Here is a situation most developers have lived through. You join a project, open the integration docs, and see the backend team calling something a "web service" while the frontend team calls the same endpoint an "API." A product manager sends an email asking you to "add the web service API." Nobody blinks. And yet these are not the same thing, and the confusion has real consequences when you are designing architecture or handing work to a new team member.
This guide is the one I wish existed when I spent two hours arguing with a senior engineer about whether our SOAP banking integration "counted" as an API or a web service. (It is both, by the way. Just not for the reasons he thought.)
Read: Types of APIs | SOA vs Microservices | API Development Guide | Microservices vs Web Services | API Testing Guide | How APIs Work
Building an API or integration?
Get a free architecture review from Decipher Zone's backend engineers.
The One Sentence That Settles This
All web services are APIs. Not all APIs are web services.
That is the entire hierarchy in one sentence. Everything else in this guide is an explanation of why that sentence is true and what it means for the decisions you make in your projects.
An API (Application Programming Interface) is any contract that allows two software components to communicate. A web service is a specific type of API that operates over a network using web protocols like HTTP or SOAP. The difference in scope is significant. APIs can be local, running entirely on one machine with no network involvement at all. Web services cannot. By definition, a web service requires a network.
The SQLite C API, for example, queries a local database entirely on the host machine without a single network packet being sent. That is an API. It is not a web service. An AWS S3 endpoint that uploads a file to a remote server is both an API and a web service.
What is an API?
An Application Programming Interface is a defined contract specifying how two software components interact. It defines what requests can be made, what format those requests take, what responses come back, and what errors might occur.
APIs are protocol-agnostic. They can communicate over HTTP for web interactions, over TCP/IP for network communication, as local function calls within the same process, as operating system system calls, or as hardware device interfaces. The protocol choice is up to the implementer.
A Real Example Worth Knowing
When Shopify merchants use third-party apps to sync inventory to Amazon, both platforms expose REST APIs. The sync app calls the Shopify Admin API to read inventory levels, then calls the Amazon Selling Partner API to update listings.
Neither company cares what server the sync app runs on, what language it uses, or how it is deployed. The API contract is the only thing that matters. That contract (the endpoint, the parameters, the authentication method, the response format) is the API.
What makes this an API and not just a "web service"? Technically both are web services too. But the distinction becomes meaningful when you compare it to a genuinely non-networked API: the same sync app might use the filesystem API to write a local log file. No network. No HTTP. Still an API.
Types of APIs
| API Type | Protocol | Data Format | Best For | Real Example |
|---|---|---|---|---|
| REST | HTTP/HTTPS | JSON, XML, HTML | Web apps, mobile apps, public APIs | Stripe payments, GitHub API, Twitter API |
| GraphQL | HTTP/HTTPS | JSON | Complex data graphs, frontend-driven queries | GitHub GraphQL API, Shopify Storefront API |
| gRPC | HTTP/2 | Protocol Buffers (binary) | High-performance internal microservices | Google internal services, Netflix, Uber |
| WebSocket | WS/WSS | JSON, binary | Real-time bidirectional communication | Slack messaging, live trading dashboards |
| SOAP | HTTP, SMTP | XML only | Enterprise, regulated industries, legacy | Banking clearing systems, government portals |
| Local/OS API | No network | Native function calls | In-process communication, hardware access | SQLite C API, Windows DirectX, POSIX |
What is a Web Service?
A web service is a software system designed to support machine-to-machine interaction over a network. As defined by the World Wide Web Consortium (W3C), it has an interface described in a machine-processable format and communicates using web protocols.
Three things define a web service and separate it from other APIs.
- Network required: Web services must operate over a network. Always. An API that works locally is not a web service.
- Standard protocols: Web services use established protocols, primarily SOAP, REST, or XML-RPC.
- Interoperability focus: Web services are specifically designed to let systems built on different platforms, languages, and frameworks communicate with each other.
When is a Web Service the Right Choice?
- You are integrating with a partner system that already exposes SOAP endpoints
- You are building in a regulated industry (banking, insurance) that mandates WS-Security and XML
- You are connecting to government tax, customs, or payment systems
- You are maintaining or extending a legacy SOA-based enterprise architecture
- You need message-level security that survives intermediate proxy routing
A Real-World Web Service Example
A UK-based insurance company processing claims through the NHS needed to connect their claims management system to the NHS Spine, the UK's national healthcare IT infrastructure. The NHS Spine uses SOAP web services with WS-Security headers for every transaction.
