# Can't retract http\_session **URL:** \ **Category:** Help!\ **Created:** [October 26, 2020, 6:46am UTC](https://swi-prolog.discourse.group/t/cant-retract-http-session/3097 "2020-10-26T06:46:36Z")\ **Posts on this page:** 10\ **Page:** 1
**Author:** ![evmusonov](https://yyz2.discourse-cdn.com/free1/user_avatar/swi-prolog.discourse.group/evmusonov/32/2967_2.png) [@evmusonov](https://swi-prolog.discourse.group/u/evmusonov)\ **Post date:** [October 26, 2020, 6:46am UTC](https://swi-prolog.discourse.group/t/cant-retract-http-session/3097/1 "2020-10-26T06:46:36Z")
I’m using: SWI-Prolog version 8.2.1 I want the code to: retract the http session using http\_session\_retract. But what I’m getting is: Internal server error My code looks like this: ```prolog http_session_assert(check_session(isset)). %creating http_session_retract(check_session(isset)). %deleting ``` check\_session(): ```prolog check_session(_) :- not(http_session_data(check_session(isset))), !. check_session(Request) :- http_redirect(moved, root(user), Request). ``` So asserting is working, because I get the redirect to /user. And one another question: how can I see what I have in the session? ---
**Author:** ![jan](https://yyz2.discourse-cdn.com/free1/user_avatar/swi-prolog.discourse.group/jan/32/4_2.png) [@jan](https://swi-prolog.discourse.group/u/jan)\ **Post date:** [October 26, 2020, 7:37am UTC](https://swi-prolog.discourse.group/t/cant-retract-http-session/3097/2 "2020-10-26T07:37:43Z")
Please create a self contained small program so we can see what you are doing precisely. Retracting facts from the session generally works, so there is something specific in what you are doing causing the problem. ---
**Author:** ![evmusonov](https://yyz2.discourse-cdn.com/free1/user_avatar/swi-prolog.discourse.group/evmusonov/32/2967_2.png) [@evmusonov](https://swi-prolog.discourse.group/u/evmusonov)\ **Post date:** [October 26, 2020, 9:51am UTC](https://swi-prolog.discourse.group/t/cant-retract-http-session/3097/3 "2020-10-26T09:51:30Z")
So I want to do this: if the session exists, you get a redirect from the index page to /user, and if not - each handler redirects you to the index page. Here is an example. ```prolog :- use_module(library(http/thread_httpd)). :- use_module(library(http/http_dispatch)). :- use_module(library(http/http_error)). :- use_module(library(http/html_write)). :- use_module(library(http/html_head)). :- use_module(library(http/http_client)). :- use_module(library(http/http_session)). server(Port) :- http_server(http_dispatch, [port(Port)]). :- http_handler(root(.), index_controller, []). :- http_handler(root(user), user_controller, []). :- http_handler(root('add-session'), session_controller, []). index_controller(Request) :- check_session(Request), format('Content-type: text/html~n~n', []), format('Hi!', []). check_session(Request) :- http_session_data(check_session(isset)), http_redirect(moved, root(user), Request). user_controller(_Request) :- format('Content-type: text/html~n~n', []), format('Username here', []). session_controller(_Request) :- http_session_assert(check_session(isset)), format('Content-type: text/html~n~n', []), format('Adding a session', []). ``` ---
**Author:** ![jan](https://yyz2.discourse-cdn.com/free1/user_avatar/swi-prolog.discourse.group/jan/32/4_2.png) [@jan](https://swi-prolog.discourse.group/u/jan)\ **Post date:** [October 26, 2020, 10:46am UTC](https://swi-prolog.discourse.group/t/cant-retract-http-session/3097/4 "2020-10-26T10:46:40Z")
Thanks. That is getting a bit clearer. The index\_controller/1 is definitely wrong. First of all, HTTP handlers must succeed: failing or raising an exception is translated into an HTTP error code. check\_session/1 may fail as http\_session\_data/1 may fail. If this succeeds though, it will redirect and never reach the remainder of index\_controller/1. Hope this helps getting started. What you really want it probably to protect the logged in part of the server using a single handler that has `prefix` and `authentication(Type)` set for the root of the protected area. Now any attempt to get to one of the handlers below will call the authentication hook. You can use one of the basic implementations or add your own to deal with the login as you please. ---
