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Kronos

Introduction

Kronos is a time series storage engine. It can store streams of data (e.g., user click streams, machine cpu utilization) and retrieve all events you've stored over a given time interval. Kronos exposes an HTTP API to send, retrieve, and delete events. We've written Kronos backends for memory (testing), Cassandra (low latency), and S3 (high throughput), with more to come.

GoDaddy's Locu team currently uses it to store all user and machine data that power our reports and analyses.

Get running in 5 minutes

First, check out Kronos, add some default settings, and launch it locally:

git clone https://github.com/Locu/chronology.git
cd chronology/kronos
sudo make installdeps
sudo pip install -r requirements.txt  # no sudo if you are in a virtualenv!!
python runserver.py --port 8151 --config settings.py.template --debug

Then, from the chronology/pykronos directory, run a python shell, and insert/retrieve some data:

from datetime import datetime
from datetime import timedelta
from dateutil.tz import tzutc
from pykronos.client import KronosClient
kc = KronosClient('http://localhost:8151', namespace='kronos')
kc.put({'yourproduct.website.clicks': [
  {'user': 35, 'num_clicks': 10}]})
for event in kc.get('yourproduct.website.clicks',
                    datetime.now(tz=tzutc()) - timedelta(minutes=5),
                    datetime.now(tz=tzutc())):
  print event

On the first line, we created a Kronos client to speak with the server we just started. On the second line, we've put a single event on the yourproduct.website.clicks clickstream. Finally, we retrieve all yourproduct.website.clicks events that happened in the last five minutes.

If you wish to see a more detailed example of the Kronos API, check out the more detailed pykronos example.

If you would like to run kronos as a daemon, run setup.py to install kronosd.

sudo python setup.py install

Configure your settings in /etc/kronos/settings.py and /etc/kronos/uwsgi.ini. Logs can be found in /var/log/kronos. When everything is configured to your liking, run

sudo /etc/init.d/kronosd start

You can also call stop, restart, or force-reload on that command.

Settings details

Take a look at settings.py.template. We tried to document all of the settings pretty thoroughly. If anything is unclear, file an issue and we'll clarify!

HTTP API

Kronos Time

Kronos time is represented as Coordinated Universal Time since the epoch. It is measured in units of 100s of nanoseconds, or 1e-7 seconds. We're not proud of our time representation, and in hindsight, we could have picked a more humane option. We'll get there. To make up for it, we've created a bunch of helper functions for converting to other time representations.

Inserting Events

Events can be sent to Kronos by sending a POST to /1.0/events/put.

The body of the POST should be a JSON-encoded object with the following format:

{namespace: namespace_name (optional),
 events: { stream_name1 : [event1, event2, ...],
           stream_name2 : [event1, event2, ...],
... }
}

where event1 and event2 are dictionaries of the event data you want to log. @id is a reserved key used for uniquely identifying events in Kronos. If it is set in the event dictionary, it will be overwritten by the Kronos server. @time is also a reserved key used for specifying the time of an event. Times are specified in Kronos time. If @time is present in the dictionary, and not a valid Kronos time, an error will be raised. If it is missing from the dictionary, it will be added with the current time.

The response is in the following format:

{stream_name: {backend1: {'num_inserted': number of events inserted
                          '@errors': optional, errors when inserting events}},
 '@success': true or false, true if insertions succeeded
 '@errors': optional, errors when validating streams or events
}

where backend1 is the name of the configured backend. num_inserted is -1 if there was an error inserting events.

Retrieving Events

Events can be retrieved from Kronos by sending a POST to /1.0/events/get.

The body of the POST should be a JSON-encoded object with the following format:

{namespace: namespace_name (optional),
 stream: stream name,
 start_time: starting time,
 end_time: ending time,
 start_id : start id,
 limit: optional maximum number of events,
 order: 'ascending' or 'descending' (default 'ascending')
}

If start_id is provided, Kronos only returns events from start_id (inclusive) to end_time (exclusive). Otherwise, Kronos returns events from start_time (inclusive) to end_time (exclusive). start_time and end_time must be given in Kronos time.

