Debugging, the Developer Way

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Debugging, the Developer Way Yarden Laifenfeld @yardenlaif

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2 Who Am I? • Software Engineer at Rookout • Go, Java, Ruby • C#, Python, JavaScript, C++ • Sewing

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Non Breaking Breakpoints 3

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4 I love debugging. wtf?

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6 1/30

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7 50%

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Why? 8 ● We don’t know how ● Intuition ● It’s not fun ● Simple code == simple bugs

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The Flow 9 ● Unexpected behavior ● What/Why/Where? ● Fix

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Unexpected Behavior 10 ● Incorrect use

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UI/UX Issue 11 CLICK ME DON’T CLICK ME

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Bad Response 12 CLICK ME DON’T CLICK ME

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Unexpected Behavior 13 ● Incorrect use ● Something went terribly wrong

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Something Went Terribly Wrong 14

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Unexpected Behavior 15 ● Incorrect use ● Something went terribly wrong ● Integration Issues

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Integration Issues 16

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Unexpected Behavior 17 ● Incorrect use ● Something went terribly wrong ● Integration Issues ● Expectation ≠ Reality

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What/Why/Where 18 ● Stare at the code ● Read all the logs ● Try to reproduce ● Run through each line of code

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Fix 19 ● Find a way to ﬁx ● Write the code ● Add a test

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The “Research Based” Flow 20 ● Our code is a mystery ● No planning ● Guessing randomly ○ print debugging

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21 Approach debugging like writing a feature

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Writing a Feature 22 ● Speciﬁcation ● Planning ● Executing

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Speciﬁcation 23 ● Get all the information ○ Understand the task ○ Edge cases ○ Protocols ● Deﬁnition of done

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Planning - Design the Feature 24 ● Components ● Language to use ● Design patterns ● Third party libraries

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Planning - Deadlines 25 ● Multiple deadlines ● What if I miss the deadline?

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Planning - Setup Your Workspace 26 ● An easy-to-run example ● Up to date code ● Comfortable environment

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Executing 27 ● Develop in iterations ● Document as you code

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Writing a Feature 28 ● Speciﬁcation ○ Get all the information ○ Deﬁnition of done ● Planning ○ Design the feature ○ Deadlines ○ Setup your workplace ● Executing ○ Develop in iterations ○ Document as you code

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Debugging a Bug 29 ● Speciﬁcation ● Planning ● Executing

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Speciﬁcation - Get All the Information ● Is it even a bug? ● Logs, Traces, Metrics ● Environment 30 ➔ Speciﬁcation ➔ Planning ➔ Executing

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Specification - Definition of Done ● Reproduction ● How do I know I fixed the bug? 31 ➔ Specification ➔ Planning ➔ Executing

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Planning - Design the Debugging Process ● If I wanted to purposefully create this bug, how would I do that? ● Multiple debugging vectors ○ Practical milestones 32 ➔ Speciﬁcation ➔ Planning ➔ Executing

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Planning - Deadlines ● Multiple deadlines ● What if I miss the deadline? 33 ➔ Speciﬁcation ➔ Planning ➔ Executing

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Planning - Setup Your Workspace 34 ● An easy-to-run example ● Relevant code version ● Comfortable environment ➔ Speciﬁcation ➔ Planning ➔ Executing

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Executing - Use the Right Tools ● Print debugging ● Debuggers ● Proﬁlers 35 ➔ Speciﬁcation ➔ Planning ➔ Executing

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Executing - Debug in Iterations ● Practical milestones ● Make sure you’re making progress ● Validate your assumptions ● Commit to git 36 ➔ Speciﬁcation ➔ Planning ➔ Executing

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Executing - Document EVERYTHING ● Document things that worked ● Document things that didn’t ● Document your thoughts ● Document consistently and clearly 37 ➔ Speciﬁcation ➔ Planning ➔ Executing

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Executing - Find and Fix Once you’ve found the bug, go back to the speciﬁcation step 38 ➔ Speciﬁcation ➔ Planning ➔ Executing

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Debugging a Bug 39 ● Speciﬁcation ○ Get all the information ○ Deﬁnition of done ● Planning ○ Design the debugging process ○ Deadlines ○ Setup your workplace ● Executing ○ Develop in iterations ○ Document as you debug ○ Use the right tools ○ Fix the bug

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40 Plan Proportionately

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41 Let’s See it in Action!

