Application Page Request To Continue To: Fill & Download for Free

(Note: this answer was written in 2011 and I’m guessing a lot has changed since then)Well, since no one else answered, I decided to break out The Google and dig a little deeper. Here's my high-level interpretation of how Heroku works. It's based on public information, but I don't think I've seen it all collected in one place before.Parts of it may be incorrect or imprecise, in which case please correct me. I've tried to note the places where I've made assumptions/inferences/guesses. Obviously there is a lot more to Heroku than what's publicly known.Request LifecycleHeroku runs DNS servers that point "appname.heroku.com" domains to a subset of their front-end reverse proxies, or you can setup a custom subdomain with a CNAME that points to proxy.heroku.com (which in turn resolves to the reverse proxy IPs), or a root domain with A records pointing directly to Heroku's reverse proxies. Multiple IPs (currently 3) will be returned to provide failover in case one or more of the reverse proxy servers is down.Your browser sends an HTTP request to ones of the reverse proxy servers (Nginx (web server)) pointed to by the DNS (if it fails to connect it should try another one of the IPs). The request is forwarded to the HTTP cache layer. When the response eventually comes back gzip encoding might be applied depending the the Content-Type header. The nginx servers also terminate SSL (i.e. HTTPS) connections from the browser. If you want a custom domain to work with SSL (not relying on SNI) Heroku must run one of these nginx server instances, with it's own IP, specifically for your application.The HTTP cache layer (Varnish) accepts requests forwarded from the reverse proxies. It will return a cached page immediately if it's in the cache, or forward the request to the "routing mesh" if not cached. Responses returned from the routing mesh / app servers are cached if the appropriate HTTP headers are set.[Information available on this piece is sparse, it's part of their secret sauce] The custom routing mesh (written in Erlang) looks for an app server (Heroku calls it a "dyno") with capacity to serve your request. If none is running it spawns one. If the load is high and the app pays for additional dynos it might spawn another dyno. The request is held until a dyno has been started or is idle, at which point it's forwarded to the new/idle dyno.Each app's dynos are spread across the "dyno grid", which consists of many servers (Heroku calls these "railgun" servers) running many applications' dynos. Dynos are spawned when necessary, which usually only takes a couple seconds. Non-responsive and excess capacity dynos are eventually killed / garbage collected, freeing up capacity for other apps' dynos. It looks like Heroku runs up to at least 60 dynos/workers per railgun instance, which appears to be an EC2 "High-Memory Extra Large Instance" with 17.1GB of RAM and two 2.67GHz CPU cores (see http://aspen-versions.heroku.com/evil)Upon deployment ("git push" to Heroku's git server) application code and dependencies are compiled into "slugs" (railgun fires off the slugs... get it?), a read-only compressed filesystem (SquashFS) that can quickly be downloaded to a railgun, mounted, and executed in a chroot sandbox with the app's configuration environment variables set. Each dyno has it's own Unix user, it can only see the files in it's own chroot jail, and cannot write to the filesystem. Security does not rely on VM sandboxing.The application server process is MRI Ruby (or Node.js) and the Ruby webserver used is Thin (based on Mongrel). The Ruby stack exposes the Rack web interface.DeploymentHeroku's git server accepts git pushes to each application's repository from permitted users. A pre-receive hook kicks off the rest of the deployment process.A git checkout of the HEAD of master is done.For Ruby apps, dependencies listed in the gem manifest (.gem) are downloaded, and native extensions are compiled if necessary.Extra files like .git, .gem, tmp, and logs are stripped.The application is compiled into the previously mentioned SquashFS "slug" with it's dependencies and configuration environment variables.The slug is tested by launching the app. If the app fails to start the deployment (including the git push) is rejected.[I'm making assumptions here.] The routing mesh stops sending requests to dynos running old slugs. Old dynos finish responding to their current request and are killed while new dynos with the new slug are started, and the routing mesh begins forwarding requests to the new dynos.Applications are also restarted after certain operations like setting configuration variables and changing add-ons so that the application has the latest configuration data.DNSIt appears Heroku runs approximately 6 nginx reverse proxies (plus the customers who pay $100/month for a dedicated IP/proxy so they can use SSL on a custom domain name).Of the domains I checked (a few hundred found through the "Find Subdomains" tool here: http://www.magic-net.info/), appname.heroku.com (including proxy.heroku.com) will return 3 of these:50.16.215.19650.16.232.13050.16.233.10275.101.145.87174.129.212.2It's