How to use HTTP outcalls: POST

Advanced

Tutorial

Overview

A minimal example to make a POST HTTPS request. The purpose of this dapp is only to show how to make HTTP requests from a canister.

The sample code is in both Motoko and Rust. This sample canister sends a POST request with some JSON to a free API where you can verify the headers and body were sent correctly.

The main intent of this canister is to show developers how to make idempotent POST requests.

This example takes less than 5 minutes to complete.

The HTTPS outcalls feature only works for sending HTTP POST requests to servers or API endpoints that support IPV6.

Candid web UI of canister

The canister in this tutorial will have only one public method which, when called, will trigger an HTTP POST request. The canister will not have a frontend (only a backend), but like all canisters, you can interact with its public methods via the Candid web UI, which will look like this:

When you call the method, the canister will send an HTTP POST request with the following JSON in the response body:

{
    "name": "Grogu",
    "force_sensitive": "true"
}

Verifying the HTTP POST request

In order to verify that your canister sent the HTTP request you expected, this canister is sending HTTP requests to a public API service where the HTTP request can be inspected. As you can see the image below, the POST request headers and body can be inspected to make sure it is what the canister sent.

Important notes on POST requests

Because HTTPS outcalls go through consensus, a developer should expect any HTTPs POST request from a canister to be sent many times to its destination. Even if you ignore the Web3 component, multiple identical POST requests is not new problem in HTTP where it is common for clients to retry requests for a variety of reasons (e.g. destination server being unavailable).

The recommended way for HTTP POST requests is to add the idempotency keys in the header so the destination server knows which POST requests from the client are the same.

Developers should be careful that the destination server understand and use idempotency keys. A canister can be coded to be send idempotency keys, but it is ultimately up to the recipient server to know what to do with then. Here is an example of an API service that uses idempotency keys.

Code structure

Before you dive in, here is the structure of the code you will touch:

Motoko

//Import some custom types from `src/backend_canister/Types.mo` file
import Types "Types";

actor {

//method that uses the HTTP outcalls feature and returns a string
  public func foo() : async Text {

    //1. DECLARE MANAGEMENT CANISTER
    let ic : Types.IC = actor ("aaaaa-aa");

    //2. SETUP ARGUMENTS FOR HTTP GET request
    let request : Types.HttpRequestArgs = {
        //construct the request
    };

    //3. ADD CYCLES TO PAY FOR HTTP REQUEST
    //code to add cycles

    //4. MAKE HTTPS REQUEST AND WAIT FOR RESPONSE
    let response : Types.HttpResponsePayload = await ic.http_request(request);

    //5. DECODE THE RESPONSE
    //code to decode response

    //6. RETURN RESPONSE OF THE BODY
    response
  };
};

You will also create some custom types in Types.mo. This will look like this:

module Types {

    //type declarations for HTTP requests, HTTP responses, management canister, etc...

}

Rust

//1. DECLARE MANAGEMENT CANISTER
use ic_cdk::api::management_canister::http_request::{
    http_request, CanisterHttpRequestArgument, HttpHeader, HttpMethod, HttpResponse, TransformArgs,
    TransformContext,
};

//Update method using the HTTPS outcalls feature
#[ic_cdk::update]
async fn foo() {
    //2. SETUP ARGUMENTS FOR HTTP GET request
    let request = CanisterHttpRequestArgument {
        //instantiate the request
    };

    //3. MAKE HTTPS REQUEST AND WAIT FOR RESPONSE
    //Note: in Rust, `http_request()` already sends the cycles needed
    //so no need for explicit Cycles.add() as in Motoko
    match http_request(request).await {

        //4. DECODE AND RETURN THE RESPONSE
        Ok((response,)) => {
            //Ok case
        }
        Err((r, m)) => {
            //error case
        }
    }
}

• Step 1: Create a new project by running the following command:

Motoko

dfx new send_http_post_motoko
cd send_http_post_motoko
npm install

Rust

dfx new --type=rust send_http_post_rust
cd send_http_post_rust
npm install
rustup target add wasm32-unknown-unknown

• Step 2: Edit the backend canister's code.

