How do I upgrade smart contract if I want to add 1 more library. Like today I am using Oraclize. After someday I want to use Chainlink to. How do I do that?
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Hi @RAHUL_SOSHTE,
Thanks for posting this question in the forum and welcome to the community 
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My (basic) understanding of oracles such as Provable and Chainlink is that your contract inherits from their API contract, you have a state variable e.g. an exchange rate and a function that you can call to get an updated value from the oracle and a callback function that the oracle calls to set the value.
I believe that there are potentially two options to support changing oracle suppliers:
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Separate the oracle functionality into a contract, and your main contract can call this contract via an interface to get the value. If you change oracle provider, you can create a new contract with the same interface and change the main contract to call this new contract.
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Include the oracle functionality in your upgradeable contract though reserve storage slots so that an upgraded version of your contract using a different oracle doesn’t mix up storage values.
See the documentation for more information on Modifying your contracts.
I am new to oracles, so feel free to ask followup questions and I will see what information I and the community can provide.
I think that this is the most robust solution! I am not familiar with the details of the Chainlink or Provable base contracts, so there is a chance that (2) may not work - changing base contracts can be somewhat tricky if you are not careful with the storage layout.
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Hi @RAHUL_SOSHTE,
I wanted to check if the above answered your question or whether you needed more information?
I am implementing the given approaches soon. I need to check which works the best for me.
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Hi @RAHUL_SOSHTE,
As per @spalladino’s reply, separating out the oracle functionality into its own contract is the more robust solution.