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Modern Java implementations do not use the old fixed segment array commonly described in older explanations. Reads are generally non-blocking; initialization and some updates use CAS, while contended updates coordinate at a bin. Map operations are thread-safe, but multi-key invariants and mutable values still need their own synchronization design.

Modern Java ConcurrentHashMap does not use the fixed lock segments from older implementations. It uses non-blocking reads and coordinates updates with CAS and synchronization at individual bins when needed. This allows unrelated keys to make progress concurrently. It protects map operations, not a multi-key transaction or mutable values stored inside it.

A hash helps select a bucket but does not establish key identity. HashMap checks equality among entries in the bucket; collisions can be stored in a list and, in sufficiently large bins, treeified under implementation conditions. Correct equals/hashCode behavior is essential, and resizing changes bucket placement without making hashes unique.

Concurrent builds use multiple phases and can leave an invalid index. Inspect the server error and pg_index readiness/validity, waits, and transactions; do not assume the operation was atomic or load-free.

Rollback restores code, not every side effect. Expand-and-contract changes preserve compatibility during overlap, and recovery assumptions must be exercised.

Derive tenant context from verified membership, apply it in lookup and mutation predicates, and check the requested action. Cache hits, downloads, and background jobs need the same scope enforcement.