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Sending HTTP requests to individual application processes
on Passenger + Nginx

This feature is currently available for Ruby apps only.

When a request is sent to Nginx, Passenger will automatically forward the request to the most suitable application process as part of its load balancing mechanism. But sometimes it is desirable to be able to directly access the individual application processes. Use cases include, but are not limited to:

  • You want to debug a memory leak or memory bloat problem that only seems to appear on certain URIs. One can send a request to a specific process to see whether that request causes the process's memory usage to rise.
  • You want to perform a health check on all application processes.
  • The application caches data in local memory, and you want to tell a specific application process to clear that local data.
  • Other debugging use cases.

All individual application processes are accessible via HTTP over a per-process, private, localhost-only address. You can use standard HTTP tools like curl to access them. The exact addresses can be obtained with the command passenger-status --verbose.

These sockets are all bound to 127.0.0.1, but the port number is dynamically assigned. As a security measure, the sockets are also protected with a process-specific random password, which you can see in the passenger-status --verbose output. This password must be sent through the X-Passenger-Connect-Password HTTP header.

Example:

# passenger-status --verbose
Version : 5.0.13
Date    : 2015-07-07 13:02:48 +0200
Instance: jyAJC5iv (nginx/1.8.0 Phusion_Passenger/5.0.13)

----------- General information -----------
Max pool size : 9
App groups    : 7
Processes     : 6
Requests in top-level queue : 0

----------- Application groups -----------
/var/www/passenger_status_service/current/public:
  App root: /var/www/passenger_status_service/current
  Requests in queue: 0
  * PID: 20387   Sessions: 0       Processed: 68369   Uptime: 6h 37m 40s
    CPU: 3%      Memory  : 110M    Last used: 0s ago
    URL     : http://127.0.0.1:41823
    Password: bsWLW5KjthaYwQyd
  * PID: 20394   Sessions: 0       Processed: 38432   Uptime: 6h 37m 30s
    CPU: 3%      Memory  : 109M    Last used: 4s ago
    URL     : http://127.0.0.1:41825
    Password: sjewUSwksSAdoUeP

...</span>

Here we see that the web application 'phusion_blog' has two processes.

  • Process 20387 is accessible via http://127.0.0.1:41823/.
  • process 20394 is accessible via http://127.0.0.1:41825/.

To access 20387 we must send its password, bsWLW5KjthaYwQyd, through the X-Passenger-Connect-Password HTTP header, like this:

$ curl -H "X-Passenger-Connect-Password: bsWLW5KjthaYwQyd" http://127.0.0.1:41823/
...