Managed accounts is a brand new concept of SharePoint 2010. They are designed to give administrators more control over the domain accounts that are used in different section of SharePoint.
When you register a account with SharePoint, administrators can maintain the account from SharePoint itself:
1) Free from worries of password change how & where in SharePoint
If a password left unchanged in any part of SharePoint it can do lot of hell J
When a domain account is registered with SharePoint as a managed account. It can be used as:
1) To run application pools
2) To run service applications.
The account which install SharePoint2010 is automatically/already registered as a managed account. More accounts can be added later by clicking è Security category in Central Administration home page and select Configure managed accounts under the subcategory General Security. While registering a managed account, you simply need to provide the
username with domain and password.
You have many option there like i.e. to have SharePoint automatically handle the password changes for you. This option is the most useful option as it removes from burden to change password everywhere in the SharePoint :
If you decide to use the automatic password change option, SharePoint will take over setting the password for the account in Active Directory for as long as the account is registered as a managed account.
If don’t allow SharePoint to change the passwords automatically; It can still be easy to manage password from within Central Administration now from the Managed Accounts page, you can edit the account whose password you like to change. There you can change the password by checking the box next to Change password now.
Imp Note: If someone change the password for the account directly in AD without the knowledge of SharePoint, your managed accounts will not work. SharePoint needs the account’s password to use it. For any password change it is recommended to change the password from SharePoint and not with AD.
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Hope this help