Scheduled tasks (cron) in Deno - Step-by-Step Execution

Concept Flow - Scheduled tasks (cron)

Start Program

↓

Define Cron Schedule

↓

Create Cron Job

↓

Cron Scheduler Waits

↓

Time Matches Schedule?

No→Wait

Yes↓

Execute Task

↓

Repeat Waiting

The program sets a schedule, waits until the time matches, then runs the task repeatedly.

Execution Sample

Deno

import { Cron } from "https://deno.land/x/croner@6.0.0/mod.ts";

const job = new Cron("*/1 * * * *", () => {
  console.log("Task runs every minute");
});

This code schedules a task to run every minute and prints a message each time.

Execution Table

Step	Time	Schedule Check	Action	Output
1	00:00	Matches */1 * * * *	Run Task	Task runs every minute
2	00:01	Matches */1 * * * *	Run Task	Task runs every minute
3	00:02	Matches */1 * * * *	Run Task	Task runs every minute
4	00:03	Matches */1 * * * *	Run Task	Task runs every minute
5	00:04	Matches */1 * * * *	Run Task	Task runs every minute
6	00:05	Matches */1 * * * *	Run Task	Task runs every minute
7	00:06	Matches */1 * * * *	Run Task	Task runs every minute
8	00:07	Matches */1 * * * *	Run Task	Task runs every minute
9	00:08	Matches */1 * * * *	Run Task	Task runs every minute
10	00:09	Matches */1 * * * *	Run Task	Task runs every minute
11	00:10	Matches */1 * * * *	Run Task	Task runs every minute
12	00:11	Matches */1 * * * *	Run Task	Task runs every minute
13	00:12	Matches */1 * * * *	Run Task	Task runs every minute
14	00:13	Matches */1 * * * *	Run Task	Task runs every minute
15	00:14	Matches */1 * * * *	Run Task	Task runs every minute
16	00:15	Matches */1 * * * *	Run Task	Task runs every minute
17	00:16	Matches */1 * * * *	Run Task	Task runs every minute
18	00:17	Matches */1 * * * *	Run Task	Task runs every minute
19	00:18	Matches */1 * * * *	Run Task	Task runs every minute
20	00:19	Matches */1 * * * *	Run Task	Task runs every minute
21	00:20	Matches */1 * * * *	Run Task	Task runs every minute
22	00:21	Matches */1 * * * *	Run Task	Task runs every minute
23	00:22	Matches */1 * * * *	Run Task	Task runs every minute
24	00:23	Matches */1 * * * *	Run Task	Task runs every minute
25	00:24	Matches */1 * * * *	Run Task	Task runs every minute
26	00:25	Matches */1 * * * *	Run Task	Task runs every minute
27	00:26	Matches */1 * * * *	Run Task	Task runs every minute
28	00:27	Matches */1 * * * *	Run Task	Task runs every minute
29	00:28	Matches */1 * * * *	Run Task	Task runs every minute
30	00:29	Matches */1 * * * *	Run Task	Task runs every minute
31	00:30	Matches */1 * * * *	Run Task	Task runs every minute
32	00:31	Matches */1 * * * *	Run Task	Task runs every minute
33	00:32	Matches */1 * * * *	Run Task	Task runs every minute
34	00:33	Matches */1 * * * *	Run Task	Task runs every minute
35	00:34	Matches */1 * * * *	Run Task	Task runs every minute
36	00:35	Matches */1 * * * *	Run Task	Task runs every minute
37	00:36	Matches */1 * * * *	Run Task	Task runs every minute
38	00:37	Matches */1 * * * *	Run Task	Task runs every minute
39	00:38	Matches */1 * * * *	Run Task	Task runs every minute
40	00:39	Matches */1 * * * *	Run Task	Task runs every minute
41	00:40	Matches */1 * * * *	Run Task	Task runs every minute
42	00:41	Matches */1 * * * *	Run Task	Task runs every minute
43	00:42	Matches */1 * * * *	Run Task	Task runs every minute
44	00:43	Matches */1 * * * *	Run Task	Task runs every minute
45	00:44	Matches */1 * * * *	Run Task	Task runs every minute
46	00:45	Matches */1 * * * *	Run Task	Task runs every minute
47	00:46	Matches */1 * * * *	Run Task	Task runs every minute
48	00:47	Matches */1 * * * *	Run Task	Task runs every minute
49	00:48	Matches */1 * * * *	Run Task	Task runs every minute
50	00:49	Matches */1 * * * *	Run Task	Task runs every minute
51	00:50	Matches */1 * * * *	Run Task	Task runs every minute
52	00:51	Matches */1 * * * *	Run Task	Task runs every minute
53	00:52	Matches */1 * * * *	Run Task	Task runs every minute
54	00:53	Matches */1 * * * *	Run Task	Task runs every minute
55	00:54	Matches */1 * * * *	Run Task	Task runs every minute
56	00:55	Matches */1 * * * *	Run Task	Task runs every minute
57	00:56	Matches */1 * * * *	Run Task	Task runs every minute
58	00:57	Matches */1 * * * *	Run Task	Task runs every minute
59	00:58	Matches */1 * * * *	Run Task	Task runs every minute
60	00:59	Matches */1 * * * *	Run Task	Task runs every minute
61	01:00	Matches */1 * * * *	Run Task	Task runs every minute

