Error 417: Expectation Failed When Using HTTP Methods in LabVIEW

Issue Details

I am trying to communicate with my server using HTTP methods, but the HTTP VI returns Error 417: Expectation Failed. What causes this error and how can I resolve it?

Solution

HTTP/1.1 Protocol defines the 100 (Continue) Status, which allows a client to check whether a server is willing to accept a request before sending the message body. The process goes as follows: 

1. Client wants to send server a request message.
2. Client sends a request header containing Expect: 100-continue.
3. Server sends back either 100 (Continue) or 417 (Expectation Failed) status.

• 100 (Continue) means the server accepts the request and the client can proceed with sending the request body.
• 417 (Expectation Failed) means the server rejects the request.

When using certain HTTP methods in the HTTP Client Palette, LabVIEW will automatically add the 100 - continue expectation to the request header when the message is 1024 characters or longer. This can cause issues if the server always responds with 417 (Expectation Failed) or does not know how to respond properly to the expectation.

Because the HTTP Client functions do not give you the option to disable 100 - continue for longer messages (1024+ characters), the solution is to create an HTTP request header from scratch that excludes it. The standards for HTTP methods are defined in the W3C HTTP Protocol Description.

The snippets below demonstrate an example that implements the HTTP POST method using basic functions and functions from the TCP Palette. When composing strings for HTTP requests, set the string display style to Backslash (\) Codes. This allows you to proofread non-displayable characters, which are necessary to terminate commands properly. 

From String Constant or String Control:

• Right-Click » '\' Codes Display, or 
• Right-Click » Properties » Display Style: Backslash (\) Codes