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Cloudflare Workers

Workers and Wrangler 4

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Module 1 · How the web works

Typing an address starts a conversation

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1 / 194 · How the web works

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Typing an address starts a conversation
, 1 to practiseA server is a program that answers
DNS turns names into numbers
, 1 to practiseAn address has parts
A request says what you want
, 1 to practiseA response says how it went
, 1 to practiseHeaders are labels on the envelope
, 1 to practiseThe project: a link shortener
Recap: how the web works

Running your own server is a job of its own
The cloud is someone else’s computers, rented
, 1 to practiseServerless: you bring code, they run it when asked
The edge: running close to the person asking
, 1 to practiseA Worker is your code on Cloudflare’s network
Workers start in milliseconds
, 1 to practiseA Worker comes with a toolbox
, 1 to practiseYou write Workers in JavaScript or TypeScript
Recap: the cloud, serverless and the edge

The terminal is where you type commands
, 1 to practiseNode.js and npm run the tools
A free Cloudflare account
, 1 to practiseOne command makes a project
, 1 to practiseWhat is in the new folder
, 1 to practiseWrangler is Cloudflare’s command-line tool
wrangler.jsonc describes your Worker
, 1 to practiseReading an error message
Recap: setting up

Four JavaScript pieces you need first
The whole Worker, line by line
Try it: be the runtime
wrangler dev runs it on your computer
, 1 to practiseMake it redirect
, 1 to practisewrangler deploy puts it on the internet
, 1 to practiseRequest, env and ctx
, 1 to practiseWhen your code throws
Recap: your first Worker

The same objects as in the browser
Reading the request
, 1 to practiseURL takes an address apart
, 1 to practiseTry it: answer from the path and the query
, 1 to practiseA body can be read once
, 1 to practiseBuilding a response
, 1 to practiseTry it: read JSON, answer JSON
Headers on a response you did not build
, 1 to practisefetch() asks another server
, 1 to practiseWhat Cloudflare adds to the request
Recap: Request, Response and URL

A route is a method and a path
Routing with if statements
, 1 to practiseOrder matters
, 1 to practisePaths arrive encoded
, 1 to practiseThe wrong method is a 405
, 1 to practiseTry it: a small router
URLPattern matches paths with parameters
, 1 to practiseCatch errors once, at the top
, 1 to practiseWhen to use a router library
Recap: routing by hand

Settings belong outside the code
, 1 to practiseVars live in wrangler.jsonc
, 1 to practiseSecrets never go in the config
Local secrets live in .dev.vars
, 1 to practiseUsing a secret to guard a route
, 1 to practiseTry it: check the token
, 1 to practisewrangler types writes the Env type
, 1 to practiseThe compatibility date pins the runtime’s behaviour
Recap: settings and secrets

A binding is a connection waiting on env
KV: a giant dictionary, everywhere
, 1 to practiseCreate a namespace and bind it
get, put and delete
, 1 to practiseTry it: read and delete
, 1 to practiseJSON values and expiry
, 1 to practiseKV is eventually consistent
Listing keys, a page at a time
, 1 to practiseCreating links through the API
, 1 to practiseTry it: create a link
Local and remote data
Recap: bindings and KV

A database keeps data in tables
, 1 to practiseD1 is SQLite, run by Cloudflare
, 1 to practiseCreate a database and bind it
, 1 to practiseMigrations change the database step by step
prepare, bind, then run
, 1 to practiseNever build SQL from strings
, 1 to practiseCount clicks in one statement
Batches run as one transaction
Looking inside a database
Recap: D1

Every upload is a version
, 1 to practiseRollback undoes code, not data
, 1 to practiseEnvironments: a staging copy from the same file
Your own domain
, 1 to practiseRoutes put a Worker in front of part of a site
, 1 to practiseGradual deployments: a little traffic first
Deploying from CI, without your laptop
Recap: deploying

