API for Reactors

This document describes the API provided by the p2p layer to the protocol layer, namely to the registered reactors.

This API consists of two interfaces: the one provided by the Switch instance, and the ones provided by multiple Peer instances, one per connected peer. The Switch instance is provided to every reactor as part of the reactor’s registration procedure. The multiple Peer instances are provided to every registered reactor whenever a new connection with a peer is established.


The practical reasons that lead to the interface to be provided in two parts, Switch and Peer instances are discussed in more datail in the knowledge-base repository.

Switch API

The Switch is the central component of the p2p layer implementation. It manages all the reactors running in a node and keeps track of the connections with peers. The table below summarizes the interaction of the standard reactors with the Switch:

Switch API method consensus block sync state sync mempool evidence PEX
Peers() IPeerSet x x       x
NumPeers() (int, int, int)   x       x
Broadcast(Envelope) chan bool x x x      
MarkPeerAsGood(Peer) x          
StopPeerForError(Peer, interface{}) x x x x x x
StopPeerGracefully(Peer)           x
Reactor(string) Reactor   x        

The above list is not exhaustive as it does not include all the Switch methods invoked by the PEX reactor, a special component that should be considered part of the p2p layer. This document does not cover the operation of the PEX reactor as a connection manager.

Peers State

The first two methods in the switch API allow reactors to query the state of the p2p layer: the set of connected peers.

func (sw *Switch) Peers() IPeerSet

The Peers() method returns the current set of connected peers. The returned IPeerSet is an immutable concurrency-safe copy of this set. Observe that the Peer handlers returned by this method were previously added to the reactor via the InitPeer(Peer) method, but not yet removed via the RemovePeer(Peer) method. Thus, a priori, reactors should already have this information.

func (sw *Switch) NumPeers() (outbound, inbound, dialing int)

The NumPeers() method returns the current number of connected peers, distinguished between outbound and inbound peers. An outbound peer is a peer the node has dialed to, while an inbound peer is a peer the node has accepted a connection from. The third field dialing reports the number of peers to which the node is currently attempting to connect, so not (yet) connected peers.


The third field returned by NumPeers(), the number of peers in dialing state, is not an information that should regard the protocol layer. In fact, with the exception of the PEX reactor, which can be considered part of the p2p layer implementation, no standard reactor actually uses this information, that could be removed when this interface is refactored.


The switch provides, mostly for historical or retro-compatibility reasons, a method for sending a message to all connected peers:

func (sw *Switch) Broadcast(e Envelope) chan bool

The Broadcast() method is not blocking and returns a channel of booleans. For every connected Peer, it starts a background thread for sending the message to that peer, using the Peer.Send() method (which is blocking, as detailed in Send Methods). The result of each unicast send operation (success or failure) is added to the returned channel, which is closed when all operations are completed.


  • The current implementation of the Switch.Broadcast(Envelope) method is not efficient, as the marshalling of the provided message is performed as part of the Peer.Send(Envelope) helper method, that is, once per connected peer.
  • The return value of the broadcast method is not considered by any of the standard reactors that employ the method. One of the reasons is that is is not possible to associate each of the boolean outputs added to the returned channel to a peer.

Vetting Peers

The p2p layer relies on the registered reactors to gauge the quality of peers. The following method can be invoked by a reactor to inform the p2p layer that a peer has presented a “good” behaviour. This information is registered in the node’s address book and influences the operation of the Peer Exchange (PEX) protocol, as node discovery adopts a bias towards “good” peers:

func (sw *Switch) MarkPeerAsGood(peer Peer)

At the moment, it is up to the consensus reactor to vet a peer. In the current logic, a peer is marked as good whenever the consensus protocol collects a multiple of votesToContributeToBecomeGoodPeer = 10000 useful votes or blocksToContributeToBecomeGoodPeer = 10000 useful block parts from that peer. By “useful”, the consensus implementation considers messages that are valid and that are received by the node when the node is expected for such information, which excludes duplicated or late received messages.


The switch doesn’t currently provide a method to mark a peer as a bad peer. In fact, the peer quality management is really implemented in the current version of the p2p layer. This topic is being discussed in the knowledge-base repository.

Stopping Peers

Reactors can instruct the p2p layer to disconnect from a peer. Using the p2p layer’s nomenclature, the reactor requests a peer to be stopped. The peer’s send and receive routines are in fact stopped, interrupting the communication with the peer. The Peer is then removed from every registered reactor, using the RemovePeer(Peer) method, and from the set of connected peers.

func (sw *Switch) StopPeerForError(peer Peer, reason interface{})

All the standard reactors employ the above method for disconnecting from a peer in case of errors. These are errors that occur when processing a message received from a Peer. The produced error is provided to the method as the reason.

The StopPeerForError() method has an important caveat: if the peer to be stopped is configured as a persistent peer, the switch will attempt reconnecting to that same peer. While this behaviour makes sense when the method is invoked by other components of the p2p layer (e.g., in the case of communication errors), it does not make sense when it is invoked by a reactor.


