id	summary	reporter	owner	description	type	status	priority	milestone	component	version	resolution	keywords	cc	launchpad_bug
2822	remove redundant read from web GET of directory	warner		"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).
"	defect	new	normal	undecided	code-frontend-web	1.11.0		dirnode cache performance tahoe-cp