[View or download sample code](https://github.com/aspnet/Docs/tree/master/aspnetcore/signalr/hubs/sample/ ) [(how to download)](xref:tutorials/index#how-to-download-a-sample)
The SignalR Hubs API enables you to call methods on connected clients from the server. In the server code, you define methods that are called by client. In the client code, you define methods that are called from the server. SignalR takes care of everything behind the scenes that makes real-time client-to-server and server-to-client communications possible.
## Configure SignalR hubs
The SignalR middleware requires some services, which are configured by calling `services.AddSignalR`.
You can specify a return type and parameters, including complex types and arrays, as you would in any C# method. SignalR handles the serialization and deserialization of complex objects and arrays in your parameters and return values.
## The Clients object
Each instance of the `Hub` class has a property named `Clients` that contains the following members for communication between server and client:
| Property | Description |
| ------ | ----------- |
| `All` | Calls a method on all connected clients |
| `Caller` | Calls a method on the client that invoked the hub method |
| `Others` | Calls a method on all connected clients except the client that invoked the method |
Each property or method in the preceding tables returns an object with a `SendAsync` method. The `SendAsync` method allows you to supply the name and parameters of the client method to call.
To make calls to specific clients, use the properties of the `Clients` object. In the following example, the `SendMessageToCaller` method demonstrates sending a message to the connection that invoked the hub method. The `SendMessageToGroups` method sends a message to the groups stored in a `List` named `groups`.
The SignalR Hubs API provides the `OnConnectedAsync` and `OnDisconnectedAsync` virtual methods to manage and track connections. Override the `OnConnectedAsync` virtual method to perform actions when a client connects to the Hub, such as adding it to a group.
Exceptions thrown in your hub methods are sent to the client that invoked the method. On the JavaScript client, the `invoke` method returns a [JavaScript Promise](https://developer.mozilla.org/docs/Web/JavaScript/Guide/Using_promises). When the client receives an error with a handler attached to the promise using `catch`, it's invoked and passed as a JavaScript `Error` object.
