Simple Database Challenge

In the Simple Database problem, you'll implement an in-memory key/value database similar to Redis. For simplicity's sake, instead of dealing with IO you only need to implement the internal functionality as if it were a database library.

Guidelines

• You have 25 minutes to work on the implementation. Get as far as you can and be prepared to discuss your approach and how you would complete the exercise afterward.

Data Commands

The database accepts the following commands to operate on keys:

• SET name value – Set the variable name to the value value. For simplicity value may be an integer.
• 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.

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

Unset a non-existant value returns an error

INPUT	OUTPUT
UNSET a	ERROR

Get a non-existant value returns no error (nil)

INPUT	OUTPUT
GET a	NULL

Set, Get, Unset, Get

INPUT	OUTPUT
SET ex 10
GET ex	10
UNSET ex
GET ex	NULL

Overwrite

INPUT	OUTPUT
SET b 10
SET b 30
GET b	30

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. Transactions can be nested; a BEGIN can be issued inside of an existing block.
• ROLLBACK – Undo commands issued in the current transaction, and closes it Returns an error if no transaction is in progress.
• COMMIT – Close all open transactions, permanently applying the changes made in them. Returns an error if no transaction is in progress. COMMIT on an empty transaction is a no-op, but not an error.

Error on commit when no transaction is in progress

INPUT	OUTPUT
COMMIT	ERROR

Ability to nest transactions

INPUT	OUTPUT
BEGIN
SET a 10
GET a	10
BEGIN
SET a 20
GET a	20
ROLLBACK
GET a	10
ROLLBACK
GET a	NULL
END

Error if rollback with no transaction in progress

INPUT	OUTPUT
BEGIN
SET a 30
BEGIN
SET a 40
COMMIT
GET a	40
ROLLBACK	ERROR
END

Immediately commit outside of transaction. Modify values inside nested transactions.

INPUT	OUTPUT
SET a 50
BEGIN
GET a	50
SET a 60
BEGIN
UNSET a
GET a	NULL
ROLLBACK
GET a	60
COMMIT
GET a	60
END