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Proposal - Fedora Messaging with 0mq (fedmsg)

This proposal reviews existing services in the Fedora infrastructure, reviews the problem of complexity in the interaction of those services, reviews previous work by the Fedora Messaging SIG (special interest group) on AMQP, and introduces an architecture-level description of a solution using 0mq.


For discussion, check https://admin.fedoraproject.org/mailman/listinfo/messaging-sig

Get (or modify) the source for this document: http://github.com/ralphbean/fedmsg

Authors:
  • Ralph Bean (threebean)

Note

This document is out of date and should just be considered a "historical" document. What you're probably looking for is the :doc:`overview` document: the start of the narrative documentation. It is the most-up-to-date version of this page.

tl;dr

We want to hook all the services in Fedora Infrastructure up to send messages to one another over a message bus instead of communicating with each other in heterogenous ways they do now.

We're writing a python library called fedmsg to help apps handle this more easily. It's built on 0mq and moksha.

Planned Stages of development/deployment

  1. Start writing fedmsg
  2. Send messages from existing services (koji, bodhi, pkgdb, fas, etc...).
  3. Consume messages for statistics, i.e. an independent statistics webapp.
  4. Consume messages for user experience, i.e. any or all of rss, email, gnome-shell notifications, javascript notifications in FI webapps.
  5. Consume messages for service interoperability, i.e. koji invalidates it's cache when it sees pkgdb messages go by on the bus. This comes last because we want to make sure that message-sending works and is reliable before we start making existing services depend on it for their functioning.

Introduction

Description of the problem

Fedora Infrastructure is composed of a number of services (koji, fedpkg, pkgdb, etc..) some of which are maintained outside the Fedora Project and some of which were built in-house by the infrastructure team. These are strung together in a pipeline. Think "how an upstream release becomes a package update", "How a new source distribution becomes a package."

At present, many of the steps in this process require the maintainer to wait and watch for a previous step to complete. For instance once a branch of a package is successfully built in koji, the maintainer must submit their update to bodhi (See the new package process for more details).

Other progressions in the pipeline are automated. For instance, AutoQA defines a set of watchers. Most watchers are run as a cron task. Each one looks for certain events and fires off tests when appropriate.

At LinuxFest Northwest (2009), jkeating gave a talk on the problem of complexity in the Fedora infrastructure and how this might be addressed with a message bus architecture. Each service in the infrastructure depends on many of the others. Some pieces directly poke others: git (fedpkg) currently pokes AutoQA from a post-update hook. Other pieces poll others' status: koji scrapes pkgdb for package-owner relationships and email aliases.

This dense coupling of services makes changing, adding, or replacing services more complicated: commits to one project require a spidering of code changes to all the others.

How messaging might address the problem

jkeating's talk on messaging in the Fedora Instructure proposed the adoption of a unified message bus to reduce the complexity of multiple interdependent services. Instead of a service interfacing with its dependencies' implementations, it could subscribe to a topic, provide a callback, and respond to events.

For instance, instead of having koji scrape pkgdb on an interval for changed email addresses, pkgdb could emit messages to the org.fedoraproject.service.pkgdb topic whenever an account's email address changes. koji could subscribe to the same topic and provide a callback that updates its local email aliases when invoked.

In another case, the git (fedpkg) post-update hook could publish messages to the org.fedoraproject.service.fedpkg.post-update topic. AutoQA could subscribe to the same. Now if we wanted to enable another service to act when updates are pushed to fedpkg, that service need only subscribe to the topic and implement its own callback instead of appending its own call to fedpkg's post-update hook (instead of coupling its own implementation with fedpkg's).

A message bus architecture, once complete, would dramatically reduce the work required to update and maintain services in the Fedora infrastructure.

Other benefits

By adopting a messaging strategy for Fedora Infrastructure we could gain:

  • A stream of data which we can watch and from which we can garner statistics about infrastructure activity.
  • The de-coupling of services from one another.
  • libnotify notifications to developers' desktops.
  • jquery.gritter.js notifications to web interfaces.
    • this could be generalized to a fedmsg.wsgi middleware layer that injects a fedora messaging dashboard header into every page served by apps X, Y, and Z.
  • An irc channel, #fedora-firehose that echoes every message on the bus.
  • An identi.ca account, @fedora-firehose, that echoes every message on the bus.