The insurance team could not choose REST. They could not choose JSON. The receiving system dictated the protocol entirely. Every claim submission was wrapped in an XML SOAP envelope, signed with a digital certificate, and transmitted over HTTPS.
Unlike integrating a payment gateway via REST which might take a day, SOAP web service integration can take weeks. The NHS Spine parsed the envelope, validated the signature, processed the claim, and returned a SOAP response confirming receipt.
This is the defining characteristic of enterprise web services: you are often not choosing between SOAP and REST. The partner system has already chosen for you.
Types of Web Services
| Web Service Type | Protocol | Format | Architecture | Typical Use |
|---|---|---|---|---|
| SOAP | HTTP, SMTP | XML only | Service-Oriented Architecture (SOA) | Banking, insurance, government, ERP integration |
| RESTful | HTTP/HTTPS | JSON, XML | Resource-based, stateless | Public APIs, mobile backends, cloud services |
| XML-RPC | HTTP | XML | Remote Procedure Call | Legacy systems, WordPress API historically |
API vs Web Service: The Complete Comparison
| Dimension | API | Web Service |
|---|---|---|
| Definition | Any contract allowing two software components to communicate | A specific type of API that communicates over a network using web protocols |
| Network requirement | Optional, can be local or networked | Always required |
| Protocols | Any: HTTP, TCP/IP, WebSocket, gRPC, local function calls, OS calls | HTTP, HTTPS, SMTP (SOAP) or HTTP only (REST web services) |
| Data formats | JSON, XML, Protocol Buffers, plain text, binary, native structs | XML mandatory for SOAP, JSON or XML for REST |
| Hosting | IIS, application server, embedded in application, or local runtime | IIS or dedicated web server, network endpoint required |
| Performance | Varies: REST moderate, gRPC very high, local very high | SOAP is heavier due to XML overhead, REST web services moderate |
| Security | OAuth 2.0, JWT, API keys, HTTPS | WS-Security for SOAP, OAuth/JWT for REST, HTTPS for both |
| Complexity | REST is low, GraphQL moderate, gRPC moderate, SOAP high | SOAP is high, REST web services moderate |
| Typical industries | Every industry (mobile, web, IoT, AI, gaming, OS) | Finance, healthcare, government, enterprise, telecoms |
REST vs SOAP: The Protocol Debate That Actually Matters
Most real decisions about APIs and web services eventually become a decision about REST versus SOAP. Getting this wrong costs weeks of rework.
REST
REST is not a protocol. It is an architectural style. Six constraints define it: stateless communication, client-server separation, a uniform interface, a layered system, cacheability, and optionally code-on-demand. REST APIs use standard HTTP verbs.
- GET , retrieve, read-only, cacheable
- POST , create a new resource
- PUT , replace an existing resource
- PATCH , partially update an existing resource
- DELETE , remove a resource
Simple. Human-readable. Works naturally with browsers and HTTP infrastructure. For most modern web and mobile projects, REST is the right default. Not because it is fashionable. Because the tooling, documentation standards, and developer ecosystem built around REST are genuinely excellent and reduce onboarding time for new team members.
SOAP
SOAP is a protocol, not a style. Every SOAP message is an XML document with a mandatory structure: an envelope wrapping the entire message, an optional header for authentication and routing metadata, and a body containing the actual request or response.
SOAP enforces a formal contract through WSDL, a machine-readable document specifying every operation, every parameter type, every possible error, and every security requirement. Nothing ambiguous. Everything validated before a request is processed.
Here is my honest take on SOAP: it is genuinely overrated as a choice for new systems. Most teams that pick SOAP for a greenfield project in 2026 do so because someone on the leadership team worked in enterprise software fifteen years ago and is pattern-matching to what they know. If you are assessing whether to adopt SOA versus microservices for a new system, the answer in 2026 is almost always microservices.
The additional security guarantees SOAP provides over a properly implemented REST API with HTTPS, OAuth 2.0, and JWT are marginal for the vast majority of use cases. The added complexity and XML verbosity are not marginal.
Where SOAP is not overrated: legacy system integration. When a bank's clearing system has been running SOAP since 2004 and processes four billion transactions a year, you are not migrating it to REST because a developer finds XML verbose. You are writing the SOAP client and moving on.
