For web development I'd like to mix rails and node.js since I want to get the best out of both worlds (rails for fast web development and node for concurrency). I know that some people choose to just use full ruby stack with eventmachine that is integrated into rails controller so that every request can be nonblocking by using fiber in event-loop model. I have been able to understand how that works in a big picture.
At this moement however I want to try doing nonblocking request processing with rails and node.js with message queue concept. I heard that this can be achieved by using redis as an intermediary. I'm still having trouble trying to figure out how that works as of now. From what I can understand: so we have 2 apps A (rails) and B (node.js) and redis. rails app will handle requests from users that go through controllers in REST manner, and then from there rails will pass that through redis, and then redis will form queues and node.js app will pick up that queue and do whatever necessary afterhand (write or read from backend db).
My questions:
So how would that improve concurrency and scalability? from what i
know since rails handle the requests through controllers
synchronously, and then write to redis, the requests will be
blocking still, even though node.js end can pickup the queue
asynchronously. (I have a feeling that it's not asynchronous yet if it's not end to end
non-blocking).
Would node.js be considered a proxy or an application here if redis
is the intermediary?
I'm new to redis and learning it still. If I'm using 100% noSQL
solution for my backend database, such as mongoDB or couchDB, are they replaceable by redis entirely or is redis more seen as a
messaging queue tool like rabbitMQ?
Is messaging queue a different concurrency concept than threading or
event-loop model or is it supposed to supplement them?
That's all my question. I'm new to message queue concept. Will appreciate any help and pointers to right direction and articles that help me learn more. thanks.
You are mixing some things here that don't go together.
Let's first make sure we are on the same page regarding the strengths/weaknesses of the involved technologies
Rails: Used for it's web-development simplicity and perfect for serving database-backed web-applications.
Not very performant when having to serve a large number of long running requests as you'd run out of threads on your Ruby workers - but well suited for anything that can scale horizontally with more web-nodes (multiple web-servers - 1 db).
Node.js: Great for high-concurrency scenarios. Not as easy as rails to write a regular web-application in it. But can handle near an insane amount of long-running low-cpu tasks efficiently.
Redis: A Key-Value Store that supports operations on it's data-structures (increment/decrement values, append/prepent push/pop to lists - all operations that make this DB work consistently with multiple clients writing at once)
Now as you can see, there is no benefit in having Rails AND Node serve the same request - communicating through Redis. Going through the Rails Stack would not provide any benefit if the requests ends up being handled by the Node server.
And even if you only offload some processing to the node server, it's still the Rails webserver that handles the requests and has to wait for a response from node - killing the desired scalability. It simply makes no sense.
Where you would a setup with Node and Rails together is in certain areas of your app that have drastically different scaling requirements.
If you are for example writing a Website that displays live stats for Football games you can easily see that there are two different concerns in your app: The "normal" Site that contains signup, billing and profile stuff that screams for a quick implementation through rails. And the "live" portion of the site where users see live results and you expect to handle a lot of clients at once - all waiting for something to happen (low cpu - high concurrency).
In such a case it may be beneficial to actually seperate the two parts of the site into a Ruby and a Node app, with then sharing data about the user through a store like Redis (but actually you just need some shared state that both can look at and write to for synchronization purposes).
So you would use for example Rails for the Signup/Login portions - once signed up write the session cookie into redis alongside with the permissions of the user (what game is he allowed to follow) and hand the user off to the Node.js app.
There the Node app can read the session information from Redis and serve the user.
Word of advice:
You don't get scalability by simply throwing Node.js into your Toolbox. You really have to find out what Node.js is good at (low-cpu high-io concurrent operations) and how you can leverage that to remedy some of the problems your currently chosen technology has.
I can answer 3 for you. Redis does not guarantee that when you perform an operation that result will actually be on disk, also transaction handling it a bit "different". It also requires for the whole database to be in memory. Depending on the situation this can be an issue or not. It is however incredibly fast. It is not a messaging queue, you can easily make a queue out of it, but it is not it's purpose. If you want to have a queuing system only you can probably do better with something else.
Related
I want to build a service that notifies me when a url returns status 200. I'm currently using a sidekiq worker, if the status == 200, it updates my database (row.available = true), if not, it raises an exception and retries the worker in n seconds, n amount of times.
Though this works, it doesn't feel efficient or scalable (1000's checks would result in 1000's of exceptions, and on certain platforms that's bad news -- JRuby), and I'm sure there is a way I can build an internal service to manage this url monitoring that doesn't rely on sidekiq (perhaps in Go, or another, more suited Ruby gem). However, I have no idea where to begin, and so I'd appreciate some general direction.
Writing and running a simple link checker is easy. Doing that for 1000s of links quickly, without redundancy, and handling dead and slow-responding links without bogging down your entire system gets harder.
I'd use three threads, plus two queues:
A dispatcher thread that only reads from the database. It is responsible for finding and queuing URLs to be checked in to a "to be checked" queue.
