[tahoe-lafs-trac-stream] [Tahoe-LAFS] #2822: remove redundant read from web GET of directory

[Tahoe-LAFS]

#2822: remove redundant read from web GET of directory
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 Reporter:  warner                              |          Owner:
     Type:  defect                              |         Status:  new
 Priority:  normal                              |      Milestone:
                                                |  undecided
Component:  code-frontend-web                   |        Version:  1.11.0
 Keywords:  dirnode cache performance tahoe-cp  |  Launchpad Bug:
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 While checking out the "recent and active operations" page, I noticed that
 doing a simple `tahoe cp` into a pre-existing top-level directory caused a
 total of 4 mapupdate operations, 4 retrieves, and 1 publish (where I was
 expecting a single retrieve and a single publish).

 It looks like we're doing some redundant operations. The `tahoe cp`
 command does two WAPI operations: `GET /uri/ALIAS/CHILD?t=json` (to see
 what we're replacing), then a `PUT /uri/ALIAS/CHILD` (to do the actual
 assignment). The WAPI GET causes two dirnode operations:

 * a `get(childname)`, called from
 `allmydata.web.directory.DirectoryNodeHandler.childFactory()` as it walks
 through the ALIAS dirnode to find CHILD
 * a `get_metadata_for(childname)`, called from
 `web.filenode.FileNodeHandler.render_GET` (in the `t=json` clause when
 `self.parentnode` and `self.name` are present). We have to retrieve the
 metadata from the parent directory, because that's how tahoe dirnodes work

 I think we should remove the `get_metadata_for` call, by changing
 `DirectoryNodeHandler.childFactory` to use `get_child_and_metadata`, and
 passing the metadata into the new `FileNodeHandler`.

 It might be possible to remove the first read that PUT does, but I'm not
 yet sure how. In general, I wonder if we should have some sort of write-
 through cache that allows us to remember the contents of dirnodes for a
 little while, until we know they've changed (because we wrote to the
 dirnode ourselves).

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Ticket URL: <https://tahoe-lafs.org/trac/tahoe-lafs/ticket/2822>
Tahoe-LAFS <https://Tahoe-LAFS.org>
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