ashdaily/cron_parser

Project Cron Parser

The solution is written in python3 utilises docker & docker-compose.

Build the solution with docker

Requirement : Docker required system-wide.

docker-compose build

Try the Cron Parser

Example command:

docker-compose run cron-parser "*/7 0 1,15 * 1-5 /usr/bin/find"

Example Result:

Creating cron-parser_run ... done
[03:36:11] INFO     Cron parser started...                                                                   main.py:28
╭─ Parsed Cron Fields ──────────────────────────────────╮
│                 Cron Parsing Results:                 │
│ ┏━━━━━━━━━━━━━━━━━━━━━━┳━━━━━━━━━━━━━━━━━━━━━━━━━━━━┓ │
│ ┃ Cron Fields          ┃ Expanded Values            ┃ │
│ ┡━━━━━━━━━━━━━━━━━━━━━━╇━━━━━━━━━━━━━━━━━━━━━━━━━━━━┩ │
│ │ minute               │ 0 7 14 21 28 35 42 49 56   │ │
│ │ hour                 │ 0                          │ │
│ │ day of month         │ 1 15                       │ │
│ │ month                │ 1 2 3 4 5 6 7 8 9 10 11 12 │ │
│ │ day of week          │ 1 2 3 4 5                  │ │
│ │ command              │ /usr/bin/find              │ │
│ └──────────────────────┴────────────────────────────┘ │
╰───────────────────────────────────────────────────────╯
           INFO     Cron parser executed !

Turn on debug logs from docker-compose.yaml file

# change LOG_LEVEL for cron-parser service in docker-compose.yaml

LOG_LEVEL=INFO # change this to DEBUG to see debug logs

# LOG_LEVEL=INFO will mute most of the logs

Try the parser again & you should see something like this:

Creating cron-parser_run ... done
[03:42:19] INFO     Cron parser started...                                                                   main.py:28
           DEBUG    Initializing CronParser, received cron_str: "*/7 0 1,15 * 1-5 /usr/bin/find"       cron_parser.py:7
           DEBUG    Initializing minute field with string: */7                                        cron_fields.py:12
           DEBUG    Initializing hour field with string: 0                                            cron_fields.py:12
           DEBUG    Initializing day of month field with string: 1,15                                 cron_fields.py:12
           DEBUG    Initializing month field with string: *                                           cron_fields.py:12
           DEBUG    Initializing day of week field with string: 1-5                                   cron_fields.py:12
           DEBUG    Parsing cron_field of type minute with value: */7                                 cron_fields.py:15
           DEBUG    Parsed cron_field of type minute */7 to [0, 7, 14, 21, 28, 35, 42, 49, 56]        cron_fields.py:17
           DEBUG    Parsing cron_field of type hour with value: 0                                     cron_fields.py:15
           DEBUG    Parsed cron_field of type hour 0 to [0]                                           cron_fields.py:17
           DEBUG    Parsing cron_field of type day of month with value: 1,15                          cron_fields.py:15
           DEBUG    Parsed cron_field of type day of month 1,15 to [1, 15]                            cron_fields.py:17
           DEBUG    Parsing cron_field of type month with value: *                                    cron_fields.py:15
           DEBUG    Parsed cron_field of type month * to [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12]      cron_fields.py:17
           DEBUG    Parsing cron_field of type day of week with value: 1-5                            cron_fields.py:15
           DEBUG    Parsed cron_field of type day of week 1-5 to [1, 2, 3, 4, 5]                      cron_fields.py:17
           DEBUG    Parsing command field: /usr/bin/find                                              cron_fields.py:75
╭─ Parsed Cron Fields ──────────────────────────────────╮
│                 Cron Parsing Results:                 │
│ ┏━━━━━━━━━━━━━━━━━━━━━━┳━━━━━━━━━━━━━━━━━━━━━━━━━━━━┓ │
│ ┃ Cron Fields          ┃ Expanded Values            ┃ │
│ ┡━━━━━━━━━━━━━━━━━━━━━━╇━━━━━━━━━━━━━━━━━━━━━━━━━━━━┩ │
│ │ minute               │ 0 7 14 21 28 35 42 49 56   │ │
│ │ hour                 │ 0                          │ │
│ │ day of month         │ 1 15                       │ │
│ │ month                │ 1 2 3 4 5 6 7 8 9 10 11 12 │ │
│ │ day of week          │ 1 2 3 4 5                  │ │
│ │ command              │ /usr/bin/find              │ │
│ └──────────────────────┴────────────────────────────┘ │
╰───────────────────────────────────────────────────────╯
           INFO     Cron parser executed !                                                                   main.py:53

Run the tests

 docker-compose run cron-parser-test

Build and run the solution without docker

Requirement : python3.7 or above and pip3

pip3 install -r requirement.txt
python3 src/main.py "*/15 0 1,15 * 1-5 /usr/bin/find"

Solution Intro

cron_parser/
│
├── Dockerfile
├── docker-compose.yml # composes cli app (cron-parser) & the unit tests (cron-parser)
├── requirements.txt    # Only using two dependencies
                        # Rich pip package (to beautify logs and parsed output in table)
                        # Parameterized pip package (For a theory style tests, to test against many
                        # different inputs for same unit tests and avoid using for loops in unit tests body)
├── src/
│   ├── main.py # will parse the cron job do very little validation and print result from cron_parser.py
│   ├── cron_fields.py # all possible cron fields like minute, day of week, day of month etc
│   ├── cron_factory.py # provides cron_field objects for minute, hour, day of week, day of month, month, cmd.
│   ├── cron_parser.py # does the parsing for all cron_fields present in cron_str by using the cron_factory to get cron_field objects and calling .parse() on them iteratively for all cron_fields present in cron_string
│   ├── logging_config.py # set logging to default INFO if not provided by ENV vars
│   └── strategies.py # bunch of different strategies to deal with cron_field syntax, support *, single integer (for eg: 1), multiple integers eg (1,10) or range (1-10), supports most cases for asterisk, comma, dash, / with one cron_field having multiple symbols (see tests)
├── tests/
│   ├── __init__.py
│   ├── test_cron_parser.py # tests variety of full command cron_string
│   └── tests_cron_fields/ # tests individual fields extensively
└── venv/

The good:

Dash, Slash, Asterisk, Comma in individual cron_field for any date time entity should work.
Each cron_field is tested properly for happy cases with many random inputs.
Cron Parser with full cron_string is also tested with many random cron strings.
Logging gives a useful insight on what happened to each cron_field
Logging level passed from OS Env so the LOG_LEVEL can be changed easily inside docker-compose.yaml.

Some Examples which are handled gracefully: Every 15 minutes:

*/15 * * * * /path/to/command

Every weekday at 8:00 AM:

0 8 * * 1-5 /path/to/command

At 12:00 AM on the first day of every month:

0 0 1 * * /path/to/command

At 5:30 PM on every Friday:

30 17 * * 5 /path/to/command

On the last day of every month at 11:59 PM:

59 23 L * * /path/to/command

On January 1st at midnight:

0 0 1 1 * /path/to/command

Some more complicated once like, every day, once in every two hours until 12pm, after that every 4 hours:

0 0-12/2,13-23/4 * * *

Future improvements

Invalid syntax for cron_string input, in general, is not handled well.
Symbols that are invalid are not handled gracefully.
Not very focused on handling errors and validations, does very basic validation around max and min value
Could better support special symbols like hash, L, W.
Could introduce test for CLI output part as well.
Month upper limit 31 is static, while 31 might not be present in a month.
More edge cases could be handled.

ashdaily / cron_parser