Multipeer P2P Replicator

The Multipeer Replicator enables lightweight, self-organizing mesh networks over Wi-Fi and Bluetooth Low Energy. This approach requires minimal setup and automates peer discovery and connectivity management, making it simpler than active-passive P2P configurations.

Couchbase Lite’s Peer-to-Peer synchronization solution offers secure storage and bidirectional data synchronization between mobile and IoT devices without needing a centralized cloud-based control point.

For small mesh topologies, Multipeer Replicator offers autodiscovery over Wi-Fi and Bluetooth Low Energy, with secure communication via TLS and certificate-based authentication. The dynamic mesh topology gives optimal peer connectivity and the lightweight and low-maintenance configuration requires less management and less code than using active-passive peer-to-peer sync.

Overview

To maintain optimal connectivity, efficient data transport, and balanced workloads, the Multipeer Replicator forms a dynamic mesh network among peers in the same group. The mesh network provides resilience through multiple communication pathways. If one connection fails, data can flow through alternative routes. It avoids redundant direct connections, evenly distributes connections across peers, and optimizes communication paths through intelligent routing.

The mesh network continuously adapts as peers join or leave, automatically healing itself by establishing new connections and rerouting data flow to maintain network integrity.

This self-organizing approach ensures reliable data synchronization even in challenging network conditions, where individual peer connections may be intermittent or unreliable.

Prerequisites

The Multipeer Replicator supports two transports for peer discovery and replication: Wi-Fi and Bluetooth Low Energy (BLE). Wi-Fi is enabled by default. The enabled transports are configured through the replicator configuration. See Transports.

Transport Support

Table 1. Transport support
Transport	Available from	Discovery	Notes
Wi-Fi	CBL 3.3	DNS-SD (Bonjour)	Peers must connect to the same Wi-Fi network. Requires `NSBonjourServices` and `NSLocalNetworkUsageDescription` in `Info.plist`. See Bluetooth Platform Configuration.
Bluetooth Low Energy	CBL 3.4	BLE advertising and scanning	Requires `NSBluetoothAlwaysUsageDescription` in `Info.plist`. See Bluetooth Platform Configuration.

When you configure both transports, the Multipeer Replicator automatically selects the best available transport for each peer and switches between them as reachability changes, preferring Wi-Fi over Bluetooth. See Automatic Transport Switching.

Supported Platforms

Table 2. iOS version requirements
Feature	Minimum iOS
Couchbase Lite general support	iOS 15
Multipeer Replicator: Wi-Fi transport	iOS 15
Multipeer Replicator: Bluetooth transport	iOS 15

See Bluetooth Platform Configuration for the required Info.plist keys.

See Supported Platforms for the full platform support matrix.

Bluetooth Platform Configuration

This section applies only if your application enables Bluetooth transport. Applications that use Wi-Fi only do not require these keys.

iOS applications must declare the required keys in Info.plist for each transport they enable. You can also configure these settings through Xcode’s Info configuration UI.

Wi-Fi Transport

To use Wi-Fi transport, declare the Bonjour service type and a local network usage description.

NSBonjourServices

<key>NSBonjourServices</key>
<array>
  <string>_couchbaseP2P._tcp</string>
</array>

NSLocalNetworkUsageDescription

<key>NSLocalNetworkUsageDescription</key>
<string>Used for discovering and connecting to peers for peer-to-peer sync.</string>

Bluetooth Transport

To use Bluetooth transport, declare a Bluetooth usage description.

NSBluetoothAlwaysUsageDescription

<key>NSBluetoothAlwaysUsageDescription</key>
<string>Used for discovering and connecting to peers for peer-to-peer sync.</string>

Configuration

Collection Configurations

You can specify one or more collections available for replication when creating a MultipeerReplicatorConfiguration. For each collection, you’ll create MultipeerCollectionConfiguration with the collection object and optionally configure a custom conflict resolver or any replication filters you want to use for the collection.

