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Microsoft.Azure.Relay

This library is built using .NET Standard 1.3. For more information on what platforms are supported see .NET Platforms Support.

Azure Relay is one of the key capability pillars of the Azure Service Bus platform. The Relay’s new "Hybrid Connections" capability is a secure, open-protocol evolution based on HTTP and WebSockets. It supersedes the former, equally named "BizTalk Services" feature that was built on a proprietary protocol foundation. The integration of Hybrid Connections into Azure App Services will continue to function as-is.

"Hybrid Connections" allows establishing bi-directional, binary stream communication between two networked applications, whereby either or both parties can reside behind NATs or Firewalls. This document describes the client-side interactions with the Hybrid Connections relay for connecting clients in listener and sender roles and how listeners accept new connections.

This repository contains samples showing how to use the Hybrid Connections capability from C# and it also holds the protocol documentation.

See our Contribution Guidelines.

For Relay Hybrid Connections samples, see the azure/azure-relay service repository.

The API discussed here is implemented in the new Microsoft.Azure.Relay.dll assembly, which can be added to your .NET project via a NuGet package.

To use the Hybrid Connection feature, you must first register a Hybrid Connection path with the Relay service. Hybrid Connection paths are string expressions that uniquely identify the entity.

To create Hybrid Connection entities, you first need a Service Bus Relay namespace that you can create either through the Azure portal, the Azure PowerShell tools, or the cross-platform Azure CLI.

Existing Azure Relay namespaces can be managed in the Azure Portal, where you can also add, edit, or remove Hybrid Connection paths interactively.

The following two settings are specific to Hybrid Connections:

Up to 25 listeners can be concurrently connected and the Relay will distribute incoming connection requests across all connected listeners, equivalent to a network load balancer.

Creating a listener requires an access token that confers the "Listen" right on the Hybrid Connection entity or at the namespace level. Creating a sender connection requires, unless the Hybrid Connection entity is configured otherwise, a token that confers the "Send" right. The follows the shared access signature authentication model that is common across all Service Bus capabilities and entities.

Access tokens are created from an Authorization rule and key using a token provider helper as described in the article linked above; the Hybrid Connections API has its own TokenProvider class, however. The TokenProvider can be initialized from a rule and key with TokenProvider.CreateSharedAccessSignatureTokenProvider(ruleName, key) or it can be initialized from an existing token string that has been issued by some other application with TokenProvider.CreateSharedAccessSignatureTokenProvider(token).

The initialized TokenProvider instance is used by the HybridConnectionListener and HybridConnectionClient API to create tokens as needed.

However, with Hybrid Connections even more than with other Service Bus features, you may have scenarios where you will want the Relay to protect your endpoint, but you also don’t want to hand the SAS rule and key to the client outright. One such case are browser-based clients. For a browser-based client that needs to connect to a resource made available via a relayed WebSocket, the server-side web site can hold on to the required SAS rule and key, and use the TokenProvider to create a short-lived token string and pass that on to the client:

var token = await TokenProvider.GetTokenAsync("http://namespace.servicebus.windows.net/path", TimeSpan.FromSeconds(30));
var tokenString = token.TokenString;

The token created in the exemplary snippet above will only be valid to establish a connection within 30 seconds of receiving it.

The Hybrid Connection API follows a very common networking design pattern. There is a listener object that is first opened to allow incoming connections to flow and from which the application can then accept these incoming connections for handling.

var listener = new HybridConnectionListener("sb://namespace.servicebus.windows.net/path", tokenProvider); 
await listener.OpenAsync(TimeSpan.FromSeconds(60)); 
do 
{ 
    var connection = await listener.AcceptConnectionAsync(); 
    Task.Run(()=>this.HandleConnection(connection)); 
} 
while( … ); 

The connections are modeled as and based on .NET streams, with the distinction that they have a Shutdown/Async() operation that cleanly signals to the connected party that this process is done sending data, and a CloseAsync() operation to cleanly close the connection.

Both of these operations also echo common networking API patterns Since the base class is System.IO.Stream, the HybridConnectionStream can be used with all .NET APIs that expect streams. This includes all standard stream readers and writers and most common stream data encoders and decoders (serializers).

The HybridConnectionListener will aggressively attempt to stay connected once opened. Should the local network connection drop or connectivity to the Relay become interrupted, the listener will patiently retry until the listener can be restored.

Listeners on clients that are location-agile and may change networks or be put into sleep mode will also reconnect automatically as circumstances permit. The application can observe the connection state through the Connecting, Online, and Offline events that fire when the network status changes. The IsOnline property reflects the current connection status, and LastError provides insight into the reason why the last connection attempt failed, if the listener transitions its state to Connecting or Offline.

Client connections are created using the HybridConnectionClient class. There are two variants of the constructor: one takes the target address and a TokenProvider that can produce a "Send" token for the target; the other omits the token provider for use with Hybrid Connections that are set up without client authorization.

New connections are created via the CreateConnectionAsync() method. When the connection has been established, the method returns a HybridConnectionStream that is connected to the remote listener. If the connection attempt fails, a RelayException will be raised that indicates the reason for why the connection could not be established.

In order to run the unit tests complete the following:

1. Click below to deploy the ARM template that is a part of this repository.

2. Add an Environment Variable called azure-relay-dotnet/connectionstring with the resulting namespace connection string.