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Practical writing about software, systems, and the small details that make products feel solid.

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Fresh from the notebook

Software Engineering 21 Sep 2026 8 min read

Write Skew Breaks Invariants Under Snapshot Isolation

Write Skew Breaks Invariants Under Snapshot Isolation Snapshot isolation gives each transaction a stable view of committed data. That property removes many anomalies caused by values changing midway through a transaction. It does not, by itself, make every concurrent execution equivalent to some serial order. Write skew is a compact example of the gap. Two transactions read the same valid state, make decisions from that state, then write different rows. Because their write sets do not overlap, both commits can succeed even though the combined result violates a rule that each transaction preserved in isolation.

Software Engineering Databases

Software Engineering 21 Sep 2026 5 min read

Write Skew Breaks Cross-Row Invariants Under Snapshot Isolation

Write Skew Breaks Cross-Row Invariants Under Snapshot Isolation Snapshot isolation gives each transaction a stable database view and commonly prevents concurrent transactions from committing conflicting writes to the same row. That is a strong concurrency property, but it does not make every application invariant serializable. Write skew appears when two transactions read overlapping state, make decisions from the same valid snapshot, then write different records. Since their write sets do not collide, both commits may succeed. The combined state can violate a rule that each transaction checked before writing.

Software Engineering Databases

Cybersecurity 21 Sep 2026 5 min read

WebAuthn RP ID Scopes Credential Use

WebAuthn RP ID Scopes Credential Use A WebAuthn credential is not a reusable key that any website can request from an authenticator. Registration and authentication are tied to a relying party identifier, or RP ID, while the browser also evaluates the calling origin. That pairing creates a domain boundary around public-key credentials. For a conventional deployment at https://login.example.com, an RP can use the host itself as the RP ID: origin: https://login.example.com RP ID: login.example.com It can also use a registrable domain suffix such as example.com when the deployment needs credentials to serve eligible subdomains:

Cybersecurity WebAuthn

Software Engineering 21 Sep 2026 7 min read

Version Vectors Separate Causality from Concurrency

Version Vectors Separate Causality from Concurrency Replicated data can receive writes at several nodes while communication between those nodes is delayed. When two versions meet later, a store has to decide whether one descends from the other or whether both were created independently. A wall-clock timestamp gives a total-looking order, but clock order is not causal order. Two replicas can write during a partition, and whichever timestamp happens to be larger does not make that write a descendant of the other.

Software Engineering Distributed Systems

Software Engineering 21 Sep 2026 4 min read

Version Vectors Distinguish Concurrent Updates from Causal Successors

Version Vectors Distinguish Concurrent Updates from Causal Successors Replicated data can receive writes at different nodes while communication between those nodes is delayed. When versions later meet, a scalar revision number can say that two values differ, but it cannot always say whether one descends from the other or both were produced independently. A version vector records progress per replica. Comparing those counters provides a partial order: one version can dominate another, the vectors can be equal, or neither can dominate. The last case identifies concurrent histories that require an explicit reconciliation rule.

Software Engineering Distributed Systems

Software Engineering 21 Sep 2026 5 min read

Version Columns Turn Lost Updates into Detectable Conflicts

Version Columns Turn Lost Updates into Detectable Conflicts A read-modify-write flow can overwrite another committed change even when every individual database statement succeeds. Two clients read the same row, compute different replacements, then write in sequence. Without a condition tying each write to the state it read, the later write can silently erase the earlier one. A version column makes that dependency explicit. The client reads both data and version, then updates only if the stored version is still the one it observed. A changed version turns the race into a failed conditional update instead of a lost update.

Software Engineering Databases

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