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What does the term sticky session mean and how is it achieved?
JBoss Enterprise Application Platform
What does the term sticky session mean and how is it achieved?
Sticky session refers to the feature of many commercial load balancing solutions for web-farms to route the requests for a particular session to the same physical machine that serviced the first request for that session. This is mainly used to ensure that a in-proc session is not lost as a result of requests for a session being routed to different servers. Since requests for a user are always routed to the same machine that first served the request for that session, sticky sessions can cause uneven load distribution across servers.
Sticky sessions is most commonly maintained by the session’s jvmRoute (used my mod_jk, mod_cluster, and mod_proxy_cluster). When JBoss creates a session, it creates it in the format “id. jvmRoute”. The jvmRoute serves as an identifier for that node so all subsequent requests for a session are stickied and sent straight to the node indicated by the jvmRoute in the session cookie. If the request fails over from one JBoss node to another, then JBoss will update the client’s session cookie so that their session’s jvmRoute matches the new node they failed over to.
Other third party load balancers may maintain sticky sessions instead by remember the client ip address and which backend server that client was sent to originally.
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Sticky Sessions: Good or Bad? – Stack Overflow
If a single request is sent to load balanced set of servers only one of the servers should get the request, typically allocated via round robin. If you are issuing a single request and it hits both your servers something else is wrong.
Otherwise I will assume that you are issuing 2 rapid request and they hit both of your load balanced servers (as round robin would), that your transaction does not complete before the second request hits the server and that you believe sticky sessions would solve this issue.
A sticky session would send all requests in this session to the same server. In your example both requests would now hit the same server and if you did nothing else, the transaction for the first request would not have been committed before the second request started, so you would get the same result i. e sticky sessions alone would not help.
If the transaction were something like placing an order then you could craft your code so that upon successful commit the contents of the cart were deleted.
The first request to complete would delete the cart, the second request would fail and you could message the user that the order had already been placed.
Sticky sessions can make it more complicated to have high availability and scalability. For the former, consider the case where one server goes down – all sessions on that server will also go down and you will have to write code to fail them over to the other server.
For the latter case, assuming your sessions last some interval e. g. 1/2 hour, if you have a N new users come to the site they will initially be evenly divided between both of your servers. If before 1/2 hour all of the users from server 1 leave and another M users come in, then you will have more load on server 2 which has original N/2 user plus new M/2 users while serer 1 only has M/2 users i. e you will have wasted capacity and will need to code to fix.
There are times when sticky sessions may be useful, but unless you have a good reason to use them I would avoid them
Sticky and NON-Sticky sessions – Stack Overflow
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 is served from server A, is served from server B and 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 load balancer may choose any webserver to serve your requests.
As an example, you may read about Amazon’s Elastic Load Balancer and sticky sessions here:
Frequently Asked Questions about sticky sessions
Are sticky sessions bad?
Sticky sessions can make it more complicated to have high availability and scalability. For the former, consider the case where one server goes down – all sessions on that server will also go down and you will have to write code to fail them over to the other server.Nov 21, 2011
What are sticky and non sticky sessions?
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 load balancer may choose any webserver to serve your requests.Sep 13, 2016
What are sticky sessions in AWS?
Sticky sessions are a mechanism to route requests from the same client to the same target. Elastic Load Balancers support sticky sessions. Stickiness is defined at a target group level.