Add instructions to migrate to Pyth from Chainlink (#250)

* migration guide

* hm

* hm

* fix docs

Author: jayantk

next.config.js

@@ -55,6 +55,10 @@ const permanentRedirectArray = [
     "/price-feeds/pythnet-price-feeds/best-practices",
     "/price-feeds/best-practices",
   ],
+  [
+    "/price-feeds/pythnet-price-feeds/pull-updates",
+    "/price-feeds/pull-updates",
+  ],
   [
     "/price-feeds/solana-price-feeds/best-practices",
     "/price-feeds/best-practices",

pages/price-feeds/_meta.json

@@ -25,6 +25,7 @@
 
   "use-real-time-data": "Use Real-Time Price Data",
   "schedule-price-updates": "Schedule Price Updates",
+  "migrate-an-app-to-pyth": "Migrate an App to Pyth",
   "publish-data": "Publish Data",
 
   "-- Reference Material": {

pages/price-feeds/migrate-an-app-to-pyth.mdx

@@ -0,0 +1,5 @@
+# Migrate to Pyth from Another Oracle
+
+The guides in this section are designed to help developers migrate an application from another oracle to use Pyth price feeds.
+
+- [Chainlink](migrate-an-app-to-pyth/chainlink.md)

pages/price-feeds/migrate-an-app-to-pyth/_meta.json

@@ -0,0 +1,3 @@
+{
+  "chainlink": "from Chainlink"
+}

pages/price-feeds/migrate-an-app-to-pyth/chainlink.md

@@ -0,0 +1,100 @@
+# Migrate from Chainlink to Pyth
+
+This guide explains how to migrate an EVM application that uses Chainlink price feeds to Pyth price feeds.
+Pyth provides a Chainlink-compatible interface for its price feeds to make this process simple.
+There are two main steps to the migration:
+
+1. Deploy the [`PythAggregatorV3`](https://github.com/pyth-network/pyth-crosschain/blob/main/target_chains/ethereum/sdk/solidity/PythAggregatorV3.sol) contract to provide a Chainlink-compatible feed interface.
+2. Schedule price updates for the feeds required by your app.
+
+## Install Pyth SDKs
+
+The `PythAggregatorV3` contract is provided in the [Pyth Price Feeds Solidity SDK](https://github.com/pyth-network/pyth-crosschain/tree/main/target_chains/ethereum/sdk/solidity).
+Add this SDK to the dependencies of your EVM contract.
+
+**Truffle/Hardhat**
+
+If you are using Truffle or Hardhat, simply install the NPM package:
+
+```bash copy
+npm install @pythnetwork/pyth-sdk-solidity
+```
+
+**Foundry**
+
+If you are using Foundry, you will need to create an NPM project if you don't already have one.
+From the root directory of your project, run:
+
+```bash copy
+npm init -y
+npm install @pythnetwork/pyth-sdk-solidity
+```
+
+Then add the following line to your `remappings.txt` file:
+
+```text copy
+@pythnetwork/pyth-sdk-solidity/=node_modules/@pythnetwork/pyth-sdk-solidity
+```
+
+## Deploy Adapter Contract
+
+First, deploy the `PythAggregatorV3` contract from `@pythnetwork/pyth-sdk-solidity` as a replacement for your application's Chainlink price feeds.
+`PythAggregatorV3` is an adapter contract that wraps the Pyth contract and implements Chainlink's `AggregatorV3Interface`.
+
+One important difference between Pyth and Chainlink is that the Pyth contract holds data for all price feeds; in contrast, Chainlink has separate instances of `AggregatorV3Interface` for each feed.
+The adapter contract resolves this discrepancy by wrapping a single Pyth price feed.
+Users should deploy an instance of this adapter for every required price feed, then point their existing app to the addresses of the deployed adapter contracts.
+
+The following `forge` deployment script demonstrates the expected deployment process:
+
+```solidity copy
+// SPDX-License-Identifier: Apache 2
+pragma solidity ^0.8.0;
+
+import "forge-std/Script.sol";
+import { PythAggregatorV3 } from "@pythnetwork/pyth-sdk-solidity/PythAggregatorV3.sol";
+import { ChainlinkApp } from "./ChainlinkApp.sol";
+
+contract PythAggregatorV3Deployment is Script {
+  function run() external {
+    uint256 deployerPrivateKey = vm.envUint("PRIVATE_KEY");
+    vm.startBroadcast(deployerPrivateKey);
+
+    // Get the address for your ecosystem from:
+    // https://docs.pyth.network/price-feeds/contract-addresses/evm
+    address pythPriceFeedsContract = 0xff1a0f4744e8582DF1aE09D5611b887B6a12925C;
+    // Get the price feed ids from:
+    // https://docs.pyth.network/price-feeds/price-feed-ids
+    bytes32 ethFeedId = 0xff61491a931112ddf1bd8147cd1b641375f79f5825126d665480874634fd0ace;
+    bytes32 solFeedId = 0xef0d8b6fda2ceba41da15d4095d1da392a0d2f8ed0c6c7bc0f4cfac8c280b56d;
+
+    // Deploy an instance of PythAggregatorV3 for every feed.
+    PythAggregatorV3 ethAggregator = new PythAggregatorV3(
+      pythPriceFeedsContract,
+      ethFeedId
+    );
+    PythAggregatorV3 solAggregator = new PythAggregatorV3(
+      pythPriceFeedsContract,
+      solFeedId
+    );
+
+    // Pass the address of the PythAggregatorV3 contract to your chainlink-compatible app.
+    ChainlinkApp app = new ChainlinkApp(
+      address(ethAggregator),
+      address(solAggregator)
+    );
+
+    vm.stopBroadcast();
+  }
+}
+
+```
+
+## Schedule Updates
+
+Chainlink-compatible applications typically expect on-chain price feeds to update on a schedule.
+When migrating to Pyth, apps may need to schedule these price updates themselves.
+This step is required because Pyth is a pull oracle; see [What is a pull oracle?](/price-feeds/pull-updates.mdx) to learn more about this topic.
+
+The [Sponsored Feeds](/price-feeds/sponsored-feeds.mdx) page shows a list of feeds that have scheduled on-chain updates.
+If the feeds your application needs are not on this list, see [Schedule Price Updates](/price-feeds/schedule-price-updates) for several options to solve this problem.