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README.md

RabbitMQ Consistent Hash Exchange Type

Introduction

This plugin adds a consistent-hash exchange type to RabbitMQ. This exchange type uses consistent hashing (intro blog posts: one, two, three) to distribute messages between the bound queues. It is recommended to get a basic understanding of the concept before evaluating this plugin and its alternatives.

rabbitmq-sharding is another plugin that provides a way to partition a stream of messages among a set of consumers while trading off total stream ordering for processing parallelism.

Problem Definition

In various scenarios it may be desired to ensure that messages sent to an exchange are reasonably uniformly distributed across a number of queues based on the routing key of the message, a nominated header, or a message property. Technically this can be accomplished using a direct or topic exchange, binding queues to that exchange and then publishing messages to that exchange that match the various binding keys.

However, arranging things this way can be problematic:

  1. It is difficult to ensure that all queues bound to the exchange will receive a (roughly) equal number of messages (distribution uniformity) without baking in to the publishers quite a lot of knowledge about the number of queues and their bindings.

  2. When the number of queues changes, it is not easy to ensure that the new topology still distributes messages between the different queues evenly.

Consistent Hashing is a hashing technique whereby each bucket appears at multiple points throughout the hash space, and the bucket selected is the nearest higher (or lower, it doesn't matter, provided it's consistent) bucket to the computed hash (and the hash space wraps around). The effect of this is that when a new bucket is added or an existing bucket removed, only a very few hashes change which bucket they are routed to.

Supported RabbitMQ Versions

This plugin ships with RabbitMQ.

Supported Erlang Versions

This plugin supports the same Erlang versions as RabbitMQ core.

Installation

This plugin ships with RabbitMQ. Like all other RabbitMQ plugins, it has to be enabled before it can be used:

rabbitmq-plugins enable rabbitmq_consistent_hash_exchange

How It Works

In the case of Consistent Hashing as an exchange type, the hash is calculated from a message property (most commonly the routing key). Thus messages that have the same routing key will have the same hash value computed for them, and thus will be routed to the same queue, assuming no bindings have changed.

Binding Weights

When a queue is bound to a Consistent Hash exchange, the binding key is a number-as-a-string which indicates the binding weight: the number of buckets (sections of the range) that will be associated with the target queue.

Consistent Hashing-based Routing

The hashing distributes routing keys among queues, not message payloads among queues; all messages with the same routing key will go the same queue. So, if you wish for queue A to receive twice as many routing keys routed to it than are routed to queue B, then you bind the queue A with a binding key of twice the number (as a string -- binding keys are always strings) of the binding key of the binding to queue B. Note this is only the case if your routing keys are evenly distributed in the hash space. If, for example, only two distinct routing keys are used on all the messages, there's a chance both keys will route (consistently!) to the same queue, even though other queues have higher values in their binding key. With a larger set of routing keys used, the statistical distribution of routing keys approaches the ratios of the binding keys.

Each message gets delivered to at most one queue. Normally, each message gets delivered to exactly one queue, but there is a race between the determination of which queue to send a message to, and the deletion/death of that queue that does permit the possibility of the message being sent to a queue which then disappears before the message is processed. Hence in general, at most one queue.

The exchange type is "x-consistent-hash".

Usage Example

The Topology

In the below example the queues q0 and q1 get bound each with the weight of 1 in the hash space to the exchange e which means they'll each get roughly the same number of routing keys. The queues q2 and q3 however, get 2 buckets each (their weight is 2) which means they'll each get roughly the same number of routing keys too, but that will be approximately twice as many as q0 and q1.

Note the routing_keys in the bindings are numbers-as-strings. This is because AMQP 0-9-1 specifies the routing_key field must be a string.

Choosing Appropriate Weight Values

The example uses low weight values intentionally. Higher values will reduce throughput of the exchange, primarily for workloads that experience a high binding churn (queues are bound to and unbound from a consistent hash exchange frequently). Single digit weight values are recommended (and usually sufficient).

Inspecting Message Counts

The example then publishes 100,000 messages to our exchange with random routing keys, the queues will get their share of messages roughly equal to the binding keys ratios. After this has completed, message distribution between queues can be inspected using RabbitMQ's management UI and rabbitmqctl list_queues.

