Active Peer

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    Description — Couchbase Lite’s Peer-to-Peer Synchronization enables edge devices to synchronize securely without consuming centralized cloud-server resources
    Abstract — How to set up a Replicator to connect with a Listener and replicate changes using peer-to-peer sync
    Related Content — API Reference | Passive Peer | Active Peer

    Enterprise Edition only
    This an Enterprise Edition feature. Purchase the Enterprise License, which includes official Couchbase Support, to use it in production (see the license and support Licensing).
    Code Snippets
    The code examples are indicative only. They demonstrate basic concepts and approaches to using a feature. Use them as inspiration and adapt these examples to best practice when developing applications for your platform.

    Introduction

    This content provides sample code and configuration examples covering the implementation of Peer-to-Peer Sync over websockets. Specifically it covers the implementation of an Active Peer.

    This active peer (also referred to as a client and-or a replicator) will initiate connection with a Passive Peer (also referred to as a server and-or listener) and participate in the replication of database changes to bring both databases into sync.

    Subsequent sections provide additional details and examples for the main configuration options.

    Secure Storage
    The use of TLS, its associated keys and certificates requires using secure storage to minimize the chances of a security breach. The implementation of this storage differs from platform to platform — see Using secure storage.

    Configuration Summary

    You should configure and initialize a replicator for each Couchbase Lite database instance you want to sync. Example 1 shows the initialization and configuration process.

    Example 1. Replication configuration and initialization
    // Set listener DB endpoint
    NSURL *url = [NSURL URLWithString:@"ws://listener.com:55990/otherDB"];
    CBLURLEndpoint *thisListener = [[CBLURLEndpoint alloc] initWithURL:url];
    
    CBLReplicatorConfiguration *thisConfig
      = [[CBLReplicatorConfiguration alloc]
          initWithDatabase:thisDB target:thisListener]; (1)
    
    thisConfig.replicatorType = kCBLReplicatorTypePush;
    
    thisConfig.continuous = YES;
    
    // Configure Server Authentication
    // Here - expect and accept self-signed certs
    thisConfig.acceptOnlySelfSignedServerCertificate = YES; (2)
    
    // Configure Client Authentication
    // Here set client to use basic authentication
    // Providing username and password credentials
    // If prompted for them by server
    thisConfig.authenticator = [[CBLBasicAuthenticator alloc] initWithUsername:@"Our Username" password:@"Our Password"]; (3)
    
    /* Optionally set custom conflict resolver call back */
    thisConfig.conflictResolver = [[LocalWinConflictResolver alloc] (4)
    
    // Apply configuration settings to the replicator
    _thisReplicator = [[CBLReplicator alloc] initWithConfig:thisConfig]; (5)
    
    // Optionally add a change listener (6)
    // Retain token for use in deletion
    id<CBLListenerToken> thisListenerToken
      = [thisReplicator addChangeListener:^(CBLReplicatorChange *thisChange) {
          if (thisChange.status.activity == kCBLReplicatorStopped) {
            NSLog(@"Replication stopped");
            } else {
            NSLog(@"Status: %d", thisChange.status.activity);
            };
        }];
    // Run the replicator using the config settings
    [thisReplicator start]; (7)

    Notes on Example

    1 Use the ReplicatorConfiguration class’s constructor — -initWithDatabase:target: — to initialize the replicator configuration with the local database — see also: Configure Target
    2 Configure how the client will authenticate the server. Here we say connect only to servers presenting a self-signed certificate. By default, clients accept only servers presenting certificates that can be verified using the OS bundled Root CA Certificates — see: Authenticating the Listener.
    3 Configure the credentials the client will present to the server. Here we say to provide Basic Authentication credentials. Other options are available — see: Client Authentication.
    4 Configure how the replication should perform Conflict Resolution.
    5 Initialize the replicator using your configuration object.
    6 Register an observer, which will notify you of changes to the replication status.
    7 Start the replicator.

