Session redirect recommendation

[satur9nine]

To be clear I am not a web developer by training, only a dabbler. I read the following on the Session documentation page https://www.playframework.com/documentation/3.0.x/JavaSessionFlash :

The last point can be a source of confusion. When you modify the cookie, you are providing information to the response, and Play must parse it again to see the updated value. If you would like to ensure the session information is current then you should always pair modification of a session with a Redirect.

I still don't understand why you "always" need to redirect. Here is a conversation I had with ChatGPT about this, and ChatGPT also believes that redirect is in fact unnecessary in most cases; virtually the opposite of "always" needed.

Am I missing something?

Jacob

---

ME:

I am unclear why they recommend "If you would like to ensure the session information is current then you should always pair modification of a session with a Redirect." Firstly they didn't say what you are redirecting the client to, I presume the same address? More importantly a redirect means the client will make another request to your server causing additional load, and a performance penalty the user might notice. Also there is a small chance you could cause a redirect loop. Furthermore it is still extremely unclear why you need a redirect at all. What do they mean "ensure the session information is current", yes you need to pass back changes to the session to the client but while the request is being processed you can track any changes to the session in memory and pass those changes to any HTML template as needed there seems to be no absolute requirement that you utilize redirect at all.

CHATGPT

Your skepticism is well-founded, and you raise excellent points about the unnecessary overhead, potential for redirect loops, and the unclear necessity for using redirects to "ensure the session information is current." Let's dissect the reasoning behind this recommendation and whether a redirect is truly needed in practice.

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What They Likely Mean by "Ensure the Session Information Is Current"

The suggestion to pair session modifications with a redirect is likely rooted in the following considerations:

1. Session Updates and Client Consistency:

   • In Play Framework, session data is stateless and stored in cookies. Any changes to the session data during a request are not reflected in the current Http.Request object because the request was initialized when it arrived, and the modified session cookie is added to the Http.Response.
   • After a response with an updated session cookie is sent, the client must include the new cookie in subsequent requests for the server to recognize the updated session.

   Implication: If you modify the session and immediately attempt to use the updated session data during the same request lifecycle (e.g., within the same request handler), you might encounter inconsistencies because the session changes are not yet "active."

2. Redirect as a Solution:

   • A redirect forces the client to send a fresh request to the server, including the updated session cookie. This ensures that any session-dependent operations in the subsequent request reflect the most up-to-date session.

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Why a Redirect Is Often Overkill

In many cases, using a redirect to ensure "session information is current" introduces unnecessary complexity and overhead. Let's address the concerns you raised:

1. Redundant Client Request:

   • A redirect forces the client to make an additional HTTP request, which increases server load and introduces latency.
   • This is unnecessary if you don't need the updated session data immediately during the same request lifecycle.

2. Redirect Loops:

   • Poorly designed redirect logic could lead to infinite loops, especially if the condition triggering the redirect isn't updated correctly after the redirect.

3. Session State Management Without Redirect:

   • You can track session changes in memory during the request and pass any modified session data to your response (e.g., HTML templates, API responses) without requiring a redirect.
   • Play Framework already handles propagating session updates to the client via the Set-Cookie header. The updated session will be available to the client and subsequent requests.

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When a Redirect Might Be Necessary

A redirect could be appropriate in these specific scenarios:

1. Flow Continuity:

   • If the updated session state is critical to completing a user interaction or redirecting the user to another step in a process (e.g., after login), then a redirect can serve a functional purpose beyond session consistency.

2. Stateless Design:

   • In some strict stateless designs, you might enforce redirect logic to ensure every new state change (e.g., a session modification) is immediately reflected in subsequent requests.

3. Inconsistent Behavior Without Redirect:

   • If your application relies on middleware or framework behaviors that depend on the updated session being included in the subsequent request, a redirect ensures consistency.

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Example Without Redirect (Recommended for Most Use Cases)

You can modify the session and return a response directly, ensuring that any session changes are included in the response's Set-Cookie header:

public Result index(Http.Request request) {
    String sessionId = request.session().getOptional("sessionId").orElse(null);

    if (sessionId == null) {
        sessionId = UUID.randomUUID().toString();
        return ok("New Session ID: " + sessionId)
                .addingToSession(request, "sessionId", sessionId);
    }

    return ok("Existing Session ID: " + sessionId);
}

In this approach:

• The session is updated and sent back in the Set-Cookie header.
• The client includes the updated session in subsequent requests.
• No redirect is required unless the updated session is critical to subsequent request processing.

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Example With Redirect (Only If Necessary)

Use a redirect only if session-dependent operations rely on the updated session cookie being sent back in the next request:

public Result index(Http.Request request) {
    String sessionId = request.session().getOptional("sessionId").orElse(null);

    if (sessionId == null) {
        sessionId = UUID.randomUUID().toString();
        return redirect(routes.HomeController.index())
                .addingToSession(request, "sessionId", sessionId);
    }

    return ok("Session ID: " + sessionId);
}

Here:

• The redirect ensures that the next request contains the updated session.
• Use this only when absolutely necessary (e.g., session changes impact application logic immediately).

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Why You Rarely Need a Redirect

You don't need a redirect in the majority of cases because:

• Session updates are automatically handled via cookies and propagated to subsequent requests.
• You can track changes to the session in memory during the request lifecycle and pass any modified data to templates or API responses as needed.

Conclusion

The recommendation to always use a redirect after modifying session data is overly cautious and unnecessary in most cases. You should only use a redirect if the updated session is essential for operations in the same workflow or if middleware depends on the updated session. Otherwise, modifying the session and returning a response directly is simpler, faster, and less error-prone.

[irumiha]

That section in the documentation is confusing and probably unnecessary. Adding or removing data from a session cookie is done at the very end of the request processing, when you generate a response. All code examples show that pattern. Modifying the session in the request makes no sense and is not even shown in the example code.

If someone is thinking of using the session as some sort of mutable request processing context that gets added to as the request is processed in multiple steps, then my response to that is: don't use the session for that :) Use your own data structure such as a Map or whatever, then at the very end add the contents of that structure to the session when creating a response object.

The only scenario where the documentation makes any kind of sense is when you need this hypothetical multi-step process separated out and logged individually by the infrastructure logging. But that's a very weird way of moving forward the request processing logic.

I hope this makes any sense to you. In short, let's just say: don't worry about it. I would never redirect for the reasons stated in the documentation. There are simpler ways to process your requests.

[satur9nine]

Makes sense, thanks!