Routing Keys and Uniformity of Distribution

It is important to ensure that the messages being published to the exchange have varying routing keys: if a very small set of routing keys are being used then there's a possibility of messages not being evenly distributed between the bound queues. With a large number of bound queues some queues may get no messages routed to them at all.

If pseudo-random or unique values such as client/session/request identifiers are used for routing keys (or another property used for hashing) then reasonably uniform distribution should be observed.

Executable Versions

Executable versions of some of the code examples can be found under ./examples.

Code Example in Python

This version of the example uses Pika, the most widely used Python client for RabbitMQ:

#!/usr/bin/env python

import pika
import time

conn = pika.BlockingConnection(pika.ConnectionParameters(host='localhost'))
ch   = conn.channel()

ch.exchange_declare(exchange="e", exchange_type="x-consistent-hash", durable=True)

for q in ["q1", "q2", "q3", "q4"]:
    ch.queue_declare(queue=q, durable=True)
    ch.queue_purge(queue=q)

for q in ["q1", "q2"]:
    ch.queue_bind(exchange="e", queue=q, routing_key="1")

for q in ["q3", "q4"]:
    ch.queue_bind(exchange="e", queue=q, routing_key="2")

n = 100000

for rk in list(map(lambda s: str(s), range(0, n))):
    ch.basic_publish(exchange="e", routing_key=rk, body="")
print("Done publishing.")

print("Waiting for routing to finish...")
# in order to keep this example simpler and focused,
# wait for a few seconds instead of using publisher confirms and waiting for those
time.sleep(5)

print("Done.")
conn.close()

Code Example in Java

Below is a version of the example that uses the official RabbitMQ Java client:

package com.rabbitmq.examples;

import com.rabbitmq.client.*;

import java.io.IOException;
import java.util.Arrays;
import java.util.concurrent.TimeoutException;

public class ConsistentHashExchangeExample1 {
  private static String CONSISTENT_HASH_EXCHANGE_TYPE = "x-consistent-hash";

  public static void main(String[] argv) throws IOException, TimeoutException, InterruptedException {
    ConnectionFactory cf = new ConnectionFactory();
    Connection conn = cf.newConnection();
    Channel ch = conn.createChannel();

    for (String q : Arrays.asList("q1", "q2", "q3", "q4")) {
      ch.queueDeclare(q, true, false, false, null);
      ch.queuePurge(q);
    }

    ch.exchangeDeclare("e1", CONSISTENT_HASH_EXCHANGE_TYPE, true, false, null);

    for (String q : Arrays.asList("q1", "q2")) {
      ch.queueBind(q, "e1", "1");
    }

    for (String q : Arrays.asList("q3", "q4")) {
      ch.queueBind(q, "e1", "2");
    }

    ch.confirmSelect();

    AMQP.BasicProperties.Builder bldr = new AMQP.BasicProperties.Builder();
    for (int i = 0; i < 100000; i++) {
      ch.basicPublish("e1", String.valueOf(i), bldr.build(), "".getBytes("UTF-8"));
    }

    ch.waitForConfirmsOrDie(10000);

    System.out.println("Done publishing!");
    System.out.println("Evaluating results...");
    // wait for one stats emission interval so that queue counters
    // are up-to-date in the management UI
    Thread.sleep(5);

    System.out.println("Done.");
    conn.close();
  }
}

Code Example in Ruby

Below is a version that uses Bunny, the most widely used Ruby client for RabbitMQ:

#!/usr/bin/env ruby

require 'bunny'

conn = Bunny.new
conn.start

ch = conn.create_channel
ch.confirm_select

q1 = ch.queue("q1", durable: true)
q2 = ch.queue("q2", durable: true)
q3 = ch.queue("q3", durable: true)
q4 = ch.queue("q4", durable: true)

[q1, q2, q3, q4]. each(&:purge)

x  = ch.exchange("chx", type: "x-consistent-hash", durable: true)

[q1, q2].each { |q| q.bind(x, routing_key: "1") }
[q3, q4].each { |q| q.bind(x, routing_key: "2") }

n = 100_000
n.times do |i|
  x.publish(i.to_s, routing_key: i.to_s)
end

ch.wait_for_confirms
puts "Done publishing!"