Specify collections without any configurations

let collections = [collection1, collection2, collection3].map {
    MultipeerCollectionConfiguration(collection: $0)
}

Specify collections with some configuration

class CustomConflictResolver: MultipeerConflictResolver {
    func resolve(peerID: PeerID, conflict: Conflict) -> Document? {
        return conflict.remoteDocument
    }
}

// Create a collection config with a conflict resolver
var config1 = MultipeerCollectionConfiguration(collection: collection1)
config1.conflictResolver = CustomConflictResolver()

// Create a collection config with a document ID filter
var config2 = MultipeerCollectionConfiguration(collection: collection2)
config2.documentIDs = ["doc1", "doc2"]

// Create a collection config with a push replication filter
var config3 = MultipeerCollectionConfiguration(collection: collection3)
config3.pushFilter = { peerID, document, flags in
    return document.int(forKey: "access-level") == 2
}

let collections = [config1, config2, config3]

Peer Identity

Each peer in the Multipeer replication is uniquely identified and authenticated by using a peer’s certificate.

Multipeer Replicator which uses TLS communication by default requires to specify a TLSIdentity object for specifying the identity.

You can use either a self-signed certificate for the identity or have an authority or issuer sign the identity’s certificate. The choice depends on your specific security requirements and deployment environment.

As each peer could be either a client or a server to the other peer in the Multipeer replication environment, you must create the identity’s certificate with the extension key usages for both client and server authentication to allow either direction to authenticate the certificate.

CA-Signed Identity

When using a certificate authority (CA) signed identity, the issuer’s certificate authenticates the connecting peer.

Get and Create an identity signed by an issuer

let persistentLabel = "com.myapp.identity"

// Retrieve the TLS identity from the keychain using the persistent label.
var identity = try TLSIdentity.identity(withLabel: persistentLabel)

// If the identity exists but is expired, delete it.
if let existing = identity, existing.expiration < Date() {
    try TLSIdentity.deleteIdentity(withLabel: persistentLabel)
    identity = nil
}

// If the identity doesn't exist or expired, create a new one.
if identity == nil {
    // Define certificate attributes and expiration date.
    let attrs: [String: String] = [certAttrCommonName: "MyApp"]
    let expiration = Calendar.current.date(byAdding: .year, value: 2, to: Date())!

    // Get issuer's private key and certificate data (DER format) for signing the identity's certificate.
    let caKey = try getIssuerPrivateKeyData()
    let caCert = try getIssuerCertificateData()
    
    // Create and store a new identity signed with the issuer in the keychain with a persistent label.
    identity = try TLSIdentity.createSignedIdentityInsecure(
        for: [.clientAuth, .serverAuth],
        attributes: attrs,
        expiration: expiration,
        caKey: caKey,
        caCertificate: caCert,
        label: persistentLabel)
}

Self-Signed Identity

For environments where certificate authority management is not feasible, you can implement peer identity using self-signed certificates. This approach is commonly used in closed network environments where devices need to authenticate with each other without external certificate authorities.

Creating a self-signed identity for peer authentication

let persistentLabel = "com.myapp.identity"

// Retrieve the TLS identity from the keychain using the persistent label.
var identity = try TLSIdentity.identity(withLabel: persistentLabel)

// If the identity exists but is expired, delete it.
if let existing = identity, existing.expiration < Date() {
    try TLSIdentity.deleteIdentity(withLabel: persistentLabel)
    identity = nil
}

// If the identity doesn't exist or expired, create a new one.
if identity == nil {
    // Define certificate attributes and expiration date.
    let attrs: [String: String] = [certAttrCommonName: "MyApp"]
    let expiration = Calendar.current.date(byAdding: .year, value: 2, to: Date())!
    
    // Create and store a new self-signed identity in the keychain with a persistent label.
    identity = try TLSIdentity.createIdentity(
        for: [.clientAuth, .serverAuth],
        attributes: attrs,
        expiration: expiration,
        label: persistentLabel)
}

When using self-signed certificates, implement your own certificate validation logic in the authenticator callback to make sure only trusted peers can join your mesh network.

