MIT OpenCourseWarehttp://ocw.mit.edu 6.033 Computer System EngineeringSpring 2009 For information about citing these materials or our Terms of Use, visit: http://ocw.mit.edu/terms.This course makes use of Athena, MIT's UNIX-based computing environment. OCW does not provide access to this environment.M.I.T. DEPARTMENT OF EECS 6.033 - Computer System Engineering WAL Hands-On Assignment Hands-on 5: Write Ahead Log System Intro to wal-sys This hands-on assignment will give you some experience using a Write Ahead Log (WAL) system. This system corresponds to the WAL scheme described in Section 9.3 of the course notes. You should carefully read that section before attempting this assignment. You can do this hands-on on any computer that has a Perl language interpreter, but we will be able to answer your questions more easily if you run this on an Athena workstation. You can download the WAL system from here (if your browser displays the file in a window instead of saving it, use "File -> Save As" to save the file). The downloaded file is a Perl script named wal-sys. Before trying to run it, change its permissions to make it executable, for example by typing: athena% chmod +x wal-sys The wal-sys script can be run as follows: athena% wal-sys [-reset] Alternatively, you can run the script as: athena% perl ./wal-sys [-reset] Wal-sys is a simple WAL system that models a bank's central database, implementing redo logging for error-recovery. Wal-sys creates and uses two files, named LOG and DB, in the current working directory. The "LOG" file contains the log entries, and the "DB" file contains all of the installed changes to the database. After you start wal-sys, you can enter commands to manage recoverable actions and accounts. There are also commands to simulate a system crash and to print the contents of the "LOG" and "DB" files. All the commands to wal-sys are case sensitive. Since wal-sys uses the standard input stream, you can use the system in batch mode. To do this, place your commands in a file ("cmd.in" for example) and redirect the file to wal-sys's standard input: athena% wal-sys -reset < cmd.in. When using batch mode, make sure that each command is followed by a newline character (including the last one). When you restart wal-sys, it will perform a log-based recovery of the "DB" file using the "LOG" file it finds in the current working directory. The -reset This course makes use of Athena, MIT's UNIX-based computing environment. OCW does not provide access to this environment.This course makes use of Athena, MIT's UNIX-based computing environment. OCW does not provide access to this environment.option tells wal-sys to discard the contents of any previous "DB" and "LOG" files so that it can start with a clean initial state. Commands interpreted by wal-sys The following commands are used for managing recoverable actions and accounts: begin action_idBegin a recoverable action denoted by acti on_id. The action_id is a positive integer that uniquely identifies a given recoverable action. create_account action_id account_name starting_balanceCreate a new account with the given account_name and starting_bal ance. The first argument specifies that this operation is part of recoverable action action_id. The account_name can be any character string with no white spaces. credit_account action_id account_name credit_amount Add credit_amount to ac count_name's balance. This command logs the credit and holds it in a buffer until an end command is executed for recoverable action action_id. debit_account action_id account_name debit_amount Reduce account_name's balance by debit_amount. Like credit, this command logs the debit and holds it in a buffer until an end command is executed for recoverable action action_id. commit action_id Commit the recoverable action action_id. This command logs a commit record. checkpointLog a checkpoint record. end action_id End recoverable action action_id. This command installs the results of recoverable action action_id to the "DB". It also logs an end record. The following commands help us understand the dynamics of the WAL system: show_state Print out the current state of the database. This command displays the contents of the "DB" and "LOG" files. crash Crash the system. In this hands-on, we are only concerned about crash recovery, so this is the only command we will use to exit the program. Using wal-sysStart wal-sys with a reset: athena% wal-sys -reset and run the following commands (sequence 1): begin 1 create_account 1 studentA 1000 commit 1 end 1 begin 2 create_account 2 studentB 2000 begin 3 create_account 3 studentC 3000 credit_account 3 studentC 100 debit_account 3 studentA 100 commit 3 show_state crash Wal-sys should print out the contents of the "DB" and "LOG" files, and then exit. Use a text editor to examine the "DB" and "LOG" files and answer the following questions (do not run wal-sys again until you have answered these questions): Question 1: Wal-sys displays the current state of the database contents after you type show_state. Why doesn't the database show studentB? Question 2: When the database recovers, which accounts should be active, and what values should they contain? Question 3: Can you explain why the "DB" file does not contain a record for studentC and contains the pre-debit balance for studentA? Recovering the database When you run wal-sys without the -reset option it recovers the database "DB" using the "LOG" file. To recover the database and then look at the results, type: athena% wal-sys > show_state > crash Question 4: What do you expect the state of "DB" to be after wal-sys recovers? Why?Question 5: Run wal-sys again to recover the database. Examine the "DB" file. Does the state of the database match your expectations? Why or why not? Question 6: During recovery, wal-sys reports the action_ids of those recoverable actions that are "Losers", "Winners", and "Done". What is the meaning of these categories? Checkpoints Start wal-sys with a reset: athena% wal-sys -reset and run the following commands (sequence 2): begin 1 create_account 1 studentA 1000 commit 1 end 1 begin 2 create_account 2 studentB 2000 checkpoint begin 3 create_account 3 studentC 3000 credit_account 3 studentC 100 debit_account 2 studentB 100 commit 3 show_state crash Note: the remainder of this assignment is only concerned with sequence 2. We will ask you to crash and recover the system a few times, but you should not run the sequence commands again. (Also
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