# wait for queue stats to be emitted so that management UI numbers
# are up-to-date
sleep 5
conn.close
puts "Done"

Code Example in Erlang

Below is a version of the example that uses the RabbitMQ Erlang client:

-include_lib("amqp_client/include/amqp_client.hrl").

test() ->
    {ok, Conn} = amqp_connection:start(#amqp_params_network{}),
    {ok, Chan} = amqp_connection:open_channel(Conn),
    Queues = [<<"q0">>, <<"q1">>, <<"q2">>, <<"q3">>],
    amqp_channel:call(Chan,
                  #'exchange.declare'{
                    exchange = <<"e">>, type = <<"x-consistent-hash">>
                  }),
    [amqp_channel:call(Chan, #'queue.declare'{queue = Q}) || Q <- Queues],
    [amqp_channel:call(Chan, #'queue.bind'{queue = Q,
                                           exchange = <<"e">>,
                                           routing_key = <<"1">>})
        || Q <- [<<"q0">>, <<"q1">>]],
    [amqp_channel:call(Chan, #'queue.bind' {queue = Q,
                                            exchange = <<"e">>,
                                            routing_key = <<"2">>})
        || Q <- [<<"q2">>, <<"q3">>]],
    RK = list_to_binary(integer_to_list(random:uniform(1000000))),
    Msg = #amqp_msg{props = #'P_basic'{}, payload = <<>>},
    [amqp_channel:call(Chan,
                   #'basic.publish'{
                     exchange = <<"e">>,
                     routing_key = RK
                   }, Msg) || _ <- lists:seq(1, 100000)],
amqp_connection:close(Conn),
ok.

Configuration

Routing on a Header

Under most circumstances the routing key is a good choice for something to hash. However, in some cases it is necessary to use the routing key for some other purpose (for example with more complex routing involving exchange to exchange bindings). In this case it is possible to configure the consistent hash exchange to route based on a named header instead. To do this, declare the exchange with a string argument called "hash-header" naming the header to be used.

When a "hash-header" is specified, the chosen header must be provided. If published messages do not contain the header, they will all get routed to the same arbitrarily chosen queue.

Code Example in Python

#!/usr/bin/env python

import pika
import time

conn = pika.BlockingConnection(pika.ConnectionParameters(host='localhost'))
ch   = conn.channel()

args = {u'hash-header': u'hash-on'}
ch.exchange_declare(exchange='e2',
                    exchange_type='x-consistent-hash',
                    arguments=args,
                    durable=True)

for q in ['q1', 'q2', 'q3', 'q4']:
    ch.queue_declare(queue=q, durable=True)
    ch.queue_purge(queue=q)

for q in ['q1', 'q2']:
    ch.queue_bind(exchange='e2', queue=q, routing_key='1')

for q in ['q3', 'q4']:
    ch.queue_bind(exchange='e2', queue=q, routing_key='2')

n = 100000

for rk in list(map(lambda s: str(s), range(0, n))):
    hdrs = {u'hash-on': rk}
    ch.basic_publish(exchange='e2',
                     routing_key='',
                     body='',
                     properties=pika.BasicProperties(content_type='text/plain',
                                                     delivery_mode=2,
                                                     headers=hdrs))
print('Done publishing.')

print('Waiting for routing to finish...')
# in order to keep this example simpler and focused,
# wait for a few seconds instead of using publisher confirms and waiting for those
time.sleep(5)

print('Done.')
conn.close()

Code Example in Java

package com.rabbitmq.examples;

import com.rabbitmq.client.*;

import java.io.IOException;
import java.util.Arrays;
import java.util.HashMap;
import java.util.Map;
import java.util.concurrent.TimeoutException;

public class ConsistentHashExchangeExample2 {
  public static final String EXCHANGE = "e2";
  private static String EXCHANGE_TYPE = "x-consistent-hash";

  public static void main(String[] argv) throws IOException, TimeoutException, InterruptedException {
    ConnectionFactory cf = new ConnectionFactory();
    Connection conn = cf.newConnection();
    Channel ch = conn.createChannel();

    for (String q : Arrays.asList("q1", "q2", "q3", "q4")) {
      ch.queueDeclare(q, true, false, false, null);
      ch.queuePurge(q);
    }

