Getting Started with Peer-to-Peer Sync on iOS

This tutorial uses a simple inventory tracker app to demonstrate Couchbase Lite’s peer-to-peer database sync functionality.

Introduction

Couchbase Lite ^[1] provides out-of-the-box support for secure Peer-to-Peer Sync, over websockets. The sync, between Couchbase Lite enabled clients in IP-based networks, does not require a centralized control point. You do not need a Sync Gateway or Couchbase Server to get peer-to-peer database sync going.

What You Will Learn

This tutorial will demonstrate how to -

Use NetService for peer discovery (i.e.to advertise and discover services/devices )
Configure a websockets listener to listen to incoming requests. We will walk through various TLS modes and client authentication modes.
Start a bi-directional replication from active peer.
Sync data between connected peers

Throughout this tutorial, these terms are used interchangeably:

"passive peer", "server" and "listener" all refer to the peer on which the websocket listener is started
"active peer" and "client" both refer to the peer on which the replicator is initialized.

We will be using a simple inventory app in swift as an example to demonstrate the peer-to-peer functionality.

You can learn more about Couchbase Lite here

Prerequisites

This tutorial assumes familiarity with building swift apps with Xcode and with Couchbase Lite.

If you are unfamiliar with the basics of Couchbase Lite, it is recommended that you follow the Getting Started guides
iOS (Xcode 12+)
- Download latest version from the Mac App Store
Wi-Fi network that the peers can communicate over
You could run your peers in multiple simulators. But if you were running the app on real devices, you will need to ensure that the devices are on the same WiFi network

App Overview

This is a simple inventory app that can be used as a passive or active peer.

The app uses a local database that is pre-populated with data. There is no Sync Gateway or Couchbase Server installed.

When used as a passive peer:

Users log in and start websockets listener for the couchbase lite database. A service corresponding to the listener is advertised over Bonjour.
View the status of connected clients
Directly sync data with connected clients

When used as an active peer, users can:

Log in and start browsing for peers
Connect to a listener
Directly sync data with connected clients.

App Installation

Clone the repo

git clone https://github.com/couchbaselabs/couchbase-lite-peer-to-peer-sync-examples

The app project does not come bundled with the Couchbase Lite framework. Run the script to pull down the framework
```
cd /path/to/cloned/repo/couchbase-lite-peer-to-peer-sync-examples/ios/list-sync
sh install_12.sh
```

Try it Out

Open the iOS project using Xcode
```
open list-sync.xcodeproj
```
Build and run the project
Verify that you see the login screen

Figure 1. App in Action

Exploring the App Project

The xcode project comes pre-bundled with some resource files that we will examine here

samplelist.json
JSON data that is loaded into the local Couchbase Lite database. It includes the data for a single document. See Data Model
userallowlist.json
List of valid client users (and passwords) in the system. This list is looked up when the server tries to authenticate credentials associated with incoming connection request.
listener-cert-pkey.p12
This is PKCS12 file archive that includes a public key cert corresponding to the listener and associated private key. The cert is a sample cert that was generated using OpenSSL tool.
listener-pinned-cert.cer
This is the public key listener cert (the same cert that is embedded in the listener-cert-pkey.p12 file) in DER encoded format. This cert is pinned on the client replicator and is used for validating server cert during connection setup

Data Model

Couchbase Lite is a JSON Document Store. A Document is a logical collection of named fields and values. The values are any valid JSON types. In addition to the standard JSON types, Couchbase Lite supports some special types like Date and Blob. While it is not required or enforced, it is a recommended practice to include a "type" property that can serve as a namespace for related.

The "List" Document

The app deals with a single Document with a "type" property of "list". This document is loaded from the samplelist.json file bundled with the project

Sample List Document

{
    "type":"list",
    "list":[
      {
         "image":{"length":16608,"digest":"sha1-LEFKeUfywGIjASSBa0l/cg5rlm8=","content_type":"image/jpeg","@type":"blob"},
          "value":10,
          "key":"Apples"
      },
      {
        "image":{"length":16608,"digest":"sha1-LEFKeUsswGIjASssSBa0l/cg5rlm8=","content_type":"image/jpeg","@type":"blob"},
        "value":110,
        "key":"oranges"
       }
    ]

}

The document is encoded as a ListRecord struct defined in the ListRecord.swift file

Initializing Local Database

The app loads the data from the JSON document named samplelist.json the first time the database is created. This is done regardless of whether the app is launched in passive or active mode.

