Service Discovery in WCF 4.0 – Part 2 z

Service Discovery in WCF 4.0 – Part 2

In the previous post I discussed about the basic usage of WCF Discovery in 4.0. I implemented a managed discovery service by inheriting from System.ServiceModel.Discovery.DiscoveryProxy. I utilized a concurrent dictionary as the endpoint repository. I created a WCF service which have the ServiceDiscoveryBehavior attached. It can send the online and offline message to the discovery service announcement endpoint. I also created a client which invoke the discovery service’s probe endpoint to retrieve the service endpoint by providing the service contract. So that the client don’t need to know where the service is.

This is a very basic usage of discovery service. As I mentioned at the end of my previous post there are something I was figured out.

1, In sample code I hard coded the binding on client. This is because by default the WCF Discovery endpoint metadata doesn’t contain binding information. So I need to make sure that service and client used the same binding. As we know when we want to call a WCF service we must understand its “ABC”, which is address, binding and contract. WCF Discovery supports “AC” but no “B”. So how can we make our discovery service store the service binding.

2, I was using a concurrent dictionary as the endpoint repository. This is good for a test project, but not for production. In most cases we might be going to save the endpoint information in database. So how to store the service endpoint metadata in a database. This might need some serialization and deserialization work.

3, By default, WCF Discovery will return all proper endpoints. But normally the client only need one endpoint to invoke. How can we leverage this feature to build a load balance enabled discovery service.

4, When we want a service to be discoverable, in previous post I need to add the ServiceDiscoveryBehavior before the service host was opened. How can we make this more flexible so that we can use an attribute on top of each discoverable service. And how about using this attribute to add more flexibility.

In this post I will discuss and demonstrate them one by one.

Add Binding Information in Endpoint Metadata

The build-in endpoint metadata doesn’t support binding. This is because binding was not in WS-Discovery and the WCF Discovery is following this specification. But WCF gives us a chance to add customized data into the endpoint metadata, which is the EndpointDiscoveryMetadata.Extensions property. It is a collection of XElement which means we can add any data here as a list of XML element. The only thing we need to do is how to serialize the value.

So now what we need is to retrieve the discoverable service’s binding, serialize it and append the value into the Extensions property when it’s online. And when client probed, we will received the binding and desterilize.

EndpointDiscoveryBehavior is the place we can put our customized data into the metadata extensions property. We need to prepare the data and insert them into the EndpointDiscoveryBehavior.Extensions property. Then add it into each service endpoint’s behavior. So when the service goes online and sends the online message, each endpoint will perform our EndpointDiscoveryBehavior and inject our customized data into the endpoint metadata extensions.

In the code below I created a service behavior. When WCF applies it, each endpoint of the service will retrieve the binding object and binding type name, serialized them to JSON, and created two XML elements and added to the EndpointDiscoveryBehavior.Extensions.

Since we cannot desterilize the binding JSON string back to an abstract Binding object, we must put the actual binding type name within the metadata extensions.

public class DiscoverableServiceBehavior : IServiceBehavior
{
    private const string CST_XELEMNAME_BINDINGTYPENAME = "bindingTypeName";
    private const string CST_XELEMNAME_BINDING = "binding";
  
    public void AddBindingParameters(ServiceDescription serviceDescription, ServiceHostBase serviceHostBase, Collection<ServiceEndpoint> endpoints, BindingParameterCollection bindingParameters)
    {
    }
  
    public void ApplyDispatchBehavior(ServiceDescription serviceDescription, ServiceHostBase serviceHostBase)
    {
        foreach (var endpoint in serviceDescription.Endpoints)
        {
            var binding = endpoint.Binding;
            var endpointDiscoveryBehavior = new EndpointDiscoveryBehavior();
            var bindingType = binding.GetType().AssemblyQualifiedName;
            var bindingJson = JsonConvert.SerializeObject(binding);
            endpointDiscoveryBehavior.Extensions.Add(new XElement(CST_XELEMNAME_BINDINGTYPENAME, bindingType));
            endpointDiscoveryBehavior.Extensions.Add(new XElement(CST_XELEMNAME_BINDING, bindingJson));
            endpoint.Behaviors.Add(endpointDiscoveryBehavior);
        }
    }
  
    public void Validate(ServiceDescription serviceDescription, ServiceHostBase serviceHostBase)
    {
    }
}

Add this new service behavior in the discoverable service host process.

static void Main(string[] args)
{
    var baseAddress = new Uri(string.Format("net.tcp://localhost:11001/stringservice/{0}/", Guid.NewGuid().ToString()));
  
    using (var host = new ServiceHost(typeof(StringService), baseAddress))
    {
        host.Opened += (sender, e) =>
        {
            Console.WriteLine("Service opened at {0}", host.Description.Endpoints.First().ListenUri);
        };
  
        host.AddServiceEndpoint(typeof(IStringService), new NetTcpBinding(), string.Empty);
  
        var announcementAddress = new EndpointAddress(ConfigurationManager.AppSettings["announcementEndpointAddress"]);
        var announcementBinding = Activator.CreateInstance(Type.GetType(ConfigurationManager.AppSettings["bindingType"], true, true)) as Binding;
        var announcementEndpoint = new AnnouncementEndpoint(announcementBinding, announcementAddress);
        var discoveryBehavior = new ServiceDiscoveryBehavior();
        discoveryBehavior.AnnouncementEndpoints.Add(announcementEndpoint);
        host.Description.Behaviors.Add(discoveryBehavior);
  
        var discoverableServiceBehavior = new DiscoverableServiceBehavior();
        host.Description.Behaviors.Add(discoverableServiceBehavior);
  
        host.Open();
  