Motoko

Open the src/send_http_post_motoko_backend/main.mo file in a text editor and replace content with:

import Debug "mo:base/Debug";
import Blob "mo:base/Blob";
import Cycles "mo:base/ExperimentalCycles";
import Array "mo:base/Array";
import Nat8 "mo:base/Nat8";
import Text "mo:base/Text";

//import the custom types you have in Types.mo
import Types "Types";

actor {

  //function to transform the response
  public query func transform(raw : Types.TransformArgs) : async Types.CanisterHttpResponsePayload {
      let transformed : Types.CanisterHttpResponsePayload = {
          status = raw.response.status;
          body = raw.response.body;
          headers = [
              {
                  name = "Content-Security-Policy";
                  value = "default-src 'self'";
              },
              { name = "Referrer-Policy"; value = "strict-origin" },
              { name = "Permissions-Policy"; value = "geolocation=(self)" },
              {
                  name = "Strict-Transport-Security";
                  value = "max-age=63072000";
              },
              { name = "X-Frame-Options"; value = "DENY" },
              { name = "X-Content-Type-Options"; value = "nosniff" },
          ];
      };
      transformed;
  };

//PUBLIC METHOD
//This method sends a POST request to a URL with a free API you can test.
  public func send_http_post_request() : async Text {

    //1. DECLARE MANAGEMENT CANISTER
    //You need this so you can use it to make the HTTP request
    let ic : Types.IC = actor ("aaaaa-aa");

    //2. SETUP ARGUMENTS FOR HTTP GET request

    // 2.1 Setup the URL and its query parameters
    //This URL is used because it allows you to inspect the HTTP request sent from the canister
    let host : Text = "putsreq.com";
    let url = "https://putsreq.com/aL1QS5IbaQd4NTqN3a81"; //HTTP that accepts IPV6

    // 2.2 prepare headers for the system http_request call

    //idempotency keys should be unique so create a function that generates them.
    let idempotency_key: Text = generateUUID();
    let request_headers = [
        { name = "Host"; value = host # ":443" },
        { name = "User-Agent"; value = "http_post_sample" },
        { name= "Content-Type"; value = "application/json" },
        { name= "Idempotency-Key"; value = idempotency_key }
    ];

    // The request body is an array of [Nat8] (see Types.mo) so do the following:
    // 1. Write a JSON string
    // 2. Convert ?Text optional into a Blob, which is an intermediate reprepresentation before you cast it as an array of [Nat8]
    // 3. Convert the Blob into an array [Nat8]
    let request_body_json: Text = "{ \"name\" : \"Grogu\", \"force_sensitive\" : \"true\" }";
    let request_body_as_Blob: Blob = Text.encodeUtf8(request_body_json);
    let request_body_as_nat8: [Nat8] = Blob.toArray(request_body_as_Blob); // e.g [34, 34,12, 0]


    // 2.2.1 Transform context
    let transform_context : Types.TransformContext = {
      function = transform;
      context = Blob.fromArray([]);
    };

    // 2.3 The HTTP request
    let http_request : Types.HttpRequestArgs = {
        url = url;
        max_response_bytes = null; //optional for request
        headers = request_headers;
        //note: type of `body` is ?[Nat8] so it is passed here as "?request_body_as_nat8" instead of "request_body_as_nat8"
        body = ?request_body_as_nat8;
        method = #post;
        transform = ?transform_context;
        // transform = null; //optional for request
    };

    //3. ADD CYCLES TO PAY FOR HTTP REQUEST

    //The management canister will make the HTTP request so it needs cycles
    //See: /docs/current/motoko/main/cycles

    //The way Cycles.add() works is that it adds those cycles to the next asynchronous call
    //See: /docs/current/references/ic-interface-spec/#ic-http_request
    Cycles.add(21_850_258_000);

    //4. MAKE HTTPS REQUEST AND WAIT FOR RESPONSE
    //Since the cycles were added above, you can just call the management canister with HTTPS outcalls below
    let http_response : Types.HttpResponsePayload = await ic.http_request(http_request);

    //5. DECODE THE RESPONSE

    //As per the type declarations in `Types.mo`, the BODY in the HTTP response
    //comes back as [Nat8s] (e.g. [2, 5, 12, 11, 23]). Type signature:

    //public type HttpResponsePayload = {
    //     status : Nat;
    //     headers : [HttpHeader];
    //     body : [Nat8];
    // };

    // You need to decode that [Na8] array that is the body into readable text.
    //To do this:
    //  1. Convert the [Nat8] into a Blob
    //  2. Use Blob.decodeUtf8() method to convert the Blob to a ?Text optional
    //  3. Use Motoko syntax "Let... else" to unwrap what is returned from Text.decodeUtf8()
    let response_body: Blob = Blob.fromArray(http_response.body);
    let decoded_text: Text = switch (Text.decodeUtf8(response_body)) {
        case (null) { "No value returned" };
        case (?y) { y };
    };

