Add ZooKeeper-based dynamic configuration system.

The dynamic configuration system has two components: the app, which
reads, and the writer script. The latter is meant for use by humans, and
converts a [live_config] section of the INI file into JSON for storage
in ZooKeeper. The app will read this data on startup and place a watch
on the node to be notified, by ZooKeeper, of changes. This means that
running the writer script with new data will automatically propagate the
changes to every app very quickly, without restart.

The writer script relies on a human-entered password to authenticate
with ZooKeeper. The reddit app uses a different set of credentials
(specified in the INI file) to obtain read-only access to the
configuration data.

Also adds a new "live_config" spec to reddit and plugins. This spec is
parsed at write-time only and the parsed values are stored as JSON in
ZooKeeper.

r2/example.ini

@@ -493,3 +493,7 @@ beaker.session_secret = somesecret
 # execute malicious code after an exception is raised.
 #set debug = false
 
+# the following configuration section makes up the "live" config. if zookeeper
+# is enabled, then this configuration will be found by the app in zookeeper. to
+# write it to zookeeper, use the writer script: scripts/write_live_config.
+[live_config]

r2/r2/lib/app_globals.py

@@ -21,6 +21,7 @@
 ###############################################################################
 
 from __future__ import with_statement
+import ConfigParser
 from pylons import config
 import pytz, os, logging, sys, socket, re, subprocess, random
 import signal
@@ -43,6 +44,26 @@
 from r2.lib.plugin import PluginLoader
 
 
+LIVE_CONFIG_NODE = "/config/live"
+
+
+def extract_live_config(config, plugins):
+    """Gets live config out of INI file and validates it according to spec."""
+
+    # ConfigParser will include every value in DEFAULT (which paste abuses)
+    # if we do this the way we're supposed to. sorry for the horribleness.
+    live_config = config._sections["live_config"].copy()
+    del live_config["__name__"]  # magic value used by ConfigParser
+
+    # parse the config data including specs from plugins
+    parsed = ConfigValueParser(live_config)
+    parsed.add_spec(Globals.live_config_spec)
+    for plugin in plugins:
+        parsed.add_spec(plugin.live_config)
+
+    return parsed
+
+
 class Globals(object):
     spec = {
 
@@ -156,6 +177,9 @@ class Globals(object):
         },
     }
 
+    live_config_spec = {
+    }
+
     def __init__(self, global_conf, app_conf, paths, **extra):
         """
         Globals acts as a container for objects available throughout
@@ -235,13 +259,20 @@ def setup(self):
         self.cache_chains = {}
 
         # for now, zookeeper will be an optional part of the stack.
+        # if it's not configured, we will grab the expected config from the
+        # [live_config] section of the ini file
         zk_hosts = self.config.get("zookeeper_connection_string")
         if zk_hosts:
-            from r2.lib.zookeeper import connect_to_zookeeper
+            from r2.lib.zookeeper import connect_to_zookeeper, LiveConfig
             zk_username = self.config["zookeeper_username"]
             zk_password = self.config["zookeeper_password"]
             self.zookeeper = connect_to_zookeeper(zk_hosts, (zk_username,
                                                              zk_password))
+            self.live_config = LiveConfig(self.zookeeper, LIVE_CONFIG_NODE)
+        else:
+            parser = ConfigParser.RawConfigParser()
+            parser.read([self.config["__file__"]])
+            self.live_config = extract_live_config(parser, self.plugins)
 
         self.lock_cache = CMemcache(self.lockcaches, num_clients=num_mc_clients)
         self.make_lock = make_lock_factory(self.lock_cache)

r2/r2/lib/plugin.py

@@ -29,6 +29,7 @@
 class Plugin(object):
     js = {}
     config = {}
+    live_config = {}
 
     @property
     def path(self):

r2/r2/lib/zookeeper.py

@@ -20,6 +20,7 @@
 # Inc. All Rights Reserved.
 ###############################################################################
 
+import json
 import functools
 
 from kazoo.client import KazooClient
@@ -45,3 +46,25 @@ def connect_to_zookeeper(hostlist, credentials):
     client.connect()
     client.add_auth("digest", ":".join(credentials))
     return client
+
+
+class LiveConfig(object):
+    """A read-only dictionary view of configuration retrieved from ZooKeeper.
+
+    The data will be parsed using the given configuration specs, exactly like
+    the ini file based configuration. When data is changed in ZooKeeper, the
+    data in this view will automatically update.
+
+    """
+    def __init__(self, client, key):
+        self.data = {}
+
+        @client.DataWatch(key)
+        def watcher(data, stat):
+            self.data = json.loads(data)
+
+    def __getitem__(self, key):
+        return self.data[key]
+
+    def __repr__(self):
+        return "<LiveConfig %r>" % self.data

