Runtime
The support machinery used while a program runs (libraries, memory management, scheduling hooks), depending on the language and platform.
Definition
A runtime is the support machinery a program relies on while it runs.
Depending on language and platform, this might include memory management, scheduling hooks, exception handling, dynamic loading, and standard libraries.
Synonyms and related terms
- Related: VM, language runtime, standard library, libc, runtime library
- Neighbor concepts: interpreter, JIT compilation, runtime environment
Examples (conceptual)
Different ecosystems use the word “runtime” differently, but common examples include:
- a language VM with JIT and garbage collection (often involving JIT compilation)
- an interpreter process (where the interpreter itself is the runtime for a language)
- a set of libraries and conventions that a compiled program expects to have available at run time
A useful analogy
If an executable is a play’s script, the runtime is the stage machinery and backstage crew that makes the play possible: lighting cues, stage changes, safety rails, and emergency procedures. The script still matters, but the runtime shapes how the script becomes a live performance.
Runtime vs runtime environment
- Runtime: the language/platform support needed during execution.
- Runtime environment: the broader external context (OS, config, filesystem, network, permissions).
What runtimes affect in production
Runtimes shape behavior that shows up as “system behavior”:
- memory allocation and reclamation (including garbage collection)
- scheduling and concurrency primitives exposed by the platform
- exception/error handling behavior and stack traces
- dynamic loading, reflection, and plugin systems
- startup/warmup characteristics (especially with JITs)
Common misconceptions
- “Runtime” is not synonymous with “runtime environment”. The environment is the world. The runtime is the language/platform support inside that world.
- “Compiled means no runtime.” Many compiled programs still depend on runtime libraries (shared libs, standard libraries, platform support).
- “Interpreted means no runtime.” Interpreter-based systems are often runtime-heavy. The interpreter is the runtime.
Mini-scenario
A service is stable in development but exhibits periodic latency spikes in production. If the service runs on a managed runtime, those spikes can come from runtime behavior such as garbage collection pauses, JIT recompilation, or runtime-level contention - even if the application logic is unchanged. Diagnosing the issue requires observing runtime signals (heap usage, GC logs, compilation activity), not only application-level metrics.
Runtime vs the artifact on disk
A compiled executable can still depend on a runtime (e.g., shared libraries, language support). Conversely, interpreter-based systems may ship mostly as source code, but the interpreter runtime must be present and correctly configured in the runtime environment.