I started with a basic ACL table, which was fine, but the interviewer kept pulling the thread.
Start by clarifying functional and non-functional requirements, including scale, consistency, and latency needs. Then design a data model that supports efficient access checks and homepage listing, likely using a graph or relational model with indexing. Finally, discuss trade-offs between different approaches and how to handle scale, caching, and consistency.
Pro tip: Emphasize the importance of defining clear access control semantics (e.g., ownership, group sharing, inheritance) early, as this drives the data model and query patterns. Also, mention how you would handle permission revocation and propagation to ensure correctness at scale.
Ask questions to understand scale (number of users, files, shares), access patterns (read/write ratio, frequency of permission checks), consistency requirements (strong vs eventual), and security constraints.
Propose a schema to represent users, files, and permissions. Consider using a relational model with a permissions table (user_id, file_id, role) or a graph model for complex sharing relationships. Discuss indexing for fast lookups.
Outline how to check if a user has access to a file, including direct ownership, explicit shares, group memberships, and inherited permissions. Consider using an access control list (ACL) or role-based access control (RBAC).
Design a query to efficiently retrieve all files a user can access. Discuss strategies like denormalization, caching, or using a search index to avoid expensive joins at scale.
Discuss how to scale the system (sharding, replication), handle consistency (e.g., eventual consistency for permission updates), and trade-offs between simplicity and performance (e.g., precomputed access lists vs on-the-fly checks).
AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.
Waved at 'eventual consistency' too early and the interviewer pushed back immediately.
Start by clarifying the system context and the business impact of stale access, then propose a layered approach combining push-based revocation with periodic validation. Define an acceptable staleness window based on risk tolerance and enforce it through caching TTLs, token expiry, and revocation propagation mechanisms.
Pro tip: Tie the staleness window to the sensitivity of the resource and the cost of false positives—e.g., for high-risk actions, enforce zero staleness with synchronous checks, while low-risk reads can tolerate minutes. This shows you balance security with availability and performance.
Ask about the system's scale, the sensitivity of resources, and the business impact of stale access. Determine whether the revocation is for a single session, a user, or a role, and identify the acceptable risk window.
Propose a mechanism to propagate revocation events, such as a pub/sub system or a centralized revocation list. Ensure it is reliable and low-latency, with fallbacks like periodic polling.
Use short-lived tokens or cached permissions with TTLs that match the acceptable staleness window. For stricter requirements, implement synchronous checks or versioning to invalidate caches immediately.
Address scenarios like network partitions, service outages, and clock skew. Consider graceful degradation, such as denying access when the revocation status cannot be verified.
Define metrics to track revocation latency and staleness, and set up alerts. Continuously review and adjust the window based on observed incidents and performance.
AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.
Start by clarifying the requirements and scale, then propose a design that separates metadata from content and uses a fan-out-on-write or fan-out-on-read strategy for sharing. Discuss trade-offs between consistency, latency, and cost, and explain how you would handle permissions and access control at scale.
Pro tip: Demonstrate awareness of Amazon's leadership principles by emphasizing customer obsession (fast access for all group members) and ownership (end-to-end design including monitoring and failure handling). Also, mention real-world constraints like S3 request limits and how to mitigate them.
Ask questions to understand the expected read/write ratio, latency requirements, consistency needs, and group membership dynamics (e.g., how often members change).
Propose a system that stores file content in a scalable object store (e.g., S3) and metadata in a database. For sharing, consider a permission model where group membership is checked at access time or precomputed.
Discuss fan-out strategies: for read-heavy workloads, use fan-out-on-read with a centralized permission service; for write-heavy, consider fan-out-on-write but beware of write amplification. Alternatively, use a hybrid approach.
Explain how to efficiently check if a user belongs to a group with tens of thousands of members. Options include caching group memberships, using a graph database, or leveraging an existing identity service.
Compare latency, consistency, cost, and complexity of different approaches. Mention optimizations like CDN for content delivery, pre-signed URLs, and asynchronous permission updates.
AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.
Start by explaining how explicit deny rules change the evaluation semantics from a simple union of allows to a deny-overrides model, then discuss the implications for policy evaluation, conflict resolution, and system design. Use concrete examples to illustrate how deny rules affect authorization decisions, performance, and maintainability.
Pro tip: Emphasize that explicit deny rules enable least privilege and defense in depth, but they also introduce complexity in policy management and debugging; show you understand the trade-offs by mentioning how to audit and test deny rules effectively.
Explain that with explicit deny, the evaluation logic must check for denies first; if any deny matches, access is denied regardless of allows. This is often called 'deny overrides'.
Describe how to handle conflicts when both allow and deny rules apply. Typically, deny takes precedence, but you should mention the need for a clear precedence order (e.g., explicit deny > explicit allow > implicit deny).
Consider how deny rules affect the evaluation algorithm: you may need to evaluate all policies (or short-circuit on deny) and ensure that denies are checked efficiently, possibly with indexing or caching.
Discuss how deny rules affect policy storage, versioning, and propagation. For example, you might need to support negative permissions in your data model and ensure consistency across distributed systems.
Mention the need for auditing, testing, and debugging tools to understand why access was denied. Also, discuss how deny rules can help meet compliance requirements like least privilege.
AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.
I described recursive expansion with a depth cap to avoid cycles.
Start by clarifying the current design and requirements, then propose a hierarchical permission model that stores permissions at the folder level and resolves effective permissions by traversing the folder ancestry. Discuss trade-offs between inheritance, overrides, and performance, and outline how to handle edge cases like moves and deletes.
Pro tip: Mention that you would cache effective permissions at the file level with invalidation on folder permission changes to balance read performance and consistency, and discuss how to handle permission propagation asynchronously for large trees.
Ask about the scale (number of folders/files, depth of hierarchy), consistency requirements, and whether permissions can be overridden at lower levels.
Propose a schema with a folders table (id, parent_id) and a permissions table (resource_id, resource_type, principal, permission). Explain how to represent inheritance, e.g., by storing permissions only at the folder level and resolving at read time.
Describe how to compute effective permissions for a file: walk up the folder hierarchy, collect permissions, and apply precedence rules (e.g., explicit deny overrides allow, closest ancestor wins).
Discuss caching effective permissions, using materialized paths or closure tables for efficient ancestor queries, and handling permission changes with asynchronous propagation.
Cover moving folders/files, deleting folders, and bulk permission updates. Explain how to maintain consistency and avoid orphaned permissions.
AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.