    Map<String, Object> args = new HashMap<>();
    args.put("hash-header", "hash-on");
    ch.exchangeDeclare(EXCHANGE, EXCHANGE_TYPE, true, false, args);

    for (String q : Arrays.asList("q1", "q2")) {
      ch.queueBind(q, EXCHANGE, "1");
    }

    for (String q : Arrays.asList("q3", "q4")) {
      ch.queueBind(q, EXCHANGE, "2");
    }

    ch.confirmSelect();


    for (int i = 0; i < 100000; i++) {
      AMQP.BasicProperties.Builder bldr = new AMQP.BasicProperties.Builder();
      Map<String, Object> hdrs = new HashMap<>();
      hdrs.put("hash-on", String.valueOf(i));
      ch.basicPublish(EXCHANGE, "", bldr.headers(hdrs).build(), "".getBytes("UTF-8"));
    }

    ch.waitForConfirmsOrDie(10000);

    System.out.println("Done publishing!");
    System.out.println("Evaluating results...");
    // wait for one stats emission interval so that queue counters
    // are up-to-date in the management UI
    Thread.sleep(5);

    System.out.println("Done.");
    conn.close();
  }
}

Code Example in Ruby

#!/usr/bin/env ruby

require 'bundler'
Bundler.setup(:default, :test)
require 'bunny'

conn = Bunny.new
conn.start

ch = conn.create_channel
ch.confirm_select

q1 = ch.queue("q1", durable: true)
q2 = ch.queue("q2", durable: true)
q3 = ch.queue("q3", durable: true)
q4 = ch.queue("q4", durable: true)

[q1, q2, q3, q4]. each(&:purge)

x  = ch.exchange("x2", type: "x-consistent-hash", durable: true, arguments: {"hash-header" => "hash-on"})

[q1, q2].each { |q| q.bind(x, routing_key: "1") }
[q3, q4].each { |q| q.bind(x, routing_key: "2") }

n = 100_000
(0..n).map(&:to_s).each do |i|
  x.publish(i.to_s, routing_key: rand.to_s, headers: {"hash-on": i})
end

ch.wait_for_confirms
puts "Done publishing!"

# wait for queue stats to be emitted so that management UI numbers
# are up-to-date
sleep 5
conn.close
puts "Done"

Code Example in Erlang

With RabbitMQ Erlang client:

-include_lib("amqp_client/include/amqp_client.hrl").

test() ->
    {ok, Conn} = amqp_connection:start(#amqp_params_network{}),
    {ok, Chan} = amqp_connection:open_channel(Conn),
    Queues = [<<"q0">>, <<"q1">>, <<"q2">>, <<"q3">>],
    amqp_channel:call(
      Chan, #'exchange.declare'{
              exchange  = <<"e">>,
              type      = <<"x-consistent-hash">>,
              arguments = [{<<"hash-header">>, longstr, <<"hash-on">>}]
            }),
    [amqp_channel:call(Chan, #'queue.declare'{queue = Q}) || Q <- Queues],
    [amqp_channel:call(Chan, #'queue.bind' {queue = Q,
                                            exchange = <<"e">>,
                                            routing_key = <<"1">>})
        || Q <- [<<"q0">>, <<"q1">>]],
    [amqp_channel:call(Chan, #'queue.bind' {queue = Q,
                                            exchange = <<"e">>,
                                            routing_key = <<"2">>})
        || Q <- [<<"q2">>, <<"q3">>]],
    RK = list_to_binary(integer_to_list(random:uniform(1000000))),
    Msg = #amqp_msg {props = #'P_basic'{headers = [{<<"hash-on">>, longstr, RK}]}, payload = <<>>},
    [amqp_channel:call(Chan,
                   #'basic.publish'{
                     exchange = <<"e">>,
                     routing_key = <<"">>,
                   }, Msg) || _ <- lists:seq(1, 100000)],
amqp_connection:close(Conn),
ok.

