Making HTTP Requests
Making HTTP Requests
Goal
Send HTTP requests offchain.
Use Cases
GET data from offchain, or POST data offchain.
Overview
Off-chain workers (OCW for short) were introduced to extend oracle-like capabilities for Substrate blockchains. Because a blockchain does not have access to data outside its own network, oracles are a useful tool to enable interactions between on and off-chain worlds.
In this guide we will look through retrieving the price of bitcoin from the cryptocompare api as well as submitting data via an OCW API.
Remember that although Rust provides various libraries for issuing HTTP requests, an OCW runs in an no-std environment.
Luckily, Substrate provides us with a few no_std libraries we can use to issue HTTP requests to an API.
The Substrate HTTP library supports the following methods:
- GET
- POST
- PUT
- PATCH
- DELETE
Steps
Create a deadline of 2 seconds
// We want to keep the offchain worker execution time reasonable, so we set a hard-coded // deadline to 2s to complete the external call. // You can also wait indefinitely for the response, however you may still get a timeout // coming from the host machine. let deadline = sp_io::offchain::timestamp().add(Duration::from_millis(2_000));Initiate an external HTTP GET request
// Initiate an external HTTP GET request. let request = http::Request::get("https://min-api.cryptocompare.com/data/price?fsym=BTC&tsyms=USD"); let pending = request.deadline(deadline).send().map_err(|_| http::Error::IoError)?; let response = pending.try_wait(deadline).map_err(|_| http::Error::DeadlineReached)??;Check the response
// Let's check the status code before we proceed to reading the response. if response.code != 200 { log::warn!("Unexpected status code: {}", response.code); return Err(http::Error::Unknown) }Read the response
let body = response.body().collect::<Vec<u8>>(); // Create a str slice from the body. let body_str = sp_std::str::from_utf8(&body).map_err(|_| { log::warn!("No UTF8 body"); http::Error::Unknown })?;Submit data to an API via a POST request
// Send a POST request let request_body = Vec::new(); let request = http::Request::post("https://reqres.in/api/users", vec![request_body.clone()]) .add_header("x-api-key", "test_api_key") .add_header("content-type", "application/json"); let pending = request .deadline(deadline) .body(vec![request_body.clone()]) .send() .map_err(|_| http::Error::IoError)?; // Wait for response let response = pending .try_wait(deadline) .map_err(|_| http::Error::DeadlineReached)??;