System Type: Distributed Systems

Foundational patterns, guarantees, and failure modes for systems that span multiple nodes.

1. Consistency Models

Most engineers say "consistent" when they mean five different things. Precision matters here because the guarantees determine what your application can and cannot assume.

Linearizability

What it guarantees. Every operation appears to take effect instantaneously at some point between its invocation and completion. All processes observe the same ordering. If operation A completes before operation B begins, B sees A's effects.

What it costs. Requires coordination on every operation. In the presence of network partitions, a linearizable system must either become unavailable or violate the guarantee. Latency is bounded by the slowest participant in the quorum. Cross-datacenter linearizability is possible (Spanner does it) but requires specialized hardware (GPS + atomic clocks) and still adds measurable latency.

When it's sufficient. Leader election, distributed locks, compare-and-swap operations, unique constraint enforcement, fencing tokens. Anywhere a stale read causes a correctness violation, not just a user inconvenience.

When it's overkill. User profile reads, product catalog browsing, analytics dashboards, social feeds. Any read where showing data from 500ms ago is indistinguishable from "correct."