JSON vs XML: What the Format Choice Actually Costs
| Dimension | JSON | XML |
|---|---|---|
| Verbosity | Compact, minimal markup | Verbose with opening and closing tags for every field |
| Payload size | Typically 30 to 40% smaller than XML for the same data | Larger due to tag overhead, noticeable at high volume |
| Browser support | Native JavaScript support, no parser needed | Requires XML parser |
| Schema validation | Optional via JSON Schema | XSD built into SOAP, mandatory validation |
| Best for | REST APIs, web apps, mobile apps, modern systems | SOAP, document exchange, strict validation requirements |
The practical implication: building a REST API for a mobile app means JSON reduces bandwidth, parses faster on device, and integrates natively with JavaScript. Integrating with a bank's payment system built in 2005 means XML whether you want it or not. The format serves the protocol, not the other way around.
Performance: What the Numbers Mean in Practice
Short version: gRPC is fastest, REST is good enough for most things, SOAP carries overhead you notice at scale.
A REST API returning a 500-byte JSON payload typically processes in under 10 milliseconds on a well-configured server. Reasonable for the vast majority of web and mobile workloads. SOAP's XML envelope adds 200 to 500 bytes of overhead to even small messages, and XML parsing is slower than JSON parsing in most runtimes. A banking system processing 100,000 transactions per hour feels that difference in infrastructure cost.
gRPC uses Protocol Buffers over HTTP/2. Binary serialization. No human-readable format. Payload sizes 3 to 10 times smaller than JSON, parsing 5 to 7 times faster. Netflix uses gRPC for internal service communication. Not because REST could not technically work. It could. Because at Netflix's volume, the efficiency gains compound into real infrastructure savings and lower latency between services.
WebSocket is a different category entirely. A REST API call opens a connection, sends a request, receives a response, closes the connection. WebSocket opens once and stays open. For a live stock ticker updating prices 20 times per second, the overhead of 20 separate HTTP connections per second per user is the problem WebSocket solves, not throughput.
Security: How Each Approach Handles It
The security difference between APIs and web services is real and matters for regulated industries.
SOAP: Message-Level Security
SOAP uses WS-Security, a specification that embeds security directly inside the message, not just the connection. XML digital signatures prevent tampering with specific message fields. XML encryption protects sensitive data within the body. SAML tokens carry identity assertions. Timestamps prevent replay attacks.
The key word is message-level. The security travels with the message through intermediate proxies and systems. Even if the message is routed through three different servers before reaching its destination, the signature is verified at the end. This is why banking and healthcare SOAP web services can be audited with complete certainty about what was sent, by whom, and when.
REST: Transport-Level Security
REST APIs rely on HTTPS for transport encryption, OAuth 2.0 for authorization, JWT for identity claims, and API keys for simple access control. Transport-level means the security lives in the connection, not the message. Once the message arrives at its destination, it is in plain text. For a direct client-to-server call, this is fine. For complex enterprise routing through multiple intermediaries, it is a design consideration worth thinking through.
For healthcare applications and telemedicine platforms, REST with HTTPS, OAuth, and JWT meets HIPAA requirements when implemented correctly. Read our GDPR and HIPAA compliance guide, cybersecurity best practices, and secure coding best practices for the specifics.
When to Use Which: A Scenario-by-Scenario Decision Framework
| Your Situation | Use This | Why |
|---|---|---|
| Building a public API for developers | REST API | Low onboarding friction, JSON, self-service documentation, standard tooling |
| Mobile app backend | REST API | Lightweight JSON, caching, stateless design suits mobile network conditions |
| Integrating with a bank or government system | SOAP Web Service | Partner system already uses SOAP, so you match what they expose |
| Real-time features (chat, live notifications) | WebSocket API | Persistent connection removes per-message HTTP overhead |
| Internal microservices at high volume | gRPC API | Binary Protocol Buffers and HTTP/2 multiplexing, 5x faster than REST for internal calls |
| Frontend needs flexible data queries | GraphQL API | Client specifies exactly what data it needs, reduces over-fetching |
| Enterprise ERP or CRM integration | Check first | SAP and Oracle often expose SOAP while modern SaaS CRMs expose REST, so determine what the system provides |
| Local library or SDK integration | Local API | No network needed. SQLite, OpenGL, OS APIs work without HTTP. |
| Healthcare data exchange | REST API (FHIR) | Modern HL7 FHIR standard is REST-based with JSON or XML |
| Legacy system integration (pre-2010 enterprise) | SOAP Web Service | Legacy systems expose SOAP, and migrating them to REST is a separate project entirely |
gRPC: The Option Most Comparisons Skip
Every comparison of API vs web service covers REST and SOAP. Most stop there. That is a gap worth closing because gRPC has become a serious production choice at many companies in the past four years.
gRPC uses Protocol Buffers, a binary serialization format, over HTTP/2. You define your service contract in a .proto file, and the gRPC toolchain generates strongly-typed client and server code in your language of choice. No JSON parsing. No XML overhead. No manual documentation of request/response formats. The proto file is the documentation..
