scylladb/scylladb
Oct 2024 – Feb 2026
15 Months active
Languages Used
C++MarkdownPythonShellrst
Technical Skills
C++C++ developmentRPC designbackend developmentcode refactoringdatabase management
Ferenc Szili
PROFILE
Ferenc Szili
Ferenc Szili engineered robust distributed database features and reliability improvements in the scylladb/scylladb repository, focusing on load balancing, topology coordination, and data integrity. He designed and implemented size-based load balancing algorithms, enhanced tablet migration workflows, and optimized metadata aggregation for scalable performance. Using C++ and Python, Ferenc addressed concurrency and race conditions, introduced cache-line aligned counters for efficient statistics, and improved observability through new metrics and documentation. His work included rigorous testing, refactoring, and documentation updates, resulting in safer rolling upgrades, more predictable resource utilization, and reduced operational risk. The solutions demonstrated deep understanding of distributed systems and backend development.
Overall Statistics
Feature vs Bugs
61%Features
Repository Contributions
64Total
Bugs
13
Commits
64
Features
20
Lines of code
4,753
Activity Months15
Your Network
196 peopleWork History
February 2026
3 Commits • 1 Features
Feb 1, 2026February 2026 (2026-02) monthly summary for scylladb/scylladb: Delivered reliability and performance improvements around tablet metadata and size statistics that directly enhance cluster stability and scalability. Key features delivered: tablet size summation performance optimization. Major bugs fixed: tablet metadata consistency across nodes and a race condition in load_stats_for_tablet_based_tables. Overall impact: more reliable cross-node tablet balancing, faster stat aggregation, and improved throughput as clusters scale. Technologies demonstrated: concurrency control, thread-safety patterns, per-shard aggregation, cache-line alignment (64-byte) to reduce false sharing, and testing hardening with read barriers.
3 Commits • 1 Features
Feb 1, 2026February 2026 (2026-02) monthly summary for scylladb/scylladb: Delivered reliability and performance improvements around tablet metadata and size statistics that directly enhance cluster stability and scalability. Key features delivered: tablet size summation performance optimization. Major bugs fixed: tablet metadata consistency across nodes and a race condition in load_stats_for_tablet_based_tables. Overall impact: more reliable cross-node tablet balancing, faster stat aggregation, and improved throughput as clusters scale. Technologies demonstrated: concurrency control, thread-safety patterns, per-shard aggregation, cache-line alignment (64-byte) to reduce false sharing, and testing hardening with read barriers.
February 2026
January 2026
6 Commits • 2 Features
Jan 1, 2026January 2026 — Focused on stabilizing Load Stats, enhancing load-balancing observability, and clarifying documentation to support reliable capacity planning. Key work hardened load_stats against migration edge-cases and table drops, introduced an effective_capacity metric to aid debugging of size-based balancing, and improved documentation for the load-balancing feature set. The changes included targeted tests and a reproducer for refresh exceptions to prevent regressions during topology changes. Deliverables improve reliability, observability, and decision quality for capacity-based routing, translating to lower operator risk and faster root-cause analysis.
January 2026
6 Commits • 2 Features
Jan 1, 2026January 2026 — Focused on stabilizing Load Stats, enhancing load-balancing observability, and clarifying documentation to support reliable capacity planning. Key work hardened load_stats against migration edge-cases and table drops, introduced an effective_capacity metric to aid debugging of size-based balancing, and improved documentation for the load-balancing feature set. The changes included targeted tests and a reproducer for refresh exceptions to prevent regressions during topology changes. Deliverables improve reliability, observability, and decision quality for capacity-based routing, translating to lower operator risk and faster root-cause analysis.
December 2025
3 Commits • 1 Features
Dec 1, 2025December 2025 monthly summary for scylladb/scylladb focusing on key business and technical outcomes: Key features delivered - Size-Based Load Balancing in the load balancer: introduced a cluster feature size_based_load_balancing, defaulting to capacity-based balancing until all nodes have complete data. This improves load distribution during rolling upgrades and enhances upgrade safety. Commits: b7ebd73e5350030aec2fd27e583033c1e8994816; 0ede8d154b03e06933e30efa3665e3c64f9c8b21. Documentation updated to reflect the feature (docs patch supporting the change). Commits: 0ede8d154b03e06933e30efa3665e3c64f9c8b21. Major bugs fixed - Tablet size reconciliation during migration and rebuild: fixed tablet size calculations by using the transition type from trinfo and by setting tablet size to 0 when rebuilding with a single replica, improving correctness during migrations. Commit: 0c9b93905efba4a675a6eb047666a2762850a63d. Overall impact and accomplishments - Increased reliability and safety of rolling upgrades by ensuring predictable load distribution even with incomplete data, reducing the risk of data skew during upgrades. - Improved correctness of load statistics during migrations, leading to more accurate migrations and resource planning. Technologies/skills demonstrated - Load balancing algorithms and feature flagging (size_based_load_balancing) - Migration/rebuild correctness (load_stats, trinfo transitions) - Documentation and developer experience improvements Business value - Safer upgrade paths with improved data distribution guarantees, lower risk of outages during rolling upgrades, and clearer operator guidance through updated docs.
