diff --git a/command.py b/command.py
index 2ff0a34..971f968 100644
--- a/command.py
+++ b/command.py
@@ -19,6 +19,7 @@
 import re
 import sys
 
+from event_log import EventLog
 from error import NoSuchProjectError
 from error import InvalidProjectGroupsError
 
@@ -28,6 +29,7 @@
   """
 
   common = False
+  event_log = EventLog()
   manifest = None
   _optparse = None
 
diff --git a/event_log.py b/event_log.py
new file mode 100644
index 0000000..d73511d
--- /dev/null
+++ b/event_log.py
@@ -0,0 +1,177 @@
+#
+# Copyright (C) 2017 The Android Open Source Project
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#      http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+from __future__ import print_function
+
+import json
+import multiprocessing
+
+TASK_COMMAND = 'command'
+TASK_SYNC_NETWORK = 'sync-network'
+TASK_SYNC_LOCAL = 'sync-local'
+
+class EventLog(object):
+  """Event log that records events that occurred during a repo invocation.
+
+  Events are written to the log as a consecutive JSON entries, one per line.
+  Each entry contains the following keys:
+  - id: A ('RepoOp', ID) tuple, suitable for storing in a datastore.
+        The ID is only unique for the invocation of the repo command.
+  - name: Name of the object being operated upon.
+  - task_name: The task that was performed.
+  - start: Timestamp of when the operation started.
+  - finish: Timestamp of when the operation finished.
+  - success: Boolean indicating if the operation was successful.
+  - try_count: A counter indicating the try count of this task.
+
+  Optionally:
+  - parent: A ('RepoOp', ID) tuple indicating the parent event for nested
+            events.
+
+  Valid task_names include:
+  - command: The invocation of a subcommand.
+  - sync-network: The network component of a sync command.
+  - sync-local: The local component of a sync command.
+
+  Specific tasks may include additional informational properties.
+  """
+
+  def __init__(self):
+    """Initializes the event log."""
+    self._log = []
+    self._next_id = _EventIdGenerator()
+    self._parent = None
+
+  def Add(self, name, task_name, start, finish=None, success=None,
+          try_count=1, kind='RepoOp'):
+    """Add an event to the log.
+
+    Args:
+      name: Name of the object being operated upon.
+      task_name: A sub-task that was performed for name.
+      start: Timestamp of when the operation started.
+      finish: Timestamp of when the operation finished.
+      success: Boolean indicating if the operation was successful.
+      try_count: A counter indicating the try count of this task.
+      kind: The kind of the object for the unique identifier.
+
+    Returns:
+      A dictionary of the event added to the log.
+    """
+    event = {
+        'id': (kind, self._next_id.next()),
+        'name': name,
+        'task_name': task_name,
+        'start_time': start,
+        'try': try_count,
+    }
+
+    if self._parent:
+      event['parent'] = self._parent['id']
+
+    if success is not None or finish is not None:
+        self.FinishEvent(event, finish, success)
+
+    self._log.append(event)
+    return event
+
+  def AddSync(self, project, task_name, start, finish, success):
+    """Add a event to the log for a sync command.
+
+    Args:
+      project: Project being synced.
+      task_name: A sub-task that was performed for name.
+                 One of (TASK_SYNC_NETWORK, TASK_SYNC_LOCAL)
+      start: Timestamp of when the operation started.
+      finish: Timestamp of when the operation finished.
+      success: Boolean indicating if the operation was successful.
+
+    Returns:
+      A dictionary of the event added to the log.
+    """
+    event = self.Add(project.relpath, success, start, finish, task_name)
+    if event is not None:
+      event['project'] = project.name
+      if project.revisionExpr:
+        event['revision'] = project.revisionExpr
+      if project.remote.url:
+        event['project_url'] = project.remote.url
+      if project.remote.fetchUrl:
+        event['remote_url'] = project.remote.fetchUrl
+      try:
+        event['git_hash'] = project.GetCommitRevisionId()
+      except Exception:
+        pass
+    return event
+
+  def GetStatusString(self, success):
+    """Converst a boolean success to a status string.
+
+    Args:
+      success: Boolean indicating if the operation was successful.
+
+    Returns:
+      status string.
+    """
+    return 'pass' if success else 'fail'
+
+  def FinishEvent(self, event, finish, success):
+    """Finishes an incomplete event.
+
+    Args:
+      event: An event that has been added to the log.
+      finish: Timestamp of when the operation finished.
+      success: Boolean indicating if the operation was successful.
+
+    Returns:
+      A dictionary of the event added to the log.
+    """
+    event['status'] =  self.GetStatusString(success)
+    event['finish_time'] = finish
+    return event
+
+  def SetParent(self, event):
+    """Set a parent event for all new entities.
+
+    Args:
+      event: The event to use as a parent.
+    """
+    self._parent = event
+
+  def Write(self, filename):
+    """Writes the log out to a file.
+
+    Args:
+      filename: The file to write the log to.
+    """
+    with open(filename, 'w+') as f:
+      for e in self._log:
+        json.dump(e, f, sort_keys=True)
+        f.write('\n')
+
+
+def _EventIdGenerator():
+  """Returns multi-process safe iterator that generates locally unique id.
+
+  Yields:
+    A unique, to this invocation of the program, integer id.
+  """
+  eid = multiprocessing.Value('i', 1)
+
+  while True:
+    with eid.get_lock():
+      val = eid.value
+      eid.value += 1
+    yield val
diff --git a/main.py b/main.py
index f965d7e..386b4f1 100755
--- a/main.py
+++ b/main.py
@@ -37,6 +37,7 @@
   kerberos = None
 