R2 stores files, called objects
, 1 to practiseCreate a bucket and bind it
Uploading: put a stream
, 1 to practiseServing a file
, 1 to practiseStream, do not buffer
, 1 to practisehead, list and delete
Conditional and partial reads
Who may read the bucket
Recap: R2

The visitor should not wait for bookkeeping
, 1 to practisectx.waitUntil keeps the Worker alive a little longer
, 1 to practiseTry it: count without waiting
, 1 to practiseWhat waitUntil does not promise
, 1 to practiseCron: code on a timetable
The scheduled handler
, 1 to practiseSeveral schedules, one handler
Running a cron on your computer
, 1 to practiseWhich background tool for which job
Recap: background work

A queue holds messages until someone handles them
, 1 to practiseCreate the queue and bind both sides
, 1 to practiseSending messages
The queue handler receives batches
, 1 to practiseSuccess, failure and retries
, 1 to practiseTry it: ack or retry each message
At least once means maybe twice
, 1 to practiseGiving up: the dead letter queue
, 1 to practiseBigger batches, or quicker ones
Recap: queues

, 1 to practiseMany copies cannot agree on their own
One instance per name, in the whole world
A class whose methods Workers call
, 1 to practiseBinding and creating the class
, 1 to practiseCalling an object by name
Two ways to store: key-value and SQL
, 1 to practiseOne at a time, until you await something else
Alarms wake an object later
, 1 to practiseWhen to use one, and how to keep it fast
Recap: Durable Objects

Static assets are served before your code runs
, 1 to practiseWho answers: a file or the Worker
Serving a file from code
Why split a system into several Workers
, 1 to practiseService bindings: Worker to Worker, privately
, 1 to practiseRPC: call methods instead of URLs
, 1 to practiseSeveral Workers on your computer
Recap: assets and services

A cache keeps answers to reuse
, 1 to practiseCache-Control says who may keep a copy, and for how long
, 1 to practiseA cached redirect is a click you never see
The Cache API: Cloudflare’s cache from code
, 1 to practiseWhat the Cache API will and will not do
, 1 to practiseChoose the cache key on purpose
, 1 to practiseTry it: normalize a cache key
Caching what you fetch
, 1 to practiseWhat not to cache
Recap: caching

Log objects, not sentences
, 1 to practiseWhat never goes in a log
, 1 to practisewrangler tail: watch it live
, 1 to practiseMetrics: the shape of the traffic
Tests prove the code still works
Vitest, running inside workerd
, 1 to practiseCalling the Worker in a test
, 1 to practiseTesting crons and the database
, 1 to practiseFake only the outside world
, 1 to practiseTry it: test against a fake fetch
Recap: observing and testing

Many isolates share one process
, 1 to practiseCold starts are short, and global scope decides how short
, 1 to practiseWhat lives between requests
, 1 to practiseWeb standards first, Node.js on request
The limits that shape a design
, 1 to practiseHow Workers are billed, in general terms
, 1 to practiseDo independent work at the same time
Run near your data when it is far away
Recap: inside the runtime

What can go wrong with a link shortener
, 1 to practiseValidate every destination
, 1 to practiseTry it: validate a new link
, 1 to practiseCompare secrets in constant time
, 1 to practiseRate limiting at the edge
Security headers on what your code returns
, 1 to practiseLeast privilege: each part gets only what it needs
The shortener, grown up
, 1 to practiseWorkers that ship separately need contracts
, 1 to practiseFail gently
Recap: security and architecture

How to take this challenge
, 1 to practiseThe staging deploy that cannot sign in
, 1 to practiseThe consumer that loses emails
, 1 to practiseChoosing the store
, 1 to practiseThe rollback that did not help
, 1 to practiseThe redirect that got slower
, 1 to practiseWrite the config from memory
, 1 to practiseDesigning a coordinator
, 1 to practiseShipping a breaking feature safely
Recap: where you are now