How to replicate session in memory on Oracle Weblogic?

Question

I want to create a high availability with Oracle Weblogic. First, I create a cluster called MyCluster and add two servers (Server1 and Server2) to MyCluster. I use Nginx as a load balancer.

I follow the tutorial from https://www.oracle.com/webfolder/technetwork/tutorials/obe/fmw/wls/12c/12-ManageSessions--4478/session.htm#t1 to replicate session in memory.

Here is my nginx config:

upstream myweb {
    server server1:38080 weight=1;
    server server2:38080 weight=1;
    server server3:38080 weight=1;
}

server {
    listen       80;
    server_name  nginxHost;

    access_log  /var/log/nginx/nginxHost.access.log  main;
    error_log   /var/log/nginx/nginxHost.error.log warn;

    location / {
        proxy_pass http://myweb/;
        proxy_set_header Host $host;
        proxy_set_header X-Real-IP $remote_addr;
        proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
    }

    error_page   500 502 503 504  /50x.html;
    location = /50x.html {
        root   /usr/share/nginx/html;
    }

}

When I test session replication, I have a problem. If Server1 is running and Server2 is shutdown now, I connect my application which is on Server1. I power on Server2 and wait it complete startup. Then I shutdown Server1 and refresh browser. The session disappear.

Finally, I find I have to refresh browser after Server2 is running. Is there any way to replicate session when servers start?