        Console.WriteLine("Press any key to exit.");
        Console.ReadKey();
    }
}

Then on the client side, when a client found the endpoint metadata from discovery service it will retrieve the binding information from the extensions. What the client need to do is to desterilize the binding object from JSON and apply to the channel factory.

private const string CST_XELEMNAME_BINDINGTYPENAME = "bindingTypeName";
private const string CST_XELEMNAME_BINDING = "binding";
  
static Tuple<EndpointAddress, Binding> FindServiceEndpoint()
{
    var probeEndpointAddress = new EndpointAddress(ConfigurationManager.AppSettings["probeEndpointAddress"]);
    var probeBinding = Activator.CreateInstance(Type.GetType(ConfigurationManager.AppSettings["bindingType"], true, true)) as Binding;
    var discoveryEndpoint = new DiscoveryEndpoint(probeBinding, probeEndpointAddress);
  
    EndpointAddress address = null;
    Binding binding = null;
    FindResponse result = null;
    using (var discoveryClient = new DiscoveryClient(discoveryEndpoint))
    {
        result = discoveryClient.Find(new FindCriteria(typeof(IStringService)));
    }
  
    if (result != null && result.Endpoints.Any())
    {
        var endpointMetadata = result.Endpoints.First();
        address = endpointMetadata.Address;
        var bindingTypeName = endpointMetadata.Extensions
            .Where(x => x.Name == CST_XELEMNAME_BINDINGTYPENAME)
            .Select(x => x.Value)
            .FirstOrDefault();
        var bindingJson = endpointMetadata.Extensions
            .Where(x => x.Name == CST_XELEMNAME_BINDING)
            .Select(x => x.Value)
            .FirstOrDefault();
        var bindingType = Type.GetType(bindingTypeName, true, true);
        binding = JsonConvert.DeserializeObject(bindingJson, bindingType) as Binding;
    }
    return new Tuple<EndpointAddress, Binding>(address, binding);
}

Then we will use the address and binding from discovery service to construct our channel factory. This will ensure that the client utilizes the same binding as the service it’s calling.

static void Main(string[] args)
{
    Console.WriteLine("Say something...");
    var content = Console.ReadLine();
    while (!string.IsNullOrWhiteSpace(content))
    {
        Console.WriteLine("Finding the service endpoint...");
        var addressAndBinding = FindServiceEndpoint();
        if (addressAndBinding == null || addressAndBinding.Item1 == null || addressAndBinding.Item2 == null)
        {
            Console.WriteLine("There is no endpoint matches the criteria.");
        }
        else
        {
            var address = addressAndBinding.Item1;
            var binding = addressAndBinding.Item2;
            Console.WriteLine("Found the endpoint {0} use binding {1}", address.Uri, binding.Name);
  
            var factory = new ChannelFactory<IStringService>(binding, address);
            factory.Opened += (sender, e) =>
            {
                Console.WriteLine("Connecting to {0}.", factory.Endpoint.ListenUri);
            };
            var proxy = factory.CreateChannel();
            using (proxy as IDisposable)
            {
                Console.WriteLine("ToUpper: {0} => {1}", content, proxy.ToUpper(content));
            }
        }
  
        Console.WriteLine("Say something...");
        content = Console.ReadLine();
    }
}

Let’s have a test. As you can see the client retrieve the binding the service specified and initialized the corresponding binding object.

Later we will see how to use the extensions to implement more features. But next, let have a look at the backend discovery endpoint repository.

Use Database as the Endpoint Repository

In our discovery service we utilized a in-memory concurrent dictionary as the endpoint repository to store all services’ endpoint metadata. This is OK as an example and could provide high performance. But in some cases, especially when we have more than one discovery service instances, we’d better put the repository to another place. Database is one of the choice. Now let’s begin to refactor our code and make the endpoint repository changeable.

From our discovery service we will find that the endpoint repository has three main functions.

1, Add a new endpoint metadata.

2, Remove an existing endpoint metadata.

3, Find the proper metadata based on the criteria.

Hence I will create an interface to cover them and then create sub classes for each repository. The interface would be like this.

public interface IEndpointMetadataProvider
{
    void AddEndpointMetadata(EndpointDiscoveryMetadata metadata);
  
    void RemoveEndpointMetadata(EndpointDiscoveryMetadata metadata);
  
    EndpointDiscoveryMetadata MatchEndpoint(FindCriteria criteria);
}

And moved our in-memory dictionary stuff from the discovery service to a separated class.

public class InProcEndpointMetadataProvider : IEndpointMetadataProvider
{
    private ConcurrentDictionary<EndpointAddress, EndpointDiscoveryMetadata> _endpoints;
  
    public InProcEndpointMetadataProvider()
    {
        _endpoints = new ConcurrentDictionary<EndpointAddress, EndpointDiscoveryMetadata>();
    }
  
    public void AddEndpointMetadata(EndpointDiscoveryMetadata metadata)
    {
        _endpoints.AddOrUpdate(metadata.Address, metadata, (key, value) => metadata);
    }
  
    public void RemoveEndpointMetadata(EndpointDiscoveryMetadata metadata)
    {
        EndpointDiscoveryMetadata value = null;
        _endpoints.TryRemove(metadata.Address, out value);
    }
  
    public EndpointDiscoveryMetadata MatchEndpoint(FindCriteria criteria)
    {
        var endpoints = _endpoints
            .Where(meta => criteria.IsMatch(meta.Value))
            .Select(meta => meta.Value)
            .ToList();
        return endpoints.FirstOrDefault();
    }
}

Then we can change our discovery service code to make it use the abstract endpoint provider interface instead of the actual repository.

