Queue events and monitoring in NestJS - Step-by-Step Execution

Concept Flow - Queue events and monitoring

Queue Initialized

↓

Job Added to Queue

↓

Event: Job Waiting

→Monitor: Log Waiting

↓

Event: Job Active

→Monitor: Log Active

↓

Event: Job Completed

→Monitor: Log Success

↓

Event: Job Failed

→Monitor: Log Failure

↓

Event: Job Stalled

→Monitor: Log Stall

↓

Repeat for next jobs or Shutdown

This flow shows how a queue processes jobs and emits events that can be monitored for status updates.

Execution Sample

NestJS

const queue = new Queue('myQueue');

queue.on('completed', job => console.log(`Job ${job.id} done`));
queue.add({ task: 'sendEmail' });

This code creates a queue, listens for job completion events, and adds a job to the queue.

Execution Table

Step	Event Triggered	Job ID	Action Taken	Monitor Output
1	Job Added	1	Job added to queue	No output
2	Job Waiting	1	Job is waiting to be processed	Log: Job 1 is waiting
3	Job Active	1	Job started processing	Log: Job 1 is active
4	Job Completed	1	Job finished successfully	Log: Job 1 done
5	Job Added	2	Job added to queue	No output
6	Job Waiting	2	Job is waiting to be processed	Log: Job 2 is waiting
7	Job Active	2	Job started processing	Log: Job 2 is active
8	Job Failed	2	Job failed during processing	Log: Job 2 failed
9	Job Added	3	Job added to queue	No output
10	Job Waiting	3	Job is waiting to be processed	Log: Job 3 is waiting
11	Job Active	3	Job started processing	Log: Job 3 is active
12	Job Stalled	3	Job stalled during processing	Log: Job 3 stalled
13	Queue Shutdown	-	Queue stopped processing	Log: Queue shutdown

💡 Queue shutdown after processing jobs and emitting all events.

Variable Tracker

Variable	Start	After Step 1	After Step 4	After Step 8	After Step 12	Final
queue.jobs	[]	[{id:1, status:'waiting'}]	[{id:1, status:'completed'}]	[{id:1, status:'completed'}, {id:2, status:'failed'}]	[{id:1, status:'completed'}, {id:2, status:'failed'}, {id:3, status:'stalled'}]	[{id:1, status:'completed'}, {id:2, status:'failed'}, {id:3, status:'stalled'}]
job.id	N/A	1	1	2	3	N/A
job.status	N/A	waiting	completed	failed	stalled	N/A

Key Moments - 3 Insights

Why do we see a 'Job Waiting' event before 'Job Active'?

The 'Job Waiting' event signals the job is queued but not yet processed. 'Job Active' means processing started. See execution_table rows 2 and 3.

What happens if a job fails? Does the queue stop?

No, the queue continues processing other jobs even if one fails. See execution_table rows 8 and 9 where job 2 fails but job 3 is still added.

What does the 'Job Stalled' event mean?

'Job Stalled' means the job was active but stopped responding temporarily. It's a warning to monitor. See execution_table row 12.

Visual Quiz - 3 Questions

Look at the execution table, what is the status of job 1 at step 4?

A completed

B active

C waiting

D failed

💡 Hint

Check the 'Event Triggered' and 'Action Taken' columns at step 4.

At which step does job 2 fail?

A Step 6

B Step 7

C Step 8

D Step 9

💡 Hint

Look for 'Job Failed' event in the 'Event Triggered' column.

If we remove the listener for 'Job Stalled', what changes in the monitor output?

A No log for stalled jobs

B No logs at all

C No logs for completed jobs

D Queue stops processing

💡 Hint

Refer to the 'Monitor Output' column for 'Job Stalled' event at step 12.

Concept Snapshot

Queue events in NestJS emit status updates like 'waiting', 'active', 'completed', 'failed', and 'stalled'.
Listeners capture these events to monitor job progress.
Add jobs with queue.add(), listen with queue.on(event).
Monitoring helps track job lifecycle and handle errors.
Queue continues processing despite failures.
Stalled jobs indicate temporary issues needing attention.

Full Transcript

In NestJS, queues emit events during job processing such as when a job is added, waiting, active, completed, failed, or stalled. We create a queue instance and add jobs to it. We listen for events like 'completed' to log or monitor job status. The execution table shows each step where events trigger and what actions happen. Variables track job IDs and statuses changing from waiting to completed or failed. Key moments clarify why waiting comes before active, that failures don't stop the queue, and what stalled means. The visual quiz tests understanding of job statuses at specific steps and effects of removing event listeners. This helps beginners see how queue events flow and how monitoring works in real time.