    //6. RETURN RESPONSE OF THE BODY
    let result: Text = decoded_text # ". See more info of the request sent at at: " # url # "/inspect";
    result
  };

  //PRIVATE HELPER FUNCTION
  //Helper method that generates a Universally Unique Identifier
  //this method is used for the Idempotency Key used in the request headers of the POST request.
  //For the purposes of this exercise, it returns a constant, but in practice it should return unique identifiers
  func generateUUID() : Text {
    "UUID-123456789";
  }
};

Rust

Open the /src/send_http_post_rust_backend/src/lib.rs file in a text editor and replace content with:

//1. IMPORT MANAGEMENT CANISTER
//This includes all methods and types needed
use ic_cdk::api::management_canister::http_request::{
    http_request, CanisterHttpRequestArgument, HttpHeader, HttpMethod, HttpResponse, TransformArgs,
    TransformContext,
};

use ic_cdk_macros::{self, query, update};
use serde::{Serialize, Deserialize};
use serde_json::{self, Value};

// This struct is legacy code and is not really used in the code.
#[derive(Serialize, Deserialize)]
struct Context {
    bucket_start_time_index: usize,
    closing_price_index: usize,
}

//Update method using the HTTPS outcalls feature
#[ic_cdk::update]
async fn send_http_post_request() -> String {
    //2. SETUP ARGUMENTS FOR HTTP GET request

    // 2.1 Setup the URL
    let host = "putsreq.com";
    let url = "https://putsreq.com/aL1QS5IbaQd4NTqN3a81";

    // 2.2 prepare headers for the system http_request call
    //Note that `HttpHeader` is declared in line 4
    let request_headers = vec![
        HttpHeader {
            name: "Host".to_string(),
            value: format!("{host}:443"),
        },
        HttpHeader {
            name: "User-Agent".to_string(),
            value: "demo_HTTP_POST_canister".to_string(),
        },
        //For the purposes of this exercise, Idempotency-Key" is hard coded, but in practice
        //it should be generated via code and unique to each POST request. Common to create helper methods for this
        HttpHeader {
            name: "Idempotency-Key".to_string(),
            value: "UUID-123456789".to_string(),
        },
        HttpHeader {
            name: "Content-Type".to_string(),
            value: "application/json".to_string(),
        },
    ];

    //note "CanisterHttpRequestArgument" and "HttpMethod" are declared in line 4.
    //CanisterHttpRequestArgument has the following types:

    // pub struct CanisterHttpRequestArgument {
    //     pub url: String,
    //     pub max_response_bytes: Option<u64>,
    //     pub method: HttpMethod,
    //     pub headers: Vec<HttpHeader>,
    //     pub body: Option<Vec<u8>>,
    //     pub transform: Option<TransformContext>,
    // }
    //see: https://docs.rs/ic-cdk/latest/ic_cdk/api/management_canister/http_request/struct.CanisterHttpRequestArgument.html

    //Where "HttpMethod" has structure:
    // pub enum HttpMethod {
    //     GET,
    //     POST,
    //     HEAD,
    // }
    //See: https://docs.rs/ic-cdk/latest/ic_cdk/api/management_canister/http_request/enum.HttpMethod.html

    //Since the body in HTTP request has type Option<Vec<u8>> it needs to look something like this: Some(vec![104, 101, 108, 108, 111]) ("hello" in ASCII)
    //where the vector of u8s are the UTF. In order to send JSON via POST, do the following:
    //1. Declare a JSON string to send
    //2. Convert that JSON string to array of UTF8 (u8)
    //3. Wrap that array in an optional
    let json_string : String = "{ \"name\" : \"Grogu\", \"force_sensitive\" : \"true\" }".to_string();

    //note: here, r#""# is used for raw strings in Rust, which allows you to include characters like " and \ without needing to escape them.
    //You could have used "serde_json" as well.
    let json_utf8: Vec<u8> = json_string.into_bytes();
    let request_body: Option<Vec<u8>> = Some(json_utf8);

    // This struct is legacy code and is not really used in the code. Need to be removed in the future
    // The "TransformContext" function does need a CONTEXT parameter, but this implementation is not necessary
    // the TransformContext(transform, context) below accepts this "context", but it does nothing with it in this implementation.
    // bucket_start_time_index and closing_price_index are meaninglesss
    let context = Context {
        bucket_start_time_index: 0,
        closing_price_index: 4,
    };

    let request = CanisterHttpRequestArgument {
        url: url.to_string(),
        max_response_bytes: None, //optional for request
        method: HttpMethod::POST,
        headers: request_headers,
        body: request_body,
        transform: Some(TransformContext::new(transform, serde_json::to_vec(&context).unwrap())),
    };