The tradeoff is real though. Protocol Buffers are binary and not human-readable. You cannot open a gRPC response in a browser or inspect it with curl the way you can with a REST API. Debugging requires specific tooling. For public-facing APIs where developer experience matters, gRPC is usually the wrong choice. For internal service communication where you control both ends and performance matters, it is often the best choice.
Read: Web Application Architecture Guide | JavaScript Frameworks
APIs in Modern Architecture: Microservices and Cloud
APIs sit at the center of modern software architecture in a way that web services never quite did.
Microservices
A microservices system decomposes a monolithic application into small, independently deployable services that communicate via APIs. Teams building these systems typically use Agile workflows to manage the complexity of multiple services shipping independently. Typically REST for external-facing endpoints and gRPC for internal service-to-service calls where latency matters. The shift from SOA (which used SOAP web services heavily) to microservices (which use REST and gRPC) reflects a broader preference for flexibility over formalism.
For software outsourcing projects, this architectural context matters practically: knowing whether the client's existing systems use SOAP or REST determines integration approach, estimated development time, and the security model you implement.
API Gateways
An API gateway sits between clients and backend services, handling authentication, rate limiting, request routing, caching, logging, and format transformation. In cloud architectures, the gateway is the single entry point for all external API calls. Major cloud API gateways, including AWS API Gateway, Azure API Management, and Google Cloud Apigee, all support both REST and SOAP integration.
Understanding where the API gateway fits in your software development lifecycle matters early in project planning and can transform between formats: taking a SOAP web service response and presenting it as JSON to modern clients.
Cloud-Native Applications
Cloud-native applications are API-first by design. Every service exposes an API. Storage, compute, database, and AI services all communicate through REST APIs. Read our cloud-native architecture guide for how this plays out in production systems.
The cloud architecture turns every infrastructure component into an API call. For mobile app development, this means the app makes REST API calls to cloud services for storage, authentication, notifications, and analytics , with no infrastructure managed directly.
Four Misconceptions Worth Correcting
Misconception 1: Web Services are Obsolete
SOAP web services are declining for new development. They are not obsolete. The global banking system, insurance industry, government payment infrastructure, and healthcare claims processing still run on SOAP and will for decades. Migration cost and risk for systems processing billions of transactions are prohibitive. Understanding SOAP is a practical requirement for enterprise integration work in 2026, not a history lesson.
Misconception 2: REST Always Outperforms SOAP
REST with JSON is faster than SOAP with XML for typical payloads. gRPC with Protocol Buffers is faster than REST. A well-optimised SOAP implementation can outperform a poorly optimised REST implementation. Performance is a function of implementation quality, not protocol choice alone.
Misconception 3: GraphQL Replaces REST
They solve different problems. REST is simpler to cache, simpler to document, and better for resource-oriented APIs. GraphQL provides superior flexibility for complex data relationships and frontend-driven data requirements. GitHub and Shopify offer both REST and GraphQL endpoints for the same services because different consumers have different needs. Not a replacement. A complement.
Misconception 4: All APIs Need a Network
The SQLite C API, Windows Registry API, POSIX file API, and hundreds of others operate entirely on the local machine. Only web services require a network. Local APIs have no network overhead, no latency, no authentication handshake. They are fast, simple, and exactly right for the use case they serve.
Three Questions That Determine Your Choice
Does the communication cross a network?
If no, you need a local API and web services are not relevant. If yes, both are candidates.
Are you integrating with a regulated industry or legacy enterprise system?
If yes, check what the partner system exposes. Their architecture determines yours. You do not get to choose.
Are you building new functionality with full technology freedom?
Choose REST for web and mobile, gRPC for high-performance internal microservices, GraphQL for flexible frontend data layers, WebSocket for real-time features. SOAP is rarely the right choice for a greenfield project in 2026 unless a regulatory requirement explicitly mandates it.
