# Will it survive Black Friday?

> Play Black Friday on a shop's checkout, find the part that falls behind first, size the fix and prove it works, in about ten minutes.

Your shop copes on a normal Tuesday. On Black Friday, seven times as many people arrive at midnight, and four times as many stay all day. Something will run out of room first. This walkthrough finds out what, by how much, and what to change, before the day comes.

You’ll use the **Checkout system** template: shoppers, a web shop, an orders API, a database, card payments, and a worker that sends the confirmation email.

## 1. Open the checkout

1. Open **Templates** and find **Checkout system**. Select **Use template**. A copy opens in the editor, so nothing you change here touches the template.

2. Press play if the map is paused. Dots move along each line, closer together where there’s more traffic.

![The Checkout system map: Shoppers and the web shop on the left, then an AWS region holding the Orders API, the Orders DB and a background jobs zone with Order events and the Email worker, and Stripe payments above.](https://docs.flozu.app/_astro/map-light.bQE_NUvi_Z2jmy8q.webp)

The map already has its numbers. Select a few things to see them in the inspector’s **Traffic** tab:

| What                  | On a normal day     | It can take    | So it’s  |
| --------------------- | ------------------- | -------------- | -------- |
| Web shop → Orders API | 40 orders a second  |                |          |
| Orders API            | 40 a second         | 400 a second   | 10% busy |
| Orders DB             | 80 queries a second | 1,000 a second | 8% busy  |
| Email worker          | 12 emails a second  | 20 a second    | 60% busy |

Every part has room to spare. The Email worker has the least: 60% busy before anything unusual happens. Keep an eye on it.

## 2. Play Black Friday

1. Open **Scenarios** and pick **Black Friday**. A timeline appears along the bottom, and the map shows the busiest moment.

2. Read the card at the top of **What happens**: two parts need more room.

3. Look at the canvas. The Email worker is outlined in red, and its strip says it’s over.

![Black Friday running on the checkout map: the scenario panel lists two parts that need more room, the email worker is outlined in red, and a timeline runs along the bottom.](https://docs.flozu.app/_astro/scenario-light.D50hHthp_2r0uL7.webp)

Here’s what happened at ten past midnight, the busiest moment:

| Part         | Arrives            | It can take    | Busy | What Flozu says                              |
| ------------ | ------------------ | -------------- | ---- | -------------------------------------------- |
| Orders API   | about 290 a second | 400 a second   | 72%  | **Tight**: past the 70% it should stay under |
| Email worker | about 88 a second  | 20 a second    | 438% | **Over**: it can’t keep up                   |
| Orders DB    | about 580 a second | 1,000 a second | 58%  | Fine                                         |

The Email worker is tight from the first minute and over by the second. That’s the part that breaks first.

> **Note**
>
> Shoppers don’t notice the Email worker falling behind. The line into it is a dashed message line: the order is placed and the shopper moves on, and the email is sent afterwards. What they’d notice is their confirmation email arriving late, or not at all. If the Orders API ran over, they’d notice straight away, because they wait for its answer.

## 3. Watch it happen

Drag the playhead along the timeline, or press `Space` to play the day in about 30 seconds. The red line along the top of the timeline marks the stretch where a part is over. **Moments** in the panel lists when things change: the Email worker gets tight at 00:00, runs over at 00:02, and the busiest moment is 00:10.

![The timeline for Black Friday: a curve of traffic across the day from 00:00 to 24:00, marked red along the top where parts need room, with the playhead at 00:10.](https://docs.flozu.app/_astro/timeline-light.B5yrEedO_ZNuc2f.webp)

## 4. Fix it

Under **Fix it**, each part that needs room comes with the fix that would let it cope:

- **Email worker:** give it 126 a second, 6.3 times what it has, or run 7 copies.
- **Orders API:** give it 417 a second, 5% more than it has, or run 2 copies.

Flozu sizes each fix so the part stays under 70% busy at its worst moment, leaving room for a surprise.

![What happens for Black Friday: a card saying 2 of 3 parts need room with a coloured bar and the outcome, then Fix it with each part that needs more room and its fix.](https://docs.flozu.app/_astro/needs-more-room-light.tXuLJjeI_2dF7W7.webp)

1. Select **Apply** beside each fix. Each is one change you can undo. (**Try it** makes the fix in a proposal instead, so you can compare before you commit. The [next walkthrough](https://docs.flozu.app/walkthroughs/what-if-it-breaks.md) uses it.)

2. Watch the canvas: the red outline goes, and the card at the top of **What happens** says everything copes. At the busiest moment the Orders API is now 70% busy and the Email worker 69%.

Not sure 126 is right? Select the Email worker and choose **Try other capacities** in its **Traffic** tab. Tiles show how busy each round number would make it, on Black Friday and on a normal day, so you can see what a smaller fix would leave you with.

![Trying other capacities for Email worker during Black Friday: tiles from 20 to 200 a second filled from red to green, 150 picked, and a slider marking healthy from 126 a second.](https://docs.flozu.app/_astro/try-capacity-light._ndGs3U3_SuvPi.webp)

## 5. Keep it

1. Choose **Save with the map** in the scenario panel. Black Friday now travels with the map, in files and share links, and you can come back to it after the next change.

2. To show someone, choose **Share a link…** from the **Actions** menu. Whoever opens it can play Black Friday too.

![The scenario panel after both fixes are applied: 0 of 3 parts need room, 69% at the busiest, and all 3 checked parts cope on Black Friday, with Save with the map at the top.](https://docs.flozu.app/_astro/black-friday-fixed-light.CnoOx8xn_ZREtCj.webp)

## What you learned

- **Rates live on lines and limits on parts.** That’s all a scenario needs to work out who runs short. See [traffic and limits](https://docs.flozu.app/guides/traffic-and-limits.md).
- **The first part to break isn’t always the obvious one.** The Orders API looked like the bottleneck, but the Email worker had the least room to start with.
- **Message lines hide problems from shoppers,** which is good for them and easy to miss for you.

## Next

- [What if something breaks?](https://docs.flozu.app/walkthroughs/what-if-it-breaks.md) takes a part down on the same map.
- [Scenarios](https://docs.flozu.app/guides/scenarios.md) covers making your own, from a TV ad to steady growth.