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Shephali SwarnkarShephali Swarnkar 

Please help me understand Apex Replay Debugger. also why we call replay debugger. what extra information we get using checkpoint rather than breakpoint

SwethaSwetha (Salesforce Developers) 
HI Shephali,

The Apex Replay Debugger is a powerful tool that allows you to see exactly what line of code you're on, the state of memory, and so on, just as you would with a live debug session, but without pausing a live server thread. This is very similar to the Developer Console's Debug Log Viewer, except coupled with an IDE so that you can easily step through, set "breakpoints", and all other kinds of nifty debug tricks on a transaction, all without having to pause and/or run the same code on the server over and over again.

Other tools are out there, such as Salesforce's Check Points, which are integrated with the Developer Console, or you can use more generic log viewers, like Microsoft Log viewer. Each tool has its own advantages and disadvantages. The Apex Replay Debugger seems to be finicky to get working correctly, but when it does, it provides a very easy-to-use, intuitive interface that any Java or C++, JavaScript, etc developer feels at home with. It's one of the best options out there for debugging Apex (it's certainly's best version of offline/postmortem debugging to date)

If this information helps, please mark the answer as best. Thank you
Shephali SwarnkarShephali Swarnkar

     Thanks for your reply... Why checkpints always says "List of size n(any number) too large to display" ??
 I saw this for even for very small size 6. I can understand it if it is for larger than 200(exceeding the batch size), but why for very small size too??