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# Subscription

A protocol representing the connection of a subscriber to a publisher.

```
protocol Subscription : Cancellable, CustomCombineIdentifierConvertible
```

## Overview

Subscriptions are class constrained because a [`Subscription`](/documentation/Combine/Subscription) has identity, defined by the moment in time a particular subscriber attached to a publisher. Canceling a [`Subscription`](/documentation/Combine/Subscription) must be thread-safe.

You can only cancel a [`Subscription`](/documentation/Combine/Subscription) once.

Canceling a subscription frees up any resources previously allocated by attaching the [`Subscriber`](/documentation/Combine/Subscriber).

## Topics

### Requesting elements

[`request(_:)`](/documentation/Combine/Subscription/request(_:))

Tells a publisher that it may send more values to the subscriber.

[`Subscribers.Demand`](/documentation/Combine/Subscribers/Demand)

A requested number of items, sent to a publisher from a subscriber through the subscription.


---

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