Side effect

As explained in API Design Patterns

A side effect is an observable state change or additional operation caused by an action beyond its primary specified result.

A side effect is an additional observable operation or state change beyond an API method’s primary specified result. Predictable standard methods avoid unexpected behavior, while custom methods provide an explicit home for consequential actions. section 7.3.2 section 9.3.1

Use these distinctions when designing or evaluating API methods:

• Keep standard methods limited to their stated resource operation; do not attach unrelated actions such as sending email, triggering background work, or updating other resources. section 7.3.2 section 9.3.1
• Separate consequential workflows into custom methods whose names and behavior make the additional operation explicit. section 9.3.1
• Check side effects alongside idempotence: hidden state changes can make repeated requests behave differently, risking data loss or corruption. section 7.3.2 section 7.6
• Treat validation requests as completely safe and repeatable: they must change no data and produce no side effects. section 27.3

Separating email creation from sending

CreateEmail should store an email as a draft and do nothing else. A custom SendEmail method can contact SMTP servers and transition the email to sent only after delivery succeeds. This keeps storage separate from the external operation and its failure modes. section 9.3.1

Figure 9.3 Sequence diagram demonstrating sending an email using custom methods

Trace the sequence from creating a draft through SMTP delivery to the final sent state. section 9.3.1

How API Design Patterns covers it

How deep it goes

Defines side effects, tests them against idempotence, and applies the distinction to email workflows and validation requests.

Examples

Uses email storage versus SMTP delivery, retrieval counters, background operations, resource updates, and validate-only requests.

What it assumes

Assumes familiarity with API methods and resources while defining side effects, idempotence, and validation behavior through concrete scenarios.

What you can do after

Designs API workflows that keep standard methods predictable and move additional operations into explicit custom methods.

How it connects

Connects side effects to standard methods, custom methods, idempotence, resource operations, and request validation.

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Also explained in

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  Functional Programming in Scala, Second EditionA side effect is an observable state change or interaction caused by evaluating an expression beyond producing its returned value.
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  Learn Haskell by ExampleA side effect is an observable interaction or state change performed by a computation beyond producing its return value.
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  F# in Action: A Revised Edition of Get Programming with F#A side effect is an observable change or interaction caused by a function beyond producing its return value, such as modifying external data or state.
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  Functional Programming in JavaScriptAn externally observable change caused by executing code, such as I/O, mutation, or interaction with shared state.
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  100 Java Mistakes and How to Avoid ThemA side effect is an observable state change or external interaction caused by evaluating an operation in addition to producing its direct result.
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  Clojure: The Essential ReferenceA side effect is an observable interaction or state change produced by evaluating a computation in addition to returning a value.
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  Deep Learning with JAXA program behavior that changes state outside the function’s explicit return value, such as mutating hidden variables or global state.
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  Functional Design and Architecture: Examples in HaskellAn observable interaction produced by a computation beyond returning its value, such as changing state or performing input and output.
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  React Hooks in Action: With Suspense and Concurrent ModeA side effect is an operation performed by code that changes state outside the value it directly computes or interacts with external systems.
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  Unit Testing Principles, Practices, and PatternsA side effect is an observable state or resource change caused by executing a method in addition to computing a return value.
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  The Well-Grounded Rubyist, Fourth EditionA side effect is an observable consequence of evaluating code beyond producing its direct return value, such as mutating state or raising an exception.
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  Modern CA side effect is an observable change to program state caused by evaluating an expression or executing an operation.
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  Modern C, Third EditionA side effect is an observable change to program state caused by evaluating an expression or executing an operation.
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  Python How-ToA side effect is an observable state change produced by evaluating or calling an operation in addition to its returned result.
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  Grokking SimplicityAn observable interaction with state or the external environment that occurs in addition to producing a computation's return value.
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  Modern AngularA side effect is an operation performed by a computation that changes state or interacts with something outside the computation's returned value.
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  The Well-Grounded Python Developer: How the pros use Python and FlaskA side effect is an observable change or external action caused by a function in addition to producing its return value.
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  React Quickly, Second EditionA side effect is an observable change or interaction caused by evaluating code beyond returning a value.
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  100 Go Mistakes and How to Avoid ThemA side effect is an observable change or interaction caused by evaluating or executing code beyond producing its direct return value.
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  Akka in Action, Second EditionA side effect is an externally observable operation, such as an HTTP request or database insertion, performed in addition to updating persistent state.
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  The Art of CodeA side effect is an observable change or interaction produced by a computation beyond returning its direct result.
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  Python Workout, Second EditionA side effect is an observable change or interaction produced by an operation in addition to its returned value.

Compare how 24 books explain side effect →

Related concepts

• pure functioncontrasts with · A pure function is a function whose output is determined solely by its explicit inputs and whose evaluation has no externally observable side effects.
• useEffectused by · useEffect is a React Hook that runs effect logic after rendering to synchronize a component with external systems and optionally perform cleanup.
• idempotencycontrasts with · Idempotency is the property of an operation whereby repeating it has the same effective result or externally observable effect as performing it once.
• bugkind of · A software bug is a defect in software or a related system component that causes behavior to differ from what is intended or expected.
• exceptionincludes · An exception is a runtime signal or object representing an abnormal condition that interrupts ordinary program flow and may be handled or propagated.
• functionproduced by · A function is a callable program unit that performs an operation, may accept inputs, and can produce a result or side effect.

This explainer was generated by Manning's knowledge graph from the text of API Design Patterns. The section links point to the parts of the book it draws on.