[ServiceBehavior(InstanceContextMode = InstanceContextMode.Single, ConcurrencyMode = ConcurrencyMode.Multiple)]
public class ManagedProxyDiscoveryService : DiscoveryProxy
{
    private IEndpointMetadataProvider _metadataProvider;
  
    public ManagedProxyDiscoveryService(IEndpointMetadataProvider metadataProvider)
    {
        _metadataProvider = metadataProvider;
    }
  
    protected override IAsyncResult OnBeginOnlineAnnouncement(DiscoveryMessageSequence messageSequence, EndpointDiscoveryMetadata endpointDiscoveryMetadata, AsyncCallback callback, object state)
    {
        _metadataProvider.AddEndpointMetadata(endpointDiscoveryMetadata);
        return new OnOnlineAnnouncementAsyncResult(callback, state);
    }
  
    protected override void OnEndOnlineAnnouncement(IAsyncResult result)
    {
        OnOnlineAnnouncementAsyncResult.End(result);
    }
  
    protected override IAsyncResult OnBeginOfflineAnnouncement(DiscoveryMessageSequence messageSequence, EndpointDiscoveryMetadata endpointDiscoveryMetadata, AsyncCallback callback, object state)
    {
        _metadataProvider.RemoveEndpointMetadata(endpointDiscoveryMetadata);
        return new OnOfflineAnnouncementAsyncResult(callback, state);
    }
  
    protected override void OnEndOfflineAnnouncement(IAsyncResult result)
    {
        OnOfflineAnnouncementAsyncResult.End(result);
    }
  
    protected override IAsyncResult OnBeginFind(FindRequestContext findRequestContext, AsyncCallback callback, object state)
    {
        var endpoint = _metadataProvider.MatchEndpoint(findRequestContext.Criteria);
        findRequestContext.AddMatchingEndpoint(endpoint);
        return new OnFindAsyncResult(callback, state);
    }
  
    protected override void OnEndFind(IAsyncResult result)
    {
        OnFindAsyncResult.End(result);
    }
  
    protected override IAsyncResult OnBeginResolve(ResolveCriteria resolveCriteria, AsyncCallback callback, object state)
    {
        throw new NotImplementedException();
    }
  
    protected override EndpointDiscoveryMetadata OnEndResolve(IAsyncResult result)
    {
        throw new NotImplementedException();
    }
}

If we using the InProcEndpointMetadataProvider as the provider then our discovery service should work as before. Just change the discovery service hosting code like this.

static void Main(string[] args)
{
    IEndpointMetadataProvider metadataProvider = new InProcEndpointMetadataProvider();
  
    using (var host = new ServiceHost(new ManagedProxyDiscoveryService(metadataProvider)))
    {
        ... ...
        ... ...
    }
}

Next, let’s start to use database as the endpoint repository. First of all we need to define the schema of the table where the endpoint metadata will be stored. Normally one service will be host at one endpoint so use the service contract name as the primary key column would be a good choice. But consider the scaling-out situation, one service might have more than one instance in the system, so that it might be more than one endpoints per service contract. So for extensibility we will use the endpoint address URI as the primary key, but add an index on the service contract column.

I also created three store procedures to add, remove and find the endpoint metadata as well. The database schema would be like this.

And here is the database creation script.

/****** Object:  StoredProcedure [dbo].[GetServiceEndpointMetadata]    Script Date: 7/4/2012 3:03:01 PM ******/
SET ANSI_NULLS ON
GO
SET QUOTED_IDENTIFIER ON
GO
CREATE PROCEDURE [dbo].[GetServiceEndpointMetadata]
    @contractType nvarchar(512)
AS
BEGIN
  
    SELECT Address, BindingType, Binding, UpdatedOn FROM EndpointMetadata WHERE ContractType = @contractType
  
END
  
GO
/****** Object:  StoredProcedure [dbo].[RegisterServiceEndpointMetadata]    Script Date: 7/4/2012 3:03:01 PM ******/
SET ANSI_NULLS ON
GO
SET QUOTED_IDENTIFIER ON
GO
CREATE PROCEDURE [dbo].[RegisterServiceEndpointMetadata]
    @uri nvarchar(256),
    @contractType nvarchar(256),
    @address nvarchar(1024),
    @bindingType nvarchar(512),
    @binding nvarchar(1024)
AS
BEGIN
  
    BEGIN TRAN
  
        EXEC UnRegisterServiceEndpointMetadata @uri, @contractType, @address, @bindingType
  
        INSERT INTO EndpointMetadata (Uri, ContractType, Address, BindingType, Binding) VALUES (@uri, @contractType, @address, @bindingType, @binding)
  
    COMMIT TRAN
  
END
  
GO
/****** Object:  StoredProcedure [dbo].[UnRegisterServiceEndpointMetadata]    Script Date: 7/4/2012 3:03:01 PM ******/
SET ANSI_NULLS ON
GO
SET QUOTED_IDENTIFIER ON
GO
CREATE PROCEDURE [dbo].[UnRegisterServiceEndpointMetadata]
    @uri nvarchar(256),
    @contractType nvarchar(256),
    @address nvarchar(1024),
    @bindingType nvarchar(512)
AS
BEGIN
  
    DELETE FROM EndpointMetadata WHERE Uri = @uri AND ContractType = @contractType AND Address = @address AND BindingType = @bindingType
  
END
  
GO
/****** Object:  Table [dbo].[EndpointMetadata]    Script Date: 7/4/2012 3:03:01 PM ******/
SET ANSI_NULLS ON
GO
SET QUOTED_IDENTIFIER ON
GO
CREATE TABLE [dbo].[EndpointMetadata](
    [Uri] [nvarchar](256) NOT NULL,
    [ContractType] [nvarchar](256) NOT NULL,
    [Address] [nvarchar](1024) NOT NULL,
    [BindingType] [nvarchar](512) NOT NULL,
    [Binding] [nvarchar](1024) NOT NULL,
    [UpdatedOn] [datetime] NOT NULL,
 CONSTRAINT [PK_EndpointMetadata] PRIMARY KEY CLUSTERED 
(
    [Uri] ASC
)WITH (STATISTICS_NORECOMPUTE = OFF, IGNORE_DUP_KEY = OFF)
)
  
