Layer 3 · Go deeper · 11 min
Go deeper
Size it, build it, then make the calls an interviewer will push you on.
Back of the envelope
Size it up
Tap through each step of the arithmetic.
Why give each process one lease instead of a TTL per lock?
How many consensus groups does the lock store need?
Can a fencing token’s counter ever run out?
Build-up · 5 steps
Build the high-level design
One functional requirement at a time, on a single lock server, the way most teams start.
Step 1 of 5
Acquire a named lock
Acquire is one atomic step: create the record only if the name is free, with an expiry, so two simultaneous requests can’t both win. The server returns a token; the worker then writes to the payouts DB.
6 interview moments
You’re the candidate
The interviewer pushes on the hard parts. Pick an approach and see how it holds up.
Deep dive 1 of 6
Interviewer
“Many teams want locks. Some guard nightly thumbnail jobs; others guard payout batches. What do you build for them?”
Your move. Pick an approach:
Deep dive 2 of 6
Interviewer
“Your lock server dies in the midnight batch rush. How do you make sure no grant is lost or doubled?”
Your move. Pick an approach:
Deep dive 3 of 6
Interviewer
“The lock service is perfect, but a holder freezes in a GC pause past its lease. How do you stop its late write?”
Meanwhile another worker has got the lock and paid the batch.
Your move. Pick an approach:
Deep dive 4 of 6
Interviewer
“NTP steps a clock, a VM resumes after migration, a network blip drops renewals. How do you time leases safely?”
Your move. Pick an approach:
Deep dive 5 of 6
Interviewer
“At the 10:00 tatkal rush, 2,000 requests a second want the lock for one train, date and class. What do you do?”
Each holder does 20 ms of work plus two lock writes of about 3 ms, so at most 38 holders a second get through.
Your move. Pick an approach:
Deep dive 6 of 6
Interviewer
“Merchant 48213’s payout jobs must run one at a time, in order. Do you need the lock service at all?”
Your move. Pick an approach:
Build-up · 7 steps
Follow one payout batch through it
The hot path is the client library, a frontend and one group leader. The protected stores do the fencing; everything else is off to the side.
Step 1 of 7
Session
The client library opens one session per group it uses: a 15 s lease, kept alive every 5 s over one stream. Locks hang off it, so a dead process loses them all at once.