If an error occurs, the response is a JSON-encoded dictionary with @success set to False and @errors is a list of error strings.

Otherwise, the response is a newline-separated stream of JSON-encoded events that match the POST parameters.

Deleting data

Events can be deleted from Kronos by sending a POST to /1.0/events/delete.

The body of the POST should be a JSON-encoded object with the following format:

{ namespace: namespace_name (optional),
  stream : stream_name,
  start_time : starting time,
  end_time : ending time,
  start_id : start id,
}

where either start_time or start_id must be specified. All other parameters are required. Kronos deletes all events from start_id (inclusive) or start_time (inclusive) to end_time (exclusive). start_time and end_time must be given in Kronos time.

The response is in the following format:

{stream_name: {backend1: {'num_deleted': number of events deleted},
                          '@errors': optional, list of error strings} }
}

Getting A List of Streams

To see a list of valid stream names, send a POST to /1.0/streams.

The body of the POST should be a JSON-encoded object with the following format:

{ namespace: namespace_name (optional),
}

If successful, the response is a newline-separated stream of stream names. If there were errors, the response is a JSON-encoded dictionary with @success set to false and @errors set to a list of error strings.

Inferred Schema

You can retrieve an inferred schema for a stream by sending a POST to /1.0/streams/infer_schema. Currently, the implementation looks at several events in the stream to infer the schema. If there are conflicts, the type will be the parent of all the conflicting types, up to Any, which encompasses all types. This implementation is subject to change.

The body of the POST should be a JSON-encoded object with the following format:

{stream: stream_name,
 namespace: namespace_name (optional)
}

The response is a JSON-encoded object with the following format:

{"@success": true if schema successfully returned,
 "@took": time in ms,
 "namespace": namespace,
 "stream": stream_name,
 "schema": see schema format below
}

The schema is represented using JSON Schema v4, where elements in the schema have the following types:

  • Null
  • String
  • Integer
  • Number
  • Boolean
  • Array
  • Object
  • Any

The schema is represented as a nested dictionary. Null, String, Integer, Number, Boolean and Any are represented by

{"type": null|string|integer|number|boolean|any}

Array is represented by

{"type": "array",
 "items": the type of the items of the array}

Object is represented by

{"type": "object",
 "required": list of required properties,
 "properties": dictionary of property names to types
}

Example response for events with fields categories, which are a list of integers, email, and username:

{"@success": true,
 "@took": "1475.893974ms",
 "namespace": "kronos",
 "stream": "user.page.visit",
 "schema": {
   "$schema": "http://json-schema.org/draft-04/schema",
   "required": ["@id", "@time", "username", "categories"],
   "type": "object",
   "properties": {"username": {"type": "string"},
                  "categories": {"items": {"type": "integer"}, "type": "array"},
                  "email": {"type": "string"}}
 }
}

Backends

Memory

The in-memory backend is mostly used for testing. Here's a sample storage configuration for it:

storage = {
  'memory': {
    'backend': 'kronos.storage.memory.InMemoryStorage',
    'max_items': 50000
  },
}

There's only one parameter, max_items, which specifies the largest number of events you can store before it starts acting funny.

Cassandra

Our Cassandra backend is targeted at low-latency event storage, but internally we use it even for relatively bursty high-throughput streams. Here's a sample storage configuration:

storage = {
  'cassandra': {
    'backend': 'kronos.storage.cassandra.CassandraStorage',
    'hosts': ['127.0.0.1'],
    'keyspace_prefix': 'kronos_test',
    'replication_factor': 3,
    'timewidth_seconds': 60 * 60, # 1 hour.
    'shards_per_bucket': 3,
    'read_size': 1000
  }
}

Our design for the Cassandra backend is heavily influenced by a great blog post with illustrations on the topic. Here are what the parameters above mean:

  • hosts is a list of Cassandra nodes to connect to.

  • keyspace_prefix is a prefix that is applied to each KeySpace Kronos creates. A KeySpace is created for each configured namespace.