    API References

    You can find Objective-C API References here.

    Device Discovery

    This phase is optional: If the listener is initialized on a well known URL endpoint (for example, a static IP Address or well known DNS address) then you can configure active peers to connect to those.

    Prior to connecting with a listener you may execute a peer discovery phase to dynamically discover peers.

    For the active peer this involves browsing-for and selecting the appropriate service using a zero-config protocol such as Bonjour-- see: https://developer.apple.com/bonjour/.

    Configure Replicator

    Configure Target

    Use the ReplicatorConfiguration class and -initWithDatabase:target: constructor to initialize the replication configuration with local and remote database locations.

    The constructor provides

    • the name of the local database to be sync’d

    • the listener’s URL (including its port and the name of the remote database to sync with) — whether pre-configured or gathered during a discovery phase.

      It is expected that the app will identify the IP address and URL and append the remote database name to the URL endpoint (this is similar to the way you would handle a Sync Gateway endpoint.

      The URL scheme for web socket URLs uses ws: (non-TLS) or wss: (SSL/TLS) prefixes.

    Example 2. Set target and database config properties
    // Set listener DB endpoint
    NSURL *url = [NSURL URLWithString:@"ws://listener.com:55990/otherDB"];
    CBLURLEndpoint *thisListener = [[CBLURLEndpoint alloc] initWithURL:url];
    
    CBLReplicatorConfiguration *thisConfig
      = [[CBLReplicatorConfiguration alloc]
          initWithDatabase:thisDB target:thisListener]; (1)

    Notes on Example

    1 Note use of the wss:// prefix to ensure TLS encryption (strongly recommended in production).

    Sync Mode

    Here we define the direction and type of replication we want to initiate.

    We use ReplicatorConfiguration class’s replicatorType and continuous parameters, to tell the replicator:

    • The direction of the replication: pushAndPull; pull; push

    • The type of replication, that is:

      • Continuous — remaining active indefinitely to replicate changed documents (continuous=true).

      • Ad-hoc — a one-shot replication of changed documents (continuous=false).

    Example 3. Configure replicator type and mode
    thisConfig.replicatorType = kCBLReplicatorTypePush;
    
    thisConfig.continuous = YES;

    Authenticating the Listener

    Define the credentials the client is expecting fom the server in order to ensure that the server (listener) is one it is prepared to interact with.

    Note that the client cannot authenticate the server if TLS is turned off. When TLS is enabled (default) the client must authenticate the server. If the server cannot provide acceptable credentials then the connection will fail.

    Use ReplicatorConfiguration methods acceptOnlySelfSignedServerCertificate and {url-api-prop-replicator-config-setPinnedServerCertificate}, to tell the replicator how to verify server-supplied TLS server certificates.

    • If there is a pinned certificate, nothing else matters, the server cert must exactly match the pinned certificate.

    • If there are no pinned certs and acceptOnlySelfSignedServerCertificate is true then any self-signed certificate is accepted. Certificates that are not self signed are rejected, no matter who signed them.

    • If there are no pinned certificates and acceptOnlySelfSignedServerCertificate is false (default), the client validates the server’s certificates against the system CA certificates. The server must supply a chain of certificates whose root is signed by one of the certificates in the system CA bundle.

    Example 4. Set Server TLS security
    • CA Cert

    • Self Signed Cert

    • Pinned Certificate

    Set the client to expect and accept only CA attested certificates.

    // Configure Server Security -- only accept CA Certs
    thisConfig.acceptOnlySelfSignedServerCertificate = NO; (1)

    Notes on Example

    1 This is the default. Only certificate chains with roots signed by a trusted CA are allowed. Self signed certificates are not allowed.

    Set the client to expect and accept only self-signed certificates

    // Configure Server Authentication
    // Here - expect and accept self-signed certs
    thisConfig.acceptOnlySelfSignedServerCertificate = YES; (1)

    Notes on Example

    1 Set this to true to accept any self signed cert. Any certificates that are not self-signed are rejected.

    Set the client to expect and accept only a pinned certificate.