    //3. MAKE HTTPS REQUEST AND WAIT FOR RESPONSE

    //Note: in Rust, `http_request()` already sends the cycles needed
    //so no need for explicit Cycles.add() as in Motoko
    match http_request(request).await {
        //4. DECODE AND RETURN THE RESPONSE

        //See:https://docs.rs/ic-cdk/latest/ic_cdk/api/management_canister/http_request/struct.HttpResponse.html
        Ok((response,)) => {
            //if successful, `HttpResponse` has this structure:
            // pub struct HttpResponse {
            //     pub status: Nat,
            //     pub headers: Vec<HttpHeader>,
            //     pub body: Vec<u8>,
            // }

            //You need to decode that Vec<u8> that is the body into readable text.
            //To do this:
            //  1. Call `String::from_utf8()` on response.body
            //  3. Use a switch to explicitly call out both cases of decoding the Blob into ?Text
            let str_body = String::from_utf8(response.body)
                .expect("Transformed response is not UTF-8 encoded.");
            ic_cdk::api::print(format!("{:?}", str_body));

            //The API response will looks like this:
            // { successful: true }

            //Return the body as a string and end the method
            let result: String = format!(
                "{}. See more info of the request sent at: {}/inspect",
                str_body, url
            );
            result
        }
        Err((r, m)) => {
            let message =
                format!("The http_request resulted into error. RejectionCode: {r:?}, Error: {m}");

            //Return the error as a string and end the method
            message
        }
    }

}

// Strips all data that is not needed from the original response.
#[query]
fn transform(raw: TransformArgs) -> HttpResponse {

    let headers = vec![
        HttpHeader {
            name: "Content-Security-Policy".to_string(),
            value: "default-src 'self'".to_string(),
        },
        HttpHeader {
            name: "Referrer-Policy".to_string(),
            value: "strict-origin".to_string(),
        },
        HttpHeader {
            name: "Permissions-Policy".to_string(),
            value: "geolocation=(self)".to_string(),
        },
        HttpHeader {
            name: "Strict-Transport-Security".to_string(),
            value: "max-age=63072000".to_string(),
        },
        HttpHeader {
            name: "X-Frame-Options".to_string(),
            value: "DENY".to_string(),
        },
        HttpHeader {
            name: "X-Content-Type-Options".to_string(),
            value: "nosniff".to_string(),
        },
    ];


    let mut res = HttpResponse {
        status: raw.response.status.clone(),
        body: raw.response.body.clone(),
        headers,
        ..Default::default()
    };

    if res.status == 200 {

        res.body = raw.response.body;
    } else {
        ic_cdk::api::print(format!("Received an error from coinbase: err = {:?}", raw));
    }
    res
}

• send_http_post_request() -> String returns a String, but this is not necessary. In this tutorial, this is done for easier testing.
• The lib.rs file used http_request which is a convenient Rust CDK method that already sends cycles to the management canister under the hood. It knows how many cycles to send for a 13-node subnet and most cases. If your HTTPS outcall needs more cycles, you should use http_request_with_cycles() method and explicitly call the cycles needed.
• The Rust CDK method http_request used above wraps the management canister method http_request, but it is not strictly the same.

• Step 3: Edit the Type or Candid files.

Motoko

Create the src/send_http_post_motoko_backend/Types.mo file in a text editor and replace content with:

module Types {

    //1. Type that describes the Request arguments for an HTTPS outcall
    //See: /docs/current/references/ic-interface-spec/#ic-http_request
    public type HttpRequestArgs = {
        url : Text;
        max_response_bytes : ?Nat64;
        headers : [HttpHeader];
        body : ?[Nat8];
        method : HttpMethod;
        transform : ?TransformRawResponseFunction;
    };

    public type HttpHeader = {
        name : Text;
        value : Text;
    };

    public type HttpMethod = {
        #get;
        #post;
        #head;
    };

    public type HttpResponsePayload = {
        status : Nat;
        headers : [HttpHeader];
        body : [Nat8];
    };

    //2. HTTPS outcalls have an optional "transform" key. These two types help describe it.
    //"The transform function may, for example, transform the body in any way, add or remove headers,
    //modify headers, etc. "
    //See: /docs/current/references/ic-interface-spec/#ic-http_request

