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The Road to Passenger 3: Technology Preview 4 – Adding new features and removing old limitations

By Hongli Lai on July 29th, 2010

In the past two years that we’ve been developing Phusion Passenger, we’ve received not only many feature requests but also many criticisms about certain limitations. Some feature requests have been implemented in Phusion Passenger 2.x, some have not. Some limitations were solved in the life time of Phusion Passenger 2, others were not because they require a lot of refactoring first. In Phusion Passenger 3 we’ve extensively refactored the code to not only make a lot of new cool features possible, but also to lift a lot of the old limitations. In this Technology Preview we are pleased to announce these changes.

Asynchronous spawning

Previously, when application processes are being spawned, Phusion Passenger is unable to handle HTTP requests until the processes are done spawning, because Phusion Passenger is holding the lock on the application pool while this is happening. Some websites have apps that need a very long time to spawn (30+ seconds) and this would be a problem for them. This behavior would also cause a “thundering herd” problem: suppose that a traffic spike appears, then the first request will cause Phusion Passenger to spawn a process. The other requests in the spike are blocked until that’s done, and then all of a sudden they are processed. Phusion Passenger thinks it needs to spawn more, so it spawns another one and blocks the rest, and so on. This can cause a large number of processes to be spawned all of a sudden, causing a long delay.

In Phusion Passenger 3 spawning happens in the background so that no clients have to be blocked. This turns out to work so well that application process spawning is now virtually unnoticeable, except when spawning the first application process.

Ability to configure minimum number of processes

Phusion Passenger automatically shuts down processes that haven’t been accessed for a while, where “a while” is configurable through the PassengerPoolIdleTime directive. Many web applications are rarely accessed during the night, so what happens is that all application processes are shut down during the night and the first person in the morning who accesses the web application has to wait for some time while the first application process is being spawned. This problem could be solved by setting PassengerPoolIdleTime to 0 or to a large number which means that processes are kept around forever or for a long time, but this also means that application processes are not shut down during the night, which might still be desirable for resource utilization reasons.

Phusion Passenger 3 introduces a new, long-awaited configuration directive: PassengerMinInstances. As its name implies, PassengerMinInstances makes sure that at least the given number of processes will be started and kept around. This, in combination with asynchronous spawning, turns out to work so well that we’ve assigned a default value of 1 for PassengerMinInstances. With Phusion Passenger 3, spawning delays should become a thing of the past!

Smart spawning support for all Rack applications

Smart spawning is a core feature of Phusion Passenger since version 1.0. It can reduce the spawning time of Rails processes by as much as 90%, and in combination with Ruby Enterprise Edition it allows you to save 33% memory on average.

However, smart spawning was limited to Rails applications only, not for Rack applications. Starting from Rails 3, all Rails 3 applications are also Rack applications, and Phusion Passenger 2 only supports smart spawning of Rails 3 applications if you remove (thereby forcing Phusion Passenger to detect it as a Rails app, not a Rack app).

Phusion Passenger 3 now supports smart spawning for all Rack applications. This works transparently and without further configuration.

Ability to access individual application processes over HTTP

When one sends a request to the web server, Phusion Passenger routes the request to one of the application processes, but one never knows up front which one that’s going to be, nor is there a way to control it. This is fine for normal requests, but sometimes one wants to send a request to a specific application process, e.g. to debug something. Another use case for accessing individual application processes directly is broadcasting messages: e.g. telling every application process to clear some local in-memory caches.

This has always been possible with reverse proxy app servers like Mongrel and Thin because each of their processes listen on their own port. Phusion Passenger 3 now also allows one to access individual application processes directly. Each application process can now be accessed through its own TCP socket port. The port numbers are randomly allocated by the operating system and the protocol is plain HTTP, so you can use existing tools like ‘curl’.

We take security very seriously. These sockets are bound to so it’s not possible to access them from remote computers. Furthermore, each socket is protected by its own unique randomly generated secure password. One can use ‘passenger-status’ to query the ports and passwords.

Global queuing now on by default

Many people with web apps that have long-running requests are familiar with the “slow Mongrel queue problem”. When one sets up Mongrel or Thin behind Nginx or Apache, it’s possible for new requests to be queued behind a Mongrel/Thin instance which is currently processing a long-running request. When other Mongrel/Thin instances become available, this new request is already queued behind the long-running request and cannot migrate to the other free instances. The more long-running requests one has the bigger of a problem this can become, resulting to very long response times for some users.

There are multiple ways to solve this problem, but Phusion Passenger has already solved this problem for a long time with a feature that we call global queuing. It was disabled by default because turning global queuing off would yield a little bit of extra performance in microbenchmarks. However for version 3.0 we’ve decided to turn it on by default: rather than saving those few milliseconds in benchmarks, we believe it’s much more important that all users get to have fair response times.

Ability to disable friendly error pages

One of the innovations that Phusion Passenger has brought us is the ability to show friendly error messages directly in the browser, e.g. when the web application fails to spawn because of a syntax error. This dramatically improves usability for developers, new and experienced alike, because one doesn’t have to manually dig into log files anymore. However this is not always desirable in production: although the error page is developer-friendly, it isn’t necessarily user-friendly. It might also expose information that the system administrator would rather not expose such as filenames.

Phusion Passenger 3 introduces a new configuration directive for controlling whether friendly error pages should be shown: ‘PassengerFriendlyErrorPages’. When turned off, Phusion Passenger will display the standard Apache/Nginx 500 Internal Server Error page instead, but all error messages are still logged to the web server error log file.

Nginx-specific improvements

In Phusion Passenger 3, Nginx is compiled with SSL support by default due to popular demand.

We’ve also introduced the following new configuration options:

passenger_set_cgi_param (name) (value)
This is the Phusion Passenger equivalent of proxy_module’s proxy_set_header. It allows you to pass arbitrary CGI environment parameters the web application.
passenger_buffer_response (on|off)
This is the Phusion Passenger equivalent of proxy_buffer. It was and still is off by default to allow streaming responses, but when streaming responses aren’t necessary one can turn this option on so that Nginx can gracefully handling clients that are slow at receiving responses.
passenger_ignore_client_abort (on|off)
This is the Phusion Passenger equivalent of proxy_ignore_client_abort.

In other news

In Technology Preview 3 we unveiled Phusion Passenger Lite. Based on various feedback that we’ve gotten since then, we’ve decided to rename this thing to Phusion Passenger Standalone in order to reduce confusion about what it is.

Towards the future

If there is one thing we’ve come to understand over time is that different businesses have different needs and constraints when it comes to deploying their applications. In order to provide the best experience and support to these businesses, we’re working on different versions of Phusion Passenger to accommodate them even better in their respective environments. In light of this, we want to underline that the technology previews we’re currently writing about will and have described the cool stuff that will be incorporated in the version intended for the most high demand environments. More information on this will follow. Having said that, almost everything we’ve blogged about up till this point will be included in the version that’s available for everybody.