Open the DatabaseManager.swift file and locate the openOrCreateDatabaseForUser() method. This method creates an instance of Couchbase Lite database for the user if one does not exist and loads the empty database with data ready from bundled sample JSON file

        var exists = false
        if Database.exists(withName: kUserDBName, inDirectory: userFolderPath) == true {
              _userDb = try? Database(name: kUserDBName, config: options)
            exists = true
 
        }
        else {
              _userDb = try? Database(name: kUserDBName, config: options)
              
        }

Open the SampleFileLoaderUtils.swift file and locate the loadSampleJSONDataForUserFromFile() method. This function parses the document in JSON and updates it to embed the "image" property into every object in the "list" array. The "image" property holds a blob entry to an image asset. The image for the blob is available in the "Assets.xcassets" folder
Open the DatabaseManager.swift file and locate the createUserDocumentWithData() method. This is where the document is saved into the database. Again, this is only done if there is no preexisting database for the user

        for (key,value) in data {
            let docId = "\(kDocPrefix)\(key)"
            print("DocId is \(docId)")
            let doc = MutableDocument(id:docId, data:value as! Dictionary<String, Any>)
            try db.saveDocument(doc)
        }

Passive Peer or Server

First, we will walk through the steps of using the app in passive peer mode

Initializing Websockets Listener

Open the DatabaseManager.swift file and locate the initWebsocketsListenerForUserDb() function. This is where the websockets listener for peer-to-peer sync is initialized

        // Include websockets listener initializer code
        var listenerConfig = URLEndpointListenerConfiguration(database: db) (1)
        
        // Configure the appropriate auth test mode
        switch listenerTLSSupportMode { (2)
            case .TLSDisabled:
                listenerConfig.disableTLS  = true
                listenerConfig.tlsIdentity = nil
            case .TLSWithAnonymousAuth:
                listenerConfig.disableTLS  = false // Use with anonymous self signed cert
                listenerConfig.tlsIdentity = nil
             case .TLSWithBundledCert:
                
                if let tlsIdentity = self.importTLSIdentityFromPKCS12DataWithCertLabel(kListenerCertLabel) {
                    listenerConfig.disableTLS  = false
                    listenerConfig.tlsIdentity = tlsIdentity
                }
                else {
                    print("Could not create identity from provided cert")
                    throw ListDocError.WebsocketsListenerNotInitialized
                }
            case .TLSWithGeneratedSelfSignedCert:
                if let tlsIdentity = self.createIdentityWithCertLabel(kListenerCertLabel) {
                    listenerConfig.disableTLS  = false
                    listenerConfig.tlsIdentity = tlsIdentity
                }
                else {
                    print("Could not create identity from generated self signed cert")
                    throw ListDocError.WebsocketsListenerNotInitialized
                }

        
        }
        
        listenerConfig.enableDeltaSync = true (3)
        
        listenerConfig.authenticator = ListenerPasswordAuthenticator.init { (4)
                   (username, password) -> Bool in
            if (self._allowlistedUsers.contains(["password" : password, "name":username])) {
                return true
            }
            return false
               }
        
        _websocketListener = URLEndpointListener(config: listenerConfig)

1	Initialize the `URLEndpointListenerConfiguration` for the specified database. There is a listener for a given database. You can specify a port to be associated with the listener. In our app, we let Couchbase Lite choose the port
2	This is where we configure the TLS mode. In the app, we have a flag named `listenerTLSSupportMode` that allows the app to switch between the various modes. You can change the mode by changing the value of the variable. See Testing Different TLS Modes
3	Enable delta sync. It is disabled by default
4	Configure the password authenticator callback function that authenticates the username/password received from the client during replication setup. The list of valid users are configured in `userallowlist.json` file bundled with the app

Testing Different TLS Modes

The app can be configured to test different TLS modes by changing the listenerTLSSupportMode setting property in the DatabaseManager.swift file — see values in Table 1.