 from color import SetDefaultColoring
+import event_log
 from trace import SetTrace
 from git_command import git, GitCommand
 from git_config import init_ssh, close_ssh
@@ -85,6 +86,9 @@
 global_options.add_option('--version',
                           dest='show_version', action='store_true',
                           help='display this version of repo')
+global_options.add_option('--event-log',
+                          dest='event_log', action='store',
+                          help='filename of event log to append timeline to')
 
 class _Repo(object):
   def __init__(self, repodir):
@@ -176,6 +180,8 @@
         RunPager(config)
 
     start = time.time()
+    cmd_event = cmd.event_log.Add(name, event_log.TASK_COMMAND, start)
+    cmd.event_log.SetParent(cmd_event)
     try:
       result = cmd.Execute(copts, cargs)
     except (DownloadError, ManifestInvalidRevisionError,
@@ -203,7 +209,8 @@
         result = e.code
       raise
     finally:
-      elapsed = time.time() - start
+      finish = time.time()
+      elapsed = finish - start
       hours, remainder = divmod(elapsed, 3600)
       minutes, seconds = divmod(remainder, 60)
       if gopts.time:
@@ -213,6 +220,12 @@
           print('real\t%dh%dm%.3fs' % (hours, minutes, seconds),
                 file=sys.stderr)
 
+      cmd.event_log.FinishEvent(cmd_event, finish,
+                                result is None or result == 0)
+      if gopts.event_log:
+        cmd.event_log.Write(os.path.abspath(
+                            os.path.expanduser(gopts.event_log)))
+
     return result
 
 
diff --git a/manifest_xml.py b/manifest_xml.py
index 4f0eb64..55d25a7 100644
--- a/manifest_xml.py
+++ b/manifest_xml.py
@@ -110,7 +110,8 @@
     return url
 
   def ToRemoteSpec(self, projectName):
-    url = self.resolvedFetchUrl.rstrip('/') + '/' + projectName
+    fetchUrl = self.resolvedFetchUrl.rstrip('/')
+    url = fetchUrl + '/' + projectName
     remoteName = self.name
     if self.remoteAlias:
       remoteName = self.remoteAlias
@@ -118,7 +119,8 @@
                       url=url,
                       pushUrl=self.pushUrl,
                       review=self.reviewUrl,
-                      orig_name=self.name)
+                      orig_name=self.name,
+                      fetchUrl=self.fetchUrl)
 
 class XmlManifest(object):
   """manages the repo configuration file"""
diff --git a/project.py b/project.py
index 45ecfe3..269fd7e 100644
--- a/project.py
+++ b/project.py
@@ -323,13 +323,15 @@
                pushUrl=None,
                review=None,
                revision=None,
-               orig_name=None):
+               orig_name=None,
+               fetchUrl=None):
     self.name = name
     self.url = url
     self.pushUrl = pushUrl
     self.review = review
     self.revision = revision
     self.orig_name = orig_name
+    self.fetchUrl = fetchUrl
 
 
 class RepoHook(object):
@@ -2876,13 +2878,14 @@
 
     self.detach_head = detach_head
     self.clean = True
+    self.recent_clean = True
 
   def info(self, project, fmt, *args):
     self._messages.append(_InfoMessage(project, fmt % args))
 
   def fail(self, project, err=None):
     self._failures.append(_Failure(project, err))
-    self.clean = False
+    self._MarkUnclean()
 
   def later1(self, project, what):
     self._later_queue1.append(_Later(project, what))
@@ -2896,6 +2899,15 @@
     self._PrintMessages()
     return self.clean
 
