Simple Database Challenge

In the Simple Database problem, you'll implement an in-memory database similar to Redis. For simplicity's sake, instead of dealing with multiple clients and communicating over the network, your program will receive commands via standard input (stdin), and should write appropriate responses to standard output (stdout).

Guidelines

  • This problem should take you between 30 and 90 minutes.

  • We recommend that you use a high-level language, like Python, Ruby, JavaScript, or Java. We're much more interested in seeing clean code and good algorithmic performance than raw throughput.

  • It is very helpful to the engineers who grade these challenges if you reduce external dependencies, make compiling your code as simple as possible, and include instructions for compiling and/or running your code.

  • Your submission must be able to detect and handle End-Of-File (EOF).

Data Commands

Your database should accept the following commands:

  • SET name value – Set the variable name to the value value. Neither variable names nor values will contain spaces.

  • GET name – Print out the value of the variable name, or NULL if that variable is not set.

  • UNSET name – Unset the variable name, making it just like that variable was never set.

  • NUMEQUALTO value – Print out the number of variables that are currently set to value. If no variables equal that value, print 0.

  • END – Exit the program. Your program will always receive this as its last command.

Commands will be fed to your program one at a time, with each command on its own line. Any output that your program generates should end with a newline character. Here are some example command sequences:

Input Output
SET ex 10
GET ex
UNSET ex
GET ex
END

10

NULL

Input Output
SET a 10
SET b 10
NUMEQUALTO 10
NUMEQUALTO 20
SET b 30
NUMEQUALTO 10
END


2
0

1

Transaction Commands

In addition to the above data commands, your program should also support database transactions by also implementing these commands:

  • BEGIN – Open a new transaction block. Transaction blocks can be nested; a BEGIN can be issued inside of an existing block.

  • ROLLBACK – Undo all of the commands issued in the most recent transaction block, and close the block. Print nothing if successful, or print NO TRANSACTION if no transaction is in progress.

  • COMMIT – Close all open transaction blocks, permanently applying the changes made in them. Print nothing if successful, or print NO TRANSACTION if no transaction is in progress.

Any data command that is run outside of a transaction block should commit immediately. Here are some example command sequences:

Input Output
BEGIN
SET a 10
GET a
BEGIN
SET a 20
GET a
ROLLBACK
GET a
ROLLBACK
GET a
END


10


20

10

NULL

Input Output
BEGIN
SET a 30
BEGIN
SET a 40
COMMIT
GET a
ROLLBACK
END





40
NO TRANSACTION

Input Output
SET a 50
BEGIN
GET a
SET a 60
BEGIN
UNSET a
GET a
ROLLBACK
GET a
COMMIT
GET a
END


50



NULL

60

60

Input Output
SET a 10
BEGIN
NUMEQUALTO 10
BEGIN
UNSET a
NUMEQUALTO 10
ROLLBACK
NUMEQUALTO 10
COMMIT
END


1


0

1


Performance Considerations

  • The most common operations are GET, SET, UNSET, and NUMEQUALTO. All of these commands should have an expected worst-case runtime of O(log N) or better, where N is the total number of variables stored in the database.

  • The vast majority of transactions will only update a small number of variables. Accordingly, your solution should be efficient about how much memory each transaction uses.