WebLogic domain consists of Servers, Machines, Clusters, Applications and Configurations related to them. Administration sever is the controlling instance for the domain. It is the keeper of all the configuration. Ideally you don't want to host (target) any application or service that clients would access on the admin instance. Managed server(s) or cluster(s) are meant to host such applications and services that clients would access (servers other than admin server in a domain are called as managed servers).
Having said that, if an admin instance fails during the middle of the day what happens to my domain? It is the controlling instance so will it halt my domain? The answer is NO. Admin server will provide all the configurations to each managed server and they will be cached locally. There are limited dependency between the servers during runtime. So an admin server failure will not affect the running managed server(s) or cluster of managed server(s).
Alright, what about the "Yes" part of the above answer?! There are few services you will lose when an admin instance is not available. You cannot introduce any new configuration or change any existing configuration in the domain. Examples are: you cannot deploy new applications, you cannot change the JNDI name of a JMS destination hosted on one or more servers etc. You will also lose the domain log entries in the Domain Log file maintained by the administration server. Still you can use the Server Log maintained on each managed server for auditing or troubleshooting purpose. Also lastly the managed server(s) in the absence of admin server can only be started or restarted in Independence Mode (using the previously cached local configuration information).
- You are not expected to introduce new configuration or change any configuration on the fly in your production setup so losing the ability to do this will have literally no or little impact.
- You can use server log file entries for auditing and troubleshooting purposes.
- The managed server can be restarted or started if required during the absence of admin server using Independence Mode (which is a default setup now in WLS 10.x - no extra configuration required).
So now you know why I said "No" and "Yes"! huh!