What is MVC Architecture of Salesforce?

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This will be a very high level introduction to salesforce and its capabilities . In my opinion, the reason for salesforce to become so popular in a short duration of time is the fact that it is a cloud based application. The puts salesforce at a huge advantage over other extremely successful CRM tools like Siebel. The trend seems to be catching up and it is in someways alarming to see the pace at with Salesforce is capturing the CRM market. Being a Multitenant application, Salesforce is able to integrate new features directly into the customers application without asking for the customer to upgrade or install anything. No doubt there are a lots of issues that Salesforce still needs to address. Many of my colleagues have come across many bugs with Salesforce and furthermore it is evident that being a shared application, there a limitations in the flexibility that Salesforce can provide as compared to other legacy applications like Siebel. This is one of the main reasons why many companies are reluctant to make a switch to Salesforce. Security and the cost of the applications are other reasons. There have been instances of phishing attack on salesforce customers and also it is quite well known that swithing to salesforce could be heavy on the pocket for a small or medium size enterprise. All said and done it is an extremely promising technology and I wouldn't think twice about investing my time learning it. Plus I find the interface very user friendly and they are always coming up with new enhancements. My favorite aspect about Salesforce is the fact that it is documented really well ( perhaps because it is so new) and also the ideas and discussion boards are great with really talented developers and moderators. I would bet on salesforce to be the next big change in the CRM world atleast for the next few years.

What is the difference between a isBLANK and an isNULL?

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Both same thing. The problem is that salesforce made isNULL but it could not check if a blank text field is null. So it used for work for most data types but not strings. So they invented IsBLANK that is capable of checking if string is empty or not. Remember however that even a space or colon is considered as not empty. Anyway in future it is always recommended that we use isBlank.

What is the difference between a cross object formula and a roll up summary?

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There are many but the main one ofcourse if that a cross object formula references fields of its parent records to perform various operation and return a result where as a roll up summary calulates the Count, sum,max, min of some fields in the child records. Also , a roll up summary is not actually calculated and stored in the database. It is calculated only at run time. Were as a formula is calculated and stored like any other field. There are a some more but these are the cream of the cake .