GO
SET ANSI_PADDING ON
  
GO
/****** Object:  Index [IX_ContractType]    Script Date: 7/4/2012 3:03:01 PM ******/
CREATE NONCLUSTERED INDEX [IX_ContractType] ON [dbo].[EndpointMetadata]
(
    [ContractType] ASC
)WITH (STATISTICS_NORECOMPUTE = OFF, DROP_EXISTING = OFF, ONLINE = OFF)
GO
ALTER TABLE [dbo].[EndpointMetadata] ADD  CONSTRAINT [DF_EndpiointMetadata_UpdatedOn]  DEFAULT (getutcdate()) FOR [UpdatedOn]
GO

Next, we will create a class implemented the IEndpointMetadataProvider interface for database. And I’m using a delegation to let the consumer pass the database connection object creator in, so that it will not bounded at an actual database.

public class DbEndpointMetadataProvider : IEndpointMetadataProvider
{
    private string _connectionString;
    private Func<string, IDbConnection> _connectionCreator;
  
    public DbEndpointMetadataProvider(Func<string, IDbConnection> connectionCreator, string connectionString)
    {
        _connectionCreator = connectionCreator;
        _connectionString = connectionString;
    }
  
    public void AddEndpointMetadata(EndpointDiscoveryMetadata metadata)
    {
        throw new NotImplementedException();
    }
  
    public void RemoveEndpointMetadata(EndpointDiscoveryMetadata metadata)
    {
        throw new NotImplementedException();
    }
  
    public EndpointDiscoveryMetadata MatchEndpoint(FindCriteria criteria)
    {
        throw new NotImplementedException();
    }
}

When need to add endpoint metadata into database, we will invoke the RegisterServiceEndpointMetadata store procedure, specify the endpoint address, contract type name, binding type, binding (in JSON format) and update date time. There are two things need to be noticed. The first one is, a service may implement more than one contracts. So that for each contract we need to add a record for its endpoint metadata. The second one is, the endpoint address and binding should be serialized before saved to database. Currently we will use JSON format.

public void AddEndpointMetadata(EndpointDiscoveryMetadata metadata)
{
    var uri = metadata.Address.Uri.AbsoluteUri;
    var addressJson = JsonConvert.SerializeObject(metadata.Address);
    var contractTypeNames = metadata.ContractTypeNames.Select(nm => string.Format("{0}, {1}", nm.Name, nm.Namespace)).ToList();
    var bindingTypeName = metadata.Extensions
        .Where(x => x.Name == CST_XELEMNAME_BINDINGTYPENAME)
        .Select(x => x.Value)
        .FirstOrDefault();
    var bindingJson = metadata.Extensions
        .Where(x => x.Name == CST_XELEMNAME_BINDING)
        .Select(x => x.Value)
        .FirstOrDefault();
    using (var conn = _connectionCreator.Invoke(_connectionString))
    {
        var cmd = conn.CreateCommand();
        cmd.CommandType = CommandType.StoredProcedure;
        cmd.CommandText = "RegisterServiceEndpointMetadata";
        cmd.AddParameter("uri", uri)
           .AddParameter("contractType", null)
           .AddParameter("address", addressJson)
           .AddParameter("bindingType", bindingTypeName)
           .AddParameter("binding", bindingJson);
  
        conn.Open();
        cmd.Transaction = conn.BeginTransaction();
        foreach (var contractTypeName in contractTypeNames)
        {
            cmd.GetParameter("contractType").Value = contractTypeName;
            cmd.ExecuteNonQuery();
        }
        cmd.Transaction.Commit();
    }
}

I also added an internal helper class for make the database parameter operation easy.

internal static class DatabaseHelpers
{
    public static IDbCommand AddParameter(this IDbCommand cmd, string name, object value)
    {
        var param = cmd.CreateParameter();
        param.ParameterName = name;
        param.Value = value;
        cmd.Parameters.Add(param);
        return cmd;
    }
  
    public static IDbDataParameter GetParameter(this IDbCommand cmd, string parameterName)
    {
        return cmd.Parameters[parameterName] as IDbDataParameter;
    }
}

Similarly, when a service goes offline, we need to remove all records based on its endpoint metadata for each contract types.

public void RemoveEndpointMetadata(EndpointDiscoveryMetadata metadata)
{
    var uri = metadata.Address.Uri.AbsoluteUri;
    var addressJson = JsonConvert.SerializeObject(metadata.Address);
    var contractTypeNames = metadata.ContractTypeNames.Select(nm => string.Format("{0}, {1}", nm.Name, nm.Namespace)).ToList();
    var bindingTypeName = metadata.Extensions
        .Where(x => x.Name == CST_XELEMNAME_BINDINGTYPENAME)
        .Select(x => x.Value)
        .FirstOrDefault();
    using (var conn = _connectionCreator.Invoke(_connectionString))
    {
        var cmd = conn.CreateCommand();
        cmd.CommandType = CommandType.StoredProcedure;
        cmd.CommandText = "UnRegisterServiceEndpointMetadata";
        cmd.AddParameter("uri", uri)
            .AddParameter("contractType", null)
            .AddParameter("address", addressJson)
            .AddParameter("bindingType", bindingTypeName);
  
        conn.Open();
        cmd.Transaction = conn.BeginTransaction();
        foreach (var contractTypeName in contractTypeNames)
        {
            cmd.GetParameter("contractType").Value = contractTypeName;
            cmd.ExecuteNonQuery();
        }
        cmd.Transaction.Commit();
    }
}

The last one is the find method, which should select the endpoint metadata and return back to the discovery service. When using in-memory dictionary this is very easy since we can store the endpoint metadata in the dictionary and return back. But when using database all information must be serialized. So now we need to find the proper endpoint records, deserialized them back to the endpoint metadata.

