oslo.messaging/doc/source/zmq_driver.rst

ZeroMQ Driver Deployment Guide

oslo_messaging

Introduction

0MQ (also known as ZeroMQ or zmq) is embeddable networking library but acts like a concurrency framework. It gives you sockets that carry atomic messages across various transports like in-process, inter-process, TCP, and multicast. You can connect sockets N-to-N with patterns like fan-out, pub-sub, task distribution, and request-reply. It's fast enough to be the fabric for clustered products. Its asynchronous I/O model gives you scalable multi-core applications, built as asynchronous message-processing tasks. It has a score of language APIs and runs on most operating systems.

Originally the zero in 0MQ was meant as "zero broker" and (as close to) "zero latency" (as possible). Since then, it has come to encompass different goals: zero administration, zero cost, and zero waste. More generally, "zero" refers to the culture of minimalism that permeates the project.

More detail regarding ZeroMQ library is available from the specification.

Abstract

Currently, ZeroMQ is one of the RPC backend drivers in oslo.messaging. ZeroMQ can be the only RPC driver across the OpenStack cluster. This document provides deployment information for this driver in oslo_messaging.

Other than AMQP-based drivers, like RabbitMQ, ZeroMQ doesn't have any central brokers in oslo.messaging, instead, each host (running OpenStack services) is both ZeroMQ client and server. As a result, each host needs to listen to a certain TCP port for incoming connections and directly connect to other hosts simultaneously.

Another option is to use a router proxy. It is not a broker because it doesn't assume any message ownership or persistence or replication etc. It performs only a redirection of messages to endpoints taking routing info from message envelope.

Topics are used to identify the destination for a ZeroMQ RPC call. There are two types of topics, bare topics and directed topics. Bare topics look like 'compute', while directed topics look like 'compute.machine1'.

Scenario

Assuming the following systems as a goal.

+--------+
| Client |
+----+---+
     |
-----+---------+-----------------------+---------------------
               |                       |
      +--------+------------+  +-------+----------------+
      | Controller Node     |  | Compute Node           |
      |  Nova               |  |  Neutron               |
      |  Keystone           |  |  Nova                  |
      |  Glance             |  |   nova-compute         |
      |  Neutron            |  |  Ceilometer            |
      |  Cinder             |  |                        |
      |  Ceilometer         |  +------------------------+
      |  zmq-proxy          |
      |  Redis              |
      |  Horizon            |
      +---------------------+

Configuration

Enabling (mandatory)

To enable the driver the 'transport_url' option must be set to 'zmq://' in the section [DEFAULT] of the conf file, the 'rpc_zmq_host' flag must be set to the hostname of the current node. :

[DEFAULT]
transport_url = "zmq://"

[oslo_messaging_zmq]
rpc_zmq_host = {hostname}

Match Making (mandatory)

The ZeroMQ driver implements a matching capability to discover hosts available for communication when sending to a bare topic. This allows broker-less communications.

The MatchMaker is pluggable and it provides two different MatchMaker classes.

DummyMatchMaker: default matchmaker driver for all-in-one scenario (messages are sent to itself).

RedisMatchMaker: loads the hash table from a remote Redis server, supports dynamic host/topic registrations, host expiration, and hooks for consuming applications to acknowledge or neg-acknowledge topic.host service availability.

For ZeroMQ driver Redis is configured in transport_url also. For using Redis specify the URL as follows:

[DEFAULT]
transport_url = "zmq+redis://127.0.0.1:6379"

In order to cleanup redis storage from expired records (e.g. target listener goes down) TTL may be applied for keys. Configure 'zmq_target_expire' option which is 120 (seconds) by default. The option is related not specifically to redis so it is also defined in [oslo_messaging_zmq] section. If option value is <= 0 then keys don't expire and live forever in the storage.

MatchMaker Data Source (mandatory)

MatchMaker data source is stored in files or Redis server discussed in the previous section. How to make up the database is the key issue for making ZeroMQ driver work.

If deploying the RedisMatchMaker, a Redis server is required. Each (K, V) pair stored in Redis is that the key is a base topic and the corresponding values are hostname arrays to be sent to.

