REST vs WebSocket APIs

When building applications that consume financial market data, one of the first architectural decisions is whether to use a REST API, a WebSocket API, or both.

The answer depends on how fresh your data needs to be and how your application uses it. In practice, many production applications combine both technologies—using REST APIs for historical and reference data while relying on WebSockets for live market updates.

What is a REST API?

A REST (Representational State Transfer) API allows your application to request data whenever it needs it. Your application sends an HTTP request to a specific endpoint, the server processes the request, returns the requested data, and then closes the connection.

Think of a REST API like asking a librarian for a specific book. You ask a question, receive the answer, and the conversation ends until you ask another question.

REST APIs are ideal for data that doesn't need to update continuously.

Common use cases include:

• Historical stock prices
• Company fundamentals
• Financial statements
• SEC filings
• Company profiles
• ETF holdings
• Economic data
• End-of-day options data

Advantages of REST APIs

• Easy to understand and implement
• Stateless, making applications easier to scale
• Well suited for historical and reference data
• Easy to cache responses
• Compatible with virtually every programming language and HTTP client

Limitations of REST APIs

If your application requests data repeatedly (for example, every second), each request creates network overhead. As polling frequency increases, applications consume more bandwidth, API calls, and server resources.

For real-time market data, repeatedly polling a REST endpoint quickly becomes inefficient.

What is a WebSocket API?

A WebSocket API creates a persistent, two-way connection between your application and the server.

Instead of repeatedly asking for updates, your application connects once. As new market data becomes available, the server immediately pushes updates to your application.

A helpful analogy is subscribing to a live news feed instead of refreshing a news website every few seconds.

This makes WebSockets ideal for applications that need continuously changing market data.

Common use cases include:

• Live stock prices
• Streaming options data
• Trades
• Quotes
• Real-time market dashboards
• Trading applications
• Alerting systems

Advantages of WebSockets

• Extremely low latency
• No polling required
• Efficient bandwidth usage
• Continuous real-time updates
• Ideal for streaming market data

Limitations of WebSockets

Because WebSocket connections remain open, applications must manage:

• Connection state
• Reconnection after network interruptions
• Heartbeats and keep-alive messages
• Subscription management
• Message ordering and processing

They are generally more complex to implement than REST APIs.

REST vs WebSocket Comparison

When Should You Use REST?

REST APIs are typically the best choice when your application needs to:

• Retrieve historical market data
• Access company financials
• Query SEC filings
• Download bulk datasets
• Look up security information
• Perform occasional data requests
• Build reports or analytics

If your data changes infrequently or users only request information on demand, REST is usually the simplest and most efficient solution.

When Should You Use WebSockets?

WebSockets are the preferred choice when your application needs:

• Real-time stock prices
• Live options data
• Streaming quotes
• Market dashboards
• Trading interfaces
• Alert notifications
• Tick-by-tick market events

Whenever users expect information to appear instantly without refreshing the page, WebSockets are generally the right solution.

Using REST and WebSockets Together

Most modern financial applications use both technologies.

For example:

1. Your application loads historical price history using the REST API.
2. It retrieves company fundamentals and metadata using REST.
3. It opens a WebSocket connection to receive live market updates.
4. Incoming WebSocket messages update the user interface in real time.

This approach minimizes network traffic while providing users with both rich historical context and live market information.

Which Should I Choose?

Ask yourself one simple question:

Do I need the newest data immediately when it changes?

If no: Use REST.

If yes: Use WebSockets.

Many professional financial platforms use both together to provide the best user experience.

Intrinio APIs

Intrinio offers both REST APIs and WebSocket APIs depending on the dataset and use case.

REST APIs provide access to a broad range of historical and reference financial data, while WebSocket APIs are available for supported streaming market data products, allowing applications to receive live market updates with minimal latency. Available datasets and streaming capabilities depend on your subscription.

To explore available REST endpoints and WebSocket-enabled datasets:

• Visit the API Documentation from your Intrinio account, where you can filter endpoints by the data feeds included in your subscription.
• Or connect to the Intrinio MCP Server, which can recommend the appropriate datasets and endpoints based on your account's entitlements and your application's goals using natural language.

For more information, see the Intrinio API documentation and the Financial Data MCP Server documentation.