We have been implemented the deserialization code for binding in previous part. Now what we need to do is to desterilize the address. This is a little bit tricky. EndpointAddress class contains some readonly property which cannot be initialized directly by the JSON converter. So I have to create some wrapper classes to make it easy to desterilize.

internal class AddressHeaderWrapper
{
    public string Name { get; set; }
  
    public string Namespace { get; set; }
  
    public AddressHeader ToAddressHeader()
    {
        return AddressHeader.CreateAddressHeader(Name, Namespace, null);
    }
}
  
internal class EndpointIdentityWrapper
{
    public Claim IdentityClaim { get; set; }
  
    public EndpointIdentity EndpointIdentity
    {
        get
        {
            return EndpointIdentity.CreateIdentity(IdentityClaim);
        }
    }
}
  
internal class EndpointAddressWrapper
{
    public AddressHeaderWrapper[] Headers { get; set; }
  
    public EndpointIdentityWrapper Identity { get; set; }
  
    public bool IsAnonymous { get; set; }
  
    public bool IsNone { get; set; }
  
    public Uri Uri { get; set; }
  
    public EndpointAddress EndpointAddress
    {
        get
        {
            var headerWrappers = Headers ?? new AddressHeaderWrapper[] { };
            var headers = headerWrappers.Select(h => h.ToAddressHeader()).ToArray();
            if (Identity == null)
            {
                return new EndpointAddress(Uri, headers);
            }
            else
            {
                return new EndpointAddress(Uri, Identity.EndpointIdentity, headers);
            }
        }
    }
}

With these wrapper classes we can easily construct the endpoint metadata from database records.

public EndpointDiscoveryMetadata MatchEndpoint(FindCriteria criteria)
{
    var endpoints = new List<EndpointDiscoveryMetadata>();
    var contractTypeNames = criteria.ContractTypeNames.Select(nm => string.Format("{0}, {1}", nm.Name, nm.Namespace)).ToList();
    using (var conn = _connectionCreator.Invoke(_connectionString))
    {
        var cmd = conn.CreateCommand();
        cmd.CommandType = CommandType.StoredProcedure;
        cmd.CommandText = "GetServiceEndpointMetadata";
        cmd.AddParameter("contractType", null);
  
        conn.Open();
        foreach (var contractTypeName in contractTypeNames)
        {
            cmd.GetParameter("contractType").Value = contractTypeName;
            using (var reader = cmd.ExecuteReader())
            {
                while (reader.Read())
                {
                    var addressJson = (string)reader["Address"];
                    var bindingTypeName = (string)reader["BindingType"];
                    var bindingJson = (string)reader["Binding"];
                    var updatedOn = (DateTime)reader["UpdatedOn"];
                    var address = JsonConvert.DeserializeObject<EndpointAddressWrapper>(addressJson).EndpointAddress;
                    var metadata = new EndpointDiscoveryMetadata();
                    metadata.Address = address;
                    metadata.Extensions.Add(new XElement(CST_XELEMNAME_BINDINGTYPENAME, bindingTypeName));
                    metadata.Extensions.Add(new XElement(CST_XELEMNAME_BINDING, bindingJson));
                    metadata.Extensions.Add(new XElement(CST_XELEMNAME_UPDATEDON, JsonConvert.SerializeObject(updatedOn)));
                    endpoints.Add(metadata);
                }
            }
        }
    }
    return endpoints.FirstOrDefault();
}

Changed the discovery service host program and have a try.

var connectionString = ConfigurationManager.ConnectionStrings["localhost"].ConnectionString;
IEndpointMetadataProvider metadataProvider = new DbEndpointMetadataProvider(connString => new SqlConnection(connString), connectionString);
  
using (var host = new ServiceHost(new ManagedProxyDiscoveryService(metadataProvider)))
{
    ... ...
}

When the discoverable service was started, in database we can see the endpoint information was added.

And our client can invoke the service through the discovery service.

And when we close the discoverable service its endpoint will be deleted from database.

Dispatcher Service Scaling-out Mode

In my Scaling-out Your Services by Message Bus-based WCF Transport Extension series I talked a lot about how to use service bus to make your service scalable. In that post I compared two scaling-out mode: dispatcher mode and pooling mode. Now with discovery service we can implement the dispatcher mode very easy.

The discovery service takes the responsible for retrieving the proper endpoint for a service by its contract. And in the discovery service procedure it’s possible that one service contract can have more than one endpoint addresses associated. This means, we can have more than one instance for a service and when the client need to invoke this service, the discovery service can find one of the endpoint of this service. This is the dispatcher mode I mentioned.

If we review our endpoint provider code, DbEndpointMetadataProvider and InProcEndpointMetadataProvider returns the first matched endpoint back. And this is where we can implement the changeable endpoint selector. We can let the discovery service return the endpoint which registered latest, or randomly select one endpoint which is the core of dispatcher mode.

