traverse path locally when possible

[aschmahmann]

Resolves most of part 3 in #62.

Note: I haven't had time to exercise this yet so might have issues

[aschmahmann]

Calling out that this will not handle the case where the data is fully local, the optimization only checks path locality in order to avoid doing the IPLD traversal twice.

We could certainly do it twice though in order to save bandwidth costs. This likely depends on how much data we think should go into the block cache and what's likely to be there.

Certainly we should consider doing this once we're further down the pipeline of changes (i.e. proper incremental verification), but could do it now too if that's of interest.

[lidel]

@aschmahmann probably too late for me to reason about traversal, do you have an example how "fully local" case would that work for loading sub-article from /ipfs/bafybeiaysi4s6lnjev27ln5icwm6tueaw2vdykrtjkwiphwekaywqhcjze/wiki ?
(fysa I've increased block cache to 16384, and we have still plenty of room – #47 (comment), so if we can save bandwidth by growing this cache, i think its worth exploring)

I think we need to pass params to fetcher.Fetch when fetching remainingPathToGet.

Reason 1: without ?depth= we will be harcore over-fetching the entire wikipedia

2023-04-05T01:13:01.065+0200	DEBUG	caboose	caboose@v0.0.0-20230331212405-5b42e9465c12/fetcher.go:75	doing fetch	{"from": "64.44.166.5", "of": "/ipfs/bafybeiaysi4s6lnjev27ln5icwm6tueaw2vdykrtjkwiphwekaywqhcjze/wiki", "mime": "application/vnd.ipld.car", "requestId": "efb63fd6-12cd-4417-99bb-fd9106d0aa96"}

Reason 2: difficult to debug, as we are missing info if the failed request was ?format=car or raw (we can fix this by adding mime to error logs in caboose, but i think if we have params, we should also have explicit format one):

2023-04-05T01:13:03.040+0200	DEBUG	caboose	caboose@v0.0.0-20230331212405-5b42e9465c12/fetcher.go:122	fetch result	{"from": "64.44.166.5", "of": "/ipfs/bafybeiaysi4s6lnjev27ln5icwm6tueaw2vdykrtjkwiphwekaywqhcjze/wiki", "status": 200, "size": 203846, "ttfb": 0, "duration": 1.975232799, "attempt": 0, "error": "context canceled"}
2023-04-05T01:13:03.040+0200	DEBUG	caboose	caboose@v0.0.0-20230331212405-5b42e9465c12/pool.go:505	fetch attempt failed	{"from": "64.44.166.5", "attempt": 0, "of": "/ipfs/bafybeiaysi4s6lnjev27ln5icwm6tueaw2vdykrtjkwiphwekaywqhcjze/wiki", "error": "context canceled"}

[aschmahmann]

Ah good catch that was a refactor bug., I meant it when I said I hadn't really tested yet 😅. It's a bug that the fetcher isn't using params. This comment was about whether the tryLoad should also do params. Right now it doesn't, but it's pretty doable to add it back in as well if we need it. WDYT?

The former is a pretty trivial fix, the later is a little more time. Not sure how much I'll be able to do before IPFS Thing, but will update if I can.

[lidel]

Correct me if we aim at different things here, but in my mind the main value of tryLoad is to avoid fetching parents over and over again, and move root CID at which we start Fetch to the right of the path.

With that framing, we don't need to pass the same params as the final Fetch, we could always use bytes=0:0 or depth=1 here (no matter what actual params would be used for nial Fetch), no?

[aschmahmann]

tryLoad is trying to reduce the amount of data we request that we already have. It's most notable/obvious for paths, but is true for things like when an entire file is stored in memory already.

Doing parents only (i.e. depth=0) happens to be easier than also dealing with the terminal element/params (e.g. byte range, depth=1, depth=all). Seems like we should deal with the others too though.

[lidel]

I think the most important optimization is to skip re-fetching parents we already have (biggest pain for websites and ranges over videos).

Could we land that first, and deal with the terminal element later, or is that the same amount of work?

[aschmahmann]

Nope parents is easier (it's already done). Will push the fixes shortly for the bugs you've found

[lidel]

opening http://en.wikipedia-on-ipfs.org.ipns.localhost:8081/wiki/Belize/ produces:

backend/graph	lib/graph_gateway.go:343	adl requested but not supported by link system: "unixfs"

[aschmahmann]

oh ... lol thanks. This is a mixing of all the tooling and coming out badly. The UnixFSPathSelector inserts requests for ADLs, but the fetcher thing uses a default reifier. Should just drop the UnixFSPathSelector and build one largely copied from boxo/path/resolver so we're only sticking to one frankenstein'd version at a time. We could also switch to using the named ADLs, but doesn't seem worth it given this is all too hacked together at the moment anyway.

[aschmahmann]

Closing as this is no longer needed now that we have backpressure #160 and no block storage.