    NSURL *certURL =
      [[NSBundle mainBundle] URLForResource: @"cert" withExtension: @"cer"];
    NSData *data =
      [[NSData alloc] initWithContentsOfURL: certURL];
    SecCertificateRef certificate =
      SecCertificateCreateWithData(NULL, (__bridge CFDataRef)data);
    
    NSURL *url =
      [NSURL URLWithString:@"ws://localhost:4984/db"];
    
    CBLURLEndpoint *target = [[CBLURLEndpoint alloc] initWithURL: url];
    
    CBLReplicatorConfiguration *thisConfig =
      [[CBLReplicatorConfiguration alloc] initWithDatabase:database
                                          target:target];
    thisConfig.pinnedServerCertificate =
      (SecCertificateRef)CFAutorelease(certificate);
    
    thisConfig.acceptOnlySelfSignedServerCertificate=false;

    Client Authentication

    Here we define the credentials that the client can present to the server if prompted to do so in order that the server can authenticate it.

    We use ReplicatorConfiguration's authenticator method to define the authentication method to the replicator - see Example 6.

    Basic Authentication

    Use the BasicAuthenticator to supply basic authentication credentials (username and password).

    Example 5. Basic Authentication

    This example shows basic authentication using user name and password:

    // Here set client to use basic authentication
    // Providing username and password credentials
    // If prompted for them by server
    thisConfig.authenticator = [[CBLBasicAuthenticator alloc] initWithUsername:@"Our Username" password:@"Our Password"]; (1)

    Certificate Authentication

    Use the ClientCertificateAuthenticator to configure the client TLS certificates to be presented to the server, on connection. This applies only to the URLEndpointListener.

    The server (listener) must have disableTLS set false and have a ClientCertificateAuthenticator configured, or it will never ask for this client’s certificate.

    The certificate to be presented to the server will need to be signed by the root certificates or be valid based on the authentication callback set to the listener via ListenerCertificateAuthenticator.

    Example 6. Client Cert Authentication

    This example shows client certificate authentication using an identity from secure storage.

    // Check if Id exists in keychain and if so, use it
    CBLTLSIdentity* identity =
      [CBLTLSIdentity identityWithLabel: @"doco-sync-server" error: &error]; (1)
    
    thisConfig.authenticator =
      [[CBLClientCertificateAuthenticator alloc] initWithIdentity: identity]; (2)

    Notes on Example <.> Get an identity from secure storage and create a TLS Identity object <.> Set the authenticator to ClientCertificateAuthenticator and configure it to use the retrieved identity

    Initialize Replicator

    Use the Replicator class’s initWith(config:) constructor, to initialize the replicator with the configuration you have defined. You can, optionally, add a change listener (see Monitor Sync) before starting the replicator running using start().

    Example 7. Initialize and run replicator
    // Apply configuration settings to the replicator
    _thisReplicator = [[CBLReplicator alloc] initWithConfig:thisConfig]; (1)
    
    // Run the replicator using the config settings
    [thisReplicator start]; (2)

    Notes on Example

    1 Initialize the replicator with the configuration
    2 Start the replicator

    Monitor Sync

    Change Listeners

    Use the Replicator class to add a change listener as a callback to the Replicator (addChangeListener(_:)) — see Example 8. You will then be asynchronously notified of state changes. Use this to monitor changes and to inform on sync progress, as shown below; this is an optional step.

    Replicator Status

    You can use the Replicator class’s status property to check the replicator status. That is, whether it is actively transferring data or if it has stopped.