AMQP, and 0mq

AMQP or "Broker? Damn near killed 'er!"

When discussions on the Fedora Messaging SIG began, AMQP was the choice by default. Since then members of the SIG have become attracted to an alternative messaging interface called 0mq.

Recommended reading:

The following is recreated from J5's Publish/Subscribe Messaging Proposal as an example of how Fedora Infrastructure could be reorganized with AMQP and a set of federated AMQP brokers (qpid).

https://github.com/ralphbean/fedmsg/raw/develop/doc/_static/reorganize-amqp-j5.png

The gist is that each service in the Fedora Infrastructure would have the address of a central message broker on hand. On startup, each service would connect to that broker, ask the broker to establish its outgoing queues, and begin publishing messages. Similarly, each service would ask the broker to establish incoming queues for them. The broker would handle the routing of messages based on routing_keys (otherwise known as topics) from each service to the others.

The downshot, in short, is that AMQP requires standing up a single central broker and thus a single-point-of-failure. In the author's work on narcissus I found that for even the most simple of AMQP configurations, my qpid brokers' queues would bloat over time until *pop*, the broker would fall over.

0mq or "Going for Broke(rless)"

0mq is developed by a team that had a hand in the original development of AMQP. It claims to be a number of things: an "intelligent transport layer", a "socket library that acts as a concurrency framework", and the sine qua non "Extra Spicy Sockets!"

Recommended reading:

The following depicts an overview of a subset of Fedora Infrastructure organized with a decentralized 0mq bus parallel to the spirit of J5's recreated diagram in the AMQP section above.

https://github.com/ralphbean/fedmsg/raw/develop/doc/_static/reorganize-0mq-overview.png

No broker. The gist is that each service will open a port and begin publishing messages ("bind to" in zmq-language). Each other service will connect to that port to begin consuming messages. Without a central broker doing all the things, 0mq can afford a high throughput. For instance, in initial tests of a 0mq-enabled moksha hub, the Fedora Engineering Team achieved a 100-fold speedup over AMQP.

Service discovery

Shortly after you begin thinking over how to enable Fedora Infrastructure to pass messages over a fabric instead of to a broker, you arrive at the problem we'll call "service discovery".

In reality, (almost) every service both produces and consumes messages. For the sake of argument, we'll talk here just about a separate producing service and some consuming services.

Scenario: the producing service starts up, producing socket (with a hidden queue), and begins producing messages. Consuming services X, Y, and Z are interested in this and they would like to connect.

With AMQP, this is simplified. You have one central broker and each consuming service need only know it's one address. They connect and the match-making is handled for them. With 0mq, each consuming service needs to somehow discover its producer(s) address(es).

There are a number of ways to address this:

  • Write our own broker; this would not be that difficult. We could (more simply) scale back the project and write our own directory lookup service that would match consumers with their providers. This could be done in surprisingly few lines of python. This issue is that we re-introduce the sticking point of AMQP, a single point of failure.
  • Use DNS; There is a helpful blog post on how to do this with djbdns. DNS is always there anyways: if DNS goes down, we have bigger things to worry about than distributing updates to our messaging topology.
  • Share a raw text file; This at first appears crude and cumbersome:
    • Maintain a list of all fedmsg-enabled producers in a text file
    • Make sure that file is accessible from every consuming service.
    • Have each consuming service read in the file and connect to every (relevant) producer in the list

In my opinion, using DNS is generally speaking the most elegant solution. However, for Fedora Infrastructure in particular, pushing updates to DNS and pushing a raw text file to every server involves much-the-same workflow: puppet. Because much of the overhead of updating the text file falls in-line with the rest of Infrastructure work, it makes more sense to go with the third option. Better not to touch DNS when we don't have to.