Peer Authenticator

MultipeerReplicator only supports certificate based authentication. You can specify the authenticator in two ways:

certificate authentication callback
root certificates.

When specifying the certificate authentication callback, the callback calls the remote peer’s identity certificate.

When specifying the root certificates, the Multipeer replicator automatically authenticates the remote peer’s identity certificate by verifying whether one of the specified root certificates signed the certificate.

Authenticator with authentication callback

let authenticator = MultipeerCertificateAuthenticator { peerID, certs in
    return true
}

Authenticator with root certificates

// Get issuer's certificate data (DER format), which was used to sign the peer's certificate.
let caCert = try getIssuerCertificateData()
let caCertRef = SecCertificateCreateWithData(nil, caCert as CFData)!
let authenticator = MultipeerCertificateAuthenticator(rootCerts: [caCertRef])

Transports

The transports property on MultipeerReplicatorConfiguration controls which transports the replicator uses for peer discovery and replication.

Wi-Fi Only

Wi-Fi is the default transport. Existing applications continue to operate on Wi-Fi only after upgrading to CBL 3.4 with no code changes required.

Default (Wi-Fi only)

Bluetooth Only

To use Bluetooth as the sole transport, set transports to [.bluetooth]. Peers discover each other using BLE advertising and scanning rather than DNS-SD.

Bluetooth only

Wi-Fi and Bluetooth with Automatic Switching

To enable both transports, set transports to include both .wifi and .bluetooth. The replicator prefers Wi-Fi and falls back to Bluetooth automatically when Wi-Fi is unavailable. See Automatic Transport Switching.

Wi-Fi and Bluetooth

Bluetooth has lower throughput and higher latency than Wi-Fi, and its reliability can decrease as more peers join the Bluetooth network. We recommend using Wi-Fi as the primary transport for multipeer sync, with Bluetooth as a fallback, rather than relying on Bluetooth alone.

Create MultipeerReplicatorConfiguration

The MultipeerReplicatorConfiguration can be created with a peerGroupID which identifies the peer-to-peer network used by the app, collection configurations, peer identity, and authenticator.

Creating MultipeerReplicatorConfiguration

let config = MultipeerReplicatorConfiguration(
    peerGroupID: "com.myapp",
    identity: identity,
    authenticator: authenticator,
    collections: collections)

Performance may vary in mesh networks depending on your specific environment and number of peers. We recommend running tests with your network configuration to assess any effects on packet loss or latency.

Automatic Transport Switching

When MultipeerReplicator is configured with both Wi-Fi and Bluetooth transports, it automatically selects the best available transport for each peer and switches transports as reachability changes.

Transport Preference

The replicator prefers Wi-Fi over Bluetooth when both transports can reach a peer. Bluetooth acts as a fallback when Wi-Fi cannot reach a peer.

Fallback to Bluetooth

For an individual peer, MultipeerReplicator falls back to Bluetooth when the peer is no longer reachable over Wi-Fi. This can occur if the peer disables Wi-Fi, becomes unreachable on the local network, or if replication over Wi-Fi fails because of a network-related error.

In cases of connection or replication failure over Wi-Fi, MultipeerReplicator performs a small number of retries before falling back to Bluetooth.

Return to Wi-Fi

If a peer becomes reachable over Wi-Fi while replication is active over Bluetooth, MultipeerReplicator establishes a Wi-Fi connection in parallel with the existing Bluetooth connection. The Bluetooth connection remains active until the Wi-Fi connection is fully established and replication has resumed over Wi-Fi. This prevents any interruption in synchronization during the transition.

Life Cycle

Create MultipeerReplicator with Configuration

Creating MultipeerReplicator

let replicator = try MultipeerReplicator(config: config)

Start

Starting MultipeerReplicator

replicator.start()

Stop

Stopping MultipeerReplicator

replicator.stop()

Events

In general, the connection should just work, and most of these optional listen events give status you may only want to use during development and testing.