💡 The cron job runs indefinitely every minute; no exit unless program stops.

Variable Tracker

Variable	Start	After 1	After 2	After 3	After 4	After 5	Final
Time (minutes)	00:00	00:01	00:02	00:03	00:04	00:05	continues...
Task Runs	Yes	Yes	Yes	Yes	Yes	Yes	continues...

Key Moments - 3 Insights

Why does the task run every minute instead of just once?

Because the cron schedule "*/1 * * * *" means every minute, so the scheduler checks every minute and runs the task each time (see execution_table rows 1-6).

What happens if the program stops running?

The scheduled tasks stop too because the cron job runs inside the program process. Without the program running, no tasks execute (implied by exit_note).

How does the scheduler know when to run the task?

It compares the current time to the cron pattern each minute. If it matches, it runs the task (see execution_table column 'Schedule Check').

Visual Quiz - 3 Questions

Look at the execution table, what is the output at step 3?

A No output

B Task runs every minute

C Error message

D Task runs every 5 minutes

💡 Hint

Check the 'Output' column at step 3 in the execution_table.

At which step does the time first become 00:05?

A Step 4

B Step 5

C Step 6

D Step 7

💡 Hint

Look at the 'Time' column in the execution_table to find when time is 00:05.

If the cron schedule changed to "*/2 * * * *", how would the execution table change?

A Task runs every 2 minutes, so fewer runs

B Task runs every minute as before

C Task runs every 5 minutes

D Task never runs

💡 Hint

Think about how the schedule "*/2 * * * *" affects the 'Schedule Check' column.

Concept Snapshot

Scheduled tasks (cron) in Deno:
- Use Cron class from croner module
- Define schedule string (e.g., "*/1 * * * *" for every minute)
- Pass a callback function to run on schedule
- Cron runs tasks repeatedly while program runs
- Scheduler checks time and runs task if matched

Full Transcript

This visual execution shows how scheduled tasks work in Deno using the croner module. The program defines a cron schedule that runs a task every minute. The scheduler waits and checks the current time against the schedule. When the time matches, it runs the task, printing a message. This repeats indefinitely as long as the program runs. Variables like current time and task run status update each minute. Key points include understanding the cron pattern meaning, the continuous running nature, and that tasks stop if the program stops. The quiz tests understanding of timing, output, and schedule changes.

Practice

1. What is the main purpose of using scheduled tasks (cron) in Deno?

easy

A. To compile Deno scripts faster

B. To manually trigger functions on user input

C. To debug code step-by-step

D. To run code automatically at specific times or intervals

Solution

1. Step 1: Understand scheduled tasks

   Scheduled tasks run code automatically without manual intervention.

2. Step 2: Identify cron job purpose in Deno

   Deno's cron jobs automate running code at set times or intervals.

