Mongrel2: State Machines, HTTP-0MQ, Events | Hacker News Reader

Mongrel2: State Machines, HTTP-0MQ, Events

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I read that post twice and created an account just to leave a comment. What a brilliant guy--saves app developers from having to hack up their webservers to support their choice of backend AND provide incredible insight into (and granular control over) connections.

Thanks! I appreciate the compliment. That's the general goal, but with all things, it remains to be seen how easy it is to actually do this. It's really easy for me to do it in C code, but the key is how to make it easy for other people without much fuss.

Zed makes a big deal about getting long-polling for free, but with evented web servers (built on EventMachine, Node, Twisted, etc), you also get long-polling for free, since requests are just objects that get thrown around until they're completed. This architecture makes long-polling trivial, because when a request comes in you can put the request object into a hash or array, then when the data or computation you're waiting on finishes you look up the request and send it back to the client.

Regardless, there are a lot of interesting ideas here, and I look forward to playing around with it when it's more done.

Well, this is long polling from any backend, not from within mongrel2. So your 0MQ handlers can respond to any connected HTTP or JSSocket, which other servers can do given the right design.

However, what I was more referring to was that by using a state machine I didn't have to actually implement anything. I just wrote the code according to the state machine, and what fell out was a ton of features simply because of design serendipity.

It seems to me that the logic behind the problem solving going on in Mongrel2 has been 'how can I make this into a problem that's already been solved?'

I think that seems like a pretty good methodology in a lot of cases actually.

Exactly, or another way to do it is, "I have problem X, how can I make it go away without solving it." Either by designing the thing so I don't need to solve X, or by finding someone else who's already solved it for me.

Does a finite state machine where you can modify the events to cause any transition you want defeat the purpose of the FSM? It's interesting that while the states are limited, by adding code you can cause a transition from any state to any other. You can look at the pretty picture to understand the default FSM, but as soon as you install any additional code, you don't know what transitions it will take. Adding modules effectively rewrites the FSM.

Is that the intent?

Also, this reminds me a bit of the design behind Varnish.

Well, you can't cause any transition you want, you can just change the events in the stream. So, if you're in the middle of proxying you can get out of it and go into a handler, but you couldn't totally screw up the FSM and go someplace weird.

So, adding modules doesn't rewrite the FSM. You'd need Ragel and a C compiler for that. ;-)

Currently I have one server running haproxy, which distributes requests to three servers running nginx (which serves static content) and proxies application requests to apache/mod_wsgi. How would Mongrel2 simplify this?

Well, knowing haproxy user, it is embed into your infrastructure like a tick, so it probably wouldn't help you. My experience has been, if it ain't broke don't fix it.

But, if I were to do an alternative, I'd basically ditch haproxy and nginx and just have Mongrel2 talking 0MQ or HTTP to arbitrary backends directly, rather than through multiple layers. With 0MQ you don't have a single point of failure by design, so you could (theoretically) run tons of Mongrel2 servers in front and have them talk to big clusters of 0MQ in back.

That's the goal anyway.

This post and comments here have convinced me I need to learn more about Finite State Machines and how to apply them.

Can anyone recommend some good educational material about using finite state machines in software development?

My Google searches find theoretical papers and not practical applications.

Check out "Practical State Charts in C/C++" by Dr. Miro Samek.

Notes:

The 2nd Edition is better than the 1st Edition, because it does not have the distracting analogies to quantum physics. For me, the best part of Samek's book was Chapter 5, State Patterns. However, he also covers Standard State machine Implementations (and their flaws) in Chapter 3. Chapter 6, Realtime Framework Concepts, covers WHY you want to take special care in implementing a state machine, such as avoiding corrupting the current event in order to preserve RTC semantics. You will not get a more coherent presentation of these issues anywhere else. You could spend years of your life reading CS papers on the subject, implementing state machines, etc. and not come to the mastery Samek has. My favorite quote from the whole book has to be pointing out deviating from a simple Moore FSM, by introducing guards, can cause big trouble. He says overuse of guards is "[the source of] architectural decay" in complex reactive systems.

The one thing I disagree with Samek about is that transitions have any state context associated with them. For me, transitions occur between states. This approximates the design of Mealy actions going between Moore actions., and is closest to the semantics of object-orientation, since the sender does not need to know the receiver directly. Samek presumes actions are dependent on the source context, but if there are two outgoing transitions from a source context, they probably will not have the same action. You move to a state before entering that state, and you move out of a state before moving to another state.

The more I read about Mongrel2 the more interested I am in it.

I have never really looked at the code of a web server before for comparison but it is kind of interesting to read about Zed's approach to building a web server.

I think Mongrel2 has some cool ideas in its design, but using sqlite for configuration seems very heavy, creates more dependencies and you can't edit the data in a text editor which moves away from how you traditionally configure a unix service. Bad choice.

The config file could be a sql script that you write in a text editor and then run.

It really isnt that big of a deal. How often are you changing your web server configuration ?

I think the traditional way of configuring a unix service manually using a random config file format that hides the underlying data structure is a bad choice. I'm trying to provide a way to automate things, and the best way to do that is with a universal data structure and langauge that modifies it accessible from every programming language.

sqlite3 fits that goal.

In other words, try doing .schema on your nginx.conf.

Initially I didn't like the idea of using sqlite for the config, but actually, this makes total sense and I agree with it as long as you don't make it into something Lovecraftian :D

I find his use of Ragel generated state machines fascinating. He uses them in ways I wouldn't typically consider, very clever stuff.

I'm on the lookout for nasty code of my own that can be replaced with a state machine.

erlang also provides a number of 'behavior' modules e.g. gen_fsm/gen_server etc. which takes out the drudgery of writing boilerplate network client/server code. w.r.t your fsm, are there timeouts also happening at some states ? might be required to avoid holding onto precious resources if either end becomes unresponsive for whatever reason...

I had them in there, but was having a problem getting the base state machine right, so took them out. Cool that you noticed though.

Hey... wait a minute ... what's up with this non-ranty, non-swearing, respectful-reply Zed... What have you done with the real Zed?!

I believe it may have something to do with the non-ranty, non-swearing, respectful questions.

Go Zed! You rock.

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