Why can’t my CakePHP app connect to MySQL through the Google Cloud SQL Proxy Docker container?

Based on your description, the most common issue here is a networking one: your CakePHP container isn’t able to reach the Cloud SQL Proxy because of how Docker handles container networking.

Understanding the Problem

• Host vs. Container Networking:
  When you run the Cloud SQL Proxy with the port mapping (‑p 127.0.0.1:5432:5432), you’re binding the proxy’s port to the host’s loopback interface. However, when you run your CakePHP app inside its own container, using “localhost” from within that container refers to itself—not the host machine.

• Connection Refused & Invalid HTTP Response:
  The error message indicates that while the Cloud SQL Proxy is listening on port 5432 on your host, your CakePHP container isn’t connecting to it correctly. This is why you see “connection refused” in CakePHP but the proxy still logs that it’s running.

Recommended Solutions

1. Connect via the Host Gateway:
   Docker provides a way for containers to connect back to the host machine. For example, on Docker for Windows or Docker for Mac, you can use the DNS name host.docker.internal. In your CakePHP configuration, update the 'host' value from 'localhost:5432' or '[::]:5432' to just the host’s address. For example:

   PHPCopied
   'Datasources' => [
       'default' => [
           'className' => Connection::class,
           'driver' => Mysql::class,
           'persistent' => false,
           'host' => 'host.docker.internal', // Updated host
           'port' => '5432',                 // Optionally specify port separately if needed
           'username' => 'myusername',
           'password' => 'mypassword',
           'database' => 'mydatabasename',
           'encoding' => 'utf8mb4',
           'timezone' => 'UTC',
           'cacheMetadata' => true,
           'flags' => [],
           'log' => false,
           'quoteIdentifiers' => false,
       ]
   ],

   By doing this, your CakePHP container will look for the proxy on the host machine rather than within itself.

2. Use a Shared Docker Network (e.g., Docker Compose):
   If you run both containers in the same user-defined network (for example, via Docker Compose), you can refer to the Cloud SQL Proxy container by its service name. For example, if your proxy service is named “sqlproxy”, your Datasource could be:

   PHPCopied
   'host' => 'sqlproxy',
   'port' => '5432',

   In this case, you’d also want to remove the port mapping that binds the proxy port to the host interface and allow Docker’s internal DNS to resolve the service name.

Summary

• The error is due to how networking is isolated between containers.
• Adjust your CakePHP configuration to use either the host gateway (host.docker.internal) or run both containers in the same custom network and refer to the proxy container by its name.
• Make sure that the Cloud SQL Proxy container is started properly with the correct port configuration and that your MySQL database instance on Google Cloud SQL is correctly set up to allow external connections via the proxy.

By addressing the networking boundary between your containers, your CakePHP app should be able to connect successfully to your Google Cloud SQL instance via the Cloud SQL Proxy.