iTechGuides is reader-supported. When you buy through links on our site, we may earn an affiliate commission. As an Amazon Associate I earn from qualifying purchases. Learn more
An interface description language (IDL) is a structured way to describe software interfaces and their data types independently of a particular programming language or platform. Tools and related standards can use that description for purposes such as code generation, API discovery, language mappings, and serialization—but what an IDL includes depends on the specific format.
What an IDL describes
An IDL records the shape of an interface in a form that people and software tools can interpret. Depending on the format, it may describe data types, operations, services, or HTTP endpoints. It acts like a contract in the sense that it documents what an interface exposes; that does not mean the IDL itself enforces runtime behavior.
For example, a service description might specify an operation and the types of data it accepts and returns. Tools in that ecosystem could use the description to generate code for different programming languages. The description can be shared across implementations, but compatible behavior still depends on the language mappings, runtime, transport, and serialization rules involved.
Free tools Windows power users keep installed
One-click scans. No signup required.
IDL is a category, not one universal language
The acronym is used for both “interface description language” and “interface definition language.” The preferred expansion varies by specification or project, so use the name chosen by the relevant documentation. Different IDLs also target different kinds of interfaces; the label does not mean they all describe the same constructs or provide the same tooling.
OMG IDL
ISO/IEC 19516:2020 defines OMG IDL as a descriptive language for defining data types and interfaces independently of programming language and operating-system or processor platform. The ISO listing says this edition was published on February 19, 2020, and reviewed and confirmed in 2025. The standard specifies syntax; related standards address language mappings, serialization, and middleware. Those boundaries are specific to OMG IDL, not a rule that every IDL must follow. [c001] [c008]
Apache Thrift IDL
In Apache Thrift 0.26.0 documentation, an IDL file defines Thrift types and is processed by a code generator to produce code for target languages. The grammar includes structures such as structs, unions, exceptions, and services; a service describes server functionality as a list of functions. See the Thrift IDL documentation and its type reference. [c002] [c003]
Protocol Buffers
Protocol Buffers uses .proto files to define data structures. Its protoc compiler generates code for working with protocol-buffer data. The definition language is one part of a larger system that also includes language runtimes and a serialization format; a .proto file alone is not the whole protocol. Google’s Protocol Buffers overview explains those components. [c004]
OpenAPI
The OpenAPI Specification 3.0.4 is a programming-language-agnostic description format for HTTP APIs. It is designed to help people and computers discover and understand an HTTP service’s capabilities. It is an IDL-like example for that domain, not a general description language for every kind of software interface. See the OpenAPI Specification 3.0.4. [c005]
Web IDL
Web IDL describes interfaces intended to be implemented in web browsers. It serves a browser-interface context rather than being a general-purpose remote procedure call (RPC) IDL. The scope is set out in the W3C Web IDL Level 1 Recommendation, published on December 15, 2016. [c006]
What an IDL does not necessarily include
An IDL is not automatically the implementation, network transport, serialization encoding, or runtime. Some ecosystems connect a description to those pieces, while others specify them separately. OMG IDL, for example, leaves language mappings, serialization, and middleware to related standards. Protocol Buffers documentation likewise treats the definition language, generated code, runtimes, and serialization as parts of a broader system. [c001] [c004]
Code generation is common in some IDL workflows, such as Thrift and Protocol Buffers, but it is not a universal property of every interface-description format. Nor does language independence by itself guarantee that two implementations will interoperate: they must also agree on the applicable mappings and behavior.
How to tell whether a format fits your interface
Start with the interface you need to describe, then check the format’s scope and what its ecosystem supplies:
- Domain: Is it intended for distributed objects, RPC services, HTTP APIs, or browser interfaces?
- Constructs: Does it describe data types, operations, services, endpoints, or the particular elements your interface needs?
- Tooling: Does its documented workflow include code generation, or is the description mainly for discovery and understanding?
- Language mappings: Are mappings specified by the format itself, by related standards, or by ecosystem tools?
- Serialization and transport: Are these included in the same ecosystem, defined separately, or outside the format’s scope?
These distinctions matter more than the shared IDL label. OpenAPI targets HTTP APIs, Web IDL targets browser interfaces, and Thrift IDL documents types and services in a code-generation workflow; none is a universal substitute for the others.
Quick Recap
Product prices and availability are accurate as of the date/time indicated and are subject to change. Any price and availability information displayed on Amazon at the time of purchase will apply.
