1xx Informational Responses

What Are 1xx Informational Responses?

HTTP 1xx Informational Responses are provisional status codes that indicate the server has received the request and is continuing to process it.

Unlike successful responses such as 200 OK, informational responses do not represent the final result of a request. They are temporary messages sent between the client and server during communication.

How 1xx Responses Work

When a client sends an HTTP request, the server may need to provide an intermediate response before returning the final result.

A typical request flow may look like this:

Client
 |
 | Request
 |
 v
Server
 |
 | 1xx Informational Response
 |
 | Processing continues
 |
 v
Final Response (2xx, 4xx, or 5xx)

Common 1xx Status Codes

The most commonly encountered 1xx responses include:

• 100 Continue - The server has received the request headers and the client can continue sending the request body.

• 101 Switching Protocols - The server agrees to switch protocols, such as upgrading from HTTP to WebSocket.

• 103 Early Hints - The server provides early information to help a browser begin loading resources before the final response.

100 Continue

The 100 Continue response is used when a client sends a request containing an Expect: 100-continue header.

It allows the server to confirm that it is willing to accept the request before the client sends a large request body.

POST /upload HTTP/1.1
Host: example.com
Expect: 100-continue

The server responds:

HTTP/1.1 100 Continue

The client can then continue uploading the data.

101 Switching Protocols

The 101 Switching Protocols response indicates that the server is changing the communication protocol as requested by the client.

A common example is upgrading an HTTP connection to WebSocket.

HTTP/1.1 101 Switching Protocols

Upgrade: websocket
Connection: Upgrade

103 Early hints

The 103 Early Hints response allows servers to provide hints about resources that should be loaded before the final response is available.

This can improve performance by allowing browsers to begin downloading assets such as stylesheets and scripts earlier.

Are 1xx Responses Important For DevOps?

1xx responses are less commonly seen during everyday troubleshooting compared to 4xx and 5xx errors, but they are still important when debugging complex systems.

They can appear when working with:

• Large file uploads

• WebSocket connections

• HTTP proxies

• Load balancers

• Performance optimisation systems

Troubleshooting 1xx Responses

When investigating unexpected informational responses, engineers should check:

• Client request headers

• Proxy and load balancer behaviour

• HTTP protocol versions

• Application server logs

Related HTTP Concepts

• HTTP Status Codes

• HTTP Headers

• HTTP Methods

• WebSockets

• Reverse Proxies