Understanding -msimode for managed servers and how it can be used with Oracle EBS r12.2

 Recently, I was asked implications of Admin server availability when connecting to Oracle EBS login Page.
When pondering further, Few scenarios that arise were -
1. What if running Admin server goes down for some reason, will this impact managed servers that are already running?
2. Can we start managed server (say oacore_server1) if admin server is down? How?

Let's start understanding relationship between admin server and managed server.

Admin server 

is the central entity for configuration of entire domain. You manage and 
configure all resources in the domain through admin server. Config.xml is loaded
by Admin server during its startup. This file is stored under -
$FMW_Home/user_projects/domains/EBS_domain/config

Managed server 

is responsible for hosting components and associated resources that comprise your application (ex. JSPs, EJBs,datasources,JMS Modules,etc).

During startup of managed server, it will reach out to admin server and get configuration and deployment settings. This pretty much the second question-- 

Can we start managed server (say oacore_server1) if admin server is down? How?

But there is a catch here, we have MSI mode for managed servers to skip this dependency. MSI stands to Managed Server Independence

During Startup -

Managed Server ---> configuration files ---> Admin server 

if Admin server is down ---> Managed server fails to start

If Managed server has MSI enabled(by default enabled) 

                                --> use -msimode option

                                    --> managed server starts by directly accessing configuration/security files

By default, MSI is enabled and below navigation can be used to check the same -

Environment 

          > Servers  

    > Select Managed server(say oacore_server1)

> Tuning>(scroll down)

       > Advanced

       >Checkbox for MSI

Using MSI mode with oacore_server1 managed server -

admanagedsrvctl.sh start oacore_server1 -msimode                        

Coming onto 1st question, 

What if running Admin server goes down for some reason, will this impact managed servers that are already running?

1. If servers(admin and managed) are on same host machine, typically the case in a single node ebs environment, then obviously all servers will have similar implications.

2. If only the admin server is down while managed servers are up and running, failure of an Administration Server itself does not interrupt the operation of Managed Servers in the domain

Ref- https://docs.oracle.com/cd/E13222_01/wls/docs90/domain_config/understand_domains.html

Note: When admin server is down, each Managed Server periodically attempts to reconnect to the Administration Server to synchronize its configuration state with that of the Administration Server.

Concluding Thoughts - 

It is important to understand dependency between admin server and managed server when starting up application services as there may be scenarios when managed servers would be waiting for admin server for EBSDomain to start and eventually result in overall availability of the system. -msimode may be used to independently start a managed server when admin server is not available and root cause of admin server can be fixed later.