How the blocks of Merkle DAG can rebuild the file?

1

Hi,

I understood how a file trough a chunking process is cut into blocks allowing the creation of a merkle DAG. My question is how from the blocks we can recreate the file ? Indeed we may have all the blocks of bytes but in which order IPFS manage to concatenate them all.

UnixFs stocks the amounts of links of a node (number of children) but does it also stock the order ?

Thanks

2

Unixfs roots looks kinda like this:

{
  "data": null,
  "links":[
    {"hash": "Qmfoo", "size": 1000},
    {"hash": "Qmbar", "size": 500},
    {"hash": "Qmbaz", "size": 1000}
  ]
}

This is an ordered list.
The size field say how much of the original file offset is covered by this block.

So you just copy each range in a DFS ordered traversal.
(note real sizes are far bigger than this)

This also allows you to do seeking and streaming, for example if I only care about the bytes from 1250 to 1350, I can skip all offsets outside this range (you need to compute the offsets by suming the sizes)
In this case I would only fetch the root block and the leaf block Qmbar (because Qmfoo and Qmbaz are outside of the range I want).

(this is a simplified version the actual spec is here specs/UNIXFS.md at main · ipfs/specs · GitHub)

1 Like
3

Thanks @Jorropo for your time