First, I created an interface named IEndpointSelector which only has one method that return one endpoint metadata from a list of metadata.

public interface IEndpointSelector
{
    EndpointDiscoveryMetadata Select(IEnumerable<EndpointDiscoveryMetadata> endpoints);
}

Then I created an abstract base class for all sub selectors. In this base class I just implemented a common feature. If the metadata list is null or empty, it will return null. If there’s only one endpoint metadata in the list, it will return directly. Otherwise, only when there are more than one candidate endpoints I will invoke the sub class OnSelect method.

public abstract class EndpointSelectorBase : IEndpointSelector
{
    public EndpointDiscoveryMetadata Select(IEnumerable<EndpointDiscoveryMetadata> endpoints)
    {
        // when no endpoints or only one endpoint then no need to invoke the actual selector, just return null or the only one
        var endpointCount = endpoints.Count();
        if (endpointCount <= 0)
        {
            return null;
        }
        else
        {
            if (endpointCount > 1)
            {
                return OnSelect(endpoints, endpointCount);
            }
            else
            {
                return endpoints.First();
            }
        }
    }
  
    protected abstract EndpointDiscoveryMetadata OnSelect(IEnumerable<EndpointDiscoveryMetadata> endpoints, int endpointCount);
}

Then we can create our own endpoint selector. For example the randomly endpoint selector.

public class RandomEndpointSelector : EndpointSelectorBase
{
    private Random _rnd;
  
    public RandomEndpointSelector()
    {
        _rnd = new Random();
    }
  
    protected override EndpointDiscoveryMetadata OnSelect(IEnumerable<EndpointDiscoveryMetadata> endpoints, int endpointCount)
    {
        var index = _rnd.Next(0, endpointCount - 1);
        return endpoints.ElementAt(index);
    }
}

Then I will inject the endpoint selector into the endpoint provider so that it will invoke the selector to return the best endpoint to the client. Create a new abstract class from the IEndpointMetadataProvider and added the selector variant.

public abstract class EndpointMetadataProviderBase : IEndpointMetadataProvider
{
    private IEndpointSelector _endpointSelector;
  
    protected EndpointMetadataProviderBase(IEndpointSelector endpointSelector)
    {
        _endpointSelector = endpointSelector;
    }
  
    public abstract void AddEndpointMetadata(System.ServiceModel.Discovery.EndpointDiscoveryMetadata metadata);
  
    public abstract void RemoveEndpointMetadata(System.ServiceModel.Discovery.EndpointDiscoveryMetadata metadata);
  
    public EndpointDiscoveryMetadata MatchEndpoint(FindCriteria criteria)
    {
        EndpointDiscoveryMetadata endpoint = null;
        var endpoints = OnMatchEndpoints(criteria);
        endpoint = _endpointSelector.Select(endpoints);
        return endpoint;
    }
  
    protected abstract IEnumerable<EndpointDiscoveryMetadata> OnMatchEndpoints(FindCriteria criteria);
}

Then we need to change the database endpoint provider and in process endpoint provider code. They now should be inherited from this base class.

The updated code can be downloaded from the link at the bottom of this post.

Now we can start the discovery service and start three instances of our discoverable service. In database we will find three records with the same service contract but different endpoint address. And as you can see my client connected different service instances.

Attribute and More Controls

Now we came to the last topic. From the sample code I provided above we can see that when we need to make a service to be discoverable, we should append the related service behavior before it was being hosted and opened. If we are using the console application to host the discoverable service we need to initialize and add the ServiceDiscoveryBehavior and DiscoverableServiceBehavior before open the host.

But this is not convenient, especially when we don’t have the service host explicitly. For example when hosting the service through IIS. As you may know, the service behavior could be specified through an attributed on the service class definition. So Let’s begin to create the attribute so that we can mark the services which we want them to be discoverable.

I renamed the DiscoverableServiceBehavior class to DiscoverableAttribute and it’s now inherited from the Attribute class, which means I can put it onto any service classes that I want it to be discoverable. Then in order to make it apply the ServiceDiscoveryBehavior I added a local variant of ServiceDiscoveryBehavior, initialized in the constructor, and perform its operations in each methods of the IServiceBehavior interface.

[AttributeUsage(AttributeTargets.Class, AllowMultiple = false, Inherited = true)]
public class DiscoverableAttribute : Attribute, IServiceBehavior
{
    private const string CST_XELEMNAME_BINDINGTYPENAME = "bindingTypeName";
    private const string CST_XELEMNAME_BINDING = "binding";
  
    private const string CST_CONFIGKEY_ANNOUNCEMENTENDPOINT = "announcementEndpointAddress";
    private const string CST_CONFIGKEY_ANNOUNCEMENTBINDING = "bindingType";
  
    private IServiceBehavior _serviceDiscoveryBehavior;
  
    public DiscoverableAttribute(string announcementEndpoint, Binding announcementBinding)
    {
        var endpoint = new AnnouncementEndpoint(announcementBinding, new EndpointAddress(announcementEndpoint));
        var serviceDiscoveryBehavior = new ServiceDiscoveryBehavior();
        serviceDiscoveryBehavior.AnnouncementEndpoints.Add(endpoint);
        _serviceDiscoveryBehavior = serviceDiscoveryBehavior;
    }
  
    public DiscoverableAttribute()
        : this(ConfigurationManager.AppSettings[CST_CONFIGKEY_ANNOUNCEMENTENDPOINT],
               Activator.CreateInstance(Type.GetType(ConfigurationManager.AppSettings[CST_CONFIGKEY_ANNOUNCEMENTBINDING], true, true)) as Binding)
    {
    }
  
    public void AddBindingParameters(ServiceDescription serviceDescription, ServiceHostBase serviceHostBase, Collection<ServiceEndpoint> endpoints, BindingParameterCollection bindingParameters)
    {
        // apply the service discovery behavior opertaion
        _serviceDiscoveryBehavior.AddBindingParameters(serviceDescription, serviceHostBase, endpoints, bindingParameters);
    }
  
