A PubSub (Publish/Subscribe) system allows peers to only receive messages of a specific type. A publisher sends messages of a specific type to subscribers of that type. For example, consider a chat application with a chat group called
music. Users interested subscribe to the group; when someone sends a message to the group, only those subscribed receive the message.
In libp2p, peers subscribe and send messages to topics. The concept is pretty similar to messaging systems (e.g., Kafka), but libp2p allows this behavior in a decentralized way. The main implementations of the protocol are Floodsub and Gossipsub.
In Floodsub, the first implementation of the pub/sub protocol, messages are delivered to all the connected nodes of a peer. For example, consider the following diagram.
Peer 1is connected to
Peer 3are connected to
Peer 1publishes a message, which is sent to
Peer 3forward the message to
FloodSub is simple, reliable, and highly resistant to malicious actors and censors, but the main problem of FloodSub is that it duplicates messages, thus using a lot of bandwidth. In the previous example,
Peer 4 receives the message twice.
Gossipsub, the protocol developed after FloodSub, tries to reduce the number of duplicate messages (i.e., bandwidth) by taking a different approach. Gossipsub is covered later in the Launchpad curriculum.
The main goal of Gossipsub is to reduce the bandwidth used in delivering messages. In the Gossipsub, peers can connect by setting up a full-message connection or a metadata-only connection. Full-message peers exchange all the information about the message; metadata-only peers exchange only data identifying the message, but not the full message itself.
With this approach, Gossipsub tries to reduce the number duplicates that a peer receives. When a metadata-only peer receives information (remember, not the full message) about a message that would like to fully receive, the connection type is changed.
Grafting: a medata-only peer converts to a full-message peer. This happens when a peer is interested in receiving full messages from a peer (mainly because the peer holds messages that are relevant). Pruning: a full-message peer converts to a medata-only peer. This happens when a peer is receiving too many full messages that are not relevant.
The number of full-message peers that a node is connected to, depends on a parameter, network peering degree,
D, which is customizable.
You can get more information about PubSub in the libp2p documentation and the in the following videos, which extensively cover the Gossipsub protocol.
Tutorial: Publish Subscribe (Pubsub) with IPFS & libp2p
In this tutorial video, you will see how you can use the publish-subscribe functionality (using the libp2p Gossipsub routing algorithm) with an IPFS node. Follow along to start an IPFS node with pubsub enabled, and send and receive messages.
Expand Your Knowledge
Demystifying libp2p Gossipsub | Ethereum Foundation - Raúl Kripalani
Gossipsub 1.1 | David Dias
A scalable, extensible & hardened P2P PubSub Router protocol | Ready Layer One - David Dias