This flag operates in conjunction with `listenerCertValidationMode — see: Testing Different Server Authentication Modes. Recommended combinations are shown in Table 2.

         fileprivate let listenerCertValidationMode:ListenerCertValidationTestMode = .TLSSkipValidation

Table 1. TLS Modes on Listener
listenerTLSSupportMode Value	Behavior
TLSDisabled	There is no TLS. All communication is plaintext (insecure mode and not recommended in production)
TLSWithAnonymousAuth	The app uses self-signed cert that is auto-generated by Couchbase Lite as `TLSIdentity` of the server. While server authentication is skipped, all encryption is still encrypted. This is the default mode of Couchbase Lite.
TLSWithBundledCert	The app generates `TLSIdentity` of the server from public key cert and private key bundled in the `listener-cert-pkey.p12` archive. Communication is encrypted
TLSWithGeneratedSelfSignedCert	The app uses Couchbase Lite `createIdentity` convenience API to generate the `TLSIdentity` of the server. Communication is encrypted

Table 2. Recommended combination of flags
TLS Mode	Validation Mode
ListenerTLSTestMode.TLSDisabled	ListenerCertValidationTestMode.TLSSkipValidation
ListenerTLSTestMode.TLSWithAnonymousAuth	ListenerCertValidationTestMode.TLSSkipValidation
ListenerTLSTestMode.TLSWithBundledCert	ListenerCertValidationTestMode.TLSEnableValidationWithCertPinning
ListenerTLSTestMode.TLSWithGeneratedSelfSignedCert	ListenerCertValidationTestMode.TLSSkipValidation (edited)

Start Websockets Listener

Open the DatabaseManager.swift file and locate the startWebsocketsListenerForUserDb() method.

        DispatchQueue.global().sync {
            do {
                try websocketListener.start()
                handler(websocketListener.urls,nil)
            }
            catch {
                handler(nil,error)
            }
    
        }

Advertising Listener Service

In the app, we use NetService to advertise the websockets listener service listening at the specified listener port. This aspect of the app has nothing to do with Couchbase Lite. In your production app, you can use any suitable mechanism including using a well known URL to advertise your service that active clients can be pre-configured to connect to.

Open the ServiceAdvertiser.swift file and look for ServiceAdvertiser class. Here, we advertise a Bonjour service with service type of `_cblistservicesync._tcp`

  /// The Bonjour service name. Setting it to an empty String will be
  /// mapped to the device's name.
  public var serviceName: String = ""
  
  /// The Bonjour service type.
  public var serviceType = "_cblistservicesync._tcp"
  
 /// The Bonjour domain type.
 public var serviceDomain = ""

The service is published as implemented in the doStart() method.

    private func doStart(database:String, _ port:UInt16) {
        let service = NetService(domain: serviceDomain, type: serviceType,
                                 name: serviceName, port:Int32(port))

        service.delegate = self
        service.includesPeerToPeer = true
        service.publish()
        services[database] = service

    }

Explore the content in the ServiceAdvertiser.swift. It includes implementation of the NetServiceDelegate delegate callback methods to accept incoming connections.

Stop Websockets Listener

Open the DatabaseManager.swift file and locate the stopWebsocketsListenerForUserDb() method. You can stop the listener at any point. If there are connected clients, it will warn you that there are active connections. If you choose to stop listener, all connected clients will be disconnected

    func stopWebsocketsListenerForUserDb() throws{
        print(#function)
        guard let websocketListener = _websocketListener else {
            throw ListDocError.WebsocketsListenerNotInitialized
        }
        websocketListener.stop()
        _websocketListener = nil
    }

Try it out

Run the app on a simulator or a real device. If its the latter, make sure you sign your app with the appropriate developer certificate
On login screen, sign in as any one of the users configured in the userallowlist.json file such as "bob" and "password"
From the "listener" tab, start the listener by clicking on "Start Listener" button
Click on the "action" button to see number of connected clients. It should be 0 if there are no connected clients
From the "listener" tab, stop the listener by clicking on "Stop Listener" button

Figure 2. App in action - start passive peer

Active Peer or Client

We will walk through the steps of using the app in active peer mode

Discovering Listeners

In the app, we use NetService to browse for devices that are advertising services with name `_cblistservicesync._tcp`. This aspect of the app has nothing to do with Couchbase Lite. In your production app, you could launch your listener at well known URL well and pre-configure your active peer to connect to the URL.