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44 Seats DB getAvailableSeats() reserve(id, seats) checkout(id) cancel(id)

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Seat Selection Flow 45 ● UserCart calls DB.reserve(id, seats) ● DB checks if seats are available: ○ Not available: throws an error ○ Available: marks seats as reserved ■ Previous seats are discarded ● UserCart adds seats to list of reserved seats

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46 func select_seat(seat) { thread { new_seats = this.seats + seat serialized = new_seats.serialize() try { db.reserve(this.id, serialized) this.seats.add(seat) } } }

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Checkout Seat Flow 47 ● UserCart calls DB.checkout (id) ● DB marks all reserved seats associated with id as taken ● UserCart sends tickets for all seats in list of reserved seats

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48 func checkout() { db.checkout(this.id) send_tickets(this.seats) }

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Bug 49 Alice and Bob arrive at the movie theater with tickets for the same seat (A3).

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Speciﬁcation - Get All the Information ● Doubt the user ● Logs 50 ➔ Speciﬁcation ➔ Planning ➔ Executing

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Speciﬁcation - Logs 51

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Specification - Definition of Done ● Reproduction ● How do I know I fixed the bug? 52 ➔ Specification ➔ Planning ➔ Executing

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Speciﬁcation - Reproduction 53

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Planning - Design the Debugging Process ● If I wanted to purposefully create this bug, how would I do that? ● Multiple debugging vectors ○ Practical milestones 54 ➔ Speciﬁcation ➔ Planning ➔ Executing

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Checkout Seat Flow 55 Seat Selection ● UserCart calls DB.reserve(id, seats) ● DB checks if seats are available: ○ Not available: throws an error ○ Available: marks seats as reserved (previous seats are discarded) ● UserCart adds seats to list of reserved seats Checkout ● UserCart calls DB.checkout(id) ● DB marks all reserved seats associated with id as taken ● UserCart sends tickets for all seats in list of reserved seats

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Planning - Debugging Vectors ● DB.reserve doesn’t reserve seats ● UserCart sends the wrong seats to reserve ● DB doesn’t checkout the correct seats ● DB doesn’t mark reserved seats as taken correctly ● UserCart sends tickets for wrong seats 56 ➔ Speciﬁcation ➔ Planning ➔ Executing

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Planning - Debugging Vectors 1. DB doesn’t mark reserved seats as taken correctly 2. DB doesn’t checkout the correct seats 3. DB.reserve doesn’t reserve seats 57 ➔ Speciﬁcation ➔ Planning ➔ Executing

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Planning - Practical Milestones 1. DB doesn’t mark reserved seats as taken correctly a. Sanity check DB.checkout() b. Stress check DB.checkout() 2. DB doesn’t checkout the correct seats a. Verify UserCart and DB mark the same seats at checkout 3. DB.reserve doesn’t reserve seats 58 ➔ Speciﬁcation ➔ Planning ➔ Executing

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Planning - Deadlines 1. DB doesn’t mark reserved seats as taken correctly a. Sanity check DB.checkout() - 2 min. b. Stress check DB.checkout() - 5 min. 2. DB doesn’t checkout the correct seats a. Verify UserCart and DB mark the same seats at checkout - 10 min. 3. DB.reserve doesn’t reserve seats 59 ➔ Speciﬁcation ➔ Planning ➔ Executing

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Planning - Setup Your Workspace 60 ● An easy-to-run example ● Relevant code version ● Comfortable environment ➔ Speciﬁcation ➔ Planning ➔ Executing

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Executing ● Use the right tools ● Debug in iterations ● Document EVERYTHING ● Find and ﬁx 61 ➔ Speciﬁcation ➔ Planning ➔ Executing

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Executing 1. DB doesn’t mark reserved seats as taken correctly a. Sanity check DB.checkout() ✔ b. Stress check DB.checkout() ✔ 2. DB doesn’t checkout the correct seats a. Verify UserCart and DB mark the same seats at checkout 3. DB.reserve doesn’t reserve seats 62 ➔ Speciﬁcation ➔ Planning ➔ Executing