From: Brian Warner <warner@allmydata.com>
Date: Fri, 9 Jan 2009 02:59:32 +0000 (-0700)
Subject: webapi/deep-manifest t=JSON: don't return the (large) manifest/SI/verifycap lists... 
X-Git-Url: https://git.rkrishnan.org/pf/content/%22news.html/running.html?a=commitdiff_plain;h=7ee336b274c311e0e7af9c5444e2bd1b3249514f;p=tahoe-lafs%2Ftahoe-lafs.git

webapi/deep-manifest t=JSON: don't return the (large) manifest/SI/verifycap lists unless the operation has completed, to avoid the considerable CPU+memory cost of creating the JSON (for 330k dirnodes, it could take two minutes to generate 275MB of JSON). They must be paid eventually, but not on every poll
---

diff --git a/docs/frontends/webapi.txt b/docs/frontends/webapi.txt
index 29fae18b..1242c6e8 100644
--- a/docs/frontends/webapi.txt
+++ b/docs/frontends/webapi.txt
@@ -213,6 +213,11 @@ GET /operations/$HANDLE?output=JSON   (same)
   * whether the operation is complete, or if it is still running
   * how much of the operation is complete, and how much is left, if possible
 
+ Note that the final status output can be quite large: a deep-manifest of a
+ directory structure with 300k directories and 200k unique files is about
+ 275MB of JSON, and might take two minutes to generate. For this reason, the
+ full status is not provided until the operation has completed.
+
  The HTML form will include a meta-refresh tag, which will cause a regular
  web browser to reload the status page about 60 seconds later. This tag will
  be removed once the operation has completed.
@@ -966,7 +971,10 @@ POST $DIRURL?t=start-manifest    (must add &ophandle=XYZ)
   by a space.
 
   If output=JSON is added to the queryargs, then the results will be a
-  JSON-formatted dictionary with six keys:
+  JSON-formatted dictionary with six keys. Note that because large directory
+  structures can result in very large JSON results, the full results will not
+  be available until the operation is complete (i.e. until output["finished"]
+  is True):
 
    finished (bool): if False then you must reload the page until True
    origin_si (base32 str): the storage index of the starting point
diff --git a/src/allmydata/web/directory.py b/src/allmydata/web/directory.py
index 0ac24bc2..07771bf8 100644
--- a/src/allmydata/web/directory.py
+++ b/src/allmydata/web/directory.py
@@ -728,13 +728,27 @@ class ManifestResults(rend.Page, ReloadMixin):
         inevow.IRequest(ctx).setHeader("content-type", "text/plain")
         m = self.monitor
         s = m.get_status()
-        status = {"manifest": s["manifest"],
-                  "verifycaps": list(s["verifycaps"]),
-                  "storage-index": list(s["storage-index"]),
-                  "stats": s["stats"],
-                  "finished": m.is_finished(),
-                  "origin": base32.b2a(m.origin_si),
-                  }
+
+        status = { "stats": s["stats"],
+                   "finished": m.is_finished(),
+                   "origin": base32.b2a(m.origin_si),
+                   }
+        if m.is_finished():
+            # don't return manifest/verifycaps/SIs unless the operation is
+            # done, to save on CPU/memory (both here and in the HTTP client
+            # who has to unpack the JSON). Tests show that the ManifestWalker
+            # needs about 1092 bytes per item, the JSON we generate here
+            # requires about 503 bytes per item, and some internal overhead
+            # (perhaps transport-layer buffers in twisted.web?) requires an
+            # additional 1047 bytes per item.
+            status.update({ "manifest": s["manifest"],
+                            "verifycaps": [i for i in s["verifycaps"]],
+                            "storage-index": [i for i in s["storage-index"]],
+                            })
+            # simplejson doesn't know how to serialize a set. We use a
+            # generator that walks the set rather than list(setofthing) to
+            # save a small amount of memory (4B*len) and a moderate amount of
+            # CPU.
         return simplejson.dumps(status, indent=1)
 
     def _si_abbrev(self):