This combines the ListStreamPositions and GetSegmentByPosition
calls with a ListSegments call that now knows how to return
only the segments within a Range, just like ListStreamPositions.
It would theoretically be possible to also include the
GetObjectLastCommitted call by having it do one of three
queries based on the incoming request range, but that would
mean duplicating the data for the object in every single
row that is returned for each segment in the range.
One gross thing that ListSegments has to do now is update the
first segment returned with the information from any ancestor
segment because GetSegmentByPosition used to do that. It only
updates the first segment so that it doesn't do O(N) database
queries. It seems difficult to have it do a single query to
update all of the segments at once. I'm not certain this change
should be merged on this basis alone.
This change has made me think a couple of things should happen:
1. Server side copy with ancestor segments strikes again
making the code less clear and potentially more buggy
or inefficient for a rare case (empirically <0.1%)
2. The download code requests individual segments from
the satellite lazily as part of its download which
requires the satellite telling it the locations of
all of the segments which requires the satellite
querying the locations of all of the segments. Instead
the download RPC could return the orders for all of
the segments for a range and the download code could
issue N download calls rather than 1 download call and
N get segment calls. I believe both sides of the code
paths would be simpler and more efficient this way.
3. In looking at the timing information for downloads when
testing this, we really need to focus on getting the
auth key and bandwidth limit verification times down.
Here's the timing I saw:
- 42ms: validate auth
- 52ms: bandwidth usage checking
- 14ms: get object info
- 26ms: get segment position info
- 26ms: getting the first segment full info
- 20ms: unaccounted for by spans
- 6ms: creating the orders
This change will remove 26ms, but there's a good 90ms
in just validation. With improved semantics hitting the
database only once and improved validation, a download
rpc taking ~30ms seems doable compared to our current
~200ms.
Change-Id: I4109dba082eaedb79e634c61dbf86efa93ab1222
7b06575f6f