update documentation for the download status page

[zooko]

Brian's New Downloader (#287, #288, #798, #800, #990), and subsequently Brian's New Visualizer (#1200) also offers a download status page with a great deal more information than the old one. Although it is really good to have all this information available, it is rather cryptic. This ticket is to update the documentation in docs/frontends/download-status.rst​ explaining what all the fields mean.

See also #1265 which is about adding labels and documentation onto the web page itself.

[warner]

I'm slowly (i.e. post-1.8.0, maybe in a 1.8.1) changing the data on this page and adding visualizations, so I don't want to put too much energy into documenting a transient/unstable data structure quite yet. But here's a quick summary of what's on the new downloader status page.

First, what's involved in a download?:

• downloads are triggered by read() calls, each with a starting offset (defaults to 0) and a length (defaults to the whole file). A regular webapi GET request will result in a whole-file read() call
• each read() call turns into an ordered sequence of get_segment() calls. A whole-file read will fetch all segments, in order, but partial reads or multiple simultaneous reads will result in random-access of segments. Segment reads always return ciphertext: the layer above that (in read()) is responsible for decryption.
• before we can satisfy any segment reads, we need to find some shares. ("DYHB" is an abbreviation for "Do You Have Block", and is the message we send to storage servers to ask them if they have any shares for us. The name is historical, from Mnet/MV, but nicely distinctive. Tahoe's actual message name is remote_get_buckets().). Responses come back eventually, or don't.
• Once we get enough positive DYHB responses, we have enough shares to start downloading. We send "block requests" for various pieces of the share. Responses come back eventually, or don't.
• When we get enough block-request responses for a given segment, we can decode the data and satisfy the segment read.
• When the segment read completes, some or all of the segment data is used to satisfy the read() call (if the read call started or ended in the middle of a segment, we'll only use part of the data, otherwise we'll use all of it).

With that background, here is the data currently on the download-status page:

• "DYHB Requests": this shows every Do-You-Have-Block query sent to storage servers and their results. Each line shows the following:
  • the serverid to which the request was sent
  • the time at which the request was sent. Note that all timestamps are relative to the start of the first read() call and indicated with a "+" sign
  • the time at which the response was received (if ever)
  • the share numbers that the server has, if any
  • the elapsed time taken by the request
  • also, each line is colored according to the serverid. This color is also used in the "Requests" section below.

• "Read Events": this shows all the FileNode read() calls and their overall results. Each line shows:
  • the range of the file that was requested (as [OFFSET:+LENGTH]). A whole-file GET will start at 0 and read the entire file.
  • the time at which the read() was made
  • the time at which the request finished, either because the last byte of data was returned to the read() caller, or because they cancelled the read by calling stopProducing (i.e. closing the HTTP connection)
  • the number of bytes returned to the caller so far
  • the time spent on the read, so far
  • the total time spent in AES decryption
  • total time spend paused by the client (pauseProducing), generally because the HTTP connection filled up, which most streaming media players will do to limit how much data they have to buffer
  • effective speed of the read(), not including paused time

• "Segment Events": this shows each get_segment() call and its resolution. This table is not well organized, and my post-1.8.0 work will clean it up a lot. In its present form, it records "request" and "delivery" events separately, indicated by the "type" column.
  • Each request shows the segment number being requested and the time at which the get_segment() call was made
  • Each delivery shows:
    • segment number
    • range of file data (as [OFFSET:+SIZE]) delivered
    • elapsed time spent doing ZFEC decoding
    • overall elapsed time fetching the segment
    • effective speed of the segment fetch

• "Requests": this shows every block-request sent to the storage servers. Each line shows:
  • the server to which the request was sent
  • which share number it is referencing
  • the portion of the share data being requested (as [OFFSET:+SIZE])
  • the time the request was sent
  • the time the response was received (if ever)
  • the amount of data that was received (which might be less than SIZE if we tried to read off the end of the share)
  • the elapsed time for the request (RTT=Round-Trip-Time)

Also note that each Request line is colored according to the serverid it was sent to. And all timestamps are shown relative to the start of the first read() call: for example the first DYHB message was sent at +0.001393s about 1.4 milliseconds after the read() call started everything off.

[zooko]

Thanks for the documentation! We should add this into the docs directory, and/or I wonder if it would be too disruptive to make it be statically served up by the wui and add a hyperlink on the download status page to it.

[warner]

heh, nice keyword :)

eh, I'm -0.. it'd be nice if folks could find out what the table means without searching for this ticket, but also we've got a lot of random little displays and it'd be a long process to provide (and maintain!) docs for each of them.

That said, it wouldn't be very hard to copy this into source:src/allmydata/web/ and include a static link to it on that page. Somebody (me?) would have to remember to change it once the changes I'm working on finally land. As a curious user poking around, I'd probably appreciate seeing that link.

[zooko]

Copied Brian's docs from comment:1 into trunk/docs/frontends/download-status.txt in 36f698b6371f82e8. I'm going to change this ticket to be about the idea of linking that document into the WUI itself with a "What does this mean?" link (as per comment:2 and comment:3).

[zooko]

Warning: we're going to integrate the new visualization tool, see #1200, which will change what is displayed on the download status page and/or will change how it is displayed. So you might want to fix this ticket to add help/documentation for the new version instead of the current version.

[zooko]

I think now that the New Visualizer has landed we need to update trunk/docs/frontends/download-status.rst​.

[warner]

not happening in 1.9

[warner]

FYI the docs in source:docs/frontends/download-status.rst​ are correct. The timeline is an alternative way of viewing that same data, but the first link the user can get to is a big HTML table, which download-status.rst (mostly) correctly describes. It might be nice to make those docs available as a hyperlinked "about this table" page, but that's not super critical.

[zooko]

Okay, this ticket is just about making sure that docs/frontends/download-status.rst​ is a correct description of the "download status" page which is the big HTML table, not the Javascript visualization. Brian, would you please review that document and fix anything that is incorrect or stale? Thank you!