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Production Debugging: Fix Real Bugs in a Live-Looking App

Course progress

0%
0 of 68 modules complete

Django

Performance0/3
  • Debugging slow pages: measure first
  • Lab: The order list page is slow for repeat customers
  • Lab: The staff dashboard hits the database too often
Data model and money0/4
  • Debugging wrong data: trace the value, not the screen
  • Lab: Invoices vanish when a customer is deleted
  • Lab: Order totals are one cent off
  • Lab: Evening orders land on tomorrow's report
Concurrency0/3
  • Debugging races: make the bug happen on purpose
  • Lab: A burst of signups crashes with IntegrityError
  • Lab: The last unit is sold twice
Migrations0/3
  • Debugging migrations: the graph and the data
  • Lab: The release is blocked by conflicting migrations
  • Lab: A migration that only fails on production data
Configuration and deployment0/2
  • Debugging configuration: what differs between here and there
  • Lab: Production has no styles or scripts
Calls to other services0/2
  • Debugging calls to other services: design for the failure
  • Lab: Checkout hangs when the payments provider is slow

FastAPI

Migrations (Alembic)0/3
  • Debugging Alembic: the revision graph and the data it meets
  • Lab: The release is blocked by multiple Alembic heads
  • Lab: A migration works in staging and crashes on production
Performance (SQLAlchemy)0/2
  • Measure first: counting statements in async SQLAlchemy
  • Lab: The order list is slow for repeat customers
Async and concurrency0/3
  • Debugging async code: the loop and the races between awaits
  • Lab: The whole API freezes whenever somebody logs in
  • Lab: Store credit goes missing under load
Service calls and errors0/3
  • Debugging outbound calls and the errors you report
  • Lab: Paying hangs when the payments provider is slow
  • Lab: Customers cannot pay but the dashboards show no errors
Data model and validation0/2
  • Debugging validation models: absent is not the same as null
  • Lab: Saving one profile field erases the others
Configuration and security0/4
  • Debugging deployment settings and authorization
  • Lab: The API docs are blank behind the gateway
  • Lab: Any customer can read any order
  • Expert lab: Double charges, then a frozen API, then a kill switch that does nothing

React

React hooks0/4
  • Debugging hooks: dependencies, closures and identity
  • Lab: The updated badge never gets past 1
  • Lab: Changing the status filter does not reload orders
  • Lab: The Reports page hammers the API
Async and races0/3
  • Debugging async UI: ordering and failure paths
  • Lab: Customer search shows results for an older query
  • Lab: An order stays flagged after the server refused it
State and identity0/3
  • Debugging state: who owns it and which component it belongs to
  • Lab: Deleting a note makes the next note show the wrong text
  • Lab: The customer editor shows the previous customer
Performance0/3
  • Debugging React performance: measure renders and leaks
  • Lab: Typing in the quick filter makes every page re-render
  • Lab: Memory and CPU grow every time Orders is opened
API and configuration0/4
  • Debugging the edges: API results, dates and build-time config
  • Lab: The UI says Saved while the server was down
  • Lab: Revenue report shows the previous day for US users
  • Lab: The staging build still calls its own host

Fullstack

Sessions, cookies and CORS0/5
  • Where the browser and the API disagree: sessions, cookies and CORS
  • Lab: Staging console cannot sign in - the form just says "Failed to fetch
  • Lab: Sign-in succeeds, then Orders and Profile say "Sign in to continue
  • Lab: Saving anything fails with "The request could not be verified
  • Lab: Saving the profile fails with "The request could not be verified
The API contract0/4
  • Contracts across the wire: error bodies, query strings and paging
  • Lab: Error messages are gone - not-found and forbidden failures show "Request failed"
  • Lab: Searching for "R&D Kit" lists everything containing an R
  • Lab: Our newest orders are missing from the order history
Data and state0/4
  • Values that change meaning on the way: time zones, money and stale writes
  • Lab: The order I placed this evening is dated tomorrow
  • Lab: Every order in the history shows a total of under a dollar
  • Lab: Saving my profile on my laptop erased the phone number I just changed on my phone

FastAPI

Latency and slow APIs0/4
  • Debugging a slow API: blocked loops, sequential awaits and throwaway clients
  • Lab: The API stalls while shoppers search
  • Lab: The storefront takes the sum of its dependencies
  • Lab: A new connection for every charge
Debugging async UI: ordering and failure paths
Course contents

