What to Expect in the Toast System Design Interview

The Toast system design interview focuses on restaurant systems that must survive bad networks. Candidates report questions about offline-capable point of sale systems, payment processing, and online ordering with live menu updates. A point of sale system, or POS, is the software that takes orders and payments in the restaurant. The round runs about 60 minutes on a virtual whiteboard, inside a five-interview onsite.

One reported follow up captures the whole round: what happens when two terminals edit the same open check. If you can answer that well, you are ready for most of what Toast asks.

The Question Types Toast Asks

• Design an offline-capable POS. The restaurant's internet fails during dinner. Terminals must keep taking orders and sync later without losing or doubling anything.
• Design payment processing for restaurants. Card data has strict handling rules called PCI compliance, the card industry's security standard. The design must move money exactly once per check.
• Design online ordering with menu synchronization. A menu edit in the back office must reach the website and apps quickly. Out of stock items must disappear fast.
• Design a kitchen display pipeline. Orders flow from terminals to kitchen screens in seconds, in the right order, during a rush.

Why These Questions Fit Toast's Product

These are Toast's daily problems, so interviewers can judge depth precisely. The environment gives the constraints: restaurant wifi is unreliable, terminals are shared, and staff cannot stop to debug. Payments add exactness: a doubled charge or a lost check is a real loss of money and trust.

So aim your effort at correctness and recovery, not at massive scale. A single restaurant is small. Tens of thousands of restaurants syncing to one cloud is where scale enters.

Walkthrough: Design an Offline-First POS

Step 1: State the core requirement. Order taking must work with zero internet. That forces a local-first design: every terminal keeps a full local copy of the menu and open checks.

Step 2: Write locally, sync in the background. Each action, such as adding an item to a check, is saved as an event on the terminal. An event is a small record of one change. A background process uploads events when the network allows.

Step 3: Resolve conflicts between terminals. Two terminals may edit the same open check while offline. Last-write-wins, keeping only the newest change, loses one server's items, so it fails here. Instead, merge at the item level: the check becomes the set of all added items, with removals recorded as their own events. Mention CRDTs (data structures that merge concurrent edits automatically) and vector clocks (counters that track which device saw which changes). Candidates report Toast interviewers know and probe these terms.

Step 4: Handle payments carefully. Card authorization normally needs the network. For offline moments, describe store-and-forward: save the card transaction locally, with a strict money cap, and process it when connectivity returns. Name the risk plainly: the card may decline later, and the restaurant accepts that small loss to keep serving.

Step 5: Make sync idempotent. Idempotent means applying the same event twice has the effect of applying it once. Give every event a unique ID so retries never double an item or a charge.

Step 6: Close the loop with reconciliation. Reconciliation means comparing terminal records, cloud records, and payment processor records to find mismatches. Run it nightly per restaurant. This is how the design proves no order or dollar was lost.

What Interviewers Evaluate

Constraint-first thinking. Strong candidates name the failure conditions before drawing boxes. Weak candidates design a clean online system and patch offline in later.

Money exactness. Every payment path needs an exactly-once story. Say "idempotency key" early.

Practical judgment. Candidates report Toast prefers workable designs over exotic ones. Choose simple parts and explain why they survive a dinner rush.

How to Prepare

• Build the fundamentals first. Queues, caches, replication, and consistency models are the vocabulary of this round. The course Grokking the System Design Interview teaches them through complete designs.
• Go deeper on sync and payment patterns. Study CRDTs, event logs, idempotency, and settlement flows. Grokking the Advanced System Design Interview covers this second layer.
• Practice the offline POS question end to end. Draw it twice on paper, 45 minutes each time. Include the two-terminal conflict, since candidates report it as a follow up.
• Know the rest of the loop. Design is one of five onsite interviews. Read What Is the Toast Interview Process Like? (Round by Round) for the full map.
• Prepare the human rounds too. See Top Toast Behavioral Interview Questions (and How to Answer Them) for the behavioral half of the onsite.