Open the ServiceBrowser.swift file and look for ServiceBrowser class. Here, we browse for a service with service type of `_cblistservicesync._tcp` using Bonjour

    public func startSearch(withDelegate delegate:ServiceBrowserDelegate? ){
         peerBrowserDelegate = delegate
        self.browser = NetServiceBrowser()
        self.browser?.delegate = self
        self.browser?.searchForServices(ofType: serviceType, inDomain: domain)
    }

Explore the content in the ServiceBrowser.swift. It includes implementation of the NetServiceDelegate delegate callback methods to resolve the service to its IP Address and port that will be used by the client to connect to the listener.

Initializing and Starting Replication

Initializing a replicator for peer-to-peer sync is fundamentally the same as the case if the Couchbase Lite client were to sync with a remote Sync Gateway.

Open the DatabaseManager.swift file and locate the startP2PReplicationWithUserDatabaseToRemotePeer() method.
If you have been using Couchbase Lite to sync data with Sync Gateway, this code should seem very familiar. In this function, we initialize a bi-directional replication to the listener peer in continuous mode. We also register a Replication Listener to be notified of status to the replication status.

        if replicatorForUserDb == nil {
            // Start replicator to connect to the URLListenerEndpoint
            var target = URL(string: "wss://\(peer)/\(kUserDBName)")
            switch listenerTLSSupportMode {
            case .TLSDisabled:
                target = URL(string: "ws://\(peer)/\(kUserDBName)")
            default:
                target = URL(string: "wss://\(peer)/\(kUserDBName)")
            }
            guard let targetUrl = target else {
                throw ListDocError.URLInvalid
            }
            print("\(#function) with \(targetUrl)")
         
            
            var config = ReplicatorConfiguration.init(database: userDb, target: URLEndpoint.init(url:targetUrl)) (1)

            config.replicatorType = .pushAndPull
            config.continuous =  true
            
            // Explicitly allows self signed certificates. By default, only
            // CA signed cert is allowed
            switch listenerCertValidationMode { (2)
                  
                case .TLSSkipValidation :
                    // Use acceptOnlySelfSignedServerCertificate set to true to only accept self signed certs.
                    // There is no cert validation
                    config.acceptOnlySelfSignedServerCertificate = true
                                
                
                case .TLSEnableValidationWithCertPinning:
                    // Set acceptOnlySelfSignedServerCertificate set to false if you are using CA signed certs
                    // Set acceptOnlySelfSignedServerCertificate when listener is setup with self signed certs
                   
                    config.acceptOnlySelfSignedServerCertificate = true // since app is bundled with self signed cert
                    
                    // Enable cert pinning to only allow certs that match pinned cert
                    
                    if let pinnedCert = self.loadSelfSignedCertForListenerFromBundle() {
                        config.pinnedServerCertificate = pinnedCert
                    }
                    else {
                        print("Failed to load server cert to pin. Will proceed without pinning")
                    }
                
                case .TLSEnableValidation:
                     // Use acceptOnlySelfSignedServerCertificate set to false to only accept CA signed certs
                     // Self signed certs will fail validation. There is no cert pinning
                    config.acceptOnlySelfSignedServerCertificate = false
                
                            
            }
             
            
            let authenticator = BasicAuthenticator(username: user, password: password)(3)
            config.authenticator = authenticator

            replicatorForUserDb = Replicator.init(config: config) (4)
            _replicatorsToPeers[peer] = replicatorForUserDb

          }
        if let pushPullReplListenerForUserDb = registerForEventsForReplicator(replicatorForUserDb,handler:handler) {
            _replicatorListenersToPeers[peer] = pushPullReplListenerForUserDb
        
        }

            
        replicatorForUserDb?.start() (5)
        handler(PeerConnectionStatus.Connecting)

1	Initialize a Repicator Configuration for the specified local database and remote listener URL endpoint
2	This is where we configure the TLS server cert validation mode - whether we enable cert validation or skip validation. This would only apply if you had enabled TLS support on listener as discussed in Table 1. If you skip server cert validation, you still get encrypted communication, but you are communicating with a untrusted listener. In the app, we have a flag named `listenerCertValidationMode` that allows you to try the various modes. You can change the mode by changing the value of the variable. See Testing Different Server Authentication Modes
3	The app uses basic client authentication to authenticate with the server
4	Initialize the Replicator
5	Start replication. The app uses the events on the Replicator Listener to listen to monitor the replication.

