# Umi's Interfaces

Umi defines a set of core interfaces that makes it easy to interact with the Solana blockchain. Namely, they are:

* [`Signer`](https://umi.typedoc.metaplex.com/interfaces/umi.Signer.html): An interface representing a wallet that can sign transactions and messages.
* [`EddsaInterface`](https://umi.typedoc.metaplex.com/interfaces/umi.EddsaInterface.html): An interface to create keypairs, find PDAs and sign/verify messages using the EdDSA algorithm.
* [`RpcInterface`](https://umi.typedoc.metaplex.com/interfaces/umi.RpcInterface.html): An interface representing a Solana RPC client.
* [`TransactionFactoryInterface`](https://umi.typedoc.metaplex.com/interfaces/umi.TransactionFactoryInterface.html): An interface allowing us to create and serialize transactions.
* [`UploaderInterface`](https://umi.typedoc.metaplex.com/interfaces/umi.UploaderInterface.html): An interface allowing us to upload files and get a URI to access them.
* [`DownloaderInterface`](https://umi.typedoc.metaplex.com/interfaces/umi.DownloaderInterface.html): An interface allowing us to download files from a given URI.
* [`HttpInterface`](https://umi.typedoc.metaplex.com/interfaces/umi.HttpInterface.html): An interface allowing us to send HTTP requests.
* [`ProgramRepositoryInterface`](https://umi.typedoc.metaplex.com/interfaces/umi.ProgramRepositoryInterface.html): An interface for registering and retrieving programs.

### The Context interface <a href="#the-context-interface" id="the-context-interface"></a>

The interfaces above are all defined in a `Context` interface that can be used to inject them in your code. The `Context` type is defined as follows:

```
interface Context {
  downloader: DownloaderInterface;
  eddsa: EddsaInterface;
  http: HttpInterface;
  identity: Signer;
  payer: Signer;
  programs: ProgramRepositoryInterface;
  rpc: RpcInterface;
  transactions: TransactionFactoryInterface;
  uploader: UploaderInterface;
};
```

As you can see the `Signer` interface is used twice in the context:

* Once for the `identity` which is the signer using your app.
* Once for the `payer` which is the signer paying for things like transaction fees and storage fees. Usually this will be the same signer as the `identity` but separating them offers more flexibility for apps – e.g. in case they wish to abstract some costs from their users to improve the user experience.

### The Umi interface <a href="#the-umi-interface" id="the-umi-interface"></a>

The `Umi` interface is built on top of this `Context` interface and simply adds a `use` method which enables end-users to register plugins. It is defined as follows:

```
interface Umi extends Context {
  use(plugin: UmiPlugin): Umi;
}
```

Therefore, end-users can add plugins to their `Umi` instance like so:

```
import { createUmi } from '@metaplex-foundation/umi-bundle-defaults';
import { walletAdapterIdentity } from '@metaplex-foundation/umi-signer-wallet-adapters';
import { awsUploader } from '@metaplex-foundation/umi-uploader-aws';
import { myProgramRepository } from '../plugins';

const umi = createUmi('https://api.mainnet-beta.solana.com')
  .use(walletAdapterIdentity(...))
  .use(awsUploader(...))
  .use(myProgramRepository());
```


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