  • replication_factor is the number of Cassandra nodes to replicate each data item to. Note that this value is set at the initialization of a keyspace, so if you change it, existing keyspaces will be at their previous replication factor. You'll have to manually change the replication factor of existing keyspaces.

  • timewidth_seconds is the number of seconds worth of data to bucket together. The Cassandra backend stores all events that happened within the same time span (e.g., one hour) together. Set this to a small timewidth to reduce latency (you won't have to look through a bunch of irrelevant events to find a small timespan), and to a high timewidth to increase throughput (you won't have to look through a bunch of buckets to aggregate all of the data of a large timespan). We tend to go for an hour as a default.

  • shards_per_bucket makes up for the white lie we told in the last bullet point. Rather than storing all events that happen during a given timespan in a single bucket (which will put pressure on a single Cassandra row key/nodes storing that key), we can shard each bucket across default_shards_per_bucket shards for each timewidth. As an example, setting default_shards_per_bucket will store each hour worth of data in one of three shards, decreasing the pressure on any one Cassandra row key.

  • read_size is probably not something you will play around with too much. If a single time width's shard contains a lot of events, the Cassandra client driver will transparently iterate through them in chunks of this size.

ElasticSearch

Our ElasticSearch backend is designed to work well with Kibana. Most implementations of a time-series storage layers on top of ElasticSearch will create a new index per day (or some time interval); e.g. Logstash does this. Our approach is a little different. We keep on writing events to an index till the number of events in it exceeds a certain limit and then rollover to a new index. In order to keep track of what indices contain data for what time ranges, we use ElasticSearch's aliasing feature to assign an alias for each day that the index might contain data for. This approach lets us be compatible with Kibana while at the same time controlling the number of indices being created over time. Here's a sample storage configuration:

storage = {
  'elasticsearch': {
    'backend': 'kronos.storage.elasticsearch.ElasticSearchStorage',
    'hosts': [{'host': 'localhost', 'port': 9200}],
    'index_template': 'kronos_test',
    'index_prefix': 'kronos_test',
    'shards': 1,
    'replicas': 0,
    'force_refresh': True,
    'read_size': 10,
    'rollover_size': 100,
    'rollover_check_period_seconds': 2
  }
}

Here are what the parameters above mean:

  • hosts is a list of ElasticSearch nodes to connect to.

  • index_template is the name of the template Kronos creates in ElasticSearch. This template is applied to all indices Kronos creates.

  • index_prefix is a name prefix for all indices Kronos creates.

  • shards is the number of shards Kronos creates for each index.

  • replicas is the number of replicas Kronos creates for each index.

  • force_refresh will flush the index being written to at the end of each put request. This shouldn't be enabled for production environments; it probably will hose your ElasticSearch cluster.

  • read_size is the scroll size when retrieving events from ElasticSearch. It amounts to the number of events fetched from ElasticSearch per request.

  • rollover_size is the number of events after which Kronos will create a new index and start writing events into the new index. This size is merely a hint. Kronos periodically checks the number of events in the index it is writing to and rolls it over when the number exceeds rollover_size.

  • rollover_check_period_seconds is the interval after which a Kronos instance checks to see if the index needs to be rolled over.

SQLite

The SQLite implementation is intended for easy initial deployment and testing, but is not suitable for production. The implementation stores all events in a single table with various indices to speed access. Here is a sample configuration:

storage = {
  'sqlite': {
    'backend': 'sqlite.SqliteStorage',
    'sqlite_database_path': '/tmp/kronos.sqlite',
  }
}

There is only one option for the SQLite backend: the path to the database file.

  • sqlite_database_path The path to the database storing the events. The database will get created if it is missing.

S3

Under construction -- check back later.

Deployment

We deploy Kronos with uWSGI. To see our starter scripts for doing this, check out install_kronosd.py, uwsgi.ini, and kronosd.init.d. If anything is unclear, file an issue and we'll clarify!

