Fix time UUID generator throughput

[dkropachev]

Fixes #505.

Summary

• Restore the legacy driver's sub-millisecond timestamp allocation instead of waiting for the wall clock after every UUID.
• Preserve monotonic generation when the system clock moves backwards.
• Use shared Rust borrows for UUID generator APIs documented as thread-safe.
• Add deterministic tests for same-millisecond capacity, clock rollback, and concurrent uniqueness.

Performance

Release build with one shared generator:

Threads	Before	After
1	~1,000 UUID/s	~10.0M UUID/s
8	~1,000 UUID/s	~10.0M UUID/s
20	~1,000 UUID/s	~10.0M UUID/s

The patched shared-generator performance matches the legacy C++ driver's approximately 10 million UUID/s ceiling from 10,000 100-nanosecond ticks per millisecond.

Validation

• cargo test --lib: 51 passed
• cargo clippy --all-targets -- -D warnings
• Release CMake build
• Optimized C API benchmark at 1, 8, and 20 threads

Tracking

• Issue: cass_uuid_gen_time() is capped at ~1K UUID/s and busy-spins concurrent callers #505
• Jira: https://scylladb.atlassian.net/browse/CUSTOMER-642

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📒 Files selected for processing (4)

• Makefile
• docs/source/topics/using/data-types/uuids.md
• scylla-rust-wrapper/src/cql_types/uuid.rs
• tests/src/integration/tests/test_uuids.cpp

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📝 Walkthrough

Walkthrough

UUID timestamp generation now uses atomic compare-and-exchange logic for sub-millisecond calls, clock rollback, stale samples, and concurrent access. cass_uuid_gen_time and cass_uuid_gen_from_time now accept shared generator pointers. Rust unit tests and a concurrent C++ integration test validate monotonicity and timestamp uniqueness. Documentation describes the per-millisecond limit and busy-wait behavior.

Sequence Diagram(s)

sequenceDiagram
  participant Caller
  participant cass_uuid_gen_time
  participant try_monotonic_timestamp
  participant AtomicTimestamp
  Caller->>cass_uuid_gen_time: request UUID timestamp
  cass_uuid_gen_time->>try_monotonic_timestamp: read current time
  try_monotonic_timestamp->>AtomicTimestamp: compare-and-exchange timestamp
  AtomicTimestamp-->>try_monotonic_timestamp: updated timestamp
  try_monotonic_timestamp-->>cass_uuid_gen_time: monotonic timestamp
  cass_uuid_gen_time-->>Caller: UUID

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Priority: ➖ Normal

Severity of issue fixed: Medium

Merge Risk: ⚪ Minimal · up to 2fa04

This change fixes UUID generation throughput by allocating sub-millisecond timestamps atomically instead of busy-waiting on the wall clock, while preserving correct ordering across clock rollbacks and concurrent callers. The previously identified rollback duplicate-timestamp bug has been corrected, and the shared-pointer API change is consistent with how the underlying atomic state is used. The remaining busy-wait behavior once a millisecond's capacity is exhausted is a known, documented limitation intentionally deferred to a follow-up improvement rather than a new risk from this change. No blocking issues were found.

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 66.67% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 12 functions across 2 files. (2 skipped: …	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly identifies the main change: improving time UUID generator throughput.
Description check	✅ Passed	The description explains the motivation, implementation, performance impact, validation, documentation, tests, and issue tracking. It omits the repository checklist, but the required information is mo…
Linked Issues check	✅ Passed	[#505] try_monotonic_timestamp allocates 100-nanosecond ticks with atomic compare-exchange, supports 10,000 timestamps per millisecond, and preserves monotonicity during clock rollback and concurren…
Out of Scope Changes check	✅ Passed	The changes stay within [#505]. The shared-borrow API changes and integration test support concurrent generation. The documentation and test-filter updates describe or validate the new behavior. The d…

Full details: Docstring Coverage

Explanation

Docstring coverage is 66.67% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 12 functions across 2 files. (2 skipped: 2 unsupported.)

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[qodo-scylladb]

Code Review by Qodo

🐞 Bugs (0) 📘 Rule violations (0) 📎 Requirement gaps (0) 🔗 Cross-repo conflicts (0) 📜 Skill insights (0)

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🔴 High

1. Clock rollback repeats identifiers ✓ Resolved 🐞 Bug ≡ Correctness

Description

try_monotonic_timestamp returns the pre-increment value from fetch_add in its rollback branch,
which is the timestamp already stored as the last allocation. When wall time moves behind the stored
millisecond, consecutive calls receive identical timestamps and combine them with the generator's
unchanged clock sequence and node, producing identical UUID values.

