Scala: Context & Observability

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Context propagation explicit parameter implicit parameter

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Thread-safe state on JVM AtomicRef ThreadLocal Every thread has its own local copy of the state which is not visible to the others Shared state safely updated by many threads using optimistic concurrency (CAS)

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Context propagation on JVM ThreadLocal InheritableThreadLocal DynamicVariable

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Scope Lexical Dynamic Depends on location in source code Depends on method call graph

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Functional context propagation ReaderT / Kleisli (cats) IOLocal (cats-effect) Context functions (scala 3)

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Context propagation with cats-mtl Ask Local Stateful read-only environment effect dynamic scope effect shared mutable state effect

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Local + log4cats demo

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Local + log4cats + IOLocal demo

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Scala 3 Context Functions

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