Testing Different Server Authentication Modes

In Initializing Websockets Listener section, we discussed the various ways the listener TLSIdentity can be configured. Here, we describe the corresponding changes on the replicator side to authenticate the server identity. The app can be configured to test different TLS modes as follows by setting the listenerCertValidationMode property in the DatabaseManager.swift file — see possible values in Table 3.

This flag operates in conjunction with listenerTLSSupportMode. Recommended combinations are shown in Table 2.

Naturally, if you initialized the listener with TLSDisabled mode, skip this section as there is no TLS.

         fileprivate let listenerCertValidationMode:ListenerCertValidationTestMode = .TLSSkipValidation

Table 3. TLS Listener Cert Authentication
listenerCertValidationMode Value	Behavior
TLSSkipValidation	There is no authentication of server cert. The server cert is a self-signed cert. This is typically in used in dev or test environments. Skipping server cert authentication is discouraged in production environments. Communication is encrypted.
TLSEnableValidation	If the listener cert is from well known CA then you will use this mode. Of course, in our sample app, the listener cert as specified in `listener-cert-pkey` is a self signed cert - so you probably will not use this mode to test. But if you have a CA signed cert, you can configure your listener with the CA signed cert and use this mode to test. Communication is encrypted
TLSEnableValidationWithCertPinning	In this mode, the app uses the pinned cert,`listener-pinned-cert.cer` that is bundled in the app to validate the listener identity. Only the server cert that exactly matches the pinned cert will be authenticated. Communication is encrypted

Stopping Replication

Open the DatabaseManager.swift file and locate the stopP2PReplicationWithUserDatabaseToRemotePeer() method.
If you have been using Couchbase Lite to sync data with Sync Gateway, this code should seem very familiar. In this function, we remove any listeners attached to the replicator and stop it. You can restart the replicator again in startP2PReplicationWithUserDatabaseToRemotePeer() method

        if let listener = _replicatorListenersToPeers[peer] {
            replicator.removeChangeListener(withToken: listener)
            _replicatorListenersToPeers.removeValue(forKey: peer)
        }
        
        replicator.stop()

Try it out

Follow instructions in "Try It Out" section of Passive Peer or Server to start app in passive mode on a simulator instance or real device.
Run the app on a separate simulator instance or a real device. If its the latter, make sure you sign your app with the appropriate developer certificate
On login screen, sign in as any one of the users configured in the userallowlist.json file such as "bob" and "password". As an exercise, try with an invalid user and ensure it fails
Tap on the "browser" tab. The app automatically browses for listener and lists it here.
Tap on the row corresponding to listener. This will start replication with the listener and it should transition to Connected state
Verify the connection count on listener
Swipe left on the the row. You should see option to remove listener and Disconnect. Try Disconnect and then reconnect again

Figure 3. App in action — start active peer

Syncing Data

Once the connection is established between the peers, you can start syncing. Couchbase Lite takes care of it.

Try it out

Run the app on two or more simulators or real devices. If it’s the latter, make sure you sign your app with the appropriate developer certificate
Start the listener on one of the app instances. You could also have multiple listeners.
Connect the other instances of the app to the listener
Tap on the "List" tab
Edit the quantity or image on any one of the instances
Watch it sync automatically to other connected clients

Figure 4. App in action — sync

What Next

As an exercise, switch between the various TLS modes and server cert validation modes and see how the app behaves. You can also try with different topologies to connect the peers.

Learn More

Congratulations on completing this tutorial!

This tutorial walked you through an example of how to directly synchronize data between Couchbase Lite enabled clients. While the tutorial is for iOS, the concepts apply equally to other Couchbase Lite platforms.