Code

scylla-rust-wrapper/src/cql_types/uuid.rs[64]

+        return Some(last_timestamp.fetch_add(1, Ordering::SeqCst));

Relevance

●●● Strong

Rollback returns the already-issued timestamp, causing duplicate UUIDs; returning the incremented
value preserves uniqueness.

ⓘ Recommendations generated based on similar findings in past PRs

Evidence

last_timestamp is the generator's sole allocation state, and the rollback condition at lines 62-64
returns fetch_add's previous value while advancing the stored state. The added rollback test
explicitly demonstrates this by expecting Some(future) from an atomic initialized to future,
while UUID generation combines that returned timestamp with the generator's constant
clock_seq_and_node, making the repeated timestamp a complete UUID collision.

scylla-rust-wrapper/src/cql_types/uuid.rs[15-18]
scylla-rust-wrapper/src/cql_types/uuid.rs[62-65]
scylla-rust-wrapper/src/cql_types/uuid.rs[166-169]
scylla-rust-wrapper/src/cql_types/uuid.rs[319-325]

Agent prompt

The issue below was found during a code review. Follow the provided context and guidance below and implement a solution

## Issue description
The rollback branch returns the old atomic value, reusing the timestamp that was allocated immediately before the clock moved backwards.

## Fix Focus Areas
- scylla-rust-wrapper/src/cql_types/uuid.rs[62-65]
- scylla-rust-wrapper/src/cql_types/uuid.rs[319-325]

## Recommended Fix
Return the value after incrementing the atomic timestamp rather than the previous value returned by `fetch_add`. Update the rollback test to expect `future + 1`, and add an assertion that an already-issued `future` timestamp is not returned again.

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Context sources

✅ Tickets: 🎫 [Mavenir] [DRIVER]cpp-rs-driver shows performance degradation compared to legacy C++ driver

✅ Compliance rules (platform): 2 rules

✅ Cross-repo context — repo relationships

✅ REVIEW.md

Review mode: ⚖️ Balanced: This changes concurrent UUID timestamp allocation and public thread-safety API borrowing semantics, creating meaningful correctness and concurrency risk, but the logic is localized rather than broad enough to warrant extended review.

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Actionable comments posted: 1

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Inline comments:
In `@scylla-rust-wrapper/src/cql_types/uuid.rs`:
- Line 64: Update the clock-rollback handling around last_timestamp so it
returns the incremented timestamp rather than fetch_add’s previous value,
ensuring the first rollback-generated UUID is unique. Update the rollback test
expectation to future + 1 and preserve the existing clock sequence and node
behavior.

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• scylla-rust-wrapper/src/cql_types/uuid.rs

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[Lorak-mmk]

Use shared Rust borrows for UUID generator APIs documented as thread-safe.

Can you extract this to a separate commit? I believe this is an important fix: previously we used &mut for something that could be shared, which is UB. cc @wprzytula

[dkropachev]

Use shared Rust borrows for UUID generator APIs documented as thread-safe.

Can you extract this to a separate commit? I believe this is an important fix: previously we used &mut for something that could be shared, which is UB. cc @wprzytula

Sure, not fully ready though

[qodo-scylladb]

Code review by qodo was updated up to the latest commit 4cbe963

[dkropachev]

@Lorak-mmk @wprzytula One issue remains: after 10,000 UUIDs in one millisecond, generator spins until next millisecond, consuming CPU and adding latency.

• Yield

  • Upside: gives CPU to other threads; preserves timestamp behavior.
  • Downside: may reduce peak throughput; CPU use can remain high.

• Advance timestamp

  • Upside: removes limit and spinning; simplest implementation.
  • Downside: timestamps may move ahead of wall clock; diverges from legacy driver.

• Sleep

  • Upside: reduces CPU use; preserves timestamp behavior.
  • Downside: scheduler may oversleep, increasing latency and reducing throughput.

• Keep current behavior

  • Upside: no added risk; matches legacy driver and current benchmarks.
  • Downside: CPU spinning remains above 10M UUID/s.

Proposal: create gh issue and defer to a follow-up PR with dedicated benchmarks.

[Lorak-mmk]

Very nice and important fix, thanks. No major issues, but some things that should be fixed.

[Lorak-mmk]

Thanks! Very good and important fixes.