A very simple in-memory key-value store with nested transactional blocks, built for the Thumbtack Programming Challenge.
$ python setup.py -q shell SET a 42 GET a 42 END
$ (echo 'SET a 42'; echo 'GET a'; echo 'END') | python setup.py -q shell 42
The DB can be invoked directly if you fancy it.
$ python vsims/db.py < test/data/basic-1-in.txt > out.txt
To test the thumbtack examples:
$ python setup.py test
Install and Python Use
You already made a virtualenv right?
$ python setup.py install
Just make a new database and you're all set!
>>> import vsims >>> db = vsims.DB() >>> db.set('a', 42) >>> db.get('a') 42
Problem 2: Simple Database
Your task is create a very simple in-memory database, which has a very limited command set. All of the commands are going to be fed to you one line at a time via stdin, and your job is the process the commands and perform whatever operation the command dictates. Here are the basic commands you need to handle:
- SET [name] [value]: Set a variable [name] to the value [value]. Neither variable names or values will ever contain spaces.
- GET [name]: Print out the value stored under the variable [name]. Print NULL if that variable name hasn't been set.
- UNSET [name]: Unset the variable [name]
- NUMEQUALTO [value]: Return the number of variables equal to [value]. If no values are equal, this should output 0.
- END: Exit the program
So here is a sample input:
SET a 10 GET a UNSET a GET a END
And its corresponding output:
And another one:
SET a 10 SET b 10 NUMEQUALTO 10 NUMEQUALTO 20 UNSET a NUMEQUALTO 10 SET b 30 NUMEQUALTO 10 END
And its corresponding output:
2 0 1 0
Now, as I said this was a database, and because of that we want to add in a few transactional features to help us maintain data integrity. So there are 3 additional commands you will need to support:
- BEGIN: Open a transactional block
- ROLLBACK: Rollback all of the commands from the most recent transaction block. If no transactional block is open, print out INVALID ROLLBACK
- COMMIT: Permanently store all of the operations from any presently open transactional blocks
Our database supports nested transactional blocks as you can tell by the above commands. Remember, ROLLBACK only rolls back the most recent transaction block, while COMMIT closes all open transactional blocks. Any command issued outside of a transactional block commits automatically.
The most commonly used commands are GET, SET, UNSET and NUMEQUALTO, and each of these commands should be faster than O(N) expected worst case, where N is the number of total variables stored in the database.
Typically, we will already have committed a lot of data when we begin a new transaction, but the transaction will only modify a few values. So, your solution should be efficient about how much memory is allocated for new transactions, i.e., it is bad if beginning a transaction nearly doubles your program's memory usage.
Here are some sample inputs and expected outputs using these commands:
BEGIN SET a 10 GET a BEGIN SET a 20 GET a ROLLBACK GET a ROLLBACK GET a END
10 20 10 NULL
BEGIN SET a 30 BEGIN SET a 40 COMMIT GET a ROLLBACK END
40 INVALID ROLLBACK
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
SET a 10 BEGIN NUMEQUALTO 10 BEGIN UNSET a NUMEQUALTO 10 ROLLBACK NUMEQUALTO 10 END
1 0 1