[tor-dev] Remote descriptor fetching

Damian Johnson atagar at torproject.org
Sun Jun 9 03:07:19 UTC 2013

> Indeed, this would be pretty bad.  I'm not convinced that moria1
> provides truncated responses though.  It could also be that it
> compresses results for every new request and that compressed responses
> randomly differ in size, but are still valid compressions of the same
> input.  Kostas, do you want to look more into this and open a ticket if
> this really turns out to be a bug?

Tor clients use the ORPort to fetch descriptors. As I understand it
the DirPort has been pretty well unused for years, in which case a
regression there doesn't seem that surprising. Guess we'll see.

If Kostas wants to lead this investigation then that would be fantastic. :)

> So, this isn't the super smart downloader that I had in mind, but maybe
> there should still be some logic left in the application using this API.
>  I can imagine how both DocTor and metrics-db-R could use this API with
> some modifications.  A few comments/suggestions:

What kind of additional smartness were you hoping for the downloader to have?

> - There could be two methods get/set_compression(compression) that
> define whether to use compression.  Assuming we get it working.

Good idea. Added.

> - If possible, the downloader should support parallel downloads, with at
> most one parallel download per directory.  But it's possible to ask
> multiple directories at the same time.  There could be two methods
> get/set_max_parallel_downloads(max) with a default of 1.

Usually I'd be all for paralleling our requests to both improve
performance and distribute load. However, tor's present interface
doesn't really encourage it. There's no way of saying "get half of the
server descriptors from location X and the other half from location
Y". You can only request specific descriptors or all of them.

Are you thinking that the get_server_descriptors() and friends should
only try to parallelize when given a set of fingerprints? If so then
that sounds like a fine idea.

> - I'd want to set a global timeout for all things requested from the
> directories, so a get/set_global_timeout(seconds) would be nice.  The
> downloader could throw an exception when the global download timeout
> elapses.  I need such a timeout for hourly running cronjobs to prevent
> them from overlapping when things are really, really slow.

How does the global timeout differ from our present set_timeout()?

> - Just to be sure, get/set_retries(tries) is meant for each endpoint, right?

Yup, clarified.

> - I don't like get_directory_mirrors() as much, because it does two
> things: make a network request and parse it.  I'd prefer a method
> use_v2dirs_as_endpoints(consensus) that takes a consensus document and
> uses the contained v2dirs as endpoints for future downloads.  The
> documentation could suggest to use this approach to move some load off
> the directory authorities and to directory mirrors.

Very good point. Changed to a use_directory_mirrors() method, callers
can then call get_endpoints() if they're really curious what the
present directory mirrors are (which I doubt they often will).

> - Related note: I always look if the Dir port is non-zero to decide
> whether a relay is a directory.  Not sure if there's a difference to
> looking at the V2Dir flag.

Sounds good. We'll go for that instead.

> - All methods starting at get_consensus() should be renamed to fetch_*
> or query_* to make it clear that these are no getters but perform actual
> network requests.

Going with fetch_*.

> - All methods starting at get_consensus() could have an additional
> parameter for the number of copies (from different directories) to
> download.  The default would be 1.  But in some cases people might be
> interested in having 2 or 3 copies of a descriptor to compare if there
> are any differences, or to compare download times (more on this below).
>  Also, a special value of -1 could mean to download every requested
> descriptor from every available directory.  That's what I'd do in DocTor
> to download the consensus from all directory authorities.
> - As for download times, is there a way to include download meta data in
> the result of get_consensus() and friends?  I'd be interested in the
> directory that a descriptor was downloaded from and in the download time
> in millis.  This is similar to how I'm interested in file meta data in
> the descriptor reader, like file name or last modified time of the file
> containing a descriptor.

This sounds really specialized. If callers cared about the download
times then that seems best done via something like...

endpoints = ['location1', 'location2'... etc]

for endpoint in endpoints:
    start_time = time.time()

    print "endpoint %s took: %0.2f" % (endpoint, time.time() - start_time)
  except IOError, exc:
    print "failed to use %s: %s" % (endpoint, exc)

> - Can you add a fetch|query_votes(fingerprints) method to request vote
> documents?

Added a fetch_vote(authority) to provide an authority's
NetworkStatusDocument vote by querying
'http://<hostname>/tor/status-vote/next/authority.z'. However, I'm not
clear from the spec how you can query for specific relays (unless you
mean fingerprints to be the authority fingerprints).

Cheers! -Damian

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