Sticky and NON-Sticky sessions

Question

I want to know the difference between sticky- and non-sticky sessions  What I understood after reading from internet   Sticky   only single session object will be there   Non-sticky session   session object for each server node

User · Answer

I ve made an answer with some more details here   https   stackoverflow com a 11045462 592477  Or you can read it there      When you use loadbalancing it means you have several instances of tomcat and you need to divide loads    If you re using session replication without sticky session   Imagine you have only one user using your web app  and you have 3 tomcat instances  This user sends several requests to your app  then the loadbalancer will send some of these requests to the first tomcat instance  and send some other of these requests to the secondth instance  and other to the third  If you re using sticky session without replication   Imagine you have only one user using your web app  and you have 3 tomcat instances  This user sends several requests to your app  then the loadbalancer will send the first user request to one of the three tomcat instances  and all the other requests that are sent by this user during his session will be sent to the same tomcat instance  During these requests  if you shutdown or restart this tomcat instance  tomcat instance which is used  the loadbalancer sends the remaining requests to one other tomcat instance that is still running  BUT as you don t use session replication  the instance tomcat which receives the remaining requests doesn t have a copy of the user session then for this tomcat the user begin a session   the user loose his session and is disconnected from the web app although the web app is still running  If you re using sticky session WITH session replication   Imagine you have only one user using your web app  and you have 3 tomcat instances  This user sends several requests to your app  then the loadbalancer will send the first user request to one of the three tomcat instances  and all the other requests that are sent by this user during his session will be sent to the same tomcat instance  During these requests  if you shutdown or restart this tomcat instance  tomcat instance which is used  the loadbalancer sends the remaining requests to one other tomcat instance that is still running  as you use session replication  the instance tomcat which receives the remaining requests has a copy of the user session then the user keeps on his session   the user continue to browse your web app without being disconnected  the shutdown of the tomcat instance doesn t impact the user navigation

User · Answer

When your website is served by only one web server  for each client-server pair  a session object is created and remains in the memory of the web server  All the requests from the client go to this web server and update this session object  If some data needs to be stored in the session object over the period of interaction  it is stored in this session object and stays there as long as the session exists   However  if your website is served by multiple web servers which sit behind a load balancer  the load balancer decides which actual  physical  web-server should each request go to  For example  if there are 3 web servers A  B and C behind the load balancer  it is possible that www mywebsite com index jsp is served from server A  www mywebsite com login jsp is served from server B and www mywebsite com accoutdetails php are served from server C   Now  if the requests are being served from  physically  3 different servers  each server has created a session object for you and because these session objects sit on three independent boxes  there s no direct way of one knowing what is there in the session object of the other  In order to synchronize between these server sessions  you may have to write read the session data into a layer which is common to all - like a DB  Now writing and reading data to from a db for this use-case may not be a good idea  Now  here comes the role of sticky-session   If the load balancer is instructed to use sticky sessions  all of your interactions will happen with the same physical server  even though other servers are present  Thus  your session object will be the same throughout your entire interaction with this website   To summarize  In case of Sticky Sessions  all your requests will be directed to the same physical web server while in case of a non-sticky loadbalancer may choose any webserver to serve your requests   As an example  you may read about Amazon s Elastic Load Balancer and sticky sessions here   http   aws typepad com aws 2010 04 new-elastic-load-balancing-feature-sticky-sessions html

[session] Sticky and NON-Sticky sessions

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