Routing on a Message Property

In addition to a value in the header property, you can also route on the message_id, correlation_id, or timestamp message properties. To do so, declare the exchange with a string argument called "hash-property" naming the property to be used.

When a "hash-property" is specified, the chosen property must be provided. If published messages do not contain the property, they will all get routed to the same arbitrarily chosen queue.

Code Example in Python

#!/usr/bin/env python

import pika
import time

conn = pika.BlockingConnection(pika.ConnectionParameters(host='localhost'))
ch   = conn.channel()

args = {u'hash-property': u'message_id'}
ch.exchange_declare(exchange='e3',
                    exchange_type='x-consistent-hash',
                    arguments=args,
                    durable=True)

for q in ['q1', 'q2', 'q3', 'q4']:
    ch.queue_declare(queue=q, durable=True)
    ch.queue_purge(queue=q)

for q in ['q1', 'q2']:
    ch.queue_bind(exchange='e3', queue=q, routing_key='1')

for q in ['q3', 'q4']:
    ch.queue_bind(exchange='e3', queue=q, routing_key='2')

n = 100000

for rk in list(map(lambda s: str(s), range(0, n))):
    ch.basic_publish(exchange='e3',
                     routing_key='',
                     body='',
                     properties=pika.BasicProperties(content_type='text/plain',
                                                     delivery_mode=2,
                                                     message_id=rk))
print('Done publishing.')

print('Waiting for routing to finish...')
# in order to keep this example simpler and focused,
# wait for a few seconds instead of using publisher confirms and waiting for those
time.sleep(5)

print('Done.')
conn.close()

Code Example in Java

package com.rabbitmq.examples;

import com.rabbitmq.client.*;

import java.io.IOException;
import java.util.Arrays;
import java.util.HashMap;
import java.util.Map;
import java.util.concurrent.TimeoutException;

public class ConsistentHashExchangeExample3 {
  public static final String EXCHANGE = "e3";
  private static String EXCHANGE_TYPE = "x-consistent-hash";

  public static void main(String[] argv) throws IOException, TimeoutException, InterruptedException {
    ConnectionFactory cf = new ConnectionFactory();
    Connection conn = cf.newConnection();
    Channel ch = conn.createChannel();

    for (String q : Arrays.asList("q1", "q2", "q3", "q4")) {
      ch.queueDeclare(q, true, false, false, null);
      ch.queuePurge(q);
    }

    Map<String, Object> args = new HashMap<>();
    args.put("hash-property", "message_id");
    ch.exchangeDeclare(EXCHANGE, EXCHANGE_TYPE, true, false, args);

    for (String q : Arrays.asList("q1", "q2")) {
      ch.queueBind(q, EXCHANGE, "1");
    }

    for (String q : Arrays.asList("q3", "q4")) {
      ch.queueBind(q, EXCHANGE, "2");
    }

    ch.confirmSelect();


    for (int i = 0; i < 100000; i++) {
      AMQP.BasicProperties.Builder bldr = new AMQP.BasicProperties.Builder();
      ch.basicPublish(EXCHANGE, "", bldr.messageId(String.valueOf(i)).build(), "".getBytes("UTF-8"));
    }

    ch.waitForConfirmsOrDie(10000);

    System.out.println("Done publishing!");
    System.out.println("Evaluating results...");
    // wait for one stats emission interval so that queue counters
    // are up-to-date in the management UI
    Thread.sleep(5);

    System.out.println("Done.");
    conn.close();
  }
}

Code Example in Ruby

#!/usr/bin/env ruby

require 'bundler'
Bundler.setup(:default, :test)
require 'bunny'

conn = Bunny.new
conn.start

ch = conn.create_channel
ch.confirm_select

q1 = ch.queue("q1", durable: true)
q2 = ch.queue("q2", durable: true)
q3 = ch.queue("q3", durable: true)
q4 = ch.queue("q4", durable: true)

[q1, q2, q3, q4].each(&:purge)

x  = ch.exchange("x3", type: "x-consistent-hash", durable: true, arguments: {"hash-property" => "message_id"})

[q1, q2].each { |q| q.bind(x, routing_key: "1") }
[q3, q4].each { |q| q.bind(x, routing_key: "2") }

n = 100_000
(0..n).map(&:to_s).each do |i|
  x.publish(i.to_s, routing_key: rand.to_s, message_id: i)
end

ch.wait_for_confirms
puts "Done publishing!"

