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Passenger vs Unicorn: What are the differences?

Key differences between Passenger and Unicorn

Passenger and Unicorn are both popular application servers used for serving Ruby on Rails applications. While they are both designed to handle incoming web requests, there are several key differences between the two. The following paragraphs outline these differences in detail.

  1. Process Management: Passenger uses a process-based approach to handle incoming requests. It spawns multiple processes to handle concurrent requests, with each process capable of serving multiple requests concurrently using multi-threading. This allows Passenger to efficiently utilize system resources and handle high traffic loads. On the other hand, Unicorn uses a multi-process, single-threaded approach. It spawns multiple worker processes, each running a single thread, to handle requests independently. This can be beneficial for certain types of applications that require a high degree of isolation between requests.

  2. Memory Overhead: Due to its process-based approach, Passenger generally has a higher memory overhead compared to Unicorn. Each Passenger process requires its own memory footprint, which can be significant for applications with a large number of concurrent requests. In contrast, Unicorn's single-threaded worker processes have a lower memory footprint, making it more memory-efficient in certain scenarios.

  3. Request Queueing: Unicorn provides built-in request queuing, allowing it to handle incoming requests even if all worker processes are busy. When a request arrives and all worker processes are occupied, Unicorn adds the request to a queue and serves it as soon as a worker becomes available. This can help prevent request failures during high traffic situations. In contrast, Passenger does not have built-in request queuing and relies on the web server (e.g., Nginx or Apache) to manage request queuing and load balancing.

  4. Ease of Configuration: Passenger offers a simpler and easier configuration process compared to Unicorn. It provides a more user-friendly interface and requires fewer manual configurations, making it more suitable for developers who prefer a streamlined setup. Unicorn, on the other hand, has a more complex configuration process that requires additional steps to set up and fine-tune the server according to specific application requirements.

  5. Integration with Web Servers: Passenger is designed to work closely with web servers like Nginx and Apache, providing tight integrations and streamlined deployment processes. It can directly communicate with the web server, allowing for efficient handling of requests and better utilization of system resources. In contrast, Unicorn operates as a separate HTTP server and requires a reverse proxy (such as Nginx or Apache) for connection to the web server, potentially adding an additional layer of complexity to the setup and configuration process.

  6. Community Adoption: Passenger has gained significant popularity and has a large and active community of developers and users. It offers comprehensive documentation, frequent updates, and strong community support, making it easier to find resources and resolve potential issues. Unicorn, while still widely used, may have a smaller community compared to Passenger, which can limit the availability of certain resources and community-driven support.

In summary, Passenger and Unicorn differ in their process management approach, memory overhead, request queuing capability, ease of configuration, integration with web servers, and community adoption. The selection between the two depends on specific application requirements, resources availability, and developer preferences.

Advice on Passenger and Unicorn
Mark Ndungu
Software Developer at Nouveta · | 4 upvotes · 28.5K views
Needs advice

I have an integration service that pulls data from third party systems saves it and returns it to the user of the service. We can pull large data sets with the service and response JSON can go up to 5MB with gzip compression. I currently use Rails 6 and Ruby 2.7.2 and Puma web server. Slow clients tend to prevent other users from accessing the system. Am considering a switch to Unicorn.

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Replies (1)

Consider trying to use puma workers first. puma -w basically. That will launch multiple puma processes to manage the requests, like unicorn, but also run threads within those processes. You can turn the number of workers and number of threads to find the right memory footprint / request per second balance.

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Pros of Passenger
Pros of Unicorn
  • 43
    Nginx integration
  • 36
    Great for rails
  • 21
    Fast web server
  • 19
  • 15
  • 14
  • 13
    Rolling restarts
  • 10
  • 9
    Out-of-process architecture
  • 6
  • 2
    Virtually infinitely scalable
  • 2
    Deployment error resistance
  • 2
    Mass deployment
  • 2
  • 1
    Many of its good features are only enterprise level
  • 1
    Apache integration
  • 1
  • 1
    Asynchronous I/O
  • 1
    Multiple programming language support
  • 81
  • 59
  • 36
    Web server
  • 30
    Very light
  • 30
    Open Source
  • 27
    Rack http server
  • 18
    Load balancing
  • 14
    Great process management

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Cons of Passenger
Cons of Unicorn
  • 0
    Cost (some features require paid/pro)
  • 4
    Not multithreaded

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What is Passenger?

Phusion Passenger is a web server and application server, designed to be fast, robust and lightweight. It takes a lot of complexity out of deploying web apps, adds powerful enterprise-grade features that are useful in production, and makes administration much easier and less complex.

What is Unicorn?

Unicorn is an HTTP server for Rack applications designed to only serve fast clients on low-latency, high-bandwidth connections and take advantage of features in Unix/Unix-like kernels. Slow clients should only be served by placing a reverse proxy capable of fully buffering both the the request and response in between Unicorn and slow clients.

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