstracted. A dht can be plugged in or a centralized
db query. The bitswap api looks as follows:
```rust
#[derive(Debug)]
pub enum BitswapEvent {
/// A get query needs a list of providers to make progress. Once the new set of
/// providers is determined the get query can be notified using the `inject_providers`
/// method.
Providers(QueryId, Cid),
/// Received a block from a peer. Includes the number of known missing blocks for a
/// sync query. When a block is received and missing blocks is not empty the counter
/// is increased. If missing blocks is empty the counter is decremented.
Progress(QueryId, usize),
/// A get or sync query completed.
Complete(QueryId, Result<()>),
}
pub trait BitswapStore: Send + Sync + 'static {
/// The store params.
type Params: StoreParams;
/// A have query needs to know if the block store contains the block.
fn contains(&mut self, cid: &Cid) -> Result<bool>;
/// A block query needs to retrieve the block from the store.
fn get(&mut self, cid: &Cid) -> Result<Option<Vec<u8>>>;
/// A block response needs to insert the block into the store.
fn insert(&mut self, block: &Block<Self::Params>) -> Result<()>;
/// A sync query needs a list of missing blocks to make progress.
fn missing_blocks(&mut self, cid: &Cid) -> Result<Vec<Cid>>;
}
pub struct BitswapConfig {
/// Timeout of a request.
pub request_timeout: Duration,
/// Time a connection is kept alive.
pub connection_keep_alive: Duration,
/// The number of concurrent requests per peer.
pub receive_limit: NonZeroU16,
}
impl<P: StoreParams> Bitswap<P> {
/// Creates a new `Bitswap` behaviour.
pub fn new(config: BitswapConfig) -> Self;
/// Adds an address for a peer.
pub fn add_address(&mut self, peer_id: &PeerId, addr: Multiaddr);
/// Removes an address for a peer.
pub fn remove_address(&mut self, peer_id: &PeerId, addr: &Multiaddr);
/// Starts a get query with an initial guess of providers.
pub fn get(&mut self, cid: Cid, initial: impl Iterator<Item = PeerId>) -> QueryId;
/// Starts a sync query with an the initial set of missing blocks.
pub fn sync(&mut self, cid: Cid, missing: impl Iterator<Item = Cid>) -> QueryId;
/// Cancels an in progress query. Returns true if a query was cancelled.
pub fn cancel(&mut self, id: QueryId) -> bool;
/// Adds a provider for a cid. Used for handling the `Providers` event.
pub fn inject_providers(&mut self, id: QueryId, providers: Vec<PeerId>);
/// Register bitswap stats in a prometheus registry.
pub fn register_metrics(&self, registry: &Registry) -> Result<()>;
/// Polls the behaviour for the next bitswap event.
pub fn poll(&mut self, cx: &mut Context) -> BitswapEvent;
}
```
So what happens when you create a get request? First all the providers in the initial set
are queried with the have request. As an optimization, in every batch of queries a block
request is sent instead. If the get query finds a block it returns a query complete. If the
block wasn't found in the initial set, a `Providers` event is emitted. This is where
the bitswap consumer tries to locate providers by for example performing a dht lookup. After
the locating of providers completes, it is signaled by calling `inject_providers`. The query
manager then performs bitswap requests using the new provider set which results in the block
being found or a `BlockNotFound` error.
Often we want to sync an entire dag of blocks. We can efficiently sync dags of blocks by adding
a sync query that runs get queries in parallel for all the references of a block. The set of
providers that had a block is used as the initial set in a reference query.
## License
MIT OR Apache-2.0