scripts/write_live_config

@@ -0,0 +1,134 @@
+#!/usr/bin/env python
+# The contents of this file are subject to the Common Public Attribution
+# License Version 1.0. (the "License"); you may not use this file except in
+# compliance with the License. You may obtain a copy of the License at
+# http://code.reddit.com/LICENSE. The License is based on the Mozilla Public
+# License Version 1.1, but Sections 14 and 15 have been added to cover use of
+# software over a computer network and provide for limited attribution for the
+# Original Developer. In addition, Exhibit A has been modified to be consistent
+# with Exhibit B.
+#
+# Software distributed under the License is distributed on an "AS IS" basis,
+# WITHOUT WARRANTY OF ANY KIND, either express or implied. See the License for
+# the specific language governing rights and limitations under the License.
+#
+# The Original Code is reddit.
+#
+# The Original Developer is the Initial Developer.  The Initial Developer of
+# the Original Code is reddit Inc.
+#
+# All portions of the code written by reddit are Copyright (c) 2006-2012 reddit
+# Inc. All Rights Reserved.
+###############################################################################
+"""Read config from an INI file and put it in ZooKeeper for instant use."""
+
+import os
+import sys
+import json
+import getpass
+import ConfigParser
+
+import kazoo.client
+from kazoo.security import make_acl, make_digest_acl
+
+from r2.lib.zookeeper import connect_to_zookeeper
+from r2.lib.app_globals import extract_live_config, LIVE_CONFIG_NODE
+from r2.lib.configparse import ConfigValue
+from r2.lib.plugin import PluginLoader
+
+
+USERNAME = "live-config"
+
+
+def write_config_to_zookeeper(node, username, password, config, live_config):
+    """Write given configuration to ZooKeeper with correct security etc."""
+
+    # read the zk configuration from the app's config
+    zk_hostlist = config.get("DEFAULT", "zookeeper_connection_string")
+    app_username = config.get("DEFAULT", "zookeeper_username")
+    app_password = config.get("DEFAULT", "zookeeper_password")
+
+    # connect to zk!
+    client = connect_to_zookeeper(zk_hostlist, (username, password))
+
+    # ensure that the path leading up to the config node exists. if it doesn't,
+    # create it with ACLs such that new stuff can be added below it, but no one
+    # but we can delete nodes.
+    parent_path = os.path.dirname(node)
+    client.ensure_path(parent_path, acl=[
+        # only we can delete children
+        make_digest_acl(username, password, delete=True),
+
+        # anyone authenticated can read/list children/create children
+        make_acl("auth", "", read=True, create=True),
+    ])
+
+    # create or update the config node ensuring that only we can write to it.
+    json_data = json.dumps(live_config)
+    try:
+        client.create(node, json_data, acl=[
+            make_digest_acl(username, password, read=True, write=True),
+            make_digest_acl(app_username, app_password, read=True),
+        ])
+    except kazoo.exceptions.NodeExistsException:
+        client.set(node, json_data)
+
+
+def confirm_config(live_config):
+    """Display the parsed live config and confirm that we should continue."""
+
+    max_key_length = max(len(k) for k in live_config.iterkeys())
+
+    print "Parsed Config:"
+    for key, value in sorted(live_config.iteritems(), key=lambda t: t[0]):
+        print " ", key.ljust(max_key_length), "=", repr(value)
+
+    answer = raw_input("Continue? [y|N] ")
+    return answer.lower() == "y"
+
+
+def main():
+    """Get and validate input from the user via CLI then write to ZK."""
+
+    progname = os.path.basename(sys.argv[0])
+
+    try:
+        ini_file_name = sys.argv[1]
+    except IndexError:
+        print >> sys.stderr, "USAGE: %s INI" % progname
+        return 1
+
+    config = ConfigParser.RawConfigParser()
+    try:
+        with open(ini_file_name, "r") as ini_file:
+            config.readfp(ini_file)
+    except (IOError, ConfigParser.Error), e:
+        print >> sys.stderr, "%s: %s: %s" % (progname, ini_file_name, e)
+        return 1
+
+    try:
+        plugin_config = config.get("DEFAULT", "plugins")
+        plugin_names = ConfigValue.tuple(plugin_config)
+        plugins = PluginLoader(plugin_names)
+        live = extract_live_config(config, plugins)
+    except ValueError as e:
+        print >> sys.stderr, "%s: %s" % (progname, e)
+        return 1
+    else:
+        if not confirm_config(live):
+            print "Oh, well, never mind then. Bye :("
+            return 0
+
+    password = getpass.getpass("Password: ")
+
+    write_config_to_zookeeper(LIVE_CONFIG_NODE,
+                              USERNAME, password,
+                              config, live)
+
+    print "Succesfully updated live config!"
+
+    return 0
+
+
+if __name__ == "__main__":
+    sys.exit(main())