That file is /etc/fedmsg-config.py. It should define a python dict called config which may look something like the following in a development environment:

# TODO -- update this.  It is out of date.
config = dict(
    # This is a dict of possible addresses from which fedmsg can send
    # messages.  fedmsg.init(...) requires that a 'name' argument be passed
    # to it which corresponds with one of the keys in this dict.
    endpoints=dict(
        # For other, more 'normal' services, fedmsg will try to guess the
        # name of it's calling module to determine which endpoint definition
        # to use.  This can be overridden by explicitly providing the name in
        # the initial call to fedmsg.init(...).
        bodhi="tcp://*:3001",
        fas="tcp://*:3002",
        fedoratagger="tcp://*:3003",

        # This is the output side of the relay to which all other
        # services can listen.
        relay_outbound="tcp://*:4001",
    ),

    # This is the address of an active->passive relay.  It is used for the
    # fedmsg-logger command which requires another service with a stable
    # listening address for it to send messages to.
    relay_inbound="tcp://127.0.0.1:2003",

    # Set this to dev if you're hacking on fedmsg or an app.
    # Set to stg or prod if running in the Fedora Infrastructure
    environment="dev",

    # Default is 0
    high_water_mark=1,

    io_threads=1,
)

fedmsg will look for a config file in /etc/, $HOME, and . (the current working directory). If it finds multiple files, it will read all of them but overwrite values from the system (/etc/) file with the more local file ($HOME).

Different buses

TODO -

  • critical and statistical buses (critical is subset of statistical).

Authn, authz

TODO -

network load

TODO -

  • calculate network load -

http://lists.zeromq.org/pipermail/zeromq-dev/2010-August/005254.html

fringe services

TODO -

Namespace considerations

In the above examples, the topic names are derived from the service names. For instance, pkgdb publishes messages to org.fedoraproject.service.pkgdb*, AutoQA presumably publishes messages to org.fedoraproject.service.autoqa*, and so on.

This convention, while clear-cut, has its limitations. Say we wanted to replace pkgdb whole-sale with our shiney new threebean-db (tm). Here, all other services are subscribed to topics that mention pkgdb explicitly. Rolling out threebean-db will require patching every other service; we find ourselves in a new flavor of the same complexity/co-dependency trap described in the first section.

The above service-oriented topic namespace is one option. Consider an object-oriented topic namespace where the objects are things like users, packages, builds, updates, tests, tickets, and composes. Having bodhi subscribe to org.fedoraproject.object.tickets and org.fedoraproject.object.builds leaves us less tied down to the current implementation of the rest of the infrastructure. We could replace bugzilla with pivotal and bodhi would never know the difference - a ticket is a ticket.

That would be nice; but there are too many objects in Fedora Infrastructure that would step on each other. For instance, Koji tags packages and Tagger tags packages; these two are very different things. Koji and Tagger cannot both emit events over org.fedoraproject.package.tag.* without widespread misery.

Consequently, our namespace follows a service-oriented pattern.

The scheme

Event topics will follow the rule:

org.fedoraproject.ENV.SERVICE.OBJECT[.SUBOBJECT].EVENT

Where:

  • ENV is one of dev, stg, or production.
  • SERVICE is something like koji, bodhi, or fedoratagger
  • OBJECT is something like package, user, or tag
  • SUBOBJECT is something like owner or build (in the case where OBJECT is package, for instance)
  • EVENT is a verb like update, create, or complete.

All 'fields' in a topic must:

  • Be singular (Use package, not packages)
  • Use existing fields as much as possible (since complete is already used by other topics, use that instead of using finished).

Code Examples - 0mq and fedmsg

This package (the package containing the docs you are reading right now) is fedmsg. It aims to be a wrapper around calls to the moksha hub API that:

  • Handles Fedora-Infra authn/authz
  • Handles Fedora-Infra service discovery
  • Helps you avoid topic and message content typos.
  • Gets in your way as little as possible

Examples of emitting events

Here's a real dummy test:

>>> import fedmsg
>>> fedmsg.publish(topic='testing', modname='test', msg={
...     'test': "Hello World",
... })

The above snippet will send the message '{test: "Hello World"}' message over the org.fedoraproject.dev.test.testing topic. The modname argument will be omitted in most use cases. By default, fedmsg will try to guess the name of the module that called it and use that to produce an intelligent topic. Specifying modname argues that fedmsg not be too smart.

Here's an example from fedora-tagger that sends the information about a new tag over org.fedoraproject.{dev,stg,prod}.fedoratagger.tag.update:

>>> import fedmsg
>>> fedmsg.publish(topic='tag.update', msg={
...     'user': user,
...     'tag': tag,
... })

Note that the tag and user objects are SQLAlchemy objects defined by tagger. They both have .__json__() methods which .publish uses to convert both objects to stringified JSON for you.

fedmsg has also guessed the module name (modname) of it's caller and inserted it into the topic for you. The code from which we stole the above snippet lives in fedoratagger.controllers.root. fedmsg figured that out and stripped it down to just fedoratagger for the final topic of org.fedoraproject.{dev,stg,prod}.fedoratagger.tag.update.