A worker thread that consumes from the first queue and pushes results into the "updated URL results" queue.
An updater/consumer thread that takes the result of a thread in #2 and updates the database.
Ruby has some built-in classes to help:
Thread
Queue
I'd highly recommend Typhoeus and Hydra for use in the middle thread. The documentation for these two classes cover a lot of what you need to do as far as handling multiple threads running in parallel.
I wouldn't write this code as part of a Rails application. There is no value added by Rails to this, nor is it necessary. I would either require Active Record and piggy-back on the existing database.yaml settings and models, or use Rails' "runner" to run the code as an adjunct to the Rails code.
Or, I'd write a small, application-specific, piece of code to run on a different server to avoid bogging down the Rails server. Using something like MySQL or PostgreSQL drivers would let you talk to the same database that Rails uses. In this case I'd use the Sequel gem to act as the ORM, but that's because I prefer it over Active Record.
There are a lot of things to consider as you write this code, including retries of failed URLs, sensing redirections and updating the source URLs to reflect them to avoid wasting time, and not beating up the hosting servers causing you to be banned.
I've written several apps for this purpose over the years and doing it right takes a lot of forethought, so think out your design up front otherwise you could end up with some major rewrites later on.
I've been working on a rails project that's unusual for me in a sense that it's not going to be using a MySQL database and instead will roll with mongoDB + Redis.
The app is pretty simple - "boot up" data from mongoDB to Redis, after which point rails will be ready to take requests from users which will consist mainly of pulling data from redis, (I was told it'd be pretty darn fast at this) doing a quick calculation and sending some of the data back out to the user.
This will be happening ~1500-4500 times per second, with any luck.
Before the might of the user army comes down on the server, I was wondering if there was a way to "simulate" the page requests somehow internally - like running a rake task to simply execute that page N times per second or something of the sort?
If not, is there a way to test that load and then time the requests to get a rough idea of the delay most users will be looking at?
Caveat
Performance testing is a very broad topic, and the right tool often depends on the type and quality of results that you need. As just one example of the issues you have to deal with, consider what happens if you write a benchmark spec for a specific controller action, and call that method 1000 times in a row. This might give a good idea of performance of that controller method, but it might be making the same redis or mongo query 1000 times, the results of which the database driver may be caching. This also ignores the time it'll take your web server to respond and serve up the static assets that are part of the request (this may be okay, especially if you have other tests for this).
Basic Tools
ab, or ApacheBench, is a simple commandline tool that you can use to test the throughput and speed of your app. I usually go to this first when I want to send a thousand requests at a web server, or test how many simultaneous requests my app can handle (e.g. when comparing mongrel, unicorn, thin, and goliath). Because all requests originate from the same server, this is good for a small number of requests, but as the number of requests grow, you'll be limited by the resources on your testing machine (network stack, cpu, and maybe memory).
Benchmark is a standard ruby class, and is great for quickly spitting out some profiling information. It can also be used with Test::Unit and RSpec. If you want a rake task for doing some quick benchmarking, this is probably the place to start
mechanize - I like using mechanize for quickly scripting an interaction with a page. It handles cookies and forms, but won't go and grab assets like images by default. It can be a good tool if you're rolling your own tests, but shouldn't be the first one to go to.
There are also some tools that will simulate actual users interacting with the site (they'll download assets as a browser would, and can be configured to simulate several different users). Most notable are The Grinder and Tsung. While still very much in development, I'm currently working on tsung-rails to make it easier to automate rails load testing with tsung, and would love some help if you choose to go in this direction :)
Rails Profiling Links
Good overview for writing performance tests
Great slide deck covering most of the latest tools for profiling at various levels
I have a Rails app hosted on Heroku. I have to do long backend calculations and queries against a mySQL database.
My understanding is that using DelayedJob or Whenever gems to invoke backend processes will still have impact on Rails (front-end) server performance. Therefore, I would like to set up two different Rails servers.
The first server is for front-end (responding to users' requests) as in a regular Rails app.
The second server (also a Rails server) is for back-end queries and calculation only. It will only read from mySQL, do calculation then write results into anothers Redis server.
My sense is that not lot of Rails developers do this. They prefer running background jobs on a Rails server and adding more workers as needed. Is my sever structure a good design, or is it an overkill? Is there any pitfall I should be aware of?
Thank you.
I don't see any reason why a background job like DelayedJob would cause any more overhead on your main application than another server would. The DelayedJob runs in it's own process so the dyno's for your main app aren't affected. The only impact could be on the database queries but that will be the same whether from a background job or another app altogether that is accessing the same database.
I would recommend using DelayedJob and workers on your primary app. It keeps things simple and shouldn't be any worse performance wise.
One other thing to consider if you are really worried about performance is to have a database "follower", this is effectively a second database that keeps itself up to date with your primary database but can only be used for reads (not writes). There may be better documentation about it, but you can get the idea here https://devcenter.heroku.com/articles/fast-database-changeovers#create_a_follower. You could then have these lengthy background jobs read data from here leaving your main database completely unaffected.
we are currently planning a rails 3.2.2 application where we use RabbitMQ. We would like to run several kind of workers (and several instances of a worker) to process messages from different queues. The workers are written in ruby and are laying in the lib directory of the rails app.
