If multiple users are trying to access one method, how to make use of it at a time one user only?

[mesameen]

If multiple users are trying to access one method, how to make use of it at a time one user only?

eg:

Meteor.methods({

getSession: function() {
var session = Session.findOne({_id:'abc'});
if(session !== undefined){
var sessionId = session._id;
return sessionId;
}
}

});

How to use lock concept in this scenario?