    public void ApplyDispatchBehavior(ServiceDescription serviceDescription, ServiceHostBase serviceHostBase)
    {
        // apply the service discovery behavior opertaion
        _serviceDiscoveryBehavior.ApplyDispatchBehavior(serviceDescription, serviceHostBase);
        // add the additional information into the endpoint metadata extension through the endpoint discovery binding for each endpoints
        foreach (var endpoint in serviceDescription.Endpoints)
        {
            var binding = endpoint.Binding;
            var endpointDiscoveryBehavior = new EndpointDiscoveryBehavior();
            var bindingType = binding.GetType().AssemblyQualifiedName;
            var bindingJson = JsonConvert.SerializeObject(binding);
            endpointDiscoveryBehavior.Extensions.Add(new XElement(CST_XELEMNAME_BINDINGTYPENAME, bindingType));
            endpointDiscoveryBehavior.Extensions.Add(new XElement(CST_XELEMNAME_BINDING, bindingJson));
            endpoint.Behaviors.Add(endpointDiscoveryBehavior);
        }
    }
  
    public void Validate(ServiceDescription serviceDescription, ServiceHostBase serviceHostBase)
    {
        // apply the service discovery behavior opertaion
        _serviceDiscoveryBehavior.Validate(serviceDescription, serviceHostBase);
    }
}

Then in my discoverable service I just need to add this attribute at the service class definition and changed the service hosting code accordingly.

[Discoverable]
public class StringService : IStringService
{
    public string ToUpper(string content)
    {
        return content.ToUpper();
    }
}

static void Main(string[] args)
{
    var baseAddress = new Uri(string.Format("http://localhost:11001/stringservice/{0}/", Guid.NewGuid().ToString()));
  
    using (var host = new ServiceHost(typeof(StringService), baseAddress))
    {
        host.Opened += (sender, e) =>
        {
            Console.WriteLine("Service opened at {0}", host.Description.Endpoints.First().ListenUri);
        };
  
        host.AddServiceEndpoint(typeof(IStringService), new BasicHttpBinding(), string.Empty);
  
        host.Open();
  
        Console.WriteLine("Press any key to exit.");
        Console.ReadKey();
    }
}

Finally let’s add a new feature of our own discovery service. By default when a service went online it will register all its endpoints for all contracts it implemented. But in some cases we don’t want all endpoints and contracts are discoverable. For example, I have IStringService and ICalculateService contracts and my MixedService class implemented both of them.

[Discoverable]
public class MixedService : IStringService, ICalculateService
{
    public string ToUpper(string content)
    {
        return content.ToUpper();
    }
  
    public int Add(int x, int y)
    {
        return x + y;
    }
}

And when hosting this service I also added a service metadata endpoint.

using (var host = new ServiceHost(typeof(MixedService), baseAddress))
{
    host.Opened += (sender, e) =>
    {
        host.Description.Endpoints.All((ep) =>
        {
            Console.WriteLine(ep.Contract.Name + ": " + ep.ListenUri);
            return true;
        });
    };
  
    var serviceMetadataBehavior = new ServiceMetadataBehavior();
    serviceMetadataBehavior.HttpGetEnabled = true;
    serviceMetadataBehavior.MetadataExporter.PolicyVersion = PolicyVersion.Policy15;
    host.Description.Behaviors.Add(serviceMetadataBehavior);
  
    host.AddServiceEndpoint(typeof(IStringService), new BasicHttpBinding(), "string");
    host.AddServiceEndpoint(typeof(ICalculateService), new BasicHttpBinding(), "calculate");
    host.AddServiceEndpoint(ServiceMetadataBehavior.MexContractName, MetadataExchangeBindings.CreateMexHttpBinding(), "mex");
  
    host.Open();
  
    Console.WriteLine("Press any key to exit.");
    Console.ReadKey();
}

So when this service was opened it will register three endpoints into my discovery service: the IStringService, ICalculateService and the IMetadataExchange. How can I not to expose the ICalculateService and the metadata endpoint.

This can be achieved by adding some more information from the attribute to the endpoint discovery behavior, then went to the discovery service. In the attribute I added two parameters which the user can specify which contract types and endpoints they don’t want to be exposed.

private IEnumerable<string> _ignoredContractTypeNames;
private IEnumerable<string> _ignoredEndpoints;
  
public DiscoverableAttribute(string announcementEndpoint, Binding announcementBinding, string[] ignoredContractTypeNames, string[] ignoredEndpoints)
{
    _ignoredContractTypeNames = ignoredContractTypeNames;
    _ignoredEndpoints = ignoredEndpoints;
  
    var endpoint = new AnnouncementEndpoint(announcementBinding, new EndpointAddress(announcementEndpoint));
    var serviceDiscoveryBehavior = new ServiceDiscoveryBehavior();
    serviceDiscoveryBehavior.AnnouncementEndpoints.Add(endpoint);
    _serviceDiscoveryBehavior = serviceDiscoveryBehavior;
}
  
public DiscoverableAttribute(string[] ignoredContractTypeNames, string[] ignoredEndpoints)
    : this(ConfigurationManager.AppSettings[CST_CONFIGKEY_ANNOUNCEMENTENDPOINT],
           Activator.CreateInstance(Type.GetType(ConfigurationManager.AppSettings[CST_CONFIGKEY_ANNOUNCEMENTBINDING], true, true)) as Binding,
           ignoredContractTypeNames, ignoredEndpoints)
{
}
  
public DiscoverableAttribute()
    : this(ConfigurationManager.AppSettings[CST_CONFIGKEY_ANNOUNCEMENTENDPOINT],
           Activator.CreateInstance(Type.GetType(ConfigurationManager.AppSettings[CST_CONFIGKEY_ANNOUNCEMENTBINDING], true, true)) as Binding,
           new string[] { }, new string[] { })
{
}

And then when we need to add the endpoint discovery behavior we will firstly check if this endpoint was in the ignored endpoint list, or the contract of this endpoint was in the ignored contract list. If yes then we will add a XML element into the metadata extension. Otherwise we will add the binding information.