HA for Redis database

Single node Redis works fine for testing, but for production there is some availability guarantees wanted. Without Redis database zmq deployment should continue working anyway, because there is no need in Redis for services when connections established already. But if you would like to restart some services or run more workers or add more hardware nodes to the deployment you will need nodes discovery mechanism to work and it requires Redis.

To provide database recovery in situations when redis node goes down for example, we use Sentinel solution and redis master-slave-slave configuration (if we have 3 controllers and run Redis on each of them).

To deploy redis with HA follow the sentinel-install instructions. From the messaging driver's side you will need to setup following configuration :

[DEFAULT]
transport_url = "zmq+redis://host1:26379,host2:26379,host3:26379"

Restrict the number of TCP sockets on controller

The most heavily used RPC pattern (CALL) may consume too many TCP sockets on controller node in directly connected configuration. To solve the issue ROUTER proxy may be used.

In order to configure driver to use ROUTER proxy set up the 'use_router_proxy' option to true in [oslo_messaging_zmq] section (false is set by default).

For example:

use_router_proxy = true

Not less than 3 proxies should be running on controllers or on stand alone nodes. The parameters for the script oslo-messaging-zmq-proxy should be:

oslo-messaging-zmq-proxy
    --config-file /etc/oslo/zeromq.conf
    --log-file /var/log/oslo/zmq-router-proxy.log

Fanout-based patterns like CAST+Fanout and notifications always use proxy as they act over PUB/SUB, 'use_pub_sub' option defaults to true. In such case publisher proxy should be running. Actually proxy does both: routing to a DEALER endpoint for direct messages and publishing to all subscribers over zmq.PUB socket.

If not using PUB/SUB (use_pub_sub = false) then fanout will be emulated over direct DEALER/ROUTER unicast which is possible but less efficient and therefore is not recommended. In a case of direct DEALER/ROUTER unicast proxy is not needed.

This option can be set in [oslo_messaging_zmq] section.

For example:

use_pub_sub = true

In case of using a proxy all publishers (clients) talk to servers over the proxy connecting to it via TCP.

You can specify ZeroMQ options in /etc/oslo/zeromq.conf if necessary.

Listening Address (optional)

All services bind to an IP address or Ethernet adapter. By default, all services bind to '*', effectively binding to 0.0.0.0. This may be changed with the option 'rpc_zmq_bind_address' which accepts a wildcard, IP address, or Ethernet adapter.

This configuration can be set in [oslo_messaging_zmq] section.

For example:

rpc_zmq_bind_address = *

Currently zmq driver uses dynamic port binding mechanism, which means that each listener will allocate port of a random number. Ports range is controlled by two options 'rpc_zmq_min_port' and 'rpc_zmq_max_port'. Change them to restrict current service's port binding range. 'rpc_zmq_bind_port_retries' controls number of retries before 'ports range exceeded' failure.

For example:

rpc_zmq_min_port = 9050
rpc_zmq_max_port = 10050
rpc_zmq_bind_port_retries = 100

DevStack Support

ZeroMQ driver has been supported by DevStack. The configuration is as follows:

ENABLED_SERVICES+=,-rabbit,zeromq
ZEROMQ_MATCHMAKER=redis

In local.conf [localrc] section need to enable zmq plugin which lives in devstack-plugin-zmq repository.

For example:

enable_plugin zmq https://github.com/openstack/devstack-plugin-zmq.git

Example of local.conf:

[[local|localrc]]
DATABASE_PASSWORD=password
ADMIN_PASSWORD=password
SERVICE_PASSWORD=password
SERVICE_TOKEN=password

enable_plugin zmq https://github.com/openstack/devstack-plugin-zmq.git

OSLOMSG_REPO=https://review.openstack.org/openstack/oslo.messaging
OSLOMSG_BRANCH=master

ZEROMQ_MATCHMAKER=redis
LIBS_FROM_GIT=oslo.messaging
ENABLE_DEBUG_LOG_LEVEL=True