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What two programs need before they can exchange anything
Software-to-software communication looks mysterious from the outside, but it rests on a small set of requirements. Picture two programs that need to share information. They need four things:
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- A way to identify the destination. One program must be able to name the thing it wants to reach.
- A message format both sides can parse. The sender must package information in a shape the receiver recognizes.
- A communication mechanism and rules. Both sides must agree on how to send, how to receive, and what order of steps is expected.
- Shared expectations about meaning. Both sides must understand what a message is for and what should happen after it arrives.
The rest of this article uses the Web as a worked example, because it is the most familiar place where these four pieces are visible. The Web is one instance of software communication, not a template for all of it.
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Consider what happens when a browser displays an image on a page. Each step maps to one of the requirements above.
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- The browser identifies the resource. The image is named by a URI (Uniform Resource Identifier), the address-style string in the page’s markup. The URI is how the browser knows what it is asking for.
- The browser sends a request. Using HTTP, it issues a GET request to the server named in the URI. The request is a structured text message with a method, a target, and header fields.
- The server responds. The reply carries a status code, a set of headers, and a body. The body is the image data.
- The browser reads the metadata. The Content-Type header tells the browser how to interpret the bytes, for example as a PNG or JPEG image rather than as a web page.
- The browser renders the representation. Having identified the data type, it decodes the body and displays it.
The W3C’s 2004 description of Web architecture makes an important addition: the request the browser sends is only the visible part. Between the browser and the server, caches, proxies, and name-resolution services may take part. The browser’s single request can therefore pass through several intermediaries before any server software handles it. Each of those parties must also handle the message correctly.
The layers that the simple request hides
A useful way to think about any software exchange is as layers, each answering a different question. The table below separates them.
| Layer | Question it answers | Web example |
|---|---|---|
| Addressing | What is being named, and where is it? | A URI identifies an image; a host name resolves to a server. |
| Message | What structure carries the information? | A request line, header fields, and a body. |
| Protocol | What rules govern sending, receiving, and the order of steps? | HTTP request/response rules. |
| Representation and format | What form does the data take, and how should it be read? | Image bytes labeled by Content-Type. |
| Mechanics and semantics | How is the exchange formed, and what does it mean? | A GET asks for a representation; the server’s response tells the client what happened. |
Each layer can fail independently. A correctly formed message can be sent to the wrong address, a valid image can be labeled with the wrong type, and a well-formed request can be misread by a receiver that expects different behavior.
Protocols: the rules of the exchange
A protocol defines the rules for sending and receiving messages. It covers the interaction pattern and the behavior each side should expect. HTTP is the familiar example.
What HTTP is
The IETF’s RFC 9110, published in June 2022, describes HTTP as “a family of stateless, application-level, request/response protocols that share a generic interface, extensible semantics, and self-descriptive messages to enable flexible interaction with network-based hypertext information systems.” Three words in that definition do most of the work:
- Application-level. HTTP defines messages between applications. It sits above the lower-level networking that moves those bytes.
- Stateless. The server does not need to remember earlier requests in order to interpret a new one. Each request carries what the server needs to understand it.
- Self-descriptive. Messages include metadata, such as headers, that tell the receiver how to handle them.
HTTP is not the whole route
HTTP does not describe the entire path a message takes. Name resolution, connection handling, and intermediaries sit beneath or beside it. An HTTP request is the part of the journey the application code sees; the network carries it through steps the application usually does not manage directly. Older Web architecture material from the W3C mentions TCP/IP as an example of lower-level transport, but it is not a guide to which transport versions a current system uses, and it should not be read that way.
Other message patterns exist
Request/response is common, but it is not the only pattern. The W3C’s 2004 Web Services Architecture Working Group Note describes one-way messages and publish-subscribe arrangements, in which a sender publishes an event and interested receivers take it from there. It also describes SOAP messages carried over different protocols. These are examples of how exchanges can be structured. Not every software interaction is synchronous, and not every one uses HTTP.
APIs, contracts, and descriptions: three different things
The words API, protocol, and contract are often used loosely. They describe different parts of the same exchange.
- An API is an interface through which one system exposes operations or data to another. It is the set of things a caller is allowed to ask for.
- A protocol defines the rules for exchanging messages, as described above. An API can be carried over a protocol, but the two are not the same thing.
- A service contract covers what the interface means and what should happen as a consequence. The W3C’s 2004 note calls this shared expectation the semantics of a service: “the shared expectation about the behavior of the service, in particular in response to messages that are sent to it.”
A machine-readable description can capture part of this. The W3C’s Web Services Architecture note explains that a service description can specify message formats, data types, protocols, serialization formats, and locations. WSDL, a description language discussed in that note, describes messages and binds them to concrete protocols and formats. A description of this kind covers mechanics well. It cannot, by itself, guarantee that both sides agree on what a message means.
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Mechanics tell each side how to form the exchange: which fields appear, what types they hold, and which order the steps follow. Semantics tell each side what the exchange is for and what must happen afterward.
A message can be syntactically valid and still be misunderstood. Consider an illustrative example, not a sourced case: a payment service expects an amount field in cents, but the calling program sends the value in dollars. The message parses correctly and the request is accepted, yet the outcome is wrong by a factor of one hundred. Nothing in the message structure signaled the mismatch. Only the shared expectation about units did.
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Other points of disagreement follow the same pattern. Two systems may disagree about what an identifier refers to, how long a value stays valid, or whether a request changes data or only reads it. These are semantic questions, and they need to be settled alongside the format.
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Interoperability is agreement, not a shared language
Programs written in different languages, or running on different platforms, can exchange information reliably. What makes this possible is compatible descriptions and compatible semantics on both sides. The implementation language is not the agreement. A client written in one language and a server written in another can work together as long as each implements the same message formats, protocol rules, and expected behavior.
Where this model stops
This model explains the parts of a software conversation that stay constant across technologies. It does not answer every practical question. The sources behind this explanation are the IETF’s RFC 9110 (June 2022) and two W3C documents from 2004: Architecture of the World Wide Web, Volume One, and the Web Services Architecture Working Group Note. They establish terminology and architecture. They do not survey which interface styles are used today, compare protocol families such as REST, RPC, GraphQL, or gRPC, or measure performance and security trade-offs among them. Those comparisons depend on the specific system and need their own sources. When choosing an approach, start with the questions this model raises: what message pattern you need, which data formats both sides will use, which protocol carries the messages, how the interface and its meaning are documented, and what reliability and security the application requires.
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