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How to use HTTPS outcalls: POST

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Tutorial

Overview

A minimal example of how to make a POST HTTP 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:

Candid web UI

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.

Public API to inspect POST request

Important notes on POST requests

Because HTTP outcalls go through consensus, a developer should expect any HTTP POST request from a canister to be sent many times to its destination. Even if you ignore the Web3 component, multiple identical POST requests are not a new problem in HTTPS 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 understands and uses idempotency keys. A canister can be coded to send idempotency keys, but it is ultimately up to the recipient server to know what to do with them. 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:


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

actor {

//method that uses the HTTPS 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 HTTP 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...

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

dfx new send_http_post_motoko
cd send_http_post_motoko
npm install
  • Step 2: Edit the backend canister's code.

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"; //HTTPS 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 representation 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/canister-maintenance/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 HTTP 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";
}
};
  • Step 3: Edit the Type or Candid files.

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;
};

}

  • Step 4: Test the dapp locally.

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 the backend and call the send_http_post_motoko_request() method:

Candid web UI

  • Step 5: Test the dapp on mainnet.

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.

Additional resources