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Design a scalable tagging system

AtlassianSoftware Engineer · Technical Screen
HardSoftware EngineerTechnical ScreenSystem Design

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This question tests a candidate's ability to design scalable data models and distributed indexing strategies for a multi-tenant tagging system. It evaluates system design competency across storage, caching, sharding, and consistency trade-offs — core skills assessed in senior software engineering interviews.

Question

Design a scalable tagging system that lets users attach multiple tags to arbitrary resources (items) and query them efficiently. The system must support creating and managing tags, attaching/removing tags, and high read/write traffic. Address the following:

  1. Tag lifecycle: Create / get-or-create tags, attach and remove tags from items, and prevent duplicate (item, tag) assignments.
  2. Single-tag query: Return all items for a given tag, with pagination and sorting (e.g., by recency).
  3. Multi-tag query (AND / OR): Return items matching a combination of tags, supporting both intersection (AND) and union (OR).
  4. Top-N per tag: Efficiently list the top-N items for a tag (e.g., by recency or score).
  5. Counts: Return the number of items per tag.
  6. Autocomplete / suggestions: Prefix-based tag autocomplete and (optionally) related-tag suggestions.
  7. Data model & storage: Specify the data model and storage choices for tags, items, and assignments.
  8. Indexing: Indexing strategy for fast writes and reads, including an inverted index (tag → items) and forward index (item → tags), plus secondary indexes to handle high-cardinality / skewed tags.
  9. Caching: What to cache, cache-invalidation strategy, and hot-key handling.
  10. Sharding / partitioning: How to partition the OLTP store and the index, including multi-tenancy isolation and hot-tag mitigation.
  11. Consistency: Choose and justify a consistency model (eventual vs. strong) and explain how to support read-your-writes.
  12. De-duplication, rename, and merge: How tag de-duplication, renaming, and merging two tags are handled.
  13. Access control: Tenant isolation and per-resource ACL filtering.
  14. Pagination: A stable, scalable pagination strategy.
  15. Backfill & cleanup: Reindex/backfill jobs and cleanup of orphan tags and drifted counters.
  16. Capacity, monitoring, and SLAs: Provide order-of-magnitude capacity estimates and describe the key metrics, alerts, and SLOs.