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How to use HTTPS outcalls: GET

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Tutorial

Overview​

A minimal example to make a GET 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 GET request to the Coinbase API and retrieves some historical data about the ICP token.

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

This example takes less than 5 minutes to complete.

Sample dapp​

The canister in this tutorial will have only one public method named get_icp_usd_exchange() which, when called, will trigger an HTTP GET request to an external service. 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

The get_icp_usd_exchange() method returns Coinbase data on the exchange rate between USD and ICP for a certain day. The data will look like this:

The API response looks like this:

  [
     [
         1682978460, <-- start timestamp
         5.714, <-- lowest price during time range
         5.718, <-- highest price during range
         5.714, <-- price at open
         5.714, <-- price at close
         243.5678 <-- volume of  traded
     ],
]

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 {

  //0. 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
  };

  //7. CREATE TRANSFORM FUNCTION
  public query func transform(raw : Types.TransformArgs) : async Types.CanisterHttpResponsePayload {
    ////code for the transform function
  }

};

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

module Types {

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

}

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

dfx new send_http_get_motoko
cd send_http_get_motoko
npm install

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

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

src/send_http_get_motoko_backend/main.mo
import Debug "mo:base/Debug";
import Blob "mo:base/Blob";
import Cycles "mo:base/ExperimentalCycles";
import Error "mo:base/Error";
import Array "mo:base/Array";
import Nat8 "mo:base/Nat8";
import Nat64 "mo:base/Nat64";
import Text "mo:base/Text";

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


//Actor
actor {

//This method sends a GET request to a URL with a free API you can test.
//This method returns Coinbase data on the exchange rate between USD and ICP
//for a certain day.
//The API response looks like this:
//  [
//     [
//         1682978460, <-- start timestamp
//         5.714, <-- lowest price during time range
//         5.718, <-- highest price during range
//         5.714, <-- price at open
//         5.714, <-- price at close
//         243.5678 <-- volume of ICP traded
//     ],
// ]

  public func get_icp_usd_exchange() : 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
    let ONE_MINUTE : Nat64 = 60;
    let start_timestamp : Types.Timestamp = 1682978460; //May 1, 2023 22:01:00 GMT
    let end_timestamp : Types.Timestamp = 1682978520;//May 1, 2023 22:02:00 GMT
    let host : Text = "api.pro.coinbase.com";
    let url = "https://" # host # "/products/ICP-USD/candles?start=" # Nat64.toText(start_timestamp) # "&end=" # Nat64.toText(start_timestamp) # "&granularity=" # Nat64.toText(ONE_MINUTE);

    // 2.2 prepare headers for the system http_request call
    let request_headers = [
        { name = "Host"; value = host # ":443" },
        { name = "User-Agent"; value = "exchange_rate_canister" },
    ];

    // 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;
        body = null; //optional for request
        method = #get;
        transform = ?transform_context;
    };

    //3. ADD CYCLES TO PAY FOR HTTP REQUEST

    //The IC specification spec says, "Cycles to pay for the call must be explicitly transferred with the call"
    //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
    //"Function add(amount) indicates the additional amount of cycles to be transferred in the next remote call"
    //See: /docs/current/references/ic-interface-spec#ic-http_request
    Cycles.add(20_949_972_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 `src/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 [Nat8] array that is the body into readable text.
    //To do this, you:
    //  1. Convert the [Nat8] into a Blob
    //  2. Use Blob.decodeUtf8() method to convert the Blob to a ?Text optional
    //  3. You use a switch to explicitly call out both cases of decoding the Blob into ?Text
    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
    //The API response will looks like this:

    // ("[[1682978460,5.714,5.718,5.714,5.714,243.5678]]")

    //Which can be formatted as this
    //  [
    //     [
    //         1682978460, <-- start/timestamp
    //         5.714, <-- low
    //         5.718, <-- high
    //         5.714, <-- open
    //         5.714, <-- close
    //         243.5678 <-- volume
    //     ],
    // ]
    decoded_text
  };

  //7. CREATE TRANSFORM FUNCTION
  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;
  };
};
  • get_icp_usd_exchange() is an update call. All methods that make HTTPS outcalls must be update calls because they go through consensus, even if the HTTPS outcall is a GET.
  • The code above adds 20_949_972_000 cycles. This is typically enough for GET requests, but this may need to change depending on your use case.
  • The code above imports Types.mo to separate the custom types from the actor file (as a best practice).

Headers in the response may not always be identical across all nodes that process the request for consensus, causing the result of the call to be "No consensus could be reached." This particular error message can be hard to debug, but one method to resolve this error is to edit the response using the transform function. The transform function is run before consensus, and can be used to remove some headers from the response. For example, the following Rust variation removes all headers aside from the body and status code of the call:

src/send_http_get_rust_backend/src/lib.rs
use ic_cdk::{
    api::management_canister::http_request::{HttpResponse, TransformArgs},
    query,
};

#[query]
fn transform(raw: TransformArgs) -> HttpResponse {
    let mut res = HttpResponse {
        status: raw.response.status.clone(),
        body: raw.response.body.clone(),
        ..Default::default()
    };

    if i32::try_from(res.status.clone().0).unwrap() == 200 {
        res.body = raw.response.body;
    } else {
        ic_cdk::api::print(format!("Received an error from proxy: err = {:?}", raw));
    }

    res
}

  • Step 3: Edit the Type or Candid files.​

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

src/send_http_get_motoko_backend/Types.mo
module Types {

  public type Timestamp = Nat64;

  //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, you 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 These types 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 you use 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:
  Frontend canister via browser
    send_http_get_motoko_frontend: http://127.0.0.1:4943/?canisterId=asrmz-lmaaa-aaaaa-qaaeq-cai
  Backend canister via Candid interface:
    send_http_get_motoko_backend: http://127.0.0.1:4943/?canisterId=a3shf-5eaaa-aaaaa-qaafa-cai&id=avqkn-guaaa-aaaaa-qaaea-cai

Open the Candid web UI for the backend (the send_http_get_motoko_backend one) and call the get_icp_usd_exchange() method:

Candid web UI

  • Step 5: Test the dapp on mainnet.​

Deploy the dapp locally:

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_get_motoko_frontend: https://ff5va-7qaaa-aaaap-qbona-cai.ic0.app/
  Backend canister via Candid interface:
    send_http_get_motoko_backend: https://a4gq6-oaaaa-aaaab-qaa4q-cai.raw.ic0.app/?id=fm664-jyaaa-aaaap-qbomq-cai

Additional resources​