Homework 1 Clarifications

Please note: The deadline for HW1 will be extended a few extra days. It is now due on Tuesday, February 13, at 10:00PM.

  • When you're interacting with the disk manager, you should not be instantiating a new DB anywhere in your BfrMgr. Access it with a call to SystemDefs.JavabaseDB (i.e. SystemDefs.JavabaseDB.read_page). This DB is created by the call to SystemDefs.initDiskMgr in the BMDriver.initBeforeTests method.
  • Use the parameter numbufs that's passed into the BfrMgr constructor to specify how many buffers are in the pool, not the global variable NUMBUF.
  • A call to freePage or flushPage in the BfrMgr class should not be allowed if the pin count is > 1. For flushPage, if the pin count is > 1, write it back to disk but do raise an exception! If it is equal to 0 or 1, it's fine. You are allowed to call freePage on a page that's not in the buffer pool. However, you should not be allowed to flush a page that's not in the buffer pool -- if that happens, throw an Exception.
  • Because the comments are confusing, please make sure that you're using read_page to read a page from disk into memory, and write_page to write the page from memory to disk.
  • For the method BfrMgr.pinPage method, you can ignore the emptyPage parameter -- always regard it as false.
  • When we are testing your code, we will be testing it as one cohesive piece. This means that we will only be making calls directly to the BfrMgr class. It doesn't matter in exactly which classes the work is being done, as long as the results of a call to any of the BfrMgr's methods are as expected.
  • Finally, to fix the problem with the BufMgrReplacer.setBufferManager() method which overrides the corresponding method in the AbstractBufMgrReplacer, try explicitly typecasting the replacer object to a BufMgrReplacer one. Normally, this setBufferManager method will be called from some initialization routine inside the BufMgr class. That call should look something like this:


  • Your code will be tested to verify that it performs like a buffer manager. For example, pin() and unpin() should correctly pin and unpin pages under different states of the buffer pool, dirty pages should be written to disk at the appropriate times, and pick_victim() should pick a frame correctly for a given replacement policy. These examples are only examples, and not meant to define the only set of tests that will be run against your code. During testing, we will only be making direct calls to the methods in the BfrMgr class. This means that a bug in your BfrMgr class could cause you to lose all points even though the replacement policies are totally correct, so be sure to test your code thoroughly with the provided BMDriver!