Problem reading channel. HTTP error code -304

when i want to read data from thingspeak, i get the below messege :

Problem reading channel. HTTP error code -304

how can i solve this?

[Walter Roberson]

https://airbrake.io/blog/http-errors/304-not-modified

[Vinod]

How are you reading the data from your ThingSpeak channel?

• A HTML page using the ThingSpeak API and Javascript?
• MATLAB code using thingSpeakRead?
• From an embedded device?
• Something else?

Answer 1

[Christopher Stapels]

The negative errors are from the communication library, not standard HTTP responses. From the ThingSpeak communication library readme, -304 is Timeout waiting for server to respond. This seems to either be a network issue, or you are sending an incomplete message to ThingSpeak. Are you using one of the standard examples form the library or your own code?

Answer 2

[Niels]

The same problem appeared here when accessing a university server, except I was writing not reading a field. I solved this by trying to write the field up to 10 times during 5 seconds. Now I always get a success with error code 200. I guess the same idea would work for reading a field. Why it works, I've got no clue.

int returnCode;
for (int i=0; i<10; ++i) {
    returnCode = ThingSpeak.writeField(channelId, field, value, pw);
    if (returnCode == 200) break;
    delay(500);
}
Serial.println("writeField " + String(field) + " " + String(value,2) + " " + String(returnCode));  

[Vinod]

The suggestion I would make is to apply some sort of (exponential?) backoff to the delay in the loop. You don't want to inadvertantly trigger a DoS attack from your device. A simple backoff might be:

delay(500 + i*(i+100));

[Niels]

That's neat, thanks. The danger of triggering a DoS attack detector did not occur to me. Thinking more about the problem, I guess that the university IoT server has gotten quite busy due to a proliferation of connected things. So, every time that I try to connect through it to ThingSpeak, there is a probability of it being busy and denying my request. Then, it is simply a matter of trying repeatedly until by chance I get through.