question.txt

Machine round question
===

Write a TCP server that provides the following functionality:

1. Be able to communicate over TCP with a client such as `nc` (netcat),
`telnet`, or a custom TCP client.

2. Provide echo functionality:
Command: "ECHO <string>"
Expected response: "<string>"

3. Allow data to be stored:
Command: "SET <key> <value>"
Expected response: "!200"
Errors:
- if store is full: "!500"
- if syntax error: "!400"

4. Allow data to be retrieved:
Command: "GET <key>"
Expected response: "<value>"
Errors:
- if key not found (ie., not previously set): "!404"
- if syntax error: "!400"

5. Allow setting tags with keys:
Command: "SET <key> <value> <tag1> <tag2> <...tagN>"
Expected behaviour is same as (3)

6. Allow data to be retrieved with tags:
Command: "GETWITHTAGS <key>"
Expected response: "<value> [<tag1> <tag2> <...tagN>]"
Other behaviour similar to (4)

7. Load into your datastore a file _on the server_:
Command: "LOAD <filename>"
Example: "LOAD scores.db"
Expected response: "!200" if load succeeds. "!500" if an error occurs

8. List keys with a specific tag:
Command: "LISTKEYSWITHTAG EXACT <tagname>"
Expected response: "[<key1> <key2> <...key3>]"
Errors:
- if tag not found (or there exists no keys with that tag): "!404"

9. Aggregation functionality:
Support COUNT, MAX, MIN, AVG, SUM.
Command: "COUNT TAG <tag>"
Response: "<result-value>", where result-value is the result of the
aggregation function.
For example, if the current state of the datastore was:
---
key1 30 tagR
key2 40 tagQ
key3 20 tagR
---
then "AVG TAG tagR" would respond with "25".

### Notes
1. It isn't necessary to preserve the values in the datastore 
2. There is a command called `checker.py` that will test
your code. You will be able to test your code by running the command,
`checker.py <port>`, where the <port> is the port number where your
TCP server is currently listening. Test this by running `checker.py 8000`. This should report
"Percentage solved: 0.00%"
3. Ensure that your program follows the spec below exactly since the
checker is automated.
4. Test often.
5. Listen for instructions on the Zoom call.


### Table of control codes
| meaning                                  | control code|
| --------                                 |   -------- |
| generic error                            |        500 |
| invalid input or parsing error           |        400 |
| input too large                          |        413 |
| key not found                            |        404 |
| error loading .db - no such file?        |        404 |
| OK (but no data response)                |        200 |