possible Heroku's DNS is returning different IPs based the load of the reverse proxies, but when querying heroku.com's 4 nameservers directly I got different subsets of those 5 IPs. A random distribution of IPs probably gives good enough load distribution.And Heroku's documentation says to point A records to these three:75.101.163.4475.101.145.87174.129.212.2It's also interesting to note that apps aren't tied to specific proxy servers. If you set the "Host" header to your app's subdomain, a request to any one of those IPs will work.SQL DatabaseA PostgreSQL database is automatically provisioned for each application.Databases are run on either shared or dedicated EC2 instances with EBS persistance.Database connection configuration details are passed to dynos via environment variables.Database backups were previously performed at periodic intervals for shared databased but they will soon roll out a continuous backup scheme for all databases.TAPS can be used to import/export your databases.MemcachedUses Membase provided by Couchbase (formerly Membase formerly NorthScale) as an add-on.WorkersWorkers (formerly Delayed Job) are also run on the railgun servers similar to app server dynos.Almost half of the dynos on Heroku's dyno grid are workers.LoggingLogs from apps servers' and workers' stdout, and even some of Heroku's internal infrastructure components (nginx, router, api, slugc) and add-ons are sent to a syslog router they developed called Logplex.For Rails applications they inject rails_log_stdout to get logging on stdout.Users can access logs via the command line tool, and even setup their own syslog endpoint.Add-onsThird-party add-ons (and some of Heroku's own services) implement implement a REST API to automate provisioningWhen a customer adds an add-on Heroku makes requests to the add-on's API.The provider receives the API request and provisions the add-on for the application.Thee provider returns a response containing configuration data (locations and credentials) that the application can use to connect to the add-on's services.The application slug is recompiled and restarted. This data is exposed to the application via environment variables like other configuration data.Misc TechDoozer, "a new, consistent, highly-available data store written in Go" for distributed coordination between their internal services. Doozer is similar to Apache ZooKeeper.Redis for "a redundant cache of shared state data, a means of tracking dynamic clusters of running instances, a container for realtime statistics data and a transient data store for high volumes of log messages".RabbitMQ plus a Ruby DSL called Minion.Splunk (product) (http://www.splunk.com/view/SP-CAAAFP4) for managing, monitoring, and troubleshooting their infrastructure.Librato's Silverline/Load Manager (https://silverline.librato.com/press_releases/20100316) for fine-grained load management.References(Sorry these aren't cited inline, but I think they're all there)http://www.heroku.com/how/architecturehttp://devcenter.heroku.com/articles/custom-domainshttp://devcenter.heroku.com/articles/http-cachinghttp://devcenter.heroku.com/articles/slug-compilerhttp://devcenter.heroku.com/articles/why-does-ip-based-ssl-cost-100-monthhttp://blog.heroku.com/archives/2010/12/13/logging/http://blog.heroku.com/archives/2010/3/16/memcached_public_beta/http://status.heroku.com/incident/151http://adam.heroku.com/http://adam.heroku.com/past/2009/9/28/background_jobs_with_rabbitmq_and_minion/http://adam.heroku.com/past/2011/4/1/logs_are_streams_not_files/http://orion.heroku.com/past/2009/7/29/io_performance_on_ebs/http://groups.google.com/group/herokuhttps://addons.heroku.com/provider/resources/technical/how/overviewhttp://blog.golang.org/2011/04/go-at-heroku.htmlhttps://github.com/heroku and https://github.com/hahttp://aspen-versions.heroku.com/evil (inspect processes and other system information on a railgun server)http://highscalability.com/heroku-simultaneously-develop-and-deploy-automatically-scalable-rails-applications-cloudhttp://pivotallabs.com/talks/30Update 5/31/2011Heroku released a major update to the platform with the "Celadon Cedar" stack: http://news.heroku.com/news_releases/heroku-announces-major-new-version-celadon-cedar-includes-new-process-model-full-nodejs-Heroku.com also got a slick update: http://www.heroku.com/howList of new devcenter documents: https://gist.github.com/1000964Procfile (http://adam.heroku.com/past/2011/5/9/applying_the_unix_process_model_to_web_apps/) is used for defining and managing processes. "web" and "worker" map to the existing dynos and worker, and you can define custom process types. "heroku scale" is used to adjust the number of each process type running on the platform.Cedar has first-class Node.js and Ruby 1.9. There appears to be unofficial support for other languages including Python (https://gist.github.com/866c79035a2d066a5850), Go, and Erlang, and potentially the ability to use custom processes/languages.All processes (web and workers) are now considered "dynos" and treated identically. (http://devcenter.heroku.com/articles/process-model)LXC (http://lxc.sourceforge.net/) is used