A more comprehensive discussion regarding this topic can be found on the knowledge-base repository.

func (sw *Switch) StopPeerGracefully(peer Peer)

The second method instructs the switch to disconnect from a peer for no particular reason. This method is only adopted by the PEX reactor of a node operating in seed mode, as seed nodes disconnect from a peer after exchanging peer addresses with it.

Reactors Table

The switch keeps track of all registered reactors, indexed by unique reactor names. A reactor can therefore use the switch to access another Reactor from its name:

func (sw *Switch) Reactor(name string) Reactor

This method is currently only used by the Block Sync reactor to access the Consensus reactor implementation, from which it uses the exported SwitchToConsensus() method. While available, this inter-reactor interaction approach is discouraged and should be avoided, as it violates the assumption that reactors are independent.

Peer API

The Peer interface represents a connected peer. A Peer instance encapsulates a multiplex connection that implements the actual communication (sending and receiving messages) with a peer. When a connection is established with a peer, the Switch provides the corresponding Peer instance to all registered reactors. From this point, reactors can use the methods of the new Peer instance.

The table below summarizes the interaction of the standard reactors with connected peers, with the Peer methods used by them:

Peer API method consensus block sync state sync mempool evidence PEX
ID() ID x x x x x x
IsRunning() bool x     x x  
Quit() <-chan struct{}       x x  
Get(string) interface{} x     x x  
Set(string, interface{}) x          
Send(Envelope) bool x x x x x x
TrySend(Envelope) bool x x        

The above list is not exhaustive as it does not include all the Peer methods invoked by the PEX reactor, a special component that should be considered part of the p2p layer. This document does not cover the operation of the PEX reactor as a connection manager.


Nodes in the p2p network are configured with a unique cryptographic key pair. The public part of this key pair is verified when establishing a connection with the peer, as part of the authentication handshake, and constitutes the peer’s ID:

func (p Peer) ID() p2p.ID

Observe that each time the node connects to a peer (e.g., after disconnecting from it), a new (distinct) Peer handler is provided to the reactors via InitPeer(Peer) method. In fact, the Peer handler is associated to a connection with a peer, not to the actual node in the network. To keep track of actual peers, the unique peer p2p.ID provided by the above method should be employed.

Peer state

The switch starts the peer’s send and receive routines before adding the peer to every registered reactor using the AddPeer(Peer) method. The reactors then usually start routines to interact with the new connected peer using the received Peer handler. For these routines it is useful to check whether the peer is still connected and its send and receive routines are still running:

func (p Peer) IsRunning() bool
func (p Peer) Quit() <-chan struct{}

The above two methods provide the same information about the state of a Peer instance in two different ways. Both of them are defined in the Service interface. The IsRunning() method is synchronous and returns whether the peer has been started and has not been stopped. The Quit() method returns a channel that is closed when the peer is stopped; it is an asynchronous state query.

Key-value store

Each Peer instance provides a synchronized key-value store that allows sharing peer-specific state between reactors:

func (p Peer) Get(key string) interface{}
func (p Peer) Set(key string, data interface{})

This key-value store can be seen as an asynchronous mechanism to exchange the state of a peer between reactors. In the current use-case of this mechanism, the Consensus reactor populates the key-value store with a PeerState instance for each connected peer. The Consensus reactor routines interacting with a peer read and update the shared peer state. The Evidence and Mempool reactors, in their turn, periodically query the key-value store of each peer for retrieving, in particular, the last height reported by the peer. This information, produced by the Consensus reactor, influences the interaction of these two reactors with their peers.


More details of how this key-value store is used to share state between reactors can be found on the knowledge-base repository.

Send methods

Finally, a Peer instance allows a reactor to send messages to companion reactors running at that peer. This is ultimately the goal of the switch when it provides Peer instances to the registered reactors. There are two methods for sending messages:

func (p Peer) Send(e Envelope) bool
func (p Peer) TrySend(e Envelope) bool

The two message-sending methods receive an Envelope, whose content should be set as follows:

The two message-sending methods attempt to add the message (e.Payload) to the send queue of the peer’s destination channel (e.ChannelID). There is a send queue for each registered channel supported by the peer, and each send queue has a capacity. The capacity of the send queues for each channel are configured by reactors via the corresponding ChannelDescriptor.

The two message-sending methods return whether it was possible to enqueue the marshalled message to the channel’s send queue. The most common reason for these methods to return false is the channel’s send queue being full. Further reasons for returning false are: the peer being stopped, providing a non-registered channel ID, or errors when marshalling the message’s payload.

The difference between the two message-sending methods is when they return false. The Send() method is a blocking method, it returns false if the message could not be enqueued, because the channel’s send queue is still full, after a 10-second timeout. The TrySend() method is a non-blocking method, it immediately returns false when the channel’s send queue is full.

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