Session Failover and Clustering

Question

          Let's say that we have two WebServers (NES) with the weblogic plugin (say WS1 and WS2) and a cluster with two WebLogicCommerce AppServers (say AS1 and AS2). Let's assume that each WebServer and AppServer runs on its own machine (total: 4 machines). Now, let's assume that the WebServer "obj.conf" files (on both WS1 and WS2) are setup so that they point to the servers in the cluster (WebLogicCluster="AS1:7601,AS2:7601").
          When a new request comes in to one of the WebServers (say WS1), the plugin will route it to one of the AppServers using Round-Robin (say AS1). A session will now be initiated in AS1 and it sends a response back to the client.
          Question 1: How does the other proxy in WS2 know that all future requests for this client need to be forwarded to AS1?
          Question 2: For failover, does the cluster automatically replicate the session state existing in AS1 onto AS2 before sending the response (does AS2 automatically become the secondary)?
          Now let's assume that AS1 crashes/dies. When the next request from the client comes to WS1 or WS2, they will forward it to AS1 (assuming that WS2 knows about the client session in AS1) . Since AS1 has crashed, will the client eventually get a timeout error message?
          Question 3: To ensure that the session failover happens so that AS2 gets the request instead (becomes the primary), do we need to setup a WebLogic Proxy Server? If so, why can't the plugins for NES provide the failover themselves?
          Thank you very much for your help!
          Giri
          

Answer 1

          Thank you very much for your responses. It has been very helpful and I am clear on the session/clustering stuff. I have new questions on EJB and clustering which I will post as a separate thread.
          Giri
          "Jason Rosenberg" <[email protected]> wrote:
          >And also, if the browser has cookies disabled, it is important for
          >the app server to embed the WebLogicSession info via url rewriting,
          >otherwise the proxy or NES will not be able to route the session
          >properly.
          >
          >So, in all http responses, be sure to pass the url string through
          >response.encodeURL(). This will do the right thing depending
          >on whether cookies are enabled or not.
          >
          >I've just only recently figured this out. Haven't actually tried it
          >all out yet, so forgive me if it is not quite this simple, but this
          >seems to be the gist of it...
          >
          >Jason
          >
          >
          >"Justin James" <[email protected]> wrote in message news:[email protected]...
          >>
          >> Giri,
          >>
          >> I'm not a weblogic representative, but I tried to replicate this proxing service inside a load balancing switch(BigIP) and I
          >discovered a few things. The weblogic server sets a cookie (WebLogicSession)that the webserver plugin uses to manage the proxying.
          >The cookie (found in the HTTP header information) contains encoded information about the primary and secondary application servers
          >that the session is bound too. Any web server can read the cookie to determine how to dispatch the request to the primary server.
          >If the primary server does not respond, the request is forwarded to the secondary server by the plugin. Regardless of cluster size,
          >the session is replicated to only one other server.
          >>
          >> <[email protected]> wrote:
          >> >Giri Alwar wrote:
          >> >
          >> >> I need a couple of clarifications. First with regard to Question 1, I understand that plugins provide load balancing and
          >failover but what I really was asking is how the plugin in WS2 knows that a session for the client has been initiated in AS1 as a
          >result of WS1 sending the initial request to AS1. If WS2 gets a future request from the client, it needs to know this to send the
          >request to AS1. Does the plugin talk to the cluster to find out if there is a primary and who it is?
          >> >>
          >> >
          >> >> I should have clarified that my other questions pertain to in-memory replication. If I do not persist the session in a database
          >then does the client get an error message (timeout) when AS1 goes down (assuming we use NES with the WebLogic plugin)?
          >> >
          >> >Plugins' know how to route requests based on cookies. If it can't reach the primary server it will automatically try secondary.
          >In your case it doesn't matter if it reaches to proxy 1 or proxy 2, it is still the same.
          >> >
          >> >- Prasad
          >> >
          >> >> To prevent this error message and achieve failover, do I need to use WebLogic as the proxy server? If so, why isn't the NES
          >plugin doing this?
          >> >
          >> >> Thanks.
          >> >> Giri
          >> >>
          >> >> Prasad Peddada <[email protected]> wrote:
          >> >> >Giri Alwar wrote:
          >> >> >
          >> >> >> Let's say that we have two WebServers (NES) with the weblogic plugin (say WS1 and WS2) and a cluster with two
          >WebLogicCommerce AppServers (say AS1 and AS2). Let's assume that each WebServer and AppServer runs on its own machine (total: 4
          >machines). Now, let's assume that the WebServer "obj.conf" files (on both WS1 and WS2) are setup so that they point to the servers
          >in the cluster (WebLogicCluster="AS1:7601,AS2:7601").
          >> >> >>
          >> >> >> When a new request comes in to one of the WebServers (say WS1), the plugin will route it to one of the AppServers using
          >Round-Robin (say AS1). A session will now be initiated in AS1 and it sends a response back to the client.
          >> >> >>
          >> >> >> Question 1: How does the other proxy in WS2 know that all future requests for this client need to be forwarded to AS1?
          >> >> >
          >> >> > Plugin takes care of load balancing and failover, it is all transparent to the client.
          >> >> >
          >> >> >
          >> >> >> Question 2: For failover, does the cluster automatically replicate the session state existing in AS1 onto AS2 before sending
          >the response (does AS2 automatically become the secondary)?
          >> >> >
          >> >> > If you have only two yes it is automatically your secondary. Yes, replication is synchronous.
          >> >> >
          >> >> >>
          >> >> >> Now let's assume that AS1 crashes/dies. When the next request from the client comes to WS1 or WS2, they will forward it to
          >AS1 (assuming that WS2 knows about the client session in AS1) . Since AS1 has crashed, will the client eventually get a timeout
          >error message?
          >> >> >
          >> >> > If you are using some kind of persistence then you will be able to retrieve the session information and since the server
          >is not available the request will automatically failover.
          >> >> >
          >> >> >> Question 3: To ensure that the session failover happens so that AS2 gets the request instead (becomes the primary), do we
          >need to setup a WebLogic Proxy Server? If so, why can't the plugins for NES provide the failover themselves?
          >> >> >
          >> >> > No, you need only one. Either NES or weblogic proxy.
          >> >> >
          >> >> >
          >> >> >> Thank you very much for your help!
          >> >> >> Giri
          >> >> >
          >> >> >- Prasad
          >> >> >
          >> >
          >>
          >
          >