for better process isolation in addition to chroot for filesystem isolation. http://devcenter.heroku.com/articles/dyno-isolationNew herokuapp.com HTTP stack has support for HTTP/1.1, long polling, chunked responses (http://devcenter.heroku.com/articles/http-routing). It also supports multiple concurrent requests per dyno. The routing mesh will send requests to the dyno immediately, or if you have multiple dynos it will select a random dyno. This is necessary to take advantage of asynchronous or multithreaded web servers such as Node.js, EventMachine, etc.The herokuapp.com stack does not include the Varnish cache layer because it is incompatible with long-polling/chunked responses (rack-cache or memcache is recommended). It also doesn't have the 30 second timeout present on the heroku.com stack, but it does have a 60 second inactivity timeout.The "dyno manifold" is apparently the new name for the "dyno grid" (http://devcenter.heroku.com/articles/dyno-manifold). Cedar gives you greater visibility into the platform through the previously discussed Logplex logging system (now with pretty colored logs!), process listings ("heroku ps": http://devcenter.heroku.com/articles/ps), easier ways for running arbitrary programs (e.x. try "heroku run bash")Processes are idled (shut down) after one hour of inactivity.Processes are terminated if they bind to incorrect ports (e.x. anything except $PORT on 0.0.0.0).Cedar apps are detected based on the presence of "Gemfile" for Ruby or "package.json" for Node.

A socket is an endpoint in a network. When applications communicate with another computer, they connect to a socket. And yes, that also includes your web browser.On the Internet, communications with sockets is generally a one-way street where every request will send back a response. Some of those requests will be followed up by more requests when e.g. a browser just loaded the HTML of a page and now needs to download the CSS files, Javascript files and all images and other media to display the page.Problem is, sometimes you want a web server to send a message back to the web browser. For example, if there's a new message, you don't want to wait for the user to click a button to send a request to see if there are new messages. Neither would you like that your web page uses Javascript to continuously send requests to the server asking: "Do you have new messages?". This is called "polling".No, you want the web server sending a message to the connected browser/site telling the browser: "Hey, you have new messages!" The Javascript on the site would pick up the the message as a request and will send a response back to the server that it did receive the message. Thus, a request-response where the browser/site is basically the server. And thus, the site needs to have a WebSocket and the browser will need to support these. This allows a "full-duplex communication" between browser/site and server.How it works? Look at Quora! You have a "Notifications" button on the top-right which will show you the number of notifications. Every time when you get new notifications, a counter will pop up and display the number that you haven't read yet.This can be done in two ways and for older browsers, the technique is basically 'polling', which generates a lot of internet traffic. But with websockets the amount of traffic is reduced enormously because it will be limited to the server sending messages back.Still, it's not just the web browser that needs to support websockets. The server needs to support them too! Websockets became a standard around 2011 so it is still reasonably new.

In the beginning of May this year, I began my application for a ‘gap year’ from practising medicine with the Ministry of Health Holdings (MOHH). (I am serving a bond with them.)I was putting my plans into place to become a full-time writer starting next year.Now, don’t get me wrong. Playing doctor is one of the most meaningful jobs in the world. But as I mentioned in many of my answers, my true passion lies in fiction writing. I wanted to be a novelist.I have worlds inside of me that cannot wait to burst forth and flesh themselves onto crisp and solid pages. And I think it would be a waste if I don’t at least give myself a chance to fulfill my dreams.I knew the application was going to take some time to be approved. In the meantime, I needed a place to practise writing after work.That’s when I found Quora.I wrote my very first answer here on May 2nd: Shanks Wang's answer to Is it okay to judge people?Yeah, I know. I have come a long way since that answer.Then followers came. For those of you who are reading this, you have no idea how much your support meant for me. Every one of you is an endless source of encouragement and motivation for me, an affirmation that I'm making the right decision.Last Monday, after numerous phone calls and meetings in past months, MOHH finally approved my request. Starting from coming November, I am allowed a break from doctoring for nine months.I hope to publish numerous short stories and one full novel in that period of time. Depending on how well I do, and how much I enjoy it, I may or may not return to being a doctor.