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Databricks·Software Engineer·Technical Phone Screen·Senior

Senior
Jun 2026

Summary

Databricks software engineering interview that went deep into networking fundamentals pretty fast. The main problem was a firewall rule evaluator and it had more layers than I expected going in.

Questions Asked (1)

Q1

Given a list of firewall rules in CIDR notation, each marked as allow or deny and evaluated in order with first match winning, write a function that determines whether a given IP address is allowed or blocked. Then discuss how you'd make the lookup efficient.

Algorithms & Data StructuresSystem DesignTechnical Trade-offs
Author's notes

I got the basic first-match logic down pretty quick but then they pushed on efficiency and I fumbled a bit.

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AI HintsAI Generated

Suggested Approach

Start by clarifying the problem: rules are evaluated in order, first match wins, and CIDR blocks can overlap. Then implement a straightforward linear scan that converts each CIDR to a range and checks if the IP falls within it, returning the action of the first matching rule. For efficiency, discuss preprocessing rules into a data structure like a binary trie or interval tree to enable faster lookups, especially for large rule sets.

Pro tip: Mention that in real systems, firewall rules are often updated frequently, so the data structure should support efficient insertions and deletions, not just lookups. Also, consider using a binary trie (prefix tree) where each node represents a bit of the IP address, which naturally handles CIDR prefixes and first-match semantics by storing the action at the node where the prefix ends.

1. Clarify requirements and edge cases

Confirm that rules are evaluated in order, first match wins, and that CIDR blocks may overlap. Ask about input format (e.g., IPv4 vs IPv6), expected rule set size, and whether rules can be updated dynamically.

2. Design a naive solution

For each rule, convert the CIDR to a range of IP addresses (start and end). Iterate through rules in order, and for each, check if the given IP is within the range. Return the action of the first matching rule, or a default (e.g., deny) if none match.

3. Analyze performance and propose optimizations

The naive solution is O(N) per lookup. For large N, preprocess rules into a more efficient structure. Discuss options like sorting rules by prefix length and using binary search, building a binary trie (radix tree) for O(W) lookup where W is bit width, or using an interval tree.

4. Discuss trade-offs and implementation details

Compare approaches: tries offer fast lookup and support dynamic updates but have memory overhead; interval trees are good for static sets; sorting with binary search is simple but requires careful handling of overlapping ranges. Mention handling of IPv6 and the importance of first-match semantics in the data structure.

5. Summarize and conclude

Recap the chosen approach, emphasizing correctness and efficiency. Mention potential extensions like caching frequent lookups or using hardware acceleration, and confirm the solution meets the requirements.

Key Points to Mention

  • CIDR notation and how to convert to IP range (e.g., using bitwise operations).
  • First-match semantics and the need to preserve rule order in any optimized structure.
  • Time complexity: naive O(N) vs optimized O(W) for trie or O(log N) for interval tree.
  • Space complexity and memory overhead of different data structures.
  • Handling of overlapping CIDR blocks and default policy.
  • Support for dynamic rule updates (insertion/deletion) if required.

AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.