You could also use the fedmsg-logger from a shell script like so:

$ echo "Hello, world." | fedmsg-logger --topic testing
$ echo '{"foo": "bar"}' | fedmsg-logger --json-input

Examples of consuming events

Consuming events is accomplished by way of the fedmsg-hub. For example, in the busmon app, all messages from the hub are processed to be formatted and displayed on a client's browser. We mark them up with a pretty-print format and use pygments to colorize them.

Here are the important parts: you must define a new class which extends moksha.api.hub:Consumer, declares a topic attribute and a consume method. The topic is used soley for constraining what messages make their way to the consumer; the consumer can send messages on any topic. You may use 'splats' ('*') in the topic and subscribe to 'org.fedoraproject.stg.koji.*' to get all of the messages from koji in the staging environment. In the example below, the MessageColorizer consumer simply subscribes to '*'; it will receive every message that hits it's local fedmsg-hub.

Here's the full example from busmon, it consumes messages from every topic, formats them in pretty colored HTML and then re-sends them out on a new topic:

import pygments.lexers
import pygments.formatters
from moksha.api.hub import Consumer

import fedmsg
import fedmsg.encoding

class MessageColorizer(Consumer):
    topic = "*"
    jsonify = False

    destination_topic = "colorized-messages"

    def consume(self, message):
        # Just so we don't create an infinite feedback loop.
        if self.destination_topic in message.topic:
            return

        # Format the incoming message
        code = pygments.highlight(
            fedmsg.encoding.pretty_dumps(fedmsg.encoding.loads(message.body)),
            pygments.lexers.JavascriptLexer(),
            pygments.formatters.HtmlFormatter(full=False)
        ).strip()

        # Ship it!
        fedmsg.publish(
            topic=self.destination_topic,
            msg=code,
        )

Now, just defining a consumer isn't enough to have it picked up by the fedmsg-hub when it runs. You must also declare the consumer as an entry-point in your app's setup.py, like this:

setup(
    ...
    entry_points={
        'moksha.consumer': (
            'colorizer = busmon.consumers:MessageColorizer',
        ),
    },
)

At initialization, fedmsg-hub looks for all the objects registered on the moksha.consumer entry point and loads them

Console Scripts

It makes sense for fedmsg to also provide a number of console scripts for use with random shell scripts or with nagios, for instance.

Currently we have implemented:

  • fedmsg-tail - watches all endpoints on the bus and prints each message to stdout.
  • fedmsg-logger - sends messages over the org.fedoraproject.dev.logger topic. This requires that an instance of fedmsg-relay be running somewhere and that it's inbound address be listed in fedmsg-config.py.
  • fedmsg-relay - a service which binds to two ports, listens for messages on one and emits them on the other. fedmsg-logger requires that an instance of fedmsg-relay be running somewhere and that it's inbound address be listed in fedmsg-config.py.
  • fedmsg-hub - the all-purpose daemon. This should be run on every host that has services which declare their own consumers. fedmsg-hub will listen to every endpoint defined in /etc/fedmsg-config.py and forward messages in-process to the locally-declared consumers.

Systems and Events

All messages will be transmitted as stringified JSON.

List of systems, their events, and associated fields

Each item here is a service followed by the list of events that it emits. Each event is followed by a list of services that will likely consume that event.

See also :doc:`status`.


  • AskBot

    • TODO - Brainstorm a list of potential message topics.
  • AutoQA

    • TODO - Add these hooks. j_dulaney is working on this.
      • org.fedoraproject.{stg,prod}.autoqa.package.tests.complete -> koji, bodhi, fcomm
  • Bodhi

  • Bugzilla

    • TODO - get AMQP messages from redhat. Run a service to translate.
      • org.fedoraproject.{stg,prod}.bugzilla.bug.create -> fcomm
      • org.fedoraproject.{stg,prod}.bugzilla.bug.update -> fcomm
  • Compose

    • TODO - Add the hooks
      • org.fedoraproject.{stg,prod}.compose.compose.complete -> mirrormanager, autoqa
  • Elections (TODO -- what is the app called?)