Some of the workers needs the rails framework (active record, active model...) and some of them don't. The first worker should be called every minute to check if updates are available. The other workers should process the messages from their queues when messages (which are send by the first worker) are present and do some (time consuming) stuff with it.
So far, so good. My problem is, that I only have little experiences with messaging systems like RabbitMQ and no experiences with the rails interaction between them. So I'm wondering what the best practices are to get the two playing with each other. Here are my requirements again:
Rails 3.2.2 app
RabbitMQ
Several kind of workers
Several instances of one worker
Control the amount of workers out of rails
Workers are doing time consuming tasks, so they have to be async
Only a few workers needs the rails framework. The others are just ruby files with some dependencies like Net or File
I was looking for some solution and came up with two possibilities:
Using amqp with EventMachine in a new thread
Of course, I don't want my rails app to be blocked when a new worker is created. The worker should run in another thread and do its work asynchronously. And furthermore, it should not start a new instance of my rails application. It should only require the things the worker needs.
But in some articles they say that there are some issues with Passenger. And another fact that I don't like is, that we are using webbrick for development and we ought to include workarounds for that too. It would be possible to switch to another webserver like thin, but I don't have any experience with that either.
Using some kind of daemonizing
Maybe its possible to run workers as a daemon, but I don't know how much overhead this would come up with, or how I can control the amount of workers.
Hope someone can advise a good solution for that (and I hope I made myself clear ;)
It seems to me that AMQP is a big shot to kill your problem. Have you tried to use Resque? The backed Redis database has some neat features (like publish/subscribe and blocking list pop) which make it very interesting as a message queue, and Resque is very easy to use in any Rails app.
The workers are daemonized, and you decide which worker of your pool listens to which queue, so you can scale each type of job as needed.
Using EM reactor inside a request/response cycle is not recommended, because it may conflict with an existing event loop (for instance if your app is served by thin), in any case you have to configure it specifically for your web server, OTOS it may be interesting to have an evented queue consumer, if your jobs have blocking IO and are not processor-bound.
If you still want to do it with AMQP, see Starting the event loop and connecting in Web applications and configure for your web server accordingly. Or use bunny to push synchronously in the queue (and whichever job consumer you deam useflu, like workling for instance)
we are running slightly different -- but similar technology stack.
daemon kit is used for eventmachine side of the system... no rails, but shared models (mongomapper & mongodb). EM is pulling messages off the queues, and doing whatever logic is required (we have ruleby in the mix, but if-then-else works too).
mulesoft ESB is our outward-facing message receiver and sender that helps us deal with the HL7/MLLP world. But in v1 of the app, we used some java code in ActiveMQ to manage HL7 messages.
the rails app then just serves up stuff for the user to see -- again, using the shared models.
I'm writing a Rails web service that interacts with various pieces of hardware scattered throughout the country.
When a call is made to the web service, the Rails app then attempts to contact the appropriate piece of hardware, get the needed information, and reply to the web client. The time between the client's call and the reply may be up to 10 seconds, depending upon lots of factors.
I do not want to split the web service call in two (ask for information, answer immediately with a pending reply, then force another api call to get the actual results).
I basically see two options. Either run JRuby and use multithreading or else run several regular Ruby instances and hope that not many people try to use the service at a time. JRuby seems like the much better solution, but it still doesn't seem to be mainstream and have out of the box support at Heroku and EngineYard. The multiple instance solution seems like a total kludge.
1) Am I right about my two options? Is there a better one I'm missing?
2) Is there an easy deployment option for JRuby?
I do not want to split the web service call in two (ask for information, answer immediately with a pending reply, then force another api call to get the actual results).
From an engineering perspective, this seems like it would be the best alternative.
Why don't you want to do it?
There's a third option: If you host your Rails app with Passenger and enable global queueing, you can do this transparently. I have some actions that take several minutes, with no issues (caveat: some browsers may time out, but that may not be a concern for you).
If you're worried about browser timeout, or you cannot control the deployment environment, you may want to process it in the background:
User requests data
You enter request into a queue
Your web service returns a "ticket" identifier to check the progress
A background process processes the jobs in the queue
The user polls back, referencing the "ticket" id
As far as hosting in JRuby, I've deployed a couple of small internal applications using the glassfish gem, but I'm not sure how much I would trust it for customer-facing apps. Just make sure you run config.threadsafe! in production.rb. I've heard good things about Trinidad, too.
You can also run the web service call in a delayed background job so that it's not hogging up a web-server and can even be run on a separate physical box. This is also a much more scaleable approach. If you make the web call using AJAX then you can ping the server every second or two to see if your results are ready, that way your client is not held in limbo while the results are being calculated and the request does not time out.