**Author:** ![evmusonov](https://yyz2.discourse-cdn.com/free1/user_avatar/swi-prolog.discourse.group/evmusonov/32/2967_2.png) [@evmusonov](https://swi-prolog.discourse.group/u/evmusonov)\ **Post date:** [October 27, 2020, 4:14am UTC](https://swi-prolog.discourse.group/t/cant-retract-http-session/3097/5 "2020-10-27T04:14:53Z")
Okay, let me get this straight. For example, I need to protect routes: /user, /user/\*, so I set `prefix` and `authentication` options like this: ```prolog :- http_handler(root(user), user_controller, [authentication(basic), prefix]). ``` Is that correct? P.S. Is it me or the official documentation is written badly? Because I’ve tried to use http\_write\_passwd\_file/2 as the documentation says `http_write_passwd_file(+File, +Data:list)`: ```prolog index_controller(_Request) :- http_write_passwd_file(passwd, [user, \crypt('12345')]). ``` And I get this: `Type error: 'passwd_entry' expected, found 'user' (an atom)` What is wrong with parameters? ---
**Author:** ![CapelliC](https://yyz2.discourse-cdn.com/free1/user_avatar/swi-prolog.discourse.group/capellic/32/17_2.png) [@CapelliC](https://swi-prolog.discourse.group/u/CapelliC)\ **Post date:** [October 27, 2020, 6:14am UTC](https://swi-prolog.discourse.group/t/cant-retract-http-session/3097/6 "2020-10-27T06:14:26Z")
> [@evmusonov](#): > > `http_write_passwd_file` Sorry I never used http\_write\_passwd\_file/2, but docs seems pretty clear to me: `Data is a list of entries as ... passwd(User, Hash, Fields)`. When in doubt, you can inspect the inlined source code of the predicate clicking on the `:-` orange button after the determinacy specifier ([det] here). ---
**Author:** ![jan](https://yyz2.discourse-cdn.com/free1/user_avatar/swi-prolog.discourse.group/jan/32/4_2.png) [@jan](https://swi-prolog.discourse.group/u/jan)\ **Post date:** [October 27, 2020, 7:39am UTC](https://swi-prolog.discourse.group/t/cant-retract-http-session/3097/7 "2020-10-27T07:39:13Z")
> [@evmusonov](#): > > Is that correct? Yes. That is, if you want classical HTTP authentication. ---
**Author:** ![evmusonov](https://yyz2.discourse-cdn.com/free1/user_avatar/swi-prolog.discourse.group/evmusonov/32/2967_2.png) [@evmusonov](https://swi-prolog.discourse.group/u/evmusonov)\ **Post date:** [October 27, 2020, 7:48am UTC](https://swi-prolog.discourse.group/t/cant-retract-http-session/3097/8 "2020-10-27T07:48:25Z")
The description of Data is a bit clear. Why there is no information about File? How should I write the path to the file? Is it an absolute or abstract path? I’ve always written imperative code and used official websites for infromation. But now it’s so hard for me to understand this logic. ---
**Author:** ![evmusonov](https://yyz2.discourse-cdn.com/free1/user_avatar/swi-prolog.discourse.group/evmusonov/32/2967_2.png) [@evmusonov](https://swi-prolog.discourse.group/u/evmusonov)\ **Post date:** [October 27, 2020, 8:26am UTC](https://swi-prolog.discourse.group/t/cant-retract-http-session/3097/9 "2020-10-27T08:26:13Z")
Thank you for your answer. Could you please tell me what I am doing wrong with `http_write_passwd_file`? ---
**Author:** ![jan](https://yyz2.discourse-cdn.com/free1/user_avatar/swi-prolog.discourse.group/jan/32/4_2.png) [@jan](https://swi-prolog.discourse.group/u/jan)\ **Post date:** [October 27, 2020, 10:37am UTC](https://swi-prolog.discourse.group/t/cant-retract-http-session/3097/10 "2020-10-27T10:37:21Z")
You find an example in the SWISH code, e.g. [https://github.com/SWI-Prolog/swish/blob/c0e750f9cc569031f91bee18733c292fa5ce5fb5/lib/plugin/http\_authenticate.pl#L297](https://github.com/SWI-Prolog/swish/blob/c0e750f9cc569031f91bee18733c292fa5ce5fb5/lib/plugin/http_authenticate.pl#L297)