    • TODO - Add the hooks
      • org.fedoraproject.{stg,prod}.elections... <-- TODO. Objects and events?
  • FAS

    • All of these hooks have been added. TODO - merge and push to stg then prod.
      • org.fedoraproject.{stg,prod}.fas.user.create -> fcomm
      • org.fedoraproject.{stg,prod}.fas.user.update -> fcomm
      • org.fedoraproject.{stg,prod}.fas.group.update -> fcomm
      • org.fedoraproject.{stg,prod}.fas.group.member.apply -> fcomm
      • org.fedoraproject.{stg,prod}.fas.group.member.sponsor -> fcomm
      • org.fedoraproject.{stg,prod}.fas.group.member.sponsor -> fcomm
      • org.fedoraproject.{stg,prod}.fas.group.create -> fcomm
      • org.fedoraproject.{stg,prod}.fas.group.update -> fcomm
      • org.fedoraproject.{stg,prod}.fas.role.update -> fcomm
  • Koji

    • TODO - Add the hooks
      • org.fedoraproject.{stg,prod}.koji.tag.build -> secondary arch koji
      • org.fedoraproject.{stg,prod}.koji.tag.create -> secondary arch koji
      • org.fedoraproject.{stg,prod}.koji.package.build.complete -> fcomm, secondary arch koji, SCM, autoqa, sigul
      • org.fedoraproject.{stg,prod}.koji.package.build.start -> fcomm
      • org.fedoraproject.{stg,prod}.koji.package.build.fail -> fcomm
  • MeetBot (supybot?)

    • TODO - Add the hooks
      • org.fedoraproject.{stg,prod}.irc.meeting.start
      • org.fedoraproject.{stg,prod}.irc.meeting.complete
  • NetApp -- FIXME, the topics from netapp should be reviewed. They seem ambiguous.

    • TODO - Add the hooks
      • org.fedoraproject.{stg,prod}.netapp.sync.stop -> mirrormanager
      • org.fedoraproject.{stg,prod}.netapp.sync.resume -> mirrormanager
  • PkgDB

    • TODO - Add the hooks
      • org.fedoraproject.{stg,prod}.pkgdb.package.create -> koji, secondary arch koji, bugzilla
      • org.fedoraproject.{stg,prod}.pkgdb.package.remove -> koji, secondary arch koji,
      • org.fedoraproject.{stg,prod}.pkgdb.package.rename -> bugzilla
      • org.fedoraproject.{stg,prod}.pkgdb.package.retire -> SCM
      • org.fedoraproject.{stg,prod}.pkgdb.package.owner.update -> koji, secondary arch koji, bugzilla
      • TODO - lots of org.fp.user... events to detail here.
  • SCM

    • TODO - Add the hooks. This is blocking on getting an instance of fedmsg-relay stood up in production. That, on the other hand, is blocking on getting the fedmsg wrapper around moksha done so that the relay doesn't eat up 100% CPU.
      • org.fedoraproject.{stg,prod}.scm.repo.checkin -> fcomm, autoqa
  • Tagger

    • These hooks have been added. Need to push to stg then prod.
      • org.fedoraproject.{stg,prod}.fedoratagger.tag.create -> fcomm, pkgdb
      • org.fedoraproject.{stg,prod}.fedoratagger.tag.remove -> fcomm, pkgdb
      • org.fedoraproject.{stg,prod}.fedoratagger.tag.update -> fcomm, pkgdb
      • org.fedoraproject.{stg,prod}.fedoratagger.user.rank.update -> fcomm, (pkgdb?)
      • org.fedoraproject.{stg,prod}.fedoratagger.login -> ??
  • Wiki. This is implemented as a mediawiki plugin in extras/mediawiki/fedmsg-mediawiki-emit.php.

    • org.fedoraproject.{stg,prod}.wiki.article.edit
    • org.fedoraproject.{stg,prod}.wiki.upload.complete
  • Zabbix

    • TODO - Add the hooks
      • org.fedoraproject.{stg,prod}.zabbix.service.update -> fcomm

Other Ideas

  • Error messages from cron jobs
  • The Nag-once script could be enhanced to send output to the bus
  • Nagios alerts
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