    //2.1 This type describes a function called "TransformRawResponse" used in line 14 above
    //"If provided, the calling canister itself must export this function."
    //In this minimal example for a GET request, declare the type for completeness, but
    //you do not use this function. You will pass "null" to the HTTP request.
    public type TransformRawResponseFunction = {
        function : shared query TransformArgs -> async HttpResponsePayload;
        context : Blob;
    };

    //2.2 This type describes the arguments the transform function needs
    public type TransformArgs = {
        response : HttpResponsePayload;
        context : Blob;
    };

    public type CanisterHttpResponsePayload = {
        status : Nat;
        headers : [HttpHeader];
        body : [Nat8];
    };

    public type TransformContext = {
        function : shared query TransformArgs -> async HttpResponsePayload;
        context : Blob;
    };


    //3. Declaring the management canister which is used to make the HTTPS outcall
    public type IC = actor {
        http_request : HttpRequestArgs -> async HttpResponsePayload;
    };

}

Rust

Open the src/hello_http_rust_backend/hello_http_rust_backend.did file in a text editor and replace content with:

Update the Candid interface file so it matches the method send_http_post_request() in lib.rs.

service : {
    "send_http_post_request": () -> (text);
}

Open the src/send_http_post_rust_backend/Cargo.toml file in a text editor and replace content with:

[package]
name = "send_http_post_rust_backend"
version = "0.1.0"
edition = "2021"

# See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html

[lib]
crate-type = ["cdylib"]

[dependencies]
candid = "0.8.2"
ic-cdk = "0.6.0"
ic-cdk-macros = "0.6.0"
serde = "1.0.152"
serde_json = "1.0.93"
serde_bytes = "0.11.9"

• Step 4: Test the dapp locally.

Motoko

Deploy the dapp locally:

dfx start --clean --background
dfx deploy

If successful, the terminal should return canister URLs you can open:

Deployed canisters.
URLs:
  Backend canister via Candid interface:
    send_http_post_motoko_backend: http://127.0.0.1:4943/?canisterId=dccg7-xmaaa-aaaaa-qaamq-cai&id=dfdal-2uaaa-aaaaa-qaama-cai

Open the candid web UI for backend and call the send_http_post_motoko_request() method:

Rust

Deploy the dapp locally:

dfx start --clean --background
dfx deploy

If successful, the terminal should return canister URLs you can open:

Deployed canisters.
URLs:
  Backend canister via Candid interface:
    send_http_post_rust_backend: http://127.0.0.1:4943/?canisterId=dxfxs-weaaa-aaaaa-qaapa-cai&id=dzh22-nuaaa-aaaaa-qaaoa-cai

Open the candid web UI for backend and call the send_http_post_request() method:

• Step 5: Test the dapp on mainnet.

Motoko

Deploy the dapp to mainnet:

dfx deploy --network ic

If successful, the terminal should return canister URLs you can open:

Committing batch.
Deployed canisters.
URLs:
  Frontend canister via browser
    send_http_post_motoko_frontend: https://fx3cz-taaaa-aaaap-qbooa-cai.ic0.app/
  Backend canister via Candid interface:
    send_http_post_motoko_backend: https://a4gq6-oaaaa-aaaab-qaa4q-cai.raw.ic0.app/?id=fc4tu-siaaa-aaaap-qbonq-cai

You can see play with the dapp's send_http_post_request method on-chain here: https://a4gq6-oaaaa-aaaab-qaa4q-cai.raw.ic0.app/?id=fc4tu-siaaa-aaaap-qbonq-cai.

Rust

Deploy the dapp to mainnet:

dfx deploy --network ic

If successful, the terminal should return canister URLs you can open:

Committing batch.
Deployed canisters.
URLs:
  Frontend canister via browser
    send_http_post_rust_frontend: https://f6yjf-fiaaa-aaaap-qbopq-cai.ic0.app/
  Backend canister via Candid interface:
    send_http_post_rust_backend: https://a4gq6-oaaaa-aaaab-qaa4q-cai.raw.ic0.app/?id=fzzpr-iqaaa-aaaap-qbopa-cai

You can see play with the dapp's send_http_post_request method on-chain here: https://a4gq6-oaaaa-aaaab-qaa4q-cai.raw.ic0.app/?id=fm664-jyaaa-aaaap-qbomq-cai.

Additional resources

• Sample code of HTTP POST requests in Rust
• Sample code of HTTP POST requests in Motoko