Make offchain HTTP requests

This guide steps through making an HTTP request using an offchain workeR, to GET or POST data offchain.

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 offchain 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

Set a deadline and instantiate an HTTP request

  1. 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));
  2. Initiate an external HTTP GET request.

    let request = http::Request::get("");
    let pending = request.deadline(deadline).send().map_err(|_| http::Error::IoError)?;
    let response = pending.try_wait(deadline).map_err(|_| http::Error::DeadlineReached)??;

Read and submit the response

  1. First, you must 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)
  2. 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");
  3. Now, here's how you can submit data to an API via a POST request.

    // Send a POST request
    let request_body = Vec::new();
    let request = http::Request::post("", vec![request_body.clone()])
     .add_header("x-api-key", "test_api_key")
     .add_header("content-type", "application/json");
    let pending = request
     .map_err(|_| http::Error::IoError)?;
    // Wait for response
    let response = pending
     .map_err(|_| http::Error::DeadlineReached)??;