diff --git a/event_log.py b/event_log.py
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--- /dev/null
+++ b/event_log.py
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+#
+# 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