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A reliability layer on top of fedmsg

Project Description

What is “Reliability”?

Every few months, a discussion comes up about fedmsg reliability. Typically, a team somewhere in the Fedora Project will be looking into how they can streamline their workflows by either using data from or sending data to the bus. We end up having a lengthy discussion in IRC until the matter gets settled. But that conversation gets lost in the ether of freenode and the next time a different team has the same questions we have to have the conversation over again.

fedmsg is a set of python tools (one part library, one part framework) that we use around Fedora Infrastructure to enable system processes to publish and listen for messages. We typically call it our “message bus”, but that is bad nomenclature; it doesn’t accurately describe what is going on. When a process wants to publish fedmsg messages, it invokes fedmsg.publish(...) which reads in some configuration from disk, binds a socket to a port if it hasn’t already, and writes the message there. When another process wants to consume fedmsg messages, it invokes fedmsg.tail_messages() (or registers itself with an already listening fedmsg-hub daemon) which then connects a socket to the bound port of the other process to recv the message(s).

A little more detail now: the call to fedmsg.publish doesn’t manage the socket binding and sending itself. It hands things off to zeromq which in turn hands things off to a number of worker threads it manages. Zeromq keeps an internal queue of messages it has been asked to send, which it sends as fast as it can. A key takeaway here is that it is “fire-and-forget”: a web application that has been enabled to publish fedmsg messages will ask the fedmsg library to do so, and then walk away to finish its database transactions or do whatever other work it is intended to do. fedmsg (really zeromq here) opaquely sends the message as soon as it has a spare moment.

(As an aside, the process involves even more than that. Outgoing messages are signed using cryptographic certificates. Python data types are serialized. Incoming messages are validated and deserialized and checked against an authorization policy about who is allowed to sign what messages, etc.. We’ll come back to this.)

Let’s quote the zguide:

Most people who speak of "reliability" don't really know what they mean. We
can only define reliability in terms of failure. That is, if we can handle
a certain set of well-defined and understood failures, then we are reliable
with respect to those failures. No more, no less. So let's look at the
possible causes of failure in a distributed ZeroMQ application

In practice, we do not “drop messages” in Fedora Infrastructure. We have run a number of experiments that demonstrate this to a degree sufficient to establish confidence with a number of other teams.

In theory, though, there’s a race condition here. You could start your publishing service and it could publish a message before anyone connects to listen for it. It would be lost. Again, in practice we have appropriate delays and reconnect intervals (with exponential backoff) configured to make this a non-issue.

Still, there could be network partitions – a listening service may suddenly find itself walled off from a publishing service by dead routers or a misconfigured firewall or a dead vpn or catastrophe, in all of such cases: messages will be lost.

Designing Reliability

The zguide provides a good description of how to think about and approach reliability for REQ-REP socket patterns. However, we’ve settled over the last few years on using only the PUB-SUB socket pattern for fedmsg. Adding another pattern (REQ-REP) alongside that smells wrong (it’s currently so simple).

The approach here in gilmsg is to layer a reliability check on top of the existing PUB-SUB fedmsg framework.

Here’s how it works broadly:

  • When gilmsg.publish(...) is invoked, you must declare a list of required recipients.
  • A background thread is started that listens for ACK messages on the whole bus.
  • If an ACK is not received from all recipients within a given timeout, then a Timeout exception is raised.

Failure cases this addresses:

  • If an intended recipient is offline when the message is published, we’ll raise an exception so that the originating process can fail and alert the operator.
  • If an intended recipient is online, but we’re suffering from a network partition, the publisher will fail loudly.

It is possible for us to have False Negatives:

  • If the intended recipient is online, but cannot publish the ACK back due to a one-way network failure or a misconfiguration, the publisher will fail loudly even though the consumer did receive the message.

It is not possible for us to have False Positives. If the producer completes execution, we can be sure the consumer received the message.

Some Details

  • We are sure that the ACKs comes from the recipients we declared because the ACKs are cryptographically signed.
  • We take care to distinguish between ACKs for the message we published and ACKs for other messages we know nothing about.
  • We start the ACK-listening socket on the producer in a thread before we publish the original message. This is necessary to avoid a race condition where we publish our original message but the ACK comes back before we get a chance to start listening for it.

Why not use gilmsg?

fedmsg has worked very well. We have a long list of integrated message producers in Fedora Infrastructure. The success is due in part to just how dumb fedmsg is. It has contributed to the rise of a loosely coupled architecture, which was the original aim. When some Fedora hacker gets an idea, then can hook onto the bus to consume messages without having to negotiate with anyone about it. They can produce messages without having to go to committee.

As an example: you can stand up Bodhi2 web app on your local box, and it will “publish to fedmsg” without you having to start any additional services or anything like that. It will publish to fedmsg on your laptop, no one will be listening, and it won’t care.

In contrast, with gilmsg-enabled services:

  • Your producers will declare their required consumers.
  • As you add more consumers, you’re going to have to patch your producers leading to an exponential number of code changes to get new things done.
  • You cannot run or test a gilmsg-enabled service on your own box without a replica of the whole production environment, since it will require that other services respond to it.
  • Your ensemble of production services becomes brittle – prone to one dead service bringing the others down.

gilmsg-enabled services are tightly coupled.

A cosmetic note:

  • The more gilmsg-enabled producer/consumer pairs you bring on line, the more spam you’ll have on the bus. It can handle the throughput(!) but it will make the datagrepper logs less readable as you add more.

Using gilmsg

It is API backwards compatible with fedmsg core. So, write your script to use fedmsg first. If at some point in the future you decide that you must have the set of guarantees that gilmsg provides, then port to gilmsg.

Publishing with .publish(..) from Python:

import gilmsg

import fedmsg.config
config = fedmsg.config.load_config()

    ack_timeout=0.25,  # 0.25 seconds

Publishing from the shell with gilmsg-logger with a timeout of 3 seconds:

echo testing | gilmsg-logger --recipients --ack-timeout 3

Compare the above with publishing with fedmsg alone.

Consuming with .tail_messages(..) in Python:

import gilmsg

import fedmsg.config
config = fedmsg.config.load_config()

target = ""
for name, ep, t, msg in gilmsg.tail_messages(topic=target, **config):
    # The ACK has already been sent at this point.
    print "Received", t, msg['msg_id']

Consuming with the “Hub-Consumer” approach:

import gilmsg

class MyConsumer(gilmsg.GilmsgConsumer):
    topic = ""

    def consume(self, message):
        # The ACK has already been sent at this point.
        print "Received", message['topic'], message['msg_id']

Compare the above with consuming with fedmsg alone.

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