Hello, I am using Plack::Session method to store some string , but session id was changing after a big string been stored in session, what is the size limitation for such methods

What are you using? Have you read Plack::Session::Store::File and Plack::Session::Store::Cache?

Hello Anonymous Monk , I am using Plack::Session::Store::Cache and store the string in PSGI env hash using a key. So do you recommend saving it in file for reuse ?

Hello Anonymous Monk , I am using Plack::Session::Store::Cache and store the string in PSGI env hash using a key. So do you recommend saving it in file for reuse ?

No. I recommend you read docs for whatever module / options you are using, so you can figure out if there are any limitations that affect you.

For whatever reason you chose the module/options you're using, so now you need to figure out if you're hitting a design limitation or a bug or if you wrote buggy code

You can use a different plack-store/cache-store/chi-store to try and diagnose where the problem lies

But at this point I can't tell whats going on. At first it sounded like you might be using Plack::Session::State::Cookie which have size limitations.

But given your new information, there are no guesses to be had, only to start looking at your code first, meaning, start dumpering data to figure out when it goes missing