Ways to release

The fix is the same for QuickCart as for anyone: stop giving everyone a new version at the same moment.

Deploying means replacing the running version with a new one while customers keep using it. There are a few standard ways to do the replacement. They all end in the same place, with everyone on the new version. They differ in how traffic moves from old to new, and that decides what happens when the new version is bad.

The four ways

QuickCart’s current method is recreate. It’s the simplest, and the only one with downtime built in.

What each one needs

The canary’s rules are called abort criteria, and they’re written down before the release starts. For example: if the canary’s error rate is more than double the old version’s for five minutes, send all traffic back. The best pipelines apply them automatically. Canary from a pipeline builds exactly that.

Old and new run side by side

In rolling and canary releases, and briefly in blue/green, both versions answer requests at the same time, and both use the same database. So every database change has to work with both versions:

This pattern is called expand, then contract: add the new thing, move the code over, and remove the old thing only when nothing uses it any more.

Choosing one

The three strategies release the same version side by side. Release a good version first. Then switch to Bad and release again. Watch how traffic moves, and how many customers the bad version reaches.

What to notice: blue/green undid the bad version fastest, but it had already reached every customer. The canary reached only one in ten, because it was stopped while it was still small. Speed of undoing and number of customers hit are different things.

Pennies — a few calls to a tiny Lambda function. Well under a cent, and inside the free allowance on most accounts.

You’ll run a real canary release on AWS, without any servers. You’ll put two versions of a small function behind one name, send 10% of calls to the new version, watch the split, then promote it, and finally switch back instantly. About 20 minutes.

The function runs on Lambda, which runs code without you managing a server. You don’t need to know Lambda yet: it’s just the simplest thing on AWS that can split traffic by percentage. The same ideas apply to servers and containers later in the course.

Check the region selector says N. Virginia (us-east-1), and that you’ve set up the AWS CLI in Set up your computer.

1. Create the function

   AWSIn the AWS console: Lambda→Functions→Create function

   Field	Enter
   Create function	Author from scratch
   Function name	quickcart-menu
   Runtime	The newest Node.js version in the list
   Architecture	arm64
   Permissions	Leave the default: Create a new role with basic Lambda permissions

   Choose Create function.

   You should see

   The function’s page, with a Code tab showing a file called index.mjs.

2. Write version 1 and publish it

   In the Code tab, replace everything in index.mjs with:

   index.mjsCopy

   export const handler = async () => {
     return { version: "1", menu: "Margherita, Pepperoni" };
   };

   Line	What it does
   export const handler	the function Lambda calls. It must be called handler, because that’s what the default settings look for
   async () =>	it takes no input we need, and may do slow work
   return { version: "1", menu: "Margherita, Pepperoni" }	the answer, including which version sent it

   Choose Deploy. Then fix this code as a numbered version that can never change:

   AWSIn the AWS console: Lambda→Functions→quickcart-menu→Actions→Publish new version

   Field	Enter
   Version description	v1

   Choose Publish.

   You should see

   A message saying Version 1 was created. You’re now looking at version 1’s page.

3. Point a name at version 1

   An alias is a fixed name that points at one or more versions. Callers always use the name; you change what it points at. That’s what makes the release strategies possible.

   AWSIn the AWS console: Lambda→Functions→quickcart-menu→Aliases→Create alias

   Field	Enter
   Name	live
   Version	1

   Leave Weighted alias empty. Choose Save.

   You should see

   The live alias, pointing at version 1.

4. Write version 2 and publish it

   Go back to the function (Lambda → Functions → quickcart-menu), open the Code tab, and change index.mjs to:

   index.mjsCopy

   export const handler = async () => {
     return { version: "2", menu: "Margherita, Pepperoni, Veggie" };
   };

   Choose Deploy, then Actions → Publish new version with the description v2, and Publish.

   You should see

   Version 2 created. Nobody is calling it yet: live still points at version 1. That’s deploy without release.

5. Start the canary: 10% to version 2

   AWSIn the AWS console: Lambda→Functions→quickcart-menu→Aliases→live→Edit

   Field	Enter
   Version	1
   Weighted alias: Additional version	2
   Weight	10

   Choose Save.

   You should see

   The alias shows version 1 at 90% and version 2 at 10%.

6. Call the alias and watch the split

   Now call live the way a real caller would. The console’s Test button is built for trying one version; to see which version answers each call through the alias, the terminal is simpler, because it prints that for you.

   TerminalCopy

   aws lambda invoke --function-name quickcart-menu:live response.json

   Part	What it means
   aws lambda invoke	call a Lambda function once and wait for the answer
   --function-name quickcart-menu:live	the function, and after the colon the alias to call through
   response.json	a file to save the function’s answer in. It’s created in the folder you’re in

   You should see

   {
       "StatusCode": 200,
       "ExecutedVersion": "1"
   }

   ExecutedVersion is the version that answered. Run the same command ten or twenty times: press ↑ then Return to repeat it.

   You should see

   Mostly "ExecutedVersion": "1", and now and then "2". Over many calls, about 1 in 10 is version 2. With so few calls, you might see none or several; that’s chance, not a fault.

7. Promote it

   The canary looks healthy, so give version 2 everyone. Edit live again:

   AWSIn the AWS console: Lambda→Functions→quickcart-menu→Aliases→live→Edit

   Field	Enter
   Version	2
   Weighted alias: Additional version	Remove it, so there is none

   Choose Save, and run the invoke command a few more times.

   You should see

   "ExecutedVersion": "2" every time. The release is complete.

8. Switch back instantly

   Say version 2 turns out to be wrong. Edit live once more and set Version to 1. Choose Save, and invoke again.

   You should see

   "ExecutedVersion": "1" every time. Version 2 still exists, so switching forward again is just as quick. This is the blue/green idea: two complete versions, and one pointer that moves between them.

9. Clean-up step: Clean up

   AWSIn the AWS console: Lambda→Functions→quickcart-menu→Actions→Delete function

   Type confirm if asked, and choose Delete. This removes every version and the alias.

   Lambda also created a log group for the function’s logs. Delete it too:

   AWSIn the AWS console: CloudWatch→Log groups→/aws/lambda/quickcart-menu→Actions→Delete log group(s)

   Finally, remove the answer file from your laptop:

   TerminalCopy

   rm response.json

   Part	What it means
   rm	remove a file
   response.json	the file aws lambda invoke wrote

   The IAM role Lambda created, named like quickcart-menu-role- followed by random letters, costs nothing. You can delete it in IAM → Roles if you like to keep things tidy.