Key Takeaways
- All web services are APIs, but not all APIs are web services
- Web services always require a network while APIs can be local or networked
- SOAP web services use XML exclusively and provide message-level security via WS-Security
- REST APIs use JSON or XML over HTTP and rely on HTTPS, OAuth, and JWT
- gRPC is the performance choice for internal microservices using Protocol Buffers over HTTP/2
- GraphQL gives frontend teams query flexibility while REST is simpler and more cacheable
- SOAP remains the standard for banking, insurance, government, and legacy enterprise systems
- REST dominates for modern web apps, mobile apps, cloud services, and public developer APIs
How Decipher Zone Handles API and Web Service Integrations
At Decipher Zone, our backend engineers have built REST APIs, SOAP integrations, gRPC services, and GraphQL endpoints across fintech, healthcare, ecommerce, and enterprise software since 2012. Every integration project starts with one question: what does the partner system actually expose?
The answer determines the protocol, the security model, and the development timeline. We do not default to REST because it is fashionable or SOAP because the client is a large enterprise.
- REST API development: Payment APIs, mobile app backends, public developer APIs
- SOAP integration: Banking system connections, insurance platform integration, government API compliance
- gRPC microservices: High-performance internal service communication for scalable architectures
- GraphQL APIs: Flexible data layers for complex frontend requirements
- API security: OAuth 2.0, JWT, WS-Security, HIPAA and GDPR compliant implementations
Get a free API architecture review from Decipher Zone.
We will assess your integration requirements and recommend the right approach, no obligation.
Frequently Asked Questions About API vs Web Services
What is the main difference between an API and a web service?
Scope. An API is any contract allowing two software components to communicate, whether over a network, locally, through operating system calls, or via hardware interfaces. A web service is a specific type of API that operates over a network using web protocols like HTTP or SOAP. All web services are APIs. Many APIs are not web services because they operate locally with no network connection required.
Are all APIs web services?
No. While all web services are APIs, many APIs are not web services. The SQLite database API, the Windows DirectX graphics API, and operating system file system APIs all operate locally without any network. They are APIs. They are not web services. A web service must communicate over a network by definition.
What is the difference between REST API and SOAP web service?
REST is an architectural style using standard HTTP methods and typically returning JSON. It is stateless, lightweight, and the dominant choice for modern web and mobile applications. SOAP is a formal protocol using XML for all messages, requiring a WSDL contract document, and supporting WS-Security for message-level encryption. REST is simpler and faster to build with. SOAP provides stricter contracts and message-level security for regulated industries like banking and healthcare.
Which is better, API or web service?
Neither is universally better. Use REST APIs for modern web applications, mobile apps, and public developer APIs. Use SOAP web services when integrating with banking, healthcare, government, or legacy enterprise systems that require strict contracts. Use gRPC for high-performance internal microservices. Use WebSocket for real-time bidirectional features. The right choice depends entirely on your specific constraints, not on general preference.
What protocols do web services use?
Web services use three primary protocols: SOAP which uses XML over HTTP or SMTP, REST which uses standard HTTP methods, and XML-RPC which uses XML over HTTP for remote procedure calls. SOAP is the most formal and comes with built-in security through WS-Security. REST is the most widely used for modern applications. XML-RPC is largely legacy. All web services require HTTP or HTTPS as the transport layer.
When should I use a REST API instead of a SOAP web service?
Use REST when you are building a new public API, a mobile backend, a web application, or any system where simplicity and developer experience are priorities. REST is lighter, faster to test, and natively supported by browsers. Use SOAP when integrating with financial systems, insurance platforms, government APIs, or legacy enterprise software that exposes SOAP endpoints. In those cases the partner system dictates the protocol and switching is not a decision you make unilaterally.
What is gRPC and how does it compare to REST and SOAP?
gRPC is a high-performance API framework using Protocol Buffers (binary serialization) over HTTP/2. It is 3 to 10 times faster than REST with JSON for the same data because binary formats are more compact and faster to parse than JSON or XML. gRPC uses .proto files as strongly-typed service contracts and generates client and server code automatically. The tradeoff: Protocol Buffers are not human-readable, so debugging requires specific tooling. Best for internal microservices where you control both ends and performance matters.
Can a web service use JSON instead of XML?
RESTful web services commonly use JSON. SOAP web services cannot. The SOAP standard mandates XML for all message formatting without exception. If you need JSON-based communication, you are using a REST web service or a REST API, not a SOAP web service. The format choice follows from the protocol choice, not the other way around.
Author Profile: Mahipal Nehra is the Digital Marketing Manager at Decipher Zone Technologies, specialising in content strategy and tech-driven marketing for software development and digital transformation. Follow on LinkedIn or explore more at Decipher Zone.