    The returned ReplicationStatus structure comprises:

    • activity enum — stopped, offline, connecting, idle or busy

    • progress enum

      • completed — the total number of changes completed

      • total — the total number of changes to be processed

    • error enum — the current error, if any

    For more on replication status, see: Replication Status

    Example 8. Monitor replication
    • Adding a Change Listener

    • Using replicator.status

    // Retain token for use in deletion
    id<CBLListenerToken> thisListenerToken
      = [thisReplicator addChangeListener:^(CBLReplicatorChange *thisChange) {
          if (thisChange.status.activity == kCBLReplicatorStopped) {
            NSLog(@"Replication stopped");
            } else {
            NSLog(@"Status: %d", thisChange.status.activity);
            };
        }];
    if (thisChange.status.activity == kCBLReplicatorStopped) {
      NSLog(@"Replication stopped");
      } else {
      NSLog(@"Status: %d", thisChange.status.activity);
      };

    Stop Sync

    If you added an optional change listener (see Monitor Sync for how) you should also remove it using the removeChangeListenerWithToken(CBLListenerToken:) method.
    Example 9. Stop replicator
    // Remove the change listener
    [thisReplicator removeChangeListenerWithToken: thisListenerToken];
    
    // Stop the replicator
    [thisReplicator stop];

    Notes on Example

    1 Stopping the replication is straightforward using the stop() method.

    Conflict Resolution

    Unless you specify otherwise, Couchbase Lite’s default conflict resolution policy is applied — see Automatic Conflict Resolution.

    To use a different policy, specify a conflict resolver using conflictResolver as shown in Example 10.

    For more complex solutions you can provide a custom conflict resolver - see: Custom Conflict Resolution.

    Example 10. Using conflict resolvers
    • Local Wins

    • Remote Wins

    • Merge

    @interface LocalWinConflictResolver: NSObject<CBLConflictResolver>
    @end
    
    @implementation LocalWinConflictResolver
    - (CBLDocument*) resolve: (CBLConflict*)conflict {
        return conflict.localDocument;
    }
    
    @end
    @interface RemoteWinConflictResolver: NSObject<CBLConflictResolver>
    @end
    
    @implementation RemoteWinConflictResolver
    - (CBLDocument*) resolve: (CBLConflict*)conflict {
        return conflict.remoteDocument;
    }
    
    @end
    @interface MergeConflictResolver: NSObject<CBLConflictResolver>
    @end
    
    @implementation MergeConflictResolver
    - (CBLDocument*) resolve: (CBLConflict*)conflict {
        NSDictionary *localDict = conflict.localDocument.toDictionary;
        NSDictionary *remoteDict = conflict.remoteDocument.toDictionary;
    
        NSMutableDictionary *result = [NSMutableDictionary dictionaryWithDictionary:localDict];
        [result addEntriesFromDictionary:remoteDict];
    
        return [[CBLMutableDocument alloc] initWithID:conflict.documentID
                                                 data:result];
    }
    
    @end

    Just as a replicator may observe a conflict — when updating a document that has changed both in the local database and in a remote database — any attempt to save a document may also observe a conflict, if a replication has taken place since the local app retrieved the document from the database. To address that possibility, a version of the Database.save() method also takes a conflict resolver as shown in [merging-document-properties].

    The following code snippet shows an example of merging properties from the existing document (current) into the one being saved (new). In the event of conflicting keys, it will pick the key value from new.

    Example 11. Merging document properties
    CBLDocument *document = [database documentWithID:@"xyz"];
    CBLMutableDocument *mutableDocument = [document toMutable];
    [mutableDocument setString:@"apples" forKey:@"name"];
    
    [database saveDocument:mutableDocument
           conflictHandler:^BOOL(CBLMutableDocument *new, CBLDocument *current) {
               NSDictionary *currentDict = current.toDictionary;
               NSDictionary *newDict = new.toDictionary;
    
               NSMutableDictionary *result = [NSMutableDictionary dictionaryWithDictionary:currentDict];
               [result addEntriesFromDictionary:newDict];
               [new setData: result];
               return YES;
           }
                     error: &error];

    For more on replicator conflict resolution see: Conflicts.

    Delta Sync

    If delta sync is enabled on the listener, then replication will use delta sync.