3 Commits • 1 Features
Dec 1, 2025December 2025 monthly summary for scylladb/scylladb focusing on key business and technical outcomes: Key features delivered - Size-Based Load Balancing in the load balancer: introduced a cluster feature size_based_load_balancing, defaulting to capacity-based balancing until all nodes have complete data. This improves load distribution during rolling upgrades and enhances upgrade safety. Commits: b7ebd73e5350030aec2fd27e583033c1e8994816; 0ede8d154b03e06933e30efa3665e3c64f9c8b21. Documentation updated to reflect the feature (docs patch supporting the change). Commits: 0ede8d154b03e06933e30efa3665e3c64f9c8b21. Major bugs fixed - Tablet size reconciliation during migration and rebuild: fixed tablet size calculations by using the transition type from trinfo and by setting tablet size to 0 when rebuilding with a single replica, improving correctness during migrations. Commit: 0c9b93905efba4a675a6eb047666a2762850a63d. Overall impact and accomplishments - Increased reliability and safety of rolling upgrades by ensuring predictable load distribution even with incomplete data, reducing the risk of data skew during upgrades. - Improved correctness of load statistics during migrations, leading to more accurate migrations and resource planning. Technologies/skills demonstrated - Load balancing algorithms and feature flagging (size_based_load_balancing) - Migration/rebuild correctness (load_stats, trinfo transitions) - Documentation and developer experience improvements Business value - Safer upgrade paths with improved data distribution guarantees, lower risk of outages during rolling upgrades, and clearer operator guidance through updated docs.
December 2025
November 2025
7 Commits • 1 Features
Nov 1, 2025November 2025 monthly wrap-up for scylladb/scylladb: Delivered Tablet Size Migration and Load Stats Improvements, introduced tablet_sizes virtual table, stabilized topology-change tests, and enhanced observability; demonstrated strong performance and reliability improvements across migration, rebuild, and bootstrap workflows.
November 2025
7 Commits • 1 Features
Nov 1, 2025November 2025 monthly wrap-up for scylladb/scylladb: Delivered Tablet Size Migration and Load Stats Improvements, introduced tablet_sizes virtual table, stabilized topology-change tests, and enhanced observability; demonstrated strong performance and reliability improvements across migration, rebuild, and bootstrap workflows.
October 2025
2 Commits • 1 Features
Oct 1, 2025Month: 2025-10 focused on strengthening load balancing accuracy and performance in scylladb/scylladb by implementing tablet size reconciliation after migrations or resizes, and revising tablet size data structures to support more granular reconciliation. These changes improve the load balancer's ability to distribute tablets effectively, reducing hot spots and migrations, and enabling more stable cluster performance during topology changes.
2 Commits • 1 Features
Oct 1, 2025Month: 2025-10 focused on strengthening load balancing accuracy and performance in scylladb/scylladb by implementing tablet size reconciliation after migrations or resizes, and revising tablet size data structures to support more granular reconciliation. These changes improve the load balancer's ability to distribute tablets effectively, reducing hot spots and migrations, and enabling more stable cluster performance during topology changes.
October 2025
September 2025
7 Commits • 1 Features
Sep 1, 2025September 2025 monthly summary focusing on business value and technical achievements for scylladb/scylladb. Key features delivered: - Size-based load balancing enhancements with simulator improvements enabling more realistic load distribution and migration planning. - Simulator and validation tooling expanded to model tablet sizes, migration deviation, and dynamic load_stats updates. - Transition of load calculations from tablet counts to tablet sizes, improving accuracy of shard and node load estimations. Major bugs fixed: - Fixed badness calculation for migrations to ensure correct decision making during balance operations. - Addressed a crash risk in node decommissioning by guaranteeing load sketch maps are initialized for all relevant nodes, preventing out-of-bounds in edge cases. Overall impact and accomplishments: - More accurate, robust load balancing under size-based metrics reduces hotspotting and migration risk, improving performance stability and capacity planning. - Enhanced test coverage and validation enable safer deployments and faster iteration on balancing strategies. - Improved resilience during node churn and decommissioning, lowering maintenance overhead and operational risk. Technologies/skills demonstrated: - C++ design for load balancer, load_sketch, and migration_badness models; advanced use of smart pointers and concurrency-friendly patterns. - Algorithmic improvements: largest-tablet-first overcommit, size-based calculations, and dynamic load_stats handling. - End-to-end testing and simulator-driven validation for stability under real-world workloads. - Debugging and incident-awareness in lifecycle events (migrations, decommissioning) to prevent crashes and improve correctness.