public void ApplyDispatchBehavior(ServiceDescription serviceDescription, ServiceHostBase serviceHostBase)
{
    // apply the service discovery behavior opertaion
    _serviceDiscoveryBehavior.ApplyDispatchBehavior(serviceDescription, serviceHostBase);
    // add the additional information into the endpoint metadata extension through the endpoint discovery binding for each endpoints
    foreach (var endpoint in serviceDescription.Endpoints)
    {
        var endpointDiscoveryBehavior = new EndpointDiscoveryBehavior();
        // check if the contract or endpoint should be ignored
        var contractTypeName = endpoint.Contract.Name;
        var endpointAddress = endpoint.Address.Uri;
        if (_ignoredContractTypeNames.Any(ctn => string.Compare(ctn, contractTypeName, true) == 0) ||
            _ignoredEndpoints.Any(ep => string.Compare(ep, endpointAddress.Segments.Last(), true) == 0))
        {
            endpointDiscoveryBehavior.Extensions.Add(new XElement(CST_XELEMNAME_IGNORED, true));
        }
        else
        {
            endpointDiscoveryBehavior.Extensions.Add(new XElement(CST_XELEMNAME_IGNORED, false));
            // add the binding infomation
            var binding = endpoint.Binding;
            var bindingType = binding.GetType().AssemblyQualifiedName;
            var bindingJson = JsonConvert.SerializeObject(binding);
            endpointDiscoveryBehavior.Extensions.Add(new XElement(CST_XELEMNAME_BINDINGTYPENAME, bindingType));
            endpointDiscoveryBehavior.Extensions.Add(new XElement(CST_XELEMNAME_BINDING, bindingJson));
        }
        endpoint.Behaviors.Add(endpointDiscoveryBehavior);
    }
}

So when the service sent the announcement message to the discovery service we will know if it should be ignored. In the discovery service we just need to simply check the ignored information from the metadata extension and then perform the register and unregister process accordingly.

protected override IAsyncResult OnBeginOnlineAnnouncement(DiscoveryMessageSequence messageSequence, EndpointDiscoveryMetadata endpointDiscoveryMetadata, AsyncCallback callback, object state)
{
    var ignored = false;
    bool.TryParse(
        endpointDiscoveryMetadata.Extensions
            .Where(x => x.Name == CST_XELEMNAME_IGNORED)
            .Select(x => x.Value)
            .FirstOrDefault(),
        out ignored);
    if (!ignored)
    {
        _metadataProvider.AddEndpointMetadata(endpointDiscoveryMetadata);
    }
    return new OnOnlineAnnouncementAsyncResult(callback, state);
}
  
protected override IAsyncResult OnBeginOfflineAnnouncement(DiscoveryMessageSequence messageSequence, EndpointDiscoveryMetadata endpointDiscoveryMetadata, AsyncCallback callback, object state)
{
    var ignored = false;
    bool.TryParse(
        endpointDiscoveryMetadata.Extensions
            .Where(x => x.Name == CST_XELEMNAME_IGNORED)
            .Select(x => x.Value)
            .FirstOrDefault(),
        out ignored);
    if (!ignored)
    {
        _metadataProvider.RemoveEndpointMetadata(endpointDiscoveryMetadata);
    }
    return new OnOfflineAnnouncementAsyncResult(callback, state);
}

The whole logic works like this. The ignored contracts and endpoints were specified on the discoverable service side through the attribute. And then when the discoverable service was opened it will load the service behavior by the attribute and check the endpoints which should be ignored or not, and add the ignored flag into the metadata extension. The metadata will be sent to the discovery service and in the discovery service it will check this flag and determined whether to insert the endpoint into the repository or not.

The WCF discovery service behavior utilized the channel dispatcher to hook the event when a service channel was opened and closed to send the service online and offline message to the discovery service. If we can move the check logic into the underlying dispatcher so that it will NOT send message if the contract or endpoint was marked as ignored, which will reduce the network usage.

But unfortunately there’s no way to modify the channel dispatchers of the ServiceDiscoveryBehavior so that we have to send the ignore information to the discovery service.

Now let’s change our discoverable service as below, mark the ICalculateService and metadata endpoint not to be discoverable.

[Discoverable(new string[] { "ICalculateService" }, new string[] { "mex" })]
public class MixedService : IStringService, ICalculateService
{
    public string ToUpper(string content)
    {
        return content.ToUpper();
    }
  
    public int Add(int x, int y)
    {
        return x + y;
    }
}

And as you can see even though there are three endpoints opened but there’s only one endpoint registered in the discovery service.

Summary

In this post I described how to add more features and flexibility to the discovery service. I demonstrated how to use database as the repository and how to make our discovery service work as a service dispatcher, which implement the dispatcher mode of service scaling-out.

I also demonstrated how to use the service behavior attribute to make the discoverable service easy to be configured. At the end I talked how to make the service contracts and endpoints configurable so that we can choose which of them should be discoverable.

Discovery service was introduced in WCF 4.0. With its functionalities we can build our service farm without having to configure all services information in each service. What we need to do is to call the discovery service and find the target endpoint, which reduced a lot of the service side configuration.

In fact when using discovery service, the service endpoints will not be changed frequently. So we can use cache before the client wanted to invoke a service. So it will firstly look for the local cache to find the endpoint. If yes then it will call the service directly. If not then it will go to the discovery service, get the endpoint, update local cache and invoke. But using cache means increased the complexity of the logic. How to deal with the cache expiration. How to deal with the service endpoint changing.

The code can be download here.

Hope this helps,

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原文地址：https://www.cnblogs.com/zeroone/p/4851984.html