Status events include a transport property that identifies which transport the event applies to. MultipeerReplicatorStatus events are delivered per enabled transport and also as an aggregated status (where transport is nil) representing the overall replicator state.

Event types include the following:

Multipeer Replicator Status

Multipeer Replicator Status Listener

let token = replicator.addStatusListener { status in
    let state = status.active ? "active" : "inactive"
    let error = status.error?.localizedDescription ?? "none"
    print("Multipeer Replicator: \(state), Error: \(error)")
}

Peer Discovery Status

Peer Discovery Status Listener

let token = replicator.addPeerDiscoveryStatusListener { status in
    let online = status.online ? "online" : "offline"
    print("Peer Discovery Status - Peer ID: \(status.peerID), Status: \(online)")
}

Peer’s Replicator Status

Peer’s Replicator Status Listener

let activities = ["stopped", "offline", "connecting", "idle", "busy"]
let token = replicator.addPeerReplicatorStatusListener { replStatus in
    let direction = replStatus.outgoing ? "outgoing" : "incoming"
    let activity = activities[Int(replStatus.status.activity.rawValue)]
    let error = replStatus.status.error?.localizedDescription ?? "none"
    print("Peer Replicator Status - Peer ID: \(replStatus.peerID), " +
          "Direction: \(direction), " +
          "Activity: \(activity), " +
          "Error: \(error)")
}

Peer’s Document Replication

Peer’s Document Replication Listener

let token = replicator.addPeerDocumentReplicationListener { docRepl in
    let direction = docRepl.isPush ? "Push" : "Pull"
    print("Peer Document Replication - Peer ID: \(docRepl.peerID), Direction: \(direction)")
    docRepl.documents.forEach { doc in
        let error = doc.error?.localizedDescription ?? "none"
        let collection = "\(doc.scope).\(doc.collection)"
        print(" Collection: \(collection)" +
              " Document ID: \(doc.id)," +
              " Flags: \(doc.flags)," +
              " Error: \(error)")
    }
}

Peer Info

Peer Identifier

A unique peerID, which is a digest of the peer’s identity certificate, identifies each peer. You can get your peerID from the peerID property of the MultipeerReplicator.

Getting peer ID

let peerID = replicator.peerID
print("Peer ID: \(peerID)")

Neighbor Peers

You can get a list of current online peers' identifiers from the MultipeerReplicator from the neighborPeers property.

Getting neighbor peers

print("Neighbor Peers:")
replicator.neighborPeers.forEach { peerID in
    print(" \(peerID)")
}

Peer Info

The PeerInfo object provides information about a peer, including its identifier, certificate, online status, replicator status, neighbor peers, the transports on which the peer was discovered, and the transport currently used for replication.

Getting peer info

let activities = ["stopped", "offline", "connecting", "idle", "busy"]

let printPeerInfo: (PeerInfo) -> Void = { info in
    print("Peer ID: \(info.peerID)")
    print(" Status: \(info.online ? "online" : "offline")")
    print(" Neighbor Peers:")
    info.neighborPeers.forEach { peerID in
        print("  \(peerID)")
    }

    let replStatus = info.replicatorStatus
    let activity = activities[Int(replStatus.activity.rawValue)]
    let error = replStatus.error?.localizedDescription ?? "none"
    print(" Replicator Status: \(activity), Error: \(error)")
}

replicator.neighborPeers.forEach { peerID in
    if let peerInfo = replicator.peerInfo(for: peerID) {
        printPeerInfo(peerInfo)
    }
}

Logging

LogDomain sets up the logging of:

Peer discovery log messages
Multipeer replication and mesh network management log messages

// Enable verbose console logging for multipeer replicator-related domains only.
LogSinks.console = ConsoleLogSink(level: .verbose, domains: [.peerDiscovery, .multipeer])

API Reference

You can find Swift API References here.