September 2025
7 Commits • 1 Features
Sep 1, 2025September 2025 monthly summary focusing on business value and technical achievements for scylladb/scylladb. Key features delivered: - Size-based load balancing enhancements with simulator improvements enabling more realistic load distribution and migration planning. - Simulator and validation tooling expanded to model tablet sizes, migration deviation, and dynamic load_stats updates. - Transition of load calculations from tablet counts to tablet sizes, improving accuracy of shard and node load estimations. Major bugs fixed: - Fixed badness calculation for migrations to ensure correct decision making during balance operations. - Addressed a crash risk in node decommissioning by guaranteeing load sketch maps are initialized for all relevant nodes, preventing out-of-bounds in edge cases. Overall impact and accomplishments: - More accurate, robust load balancing under size-based metrics reduces hotspotting and migration risk, improving performance stability and capacity planning. - Enhanced test coverage and validation enable safer deployments and faster iteration on balancing strategies. - Improved resilience during node churn and decommissioning, lowering maintenance overhead and operational risk. Technologies/skills demonstrated: - C++ design for load balancer, load_sketch, and migration_badness models; advanced use of smart pointers and concurrency-friendly patterns. - Algorithmic improvements: largest-tablet-first overcommit, size-based calculations, and dynamic load_stats handling. - End-to-end testing and simulator-driven validation for stability under real-world workloads. - Debugging and incident-awareness in lifecycle events (migrations, decommissioning) to prevent crashes and improve correctness.
August 2025
4 Commits • 1 Features
Aug 1, 2025Monthly summary for 2025-08: Focused on customer-facing clarity and system stability. Delivered documentation improvements explaining capacity-based balancing and tablet allocation for ScyllaDB, fixed critical concurrency bugs affecting truncation and concurrent table drop during migration cleanup, and introduced reproducer and tests to prevent regressions. These efforts improve operator onboarding, reduce support overhead, and enhance the reliability of the tablet allocator and storage governance.
4 Commits • 1 Features
Aug 1, 2025Monthly summary for 2025-08: Focused on customer-facing clarity and system stability. Delivered documentation improvements explaining capacity-based balancing and tablet allocation for ScyllaDB, fixed critical concurrency bugs affecting truncation and concurrent table drop during migration cleanup, and introduced reproducer and tests to prevent regressions. These efforts improve operator onboarding, reduce support overhead, and enhance the reliability of the tablet allocator and storage governance.
August 2025
July 2025
4 Commits • 1 Features
Jul 1, 2025July 2025: Delivered focused improvements to scylladb/scylladb including configurable load balancing, hardened TRUNCATE handling under concurrent writes, and stability improvements through test cleanup after Enterprise/OSS merge. The work reduces operational risk during scale-outs and maintenance, while boosting resource utilization and reliability.
July 2025
4 Commits • 1 Features
Jul 1, 2025July 2025: Delivered focused improvements to scylladb/scylladb including configurable load balancing, hardened TRUNCATE handling under concurrent writes, and stability improvements through test cleanup after Enterprise/OSS merge. The work reduces operational risk during scale-outs and maintenance, while boosting resource utilization and reliability.
June 2025
2 Commits • 2 Features
Jun 1, 20252025-06 monthly summary for scylladb/scylladb focusing on observability, efficiency, and stability gains. Delivered enhanced logging for large partitions and size-based load balancing, improving diagnostics and distribution efficiency across tablets.
2 Commits • 2 Features
Jun 1, 20252025-06 monthly summary for scylladb/scylladb focusing on observability, efficiency, and stability gains. Delivered enhanced logging for large partitions and size-based load balancing, improving diagnostics and distribution efficiency across tablets.
June 2025
May 2025
3 Commits • 1 Features
May 1, 2025May 2025 monthly summary for scylladb/scylladb focused on reliability improvements and LB groundwork to enable better resource utilization and stability.
May 2025
3 Commits • 1 Features
May 1, 2025May 2025 monthly summary for scylladb/scylladb focused on reliability improvements and LB groundwork to enable better resource utilization and stability.
March 2025
1 Commits
Mar 1, 2025Monthly work summary for 2025-03 focusing on tombstone GC stability during file-based tablet streaming in scylladb/scylladb. Reproduced a bug using an enterprise test ported to master (commit 2c9b312b58c9e2e3d6ca99b3c4d19fae8c5c4725) to illustrate tombstones could be garbage collected prematurely, risking data resurrection on pending replicas. Implemented a fix to preserve tombstones until streaming/replication safety is ensured, and validated via an ordered SSTable streaming test. Result: improved data consistency and reliability in streaming paths; reduced risk of data reversion across replicas.
