Assuming passenger has started many instances of your node.js app and given the output from passenger-status --show=xml, is there a way to curl a specific instance?
Example output from passenger-status --show=xml:
$ sudo passenger-status --show=xml
<?xml version="1.0" encoding="iso8859-1"?>
<info version="3">
<passenger_version>5.0.22</passenger_version>
<group_count>1</group_count>
<process_count>1</process_count>
<max>1</max>
<capacity_used>1</capacity_used>
<get_wait_list_size>0</get_wait_list_size>
<supergroups>
<supergroup>
<name>/opt/testApp/node_modules/testApp/public (production)</name>
<state>READY</state>
<get_wait_list_size>0</get_wait_list_size>
<capacity_used>1</capacity_used>
<group default="true">
<name>/opt/testApp/node_modules/testApp/public (production)</name>
<component_name>/opt/testApp/node_modules/testApp/public (production)</component_name>
<app_root>/opt/testApp/node_modules/testApp</app_root>
<app_type>node</app_type>
<environment>production</environment>
<uuid>Ye3PSGsJiXL7LLQxjZxY</uuid>
<enabled_process_count>1</enabled_process_count>
<disabling_process_count>0</disabling_process_count>
<disabled_process_count>0</disabled_process_count>
<capacity_used>1</capacity_used>
<get_wait_list_size>0</get_wait_list_size>
<disable_wait_list_size>0</disable_wait_list_size>
<processes_being_spawned>0</processes_being_spawned>
<life_status>ALIVE</life_status>
<user>testUser</user>
<uid>503</uid>
<group>testUser</group>
<gid>504</gid>
<options>
<app_root>/opt/testApp/node_modules/testApp</app_root>
<app_group_name>/opt/testApp/node_modules/testApp/public (production)</app_group_name>
<app_type>node</app_type>
<start_command>node /usr/share/passenger/helper-scripts/node-loader.js</start_command>
<startup_file>/opt/testApp/node_modules/testApp/server.js</startup_file>
<process_title>Passenger NodeApp</process_title>
<log_level>3</log_level>
<start_timeout>90000</start_timeout>
<environment>production</environment>
<base_uri>/</base_uri>
<spawn_method>smart</spawn_method>
<user>testUser</user>
<default_user>testUser</default_user>
<default_group>testUser</default_group>
<integration_mode>standalone</integration_mode>
<ruby>/usr/bin/ruby</ruby>
<python>python</python>
<nodejs>node</nodejs>
<ust_router_address>unix:/tmp/passenger.cEAtrKS/agents.s/ust_router</ust_router_address>
<ust_router_username>logging</ust_router_username>
<ust_router_password>USTPASSWORD</ust_router_password>
<debugger>false</debugger>
<analytics>false</analytics>
<api_key>APIKEY</api_key>
<min_processes>1</min_processes>
<max_processes>0</max_processes>
<max_preloader_idle_time>300</max_preloader_idle_time>
<max_out_of_band_work_instances>1</max_out_of_band_work_instances>
</options>
<processes>
<process>
<pid>23090</pid>
<sticky_session_id>STICKYSESSIONID</sticky_session_id>
<gupid>GUPID</gupid>
<concurrency>0</concurrency>
<sessions>0</sessions>
<busyness>0</busyness>
<processed>12</processed>
<spawner_creation_time>1451496406616398</spawner_creation_time>
<spawn_start_time>1451496406629729</spawn_start_time>
<spawn_end_time>1451496407909872</spawn_end_time>
<last_used>1451506377943256</last_used>
<last_used_desc>53m 6s ago</last_used_desc>
<uptime>3h 39m 16s</uptime>
<life_status>ALIVE</life_status>
<enabled>ENABLED</enabled>
<has_metrics>true</has_metrics>
<cpu>0</cpu>
<rss>44340</rss>
<pss>43310</pss>
<private_dirty>36984</private_dirty>
<swap>0</swap>
<real_memory>36984</real_memory>
<vmsize>653728</vmsize>
<process_group_id>23063</process_group_id>
<command>Passenger NodeApp: /opt/testApp/node_modules/testApp</command>
</process>
</processes>
</group>
</supergroup>
</supergroups>
</info>
Looking at this documentation, you would think that you can use passenger-status -v to get url's and passwords for each instance. As the note on that page states, this feature is not available for Node.js instances.
The best solution I found was using passenger-status --show=xml -v to get the linux socket for each instance, then using a command line http client to get a response
Here is a working example of doing a simple GET request to the /health endpoint of a specific node.js instance:
$ passenger-status --show=xml -v
...
<address>unix:/tmp/passenger.30Ge2aS/apps.s/node.34l02e</address>
...
$ echo -e "GET /health HTTP/1.1\r\n" | sudo socat unix:/tmp/passenger.30Ge2aS/apps.s/node.34l02e STDIO
HTTP/1.1 200 OK
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ETag: W/"10-8MtBq0wHBRF66BmScI50YQ"
Date: Mon, 04 Jan 2016 17:42:33 GMT
Connection: keep-alive
All systems go!