+  def Recently(self):
+    recent_clean = self.recent_clean
+    self.recent_clean = True
+    return recent_clean
+
+  def _MarkUnclean(self):
+    self.clean = False
+    self.recent_clean = False
+
   def _RunLater(self):
     for q in ['_later_queue1', '_later_queue2']:
       if not self._RunQueue(q):
@@ -2904,7 +2916,7 @@
   def _RunQueue(self, queue):
     for m in getattr(self, queue):
       if not m.Run(self):
-        self.clean = False
+        self._MarkUnclean()
         return False
     setattr(self, queue, [])
     return True
diff --git a/subcmds/sync.py b/subcmds/sync.py
index 82056f3..ef02327 100644
--- a/subcmds/sync.py
+++ b/subcmds/sync.py
@@ -64,6 +64,7 @@
 except ImportError:
   multiprocessing = None
 
+import event_log
 from git_command import GIT, git_require
 from git_config import GetUrlCookieFile
 from git_refs import R_HEADS, HEAD
@@ -304,9 +305,10 @@
     # - We always set err_event in the case of an exception.
     # - We always make sure we call sem.release().
     # - We always make sure we unlock the lock if we locked it.
+    start = time.time()
+    success = False
     try:
       try:
-        start = time.time()
         success = project.Sync_NetworkHalf(
           quiet=opt.quiet,
           current_branch_only=opt.current_branch_only,
@@ -345,6 +347,9 @@
     finally:
       if did_lock:
         lock.release()
+      finish = time.time()
+      self.event_log.AddSync(project, event_log.TASK_SYNC_NETWORK,
+                             start, finish, success)
 
     return success
 
@@ -720,16 +725,24 @@
       _PostRepoUpgrade(self.manifest, quiet=opt.quiet)
 
     if not opt.local_only:
-      mp.Sync_NetworkHalf(quiet=opt.quiet,
-                          current_branch_only=opt.current_branch_only,
-                          no_tags=opt.no_tags,
-                          optimized_fetch=opt.optimized_fetch,
-                          submodules=self.manifest.HasSubmodules)
+      start = time.time()
+      success = mp.Sync_NetworkHalf(quiet=opt.quiet,
+                                    current_branch_only=opt.current_branch_only,
+                                    no_tags=opt.no_tags,
+                                    optimized_fetch=opt.optimized_fetch,
+                                    submodules=self.manifest.HasSubmodules)
+      finish = time.time()
+      self.event_log.AddSync(mp, event_log.TASK_SYNC_NETWORK,
+                             start, finish, success)
 
     if mp.HasChanges:
       syncbuf = SyncBuffer(mp.config)
+      start = time.time()
       mp.Sync_LocalHalf(syncbuf, submodules=self.manifest.HasSubmodules)
-      if not syncbuf.Finish():
+      clean = syncbuf.Finish()
+      self.event_log.AddSync(mp, event_log.TASK_SYNC_LOCAL,
+                             start, time.time(), clean)
+      if not clean:
         sys.exit(1)
       self._ReloadManifest(manifest_name)
       if opt.jobs is None:
@@ -823,7 +836,10 @@
     for project in all_projects:
       pm.update()
       if project.worktree:
+        start = time.time()
         project.Sync_LocalHalf(syncbuf, force_sync=opt.force_sync)
+        self.event_log.AddSync(project, event_log.TASK_SYNC_LOCAL,
+                               start, time.time(), syncbuf.Recently())
     pm.end()
     print(file=sys.stderr)
     if not syncbuf.Finish():
@@ -907,6 +923,7 @@
     return False
   return True
 
+
 class _FetchTimes(object):
   _ALPHA = 0.5