1 Commits
Mar 1, 2025Monthly work summary for 2025-03 focusing on tombstone GC stability during file-based tablet streaming in scylladb/scylladb. Reproduced a bug using an enterprise test ported to master (commit 2c9b312b58c9e2e3d6ca99b3c4d19fae8c5c4725) to illustrate tombstones could be garbage collected prematurely, risking data resurrection on pending replicas. Implemented a fix to preserve tombstones until streaming/replication safety is ensured, and validated via an ordered SSTable streaming test. Result: improved data consistency and reliability in streaming paths; reduced risk of data reversion across replicas.
March 2025
January 2025
9 Commits • 1 Features
Jan 1, 2025January 2025 focused on reliability, concurrency, and topology-aware improvements in scylladb/scylladb. Key deliverables include a bug fix for reliable split-ready compaction group creation across all storage groups and a comprehensive TRUNCATE TABLE enhancement that enables parallel truncates per shard, synchronizes on queued truncates for the same table, improves logging and error messages, and integrates truncate handling with the topology state machine (new transition state). These changes enhance data integrity during DDL operations, reduce timeout/retry friction, and improve observability and developer ergonomics.
January 2025
9 Commits • 1 Features
Jan 1, 2025January 2025 focused on reliability, concurrency, and topology-aware improvements in scylladb/scylladb. Key deliverables include a bug fix for reliable split-ready compaction group creation across all storage groups and a comprehensive TRUNCATE TABLE enhancement that enables parallel truncates per shard, synchronizes on queued truncates for the same table, improves logging and error messages, and integrates truncate handling with the topology state machine (new transition state). These changes enhance data integrity during DDL operations, reduce timeout/retry friction, and improve observability and developer ergonomics.
December 2024
3 Commits • 3 Features
Dec 1, 2024December 2024 monthly summary for scylladb/scylladb focusing on reliability and robustness of TRUNCATE TABLE operations and topology coordination. Delivered a comprehensive unit test suite for TRUNCATE TABLE with tablets, refactored topology request completion into topology_state_machine for better maintainability, and enabled crash scenario coverage by fixing a replay-position bug. These efforts increased data safety during truncate operations, improved resilience during migrations and topology changes, and expanded test coverage with clear business value.
3 Commits • 3 Features
Dec 1, 2024December 2024 monthly summary for scylladb/scylladb focusing on reliability and robustness of TRUNCATE TABLE operations and topology coordination. Delivered a comprehensive unit test suite for TRUNCATE TABLE with tablets, refactored topology request completion into topology_state_machine for better maintainability, and enabled crash scenario coverage by fixing a replay-position bug. These efforts increased data safety during truncate operations, improved resilience during migrations and topology changes, and expanded test coverage with clear business value.
December 2024
November 2024
6 Commits • 1 Features
Nov 1, 2024November 2024 performance summary for scylladb/scylladb focused on enabling safe, cluster-wide data lifecycle operations and improving truncation reliability. Delivered a centralized, topology-coordinated TRUNCATE TABLE across the cluster with tablet-level optimization, supported by topology schema changes and updated documentation. Fixed replay position correctness for truncate with regression tests to ensure shard replay positions align with the correct shard IDs. Documented truncate_table workflow in topology-over-raft and reinforced operational safety around truncation across nodes.
November 2024
6 Commits • 1 Features
Nov 1, 2024November 2024 performance summary for scylladb/scylladb focused on enabling safe, cluster-wide data lifecycle operations and improving truncation reliability. Delivered a centralized, topology-coordinated TRUNCATE TABLE across the cluster with tablet-level optimization, supported by topology schema changes and updated documentation. Fixed replay position correctness for truncate with regression tests to ensure shard replay positions align with the correct shard IDs. Documented truncate_table workflow in topology-over-raft and reinforced operational safety around truncation across nodes.
October 2024
4 Commits • 3 Features
Oct 1, 2024October 2024: Delivered core features for distributed query reliability, topology-change safety, and maintainability in scylladb/scylladb. The work strengthens distributed query management, guards against stale topology operations during upgrades, and improves code quality for long-term maintainability.
4 Commits • 3 Features
Oct 1, 2024October 2024: Delivered core features for distributed query reliability, topology-change safety, and maintainability in scylladb/scylladb. The work strengthens distributed query management, guards against stale topology operations during upgrades, and improves code quality for long-term maintainability.
October 2024
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Quality Metrics
Correctness94.6%
Maintainability87.8%
Architecture89.6%
Performance85.6%
AI Usage20.4%
Skills & Technologies
Programming Languages
C++MarkdownPythonShellrst
Technical Skills
API developmentAsynchronous ProgrammingBackend DevelopmentBug FixingC++C++ developmentC++ programmingCassandraCassandra Query Language (CQL)Code OrganizationConcurrencyConcurrency ControlData StructuresDatabaseDatabase Internals
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