Explain common pitfalls surrounding CIDs and how to "update" them.

[mishmosh]

The IPFS discord has been seeing multiple questions about updating CIDs and IPNS, and @Jorropo has written an outline addressing these Qs.

https://hackmd.io/SuF_fe82Qeamj2nSXmMWiw?both

Can we figure out the best place to incorporate this content? These 2 articles seem like the closest I can find:

• https://docs.ipfs.io/concepts/immutability/ (expand last paragraph, and link to...)
• https://docs.ipfs.io/concepts/ipns/ (maybe add "IPNS with Smart Contracts" as a new section there?)

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[johnnymatthews]

Here's a copy of the above-linked post, just in case it goes missing/get's deleted:

# Why you can't update a CID and how to do it.

A CID (Content IDentifier) is the hash of the content of the files so if you change the content the CID must change.

# But how to do it anyway

The goal should to not share a CID but share something else that points to a CID that is updatable.

## IPNS

IPNS (InterPlanetary Naming System) is a stack that allows to create things that points to a CID, so you instead of updating the CID you update the IPNS address, 
IPNS supports multiple resolving technology.
- [IPNS records](https://docs.ipfs.io/concepts/ipns/#interplanetary-name-system-ipns)
- [ENS](https://docs.ipfs.io/how-to/websites-on-ipfs/link-a-domain/#ethereum-naming-service-ens)
- [DNSLink](https://docs.ipfs.io/how-to/websites-on-ipfs/link-a-domain/#domain-name-service-dns)

## Using CIDs with Smart Contracts

If your application is already receiving the CID from a smart contract (like an NFT for example).

You can just use smart contract logic to update the CID you give to peoples.

For example many NFTs has a function looking like this (python pseudocode) :
```py
def tokenURI(this, tokenID: uint256) -> string:
    return this.baseURI + str(tokenID) + ".json"
```

So this function just grabs `baseURI` from the storage of the contract and appends the token id and `.json` extension, which is a metadata file for the NFT.

This will return something like this : `ipfs://Qmfoo/1234.json` for the NFT ID `1234`.

So you can just add a setter function that updates `this.baseURI`:
```py
def setBaseURI(this, newBaseURI: string):
    require(msg.sender == this.owner)
    this.baseURI = newBaseURI
```

So basically we just updated what we use to construct our tokenURI for our NFT returning a **new CID**.

So if I do `contract.setBaseURI("ipfs://Qmbar/")`, `tokenURI` will now return `ipfs://Qmbar/1234.json` instead of `Qmfoo`.

### --- Link the video NFT example about how to do it in remix ! ---