CORS configuration in Ruby on Rails - Step-by-Step Execution

Concept Flow - CORS configuration

Client sends request

↓

Server receives request

↓

Check Origin header

↓

Is Origin allowed?

No→Reject request or no CORS headers

Yes↓

Add CORS headers to response

↓

Send response back to client

The server checks the request's origin and adds CORS headers if allowed, enabling cross-origin access.

Execution Sample

Ruby on Rails

Rails.application.config.middleware.insert_before 0, Rack::Cors do
  allow do
    origins 'https://example.com'
    resource '*', headers: :any, methods: [:get, :post]
  end
end

This code configures Rails to allow CORS requests from https://example.com for GET and POST methods.

Execution Table

Step	Action	Request Origin	Allowed Origins	CORS Headers Added	Response Sent
1	Client sends request	https://example.com	https://example.com	No	No
2	Server receives request	https://example.com	https://example.com	No	No
3	Check if origin is allowed	https://example.com	https://example.com	No	No
4	Add CORS headers to response	https://example.com	https://example.com	Yes	No
5	Send response back to client	https://example.com	https://example.com	Yes	Yes
6	Client sends request	https://notallowed.com	https://example.com	No	No
7	Server receives request	https://notallowed.com	https://example.com	No	No
8	Check if origin is allowed	https://notallowed.com	https://example.com	No	No
9	Send response back to client	https://notallowed.com	https://example.com	No	Yes

💡 Requests from allowed origins get CORS headers; others do not but still get a response.

Variable Tracker

Variable	Start	After Step 3	After Step 4	After Step 5	After Step 8	After Step 9
request_origin	N/A	https://example.com	https://example.com	https://example.com	https://notallowed.com	https://notallowed.com
allowed_origins	['https://example.com']	['https://example.com']	['https://example.com']	['https://example.com']	['https://example.com']	['https://example.com']
cors_headers_added	false	false	true	true	false	false
response_sent	false	false	false	true	false	true

Key Moments - 2 Insights

Why does the server add CORS headers only for some origins?

The server checks if the request's origin matches the allowed origins list (see execution_table step 3). Only if it matches, CORS headers are added (step 4). This prevents unauthorized cross-origin access.

What happens if the origin is not allowed?

The server does not add CORS headers but still sends a response (see execution_table steps 8 and 9). The browser will block access to the response due to missing CORS headers.

Visual Quiz - 3 Questions

Look at the execution table, at which step are CORS headers first added for an allowed origin?

A Step 3

B Step 4

C Step 5

D Step 2

💡 Hint

Check the 'CORS Headers Added' column for the first 'Yes' value for https://example.com.

According to the variable tracker, what is the value of 'cors_headers_added' after step 8 for a disallowed origin?

A false

B true

C undefined

D null

💡 Hint

Look at the 'cors_headers_added' row under 'After Step 8' column.

If we add 'https://notallowed.com' to allowed origins, how would the execution table change at step 8?

A Response would not be sent at step 9

B Request origin would change to https://example.com

C CORS headers would be added at step 8

D No change in CORS headers

💡 Hint

Adding origin to allowed list means CORS headers get added when that origin requests (see step 3 logic).

Concept Snapshot

CORS configuration in Rails:
- Use Rack::Cors middleware in config/application.rb
- Specify allowed origins with origins 'https://example.com'
- Define allowed resources and HTTP methods
- Server checks Origin header and adds CORS headers if allowed
- Browsers enforce CORS based on these headers

Full Transcript

CORS configuration in Rails works by adding middleware that checks the origin of incoming requests. If the origin matches the allowed list, the server adds special headers to the response. These headers tell the browser it is safe to share the response with the requesting site. If the origin is not allowed, the server sends the response without these headers, so the browser blocks access. The example code uses Rack::Cors middleware to allow requests from https://example.com for GET and POST methods. The execution table shows how requests from allowed and disallowed origins are handled step-by-step, including when headers are added and responses sent. The variable tracker follows key variables like request origin and whether CORS headers were added. Key moments clarify why headers are only added for allowed origins and what happens otherwise. The quiz tests understanding of when headers are added and how changing allowed origins affects behavior.