Design goals, limitations, and comparisons to other systems

Put + Get + Delete

There's a class of useful event logging/streaming systems allow you to programmatically put events onto a stream via another log collector (e.g., apache logs, syslogd), or an API, filter/transform the data in some way, and send it off to another backend (e.g., S3, ElasticSearch, etc.). If you wish to retrieve the data you just logged, you must interact with the system you sent it off to (e.g., query ElasticSearch directly). Logstache and Fluentd are good examples of such systems.

In the Kronos API, we've included the ability to retrieve and delete your data from the data source after it's been sent to a backend. This decision has its pros and cons.

The key benefit of supporting get and delete endpoints is that Kronos is closer to a one-stop shop for your time series data. After inserting an event to a Kronos datastore, you can use the same Kronos client API to retrieve events. Because Kronos allows developers to retrieve events based on the time they occur, the developer need not understand the underlying storage format, providing them with physical data independence. With the delete endpoint, a developer that accidentally writes faulty log data to a stream can quickly delete that data before it muddies up downstream processes.

If Kronos didn't support get and delete, it wouldn't be the end of the world, but it would put more onus on the developer to understand multiple systems. Consider an S3 backend that can only store events as they occur, but not retrieve them based on when they occur. A developer wishing to crunch data that happened in a particular week now has to programmatically list all files in an S3 bucket, identify file names with the appropriate dates for that week, and then filter for events in those files outside of the time frame they wish to analyze.

Allowing developers to retrieve events from storage backends through Kronos has its drawbacks. First, data must flow through an extra hop: Kronos retrieves data from a storage backend before sending it off to the user. Second, building get and delete endpoints for storage backends requires more work on behalf of people writing new Kronos backends.

Finally, and perhaps most controvertially, providing a delete endpoint on an append-mostly system, while useful, has performance implications on some backends. While implementing event deletion in the Kronos Cassandra backend was straightforward, deleting a single event from a several hundred megabyte log file in S3 required making difficult consistency and latency tradeoffs. We imagine that many delete implementations, if implemented, will not be as performant or consistent as their put and get cousins.

Time-based

Every event happens at a particular time. You can insert events that happen at particular times (regardless of the current wall clock time) and retrieve all events that happened between two times. Developers do not have to think about how these events will be stored or retrieved, or the order in which events arrive.

Making time a first class citizen in a time series storage engine was a natural fit. Developers using Kronos appreciate that they can quickly retrieve last month's data to do a quick analysis, and the learning time to store and retrieve events is low. Since developers can store events that happened last month with the same ease that they can store events that just happened, there's not a lot of mental overhead in doing things like migrating or restoring data.

Providing a time-based API has its challenges. In developing a Kronos backend, one has to consider events that arrive out of order, and in returning events to users, backends have to ensure time ordering on events. Kyle Kingsbury has a great writeup on the challenges of using timestamps in distributed systems design, and Kronos is no exception to this great analysis.

In particular, be careful in using Kronos for publish-subscribe messaging applications that a system like Kafka might be better suited for. Just because Kronos allows you to query for events that happened in the past minute doesn't mean all events that happened in the last minute have made their way across the network and into a storage system. While we use Kronos for operational monitoring applications that require us to look at recent data, we only perform analyses that inform business decisions after giving events enough time (e.g., 5-20 minutes, or even an hour) to make their way to their final destination.

Multiple backends

Unlike systems such as InfluxDB or Kafka, Kronos does not have a single backend that it relies on. This allows us to provide different backends for different use cases. For example, while the Cassandra backend provides relatively low latency access, the S3 backend optimizes for write and read throughput. The breadth of storage options places more effort on Kronos users to decide which backends to store and retrieve data from for different use-cases.

Avoid more complex operations

We use Kronos in conjunction with tools like Spark and PANDAS to do data analysis on several streams, and leave complex operations (e.g., filters, aggregates, joins) to those systems. The Metis project facilitates time series analyses of this form on top of Kronos, and a natural question would be whether Kronos could perform some amount of projection, filtering, or aggregation on behalf of a user to speed up these queries.

At this point, the Kronos get endpoint does not support anything other than time-based event retrieval to keep our design simple. In the future, we imagine pushing down operations such as filters or projections into the Kronos API, but want to tread lightly until we have a good design for this.