[SalesForce] How do i update user alias in Salesforce

You should be able to populate the fields from an external system without a problem.

However, there's one thing to keep in mind, the community nickname is a unique name used on the salesforce community.

• Alias <= 8 chars
• Community nickname <= 40 chars

If your action is tripped by a DML on Contact, then you're mixing a DML operation of a non-setup object (Contact) with a setup object (User) and therefore you have to kick your trigger action to be handled by an @future method. There are no special limits here (other than apex trigger limits) as it applies to portal or communities context. Some things to consider though as you architect this code:

• User context is lost in the @future method. That is to say, future methods run in system context. If you want to preserve something about the running user, you will need to pass it in as an argument to your future method.
• How are you planning handle deactivated users attempting to login again? Ideally, the login page would surface a helpful message about why a user is deactivated, or you would handle reactivation in your registration handler class. Otherwise it could just be confusing for deactivated users attempting to login after a lapse.