I think we’ve got a serious race condition
I'm a passionate software developer.
Brian:
I was checking the logs from last night, and I think we’ve got a serious race condition. Some of the payment statuses are being overwritten.
Ingrid:
Overwritten? As in, two processes updating the same row at the same time?
Brian:
Exactly. The jobs run in parallel, and the updates aren’t atomic. One thread marks the payment as completed, and almost immediately another marks it as failed.
Ingrid:
Classic data race. So we’re basically losing consistency because the order of execution isn’t guaranteed.
Brian:
Yep. It’s completely non-deterministic. Sometimes it passes, sometimes it breaks, depending on how the scheduler interleaves the operations.
Ingrid:That’s messy. Are we using any synchronization at all? Mutex, semaphore, even a simple database-level lock?
Brian:
Nope. Right now it’s just raw updates against the shared state. No lock, no transaction, nothing.
Ingrid:
Okay, then it’s not thread-safe at all. We should at least wrap that code in a critical section.
Brian:
My only concern is performance. If we put a lock around the whole thing, we might create contention or even deadlocks.
Ingrid:
True. But we don’t need to lock the whole workflow, just the part where the shared record gets updated. Keep the critical section as small as possible.
Vocabulary
atomic | deadlocks | contention |
raw updates | wrap that code | thread-safe |
That’s messy | non-deterministic | interleaves the operations |
race condition | overwritten | run in parallel |
Vocabulary
Atomic
Meaning: An operation that happens completely or not at all. Nothing can interrupt it midway.
Example: “The update must be atomic so that no other thread can mess with it while it’s running.”
Deadlocks
Meaning: A situation where two or more processes are waiting for each other forever, and nothing moves forward.
Example: “If thread A holds Lock 1 and waits for Lock 2 while thread B holds Lock 2 and waits for Lock 1, you’ve got a deadlock.”
Contention
Meaning: When multiple processes or threads are trying to use the same resource at the same time, causing slowdowns.
Example: “Too many threads writing to the database creates contention.”
Raw updates
Meaning: Direct changes to the data without any safeguards like transactions or locks.
Example: “We’re doing raw updates on the table, so nothing is protecting us from conflicts.”
Wrap that code
Meaning: Put the code inside a structure (like a lock or transaction) to make it safe.
Example: “We should wrap that code in a mutex to avoid race conditions.”
Thread-safe
Meaning: Code that works correctly even when multiple threads run it at the same time.
Example: “This function isn’t thread-safe—it fails when two users hit it simultaneously.”
That’s messy
Meaning: Informal way of saying something is confusing, hard to manage, or poorly controlled.
Example: “Two different states being written at the same time? That’s messy.”
Non-deterministic
Meaning: The result is unpredictable and can change depending on timing or execution order.
Example: “The test is non-deterministic—it passes sometimes and fails other times.”
Interleaves the operations
Meaning: When the system mixes pieces of different processes or threads together during execution.
Example: “The scheduler interleaves the operations, so thread A and thread B take turns running.”
Race condition
Meaning: A bug that happens when two processes or threads try to use the same resource at the same time, and the result depends on who “wins the race.”
Example: “Updating the counter without a lock caused a race condition.”
Overwritten
Meaning: When new data replaces old data, sometimes unintentionally.
Example: “The first update marked the payment as complete, but it got overwritten by the second update.”
Exercises
Fill in the blank
An ____________ operation cannot be interrupted; it either completes fully or not at all.
When two threads wait for each other forever, it causes a ____________.
Multiple processes trying to access the same resource at the same time creates ____________.
Directly updating data without using locks or transactions is known as ____________.
Developers often say “__________” when they want to put risky code inside a safe structure like a lock.
If a method works reliably even when multiple threads call it, we say it is ____________.
“__________” is an informal way to describe code that produces confusing or inconsistent results.
A process is ____________ when its result is unpredictable and changes depending on timing.
The scheduler sometimes ____________ the operations, letting pieces of different threads run in sequence.
A ____________ happens when the outcome depends on who “wins” access to a shared resource first.
When one update accidentally replaces another, the data is said to be ____________.
To avoid a ____________, it’s best to acquire locks in a consistent order across all threads.
Using a database transaction can make an update behave like an ____________ operation.
The performance issue that occurs when too many threads wait for the same lock is called ____________.
Writing to shared state without synchronization often leads to a ____________ condition.
A result that is not consistent between runs, even with the same inputs, is ____________.
To protect shared variables, engineers often ____________ the code that modifies them.
If a system is not ____________, it may crash or corrupt data under concurrent access.
When log entries show two different values being written at the same time, it’s a sign of being ____________.
Debugging is harder when execution is ____________, because you can’t predict the exact order of steps.
Reading comprehension
1. What problem did Brian find in the logs?
a) The system was too slow
b) Payment statuses were being overwritten
c) Users couldn’t log in
d) The database was offline
2. How does Ingrid describe the situation where two processes update the same row at the same time?
a) A deadlock
b) A race condition
c) Contention
d) Non-determinism
3. Why does Brian call the results “non-deterministic”?
a) Because they depend on who logs in first
b) Because they are predictable
c) Because the outcome changes depending on execution order
d) Because the database is corrupted
4. What does Ingrid ask about synchronization?
a) If they are using caching
b) If they are using locks or other synchronization tools
c) If they are using AI to optimize scheduling
d) If they are using a new payment API
5. What is Brian’s reply about synchronization?
a) They are using transactions for every update
b) They only use semaphores
c) They are doing raw updates with no protection
d) They disabled synchronization for performance
6. What does Ingrid suggest to make the code safer?
a) Rewrite the system from scratch
b) Wrap the update in a critical section
c) Add more servers
d) Disable parallel processing
7. What concern does Brian raise about using locks?
a) It might cause data loss
b) It might create deadlocks or slowdowns
c) It might make the UI ugly
d) It might reduce test coverage
8. How does Ingrid respond to Brian’s concern?
a) She says to avoid locks completely
b) She suggests locking only the critical section
c) She suggests running the code without synchronization
d) She suggests adding more threads
9. What alternative solution does Brian propose?
a) Making the operation idempotent
b) Restarting the system every night
c) Removing database indexes
d) Using only one thread for all jobs
10. What final step does Brian decide to take?
a) Test it with a lock around the update first
b) Do nothing and ignore the issue
c) Rewrite the database schema
d) Remove the payment feature
Homework
Produce
Write a short dialogue between two developers who discover a bug caused by a race condition.
- Use at least 5 of these words/phrases:
[atomic, deadlock, contention, raw updates, thread-safe, non-deterministic, overwritten, race condition, interleaves the operations, That’s messy, wrap that code ] atomic
Personal connection
Think about any real project or exercise (class project, homework, internship, or job). Describe your personal experience in the topic.
