# Subscriber (/docs/protocols/v4/guides/subscriber)

Implement subscriber contracts that receive Uniswap v4 position lifecycle notifications.

## Context
For developers looking to support custom *liquidity mining*, Subscriber contracts can be used to receive notifications about position modifications or transfers.

## Guide
## 1. Implement the ISubscriber interface
Can also refer to [MockSubscriber](https://github.com/Uniswap/v4-periphery/blob/main/test/mocks/MockSubscriber.sol) for an actual implementation example.

```solidity
import {ISubscriber} from "v4-periphery/src/interfaces/ISubscriber.sol";

contract MySubscriber is ISubscriber {
    uint256 public notifySubscribeCount;
    uint256 public notifyUnsubscribeCount;
    uint256 public notifyModifyLiquidityCount;
    uint256 public notifyBurnCount;
    // other implementations...

    function notifySubscribe(uint256, bytes memory) external onlyByPosm {
        notifySubscribeCount++;
    }

    function notifyUnsubscribe(uint256) external onlyByPosm {
        notifyUnsubscribeCount++;
    }

    function notifyModifyLiquidity(uint256, int256, BalanceDelta) external onlyByPosm {
        notifyModifyLiquidityCount++;
    }

    function notifyBurn(uint256, address, PositionInfo, uint256, BalanceDelta)
        external
        onlyByPosm
    {
        notifyBurnCount++;
    }
}
```

## 2. A caveat on unsubscribe()
To prevent gas griefing during unsubscription, Uniswap v4 sets a fixed variable `unsubscribeGasLimit` when calling a subscriber’s `notifyUnsubscribe()` function.

Without this limit, malicious subscribers could prevent liquidity providers from unsubscribing. If `notifyUnsubscribe()` were to consume too much gas, it would cause the unsubscription transaction to revert, thus leading to a denial-of-service

With the gas limit in place, if the subscriber’s notification fails, the unsubscription will still succeed and only the notification to the subscriber is skipped.

From [`_unsubscribe()`](https://github.com/Uniswap/v4-periphery/blob/main/src/base/Notifier.sol#L80) on `Notifier`:

```solidity
if (address(_subscriber).code.length > 0) {
    // require that the remaining gas is sufficient to notify the subscriber
    // otherwise, users can select a gas limit where .notifyUnsubscribe hits OutOfGas yet the
    // transaction/unsubscription can still succee
    if (gasleft() < unsubscribeGasLimit) GasLimitTooLow.selector.revertWith();
    try _subscriber.notifyUnsubscribe{gas: unsubscribeGasLimit}(tokenId) {} catch {}
}
```

## 3. Opt-in to a subscriber contract
To opt-in to a subscriber contract, call `subscribe()` on `PositionManager`.

```solidity
import {IPositionManager} from "v4-periphery/src/interfaces/IPositionManager.sol";

IPositionManager posm = IPositionManager(<address>);
ISubscriber mySubscriber = ISubscriber(<address>);

bytes memory optionalData = ...;
posm.subscribe(tokenId, mySubscriber, optionalData);
```