Production Debugging: Fix Real Bugs in a Live-Looking App

Course progress

0%
0 of 68 modules complete

Django

Performance0/3
  • Debugging slow pages: measure first
  • Lab: The order list page is slow for repeat customers
  • Lab: The staff dashboard hits the database too often
Data model and money0/4
  • Debugging wrong data: trace the value, not the screen
  • Lab: Invoices vanish when a customer is deleted
  • Lab: Order totals are one cent off
  • Lab: Evening orders land on tomorrow's report
Concurrency0/3
  • Debugging races: make the bug happen on purpose
  • Lab: A burst of signups crashes with IntegrityError
  • Lab: The last unit is sold twice
Migrations0/3
  • Debugging migrations: the graph and the data
  • Lab: The release is blocked by conflicting migrations
  • Lab: A migration that only fails on production data
Configuration and deployment0/2
  • Debugging configuration: what differs between here and there
  • Lab: Production has no styles or scripts
Calls to other services0/2
  • Debugging calls to other services: design for the failure
  • Lab: Checkout hangs when the payments provider is slow

FastAPI

Migrations (Alembic)0/3
  • Debugging Alembic: the revision graph and the data it meets
  • Lab: The release is blocked by multiple Alembic heads
  • Lab: A migration works in staging and crashes on production
Performance (SQLAlchemy)0/2
  • Measure first: counting statements in async SQLAlchemy
  • Lab: The order list is slow for repeat customers
Async and concurrency0/3
  • Debugging async code: the loop and the races between awaits
  • Lab: The whole API freezes whenever somebody logs in
  • Lab: Store credit goes missing under load
Service calls and errors0/3
  • Debugging outbound calls and the errors you report
  • Lab: Paying hangs when the payments provider is slow
  • Lab: Customers cannot pay but the dashboards show no errors
Data model and validation0/2
  • Debugging validation models: absent is not the same as null
  • Lab: Saving one profile field erases the others
Configuration and security0/4
  • Debugging deployment settings and authorization
  • Lab: The API docs are blank behind the gateway
  • Lab: Any customer can read any order
  • Expert lab: Double charges, then a frozen API, then a kill switch that does nothing

React

React hooks0/4
  • Debugging hooks: dependencies, closures and identity
  • Lab: The updated badge never gets past 1
  • Lab: Changing the status filter does not reload orders
  • Lab: The Reports page hammers the API
Async and races0/3
  • Debugging async UI: ordering and failure paths
  • Lab: Customer search shows results for an older query
  • Lab: An order stays flagged after the server refused it
State and identity0/3
  • Debugging state: who owns it and which component it belongs to
  • Lab: Deleting a note makes the next note show the wrong text
  • Lab: The customer editor shows the previous customer
Performance0/3
  • Debugging React performance: measure renders and leaks
  • Lab: Typing in the quick filter makes every page re-render
  • Lab: Memory and CPU grow every time Orders is opened
API and configuration0/4
  • Debugging the edges: API results, dates and build-time config
  • Lab: The UI says Saved while the server was down
  • Lab: Revenue report shows the previous day for US users
  • Lab: The staging build still calls its own host

Fullstack

Sessions, cookies and CORS0/5
  • Where the browser and the API disagree: sessions, cookies and CORS
  • Lab: Staging console cannot sign in - the form just says "Failed to fetch
  • Lab: Sign-in succeeds, then Orders and Profile say "Sign in to continue
  • Lab: Saving anything fails with "The request could not be verified
  • Lab: Saving the profile fails with "The request could not be verified
The API contract0/4
  • Contracts across the wire: error bodies, query strings and paging
  • Lab: Error messages are gone - not-found and forbidden failures show "Request failed"
  • Lab: Searching for "R&D Kit" lists everything containing an R
  • Lab: Our newest orders are missing from the order history
Data and state0/4
  • Values that change meaning on the way: time zones, money and stale writes
  • Lab: The order I placed this evening is dated tomorrow
  • Lab: Every order in the history shows a total of under a dollar
  • Lab: Saving my profile on my laptop erased the phone number I just changed on my phone

FastAPI

Latency and slow APIs0/4
  • Debugging a slow API: blocked loops, sequential awaits and throwaway clients
  • Lab: The API stalls while shoppers search
  • Lab: The storefront takes the sum of its dependencies
  • Lab: A new connection for every charge
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Debugging async UI: ordering and failure paths

Reading15 min

Responses do not arrive in the order requests were sent

A typeahead that applies every response will show the answer to an older query whenever that request is slower. Reproduce it by delaying the first response in a test, then make the effect ignore stale results or abort the previous request in its cleanup.

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Optimistic updates need a rollback

Updating the UI before the server confirms is fine, but the failure branch must restore the previous state as well as show a message. Read the catch block and ask what state the screen is left in.

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PreviousLab: The Reports page hammers the APINextLab: Customer search shows results for an older query

Course progress

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On this page

  • Responses do not arrive in the order requests were sent
  • Optimistic updates need a rollback