Exercise: K/V REPL with nested transactions

In this exercise we ask you to write a command line REPL (read-eval-print loop) that drives a simple in-memory key/value storage system. This system should also allow for nested transactions. A transaction can then be committed or aborted.

Submitting Code: Please see instructions here on how to submit your code.

We realize that your time is valuable. Please try to limit this to ~2 hours. If you can't complete the exercise in this time, please share what you have as a basis for a discussion.

Example Run

$ my-program
> WRITE a hello
> READ a
> WRITE a hello-again
> READ a
> READ a
Key not found: a
> READ a
Key not found: a
> WRITE a once-more
> READ a
> READ a


  • READ <key> Reads and prints, to stdout, the val associated with key. If the value is not present an error is printed to stderr.
  • WRITE <key> <val> Stores val in key.
  • DELETE <key> Removes all key from store. Future READ commands on that key will return an error.
  • START Start a transaction.
  • COMMIT Commit a transaction. All actions in the current transaction are committed to the parent transaction or the root store. If there is no current transaction an error is output to stderr.
  • ABORT Abort a transaction. All actions in the current transaction are discarded.
  • QUIT Exit the REPL cleanly. A message to stderr may be output.

Other details

  • For simplicity, all keys and values are simple ASCII strings delimited by whitespace. No quoting is needed.
  • All errors are output to stderr.
  • Commands are case-insensitive.
  • As this is a simple command line program with no networking, there is only one "client" at a time. There is no need for locking or multiple threads.

Live Example

There is an SSH server you can hit that has an implementation of this exercies. Run the following to access it:

ssh simple-kv.heptio.com -p 2258