Hi Mikolaj,
On 2/7/19 4:46 AM, Mikolaj Izdebski wrote:
Hello,
Today I've looked into porting Koschei from fedmsg to
fedora-messaging. Unfortunately I've encountered problems that prevent
me from progressing. Most importantly our RabbitMQ servers are not
accessible from outsides of Fedora infrastructure VPN. Moreover, all
clients must authenticate with a TLS certificate, even pure consumers
that don't intend to publish any messages. For these reasons I've
decided that upstream Koschei project will keep using fedmsg for
consuming messages. For publishing I'm still considering
fedora-messaging as authentication in this case is expected.
My main questions are:
What are the plans regarding our fedmsg deployment?
Are we planning to have it continue to run in parallel with
fedora-messaging in foreseeable future?
If not, then will at least fedora-messaging to fedmsg relay keep to be
maintained?
The goal is to get rid of fedmsg within Fedora infrastructure. However,
there's no deadline at the moment to migrate, so for the "foreseeable
future" you're okay.
When it comes to making messages available to users outside the
infrastructure VPN, we have a number of options:
1. Maintain an AMQP (fedora-messaging) to ZeroMQ PUB socket (fedmsg)
bridge with no wire format change. Users outside the VPN continue to
use the fedmsg library.
The downsides here are fedmsg isn't really maintained by anyone,
fedora_fedmsg_meta_infrastructure also won't keep up with new message
formats (not that it really does now), and ZeroMQ PUB sockets make no
delivery guarantees so if missing messages is bad for your
application...
The upside is clients don't have to make any changes, but what they're
using is going to bitrot.
2. Maintain an AMQP to ZeroMQ PUB socket which isn't the same format
as fedmsg and write a consumer API for it which is compatible with
the fedora-messaging API.
The advantage of this option over 1. is that the API would be maintained
and give users access to the functionality offered by
fedora_fedmsg_meta_infrastructure. The same downsides of ZeroMQ PUB
sockets apply, plus clients need to change, and we have to maintain
more stuff.
3. Expose a consume-only AMQP broker to the public.
The advantage here is everything can use the same fedora-messaging
APIs (or any AMQP client) inside or outside infrastructure. The downside
is the service is exposed to the harsh world of the Internet.
Talking with Patrick before DevConf, I think this is the direction we
will ultimately go, but there are a number of deployment configurations
to consider so this isn't something you'll see tomorrow. I plan to do
some more research and present the options in detail in the coming
weeks.
Also, is there a list of advantages of fedora-messaging over fedmsg
available somewhere? I'm wondering if there are any significant
advantages that would justify the effort to port Koschei and other
software I maintain away form fedmsg.
There is some discussion in passing[0] in the migration tools package,
but you're not the first person to ask for a clear comparison with
fedmsg so I think I'm going to write up a "History" section in at
https://fedora-messaging.readthedocs.io/. The short version is:
* "At least once" message delivery guarantees. Publishers know when
they've successfully published a message (or not). Messages will
be re-delivered to consumers that fail to acknowledge messages.
If fedmsg the delivery guarantee is "at most once".
* Something covered by above, but is worth noting is consumers receive
messages that were published while they were offline assuming their
queue has been created in the broker. fedmsg has an approximation of
this by using datagrepper, but this is not reliable because of the "at
most once" semantic: datagrepper misses messages, too. It's also slow.
* A focus on message schema. Consumer-breaking schema changes regularly
make it to production with fedmsg, and go unnoticed for a long time.
Consumers are not typically written to handle this robustly.
* Rather than a painfully fragile message signing scheme, message
authenticity is enforced with the AMQP broker's authentication and
authorization.
* At an API and configuration level (this is Python-only, of course), it
is significantly simpler. Consumers don't need to inherit a class and
implement an undocumented interface; they need only be a callable that
accepts a single positional argument (the message). Topics can be
defined in the configuration file or on the command line instead of in
the Python code.
[0]
https://fedmsg-migration-tools.readthedocs.io/en/latest/migration/overview.html
To summarize: I understand there's no path forward for consumers outside
Fedora's infrastructure and that is not nice. Until there is, the fedmsg
gateway isn't going anywhere. Once there is, you'll have plenty of time
to migrate, and migrating is trivial if you opt to not take advantage of
a lot of the new features