3. Final Answer:

   To run code automatically at specific times or intervals -> Option D

4. Quick Check:

   Scheduled tasks = automatic timed code run [OK]

Hint: Scheduled tasks run code on a timer, not manually [OK]

Common Mistakes:

• Confusing scheduled tasks with manual triggers
• Thinking cron is for debugging
• Assuming cron speeds up compilation

2. Which of the following is the correct way to import the croner library in a Deno script?

easy

A. import Cron from "croner";

B. require('croner');

C. import { Cron } from "https://deno.land/x/croner@6.0.0/mod.ts";

D. import * as Cron from 'cron';

Solution

1. Step 1: Check Deno import style

   Deno uses ES module imports with URLs, not CommonJS require.

2. Step 2: Identify correct croner import

   The official croner import uses a URL with version and named import.

3. Final Answer:

   import { Cron } from "https://deno.land/x/croner@6.0.0/mod.ts"; -> Option C

4. Quick Check:

   Deno imports use URLs and named imports [OK]

Hint: Deno imports use full URLs, not short names or require [OK]

Common Mistakes:

• Using require() which is Node.js style
• Importing from 'cron' instead of 'croner'
• Using wildcard import without URL

3. What will the following Deno croner code output when run?

import { Cron } from "https://deno.land/x/croner@6.0.0/mod.ts";
const cron = new Cron("* * * * *", () => {
  console.log("Hello every minute");
});
cron.start();

medium

A. Prints "Hello every minute" every second

B. Prints "Hello every minute" every minute

C. Prints "Hello every minute" once immediately

D. Throws a syntax error

Solution

1. Step 1: Understand the cron pattern "* * * * *"

   This pattern means the task runs every minute.

2. Step 2: Analyze the callback behavior

   The callback logs "Hello every minute" each time the cron triggers.

3. Final Answer:

   Prints "Hello every minute" every minute -> Option B

4. Quick Check:

   Cron pattern "* * * * *" = every minute [OK]

Hint: Five stars in cron means every minute [OK]

Common Mistakes:

• Thinking it runs every second
• Expecting immediate output without waiting
• Confusing cron syntax with other formats

4. Identify the error in this Deno croner code snippet:

import { Cron } from "https://deno.land/x/croner@6.0.0/mod.ts";
const cron = new Cron("60 * * * *", () => {
  console.log("This runs every hour");
});
cron.start();

medium

A. The cron pattern "60 * * * *" is invalid because minutes must be 0-59

B. Missing await before cron.start()

C. Callback function should be async

D. Cron library does not support hourly tasks

Solution

1. Step 1: Check the cron pattern validity

   Minutes field must be between 0 and 59; "60" is invalid.

2. Step 2: Confirm other code parts

   cron.start() does not require await; callback can be sync; hourly tasks are supported.

3. Final Answer:

   The cron pattern "60 * * * *" is invalid because minutes must be 0-59 -> Option A

4. Quick Check:

   Minutes range = 0-59, 60 is invalid [OK]

Hint: Minutes in cron must be 0-59, not 60 or above [OK]

Common Mistakes:

• Using invalid minute value 60
• Thinking cron.start() needs await
• Assuming callback must be async

5. You want to schedule a Deno task to run every weekday at 9:30 AM using croner. Which cron pattern is correct?

hard

A. 30 9 * * 1-5

B. 9 30 * * 1-5

C. 30 9 * * 6,7

D. 0 9 * * 1-5

Solution

1. Step 1: Understand cron fields order

   Cron fields are: minute, hour, day of month, month, day of week.

2. Step 2: Match 9:30 AM on weekdays

   Minute = 30, hour = 9, day of week = 1-5 (Monday to Friday).

3. Final Answer:

   "30 9 * * 1-5" -> Option A

4. Quick Check:

   Minute=30, Hour=9, Weekdays=1-5 [OK]

Hint: Minute first, hour second, weekdays 1-5 for Mon-Fri [OK]

Common Mistakes:

• Swapping hour and minute fields
• Using weekend days 6,7 instead of weekdays
• Setting minute or hour to zero incorrectly