# wait for queue stats to be emitted so that management UI numbers
# are up-to-date
sleep 5
conn.close
puts "Done"

Code Example in Erlang

-include_lib("amqp_client/include/amqp_client.hrl").

test() ->
    {ok, Conn} = amqp_connection:start(#amqp_params_network{}),
    {ok, Chan} = amqp_connection:open_channel(Conn),
    Queues = [<<"q0">>, <<"q1">>, <<"q2">>, <<"q3">>],
    amqp_channel:call(Chan,
                  #'exchange.declare'{
                    exchange = <<"e">>, type = <<"x-consistent-hash">>,
                    arguments = {<<"hash-property">>, longstr, <<"message_id">>}                    
                  }),
    [amqp_channel:call(Chan, #'queue.declare'{queue = Q}) || Q <- Queues],
    [amqp_channel:call(Chan, #'queue.bind'{queue = Q,
                                           exchange = <<"e">>,
                                           routing_key = <<"1">>})
        || Q <- [<<"q0">>, <<"q1">>]],
    [amqp_channel:call(Chan, #'queue.bind' {queue = Q,
                                            exchange = <<"e">>,
                                            routing_key = <<"2">>})
        || Q <- [<<"q2">>, <<"q3">>]],
    RK = list_to_binary(integer_to_list(random:uniform(1000000)),
    Msg = #amqp_msg{props = #'P_basic'{message_id = RK}, payload = <<>>},
    [amqp_channel:call(Chan,
                   #'basic.publish'{
                     exchange = <<"e">>,
                     routing_key = <<"">>,
                     )
                   }, Msg) || _ <- lists:seq(1, 100000)],
amqp_connection:close(Conn),
ok.

Getting Help

If you have questions or need help, feel free to ask on the RabbitMQ mailing list.

Implementation Details

The hash function used in this plugin as of RabbitMQ 3.7.8 is A Fast, Minimal Memory, Consistent Hash Algorithm by Lamping and Veach. Erlang's phash2 function is used to convert non-integer values to an integer one that can be used by the jump consistent hash function by Lamping and Veach.

Distribution Uniformity

A Chi-squared test was used to evaluate distribution uniformity. Below are the results for 18 bucket counts and how they compare to two commonly used p-value thresholds:

Number of buckets Chi-squared test result Degrees of freedom p-value = 0.05 p-value = 0.01
2 0.5 1 3.84 6.64
3 0.946 2 5.99 9.21
4 2.939 3 7.81 11.35
5 2.163 4 3.49 13.28
6 2.592 5 11.07 15.09
7 4.654 6 12.59 16.81
8 7.566 7 14.07 18.48
9 5.847 8 15.51 20.09
10 9.790 9 16.92 21.67
11 13.448 10 18.31 23.21
12 12.432 11 19.68 24.73
13 12.338 12 21.02 26.22
14 9.898 13 22.36 27.69
15 8.513 14 23.69 29.14
16 6.997 15 24.99 30.58
17 6.279 16 26.30 32.00
18 10.373 17 28.87 34.81
19 12.935 18 30.14 36.19
20 11.895 19 31.41 37.57

Binding Operations and Bucket Management

When a queue is bound to a consistent hash exchange, the protocol method, queue.bind, carries a weight in the routing (binding) key. The binding is given a number of buckets on the hash ring (hash space) equal to the weight. When a queue is unbound, the buckets added for the binding are deleted. These two operations use linear algorithms to update the ring.

To perform routing the exchange extract the appropriate value for hashing, hashes it and retrieves a bucket number from the ring, then the bucket and its associated queue.

The implementation assumes there is only one binding between a consistent hash exchange and a queue. Having more than one binding is unnecessary because queue weight can be provided at the time of binding.

Clustered Environments

The state of the hash space is distributed across all cluster nodes.

Continuous Integration

Build Status

Copyright and License

(c) 2013-2018 Pivotal Software Inc.

Released under the Mozilla Public License 1.1, same as RabbitMQ. See LICENSE for details.

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