Composite queries

Advanced

Tutorial

The Internet Computer Protocol supports two types of messages: updates and queries. An update message is executed on all nodes and persists canister state changes. A query message discards state changes and typically executes on a single node. It is possible to execute a query message as an update. In such a case, the query still discards the state changes, but the execution happens on all nodes and the result of execution goes through consensus. This “query-as-update” execution mode is also known as replicated query.

An update can call other updates and queries. However a query cannot make any calls, which can hinder development of scalable decentralized applications, especially those that shard data across multiple canisters.

Composite queries solve this problem. You can add composite queries to your canister using the following annotations:

• Candid: composite_query
• Motoko: composite query
• Rust: #[query(composite = true)]

Users and the client-side JavaScript code can invoke a composite query endpoint of a canister using the same query URL for existing regular queries. In contrast to regular queries, a composite query can call other composite and regular queries. Due to limitations of the current implementation, composite queries have two restrictions:

Query	Update	Composite query
Cannot call other queries or composite queries	Can call other updates and queries ; Cannot call composite queries	Can call other queries and composite queries
Can be called as an update	Cannot be called as a query	Cannot be called as an update
Can call canisters on another subnet	Can call canisters on another subnet	Cannot call canisters on another subnet

Composite queries were enabled in the following releases:

Platform / Language	Version
Internet computer mainnet	Release 7742d96ddd30aa6b607c9d2d4093a7b714f5b25b
Candid	2023-06-30 (Rust 0.9.0)
Motoko	0.9.4, revision: 2d9902f
Rust	0.6.8

Sample code

As an example, consider a partitioned key-value store, where a single frontend does the following for a put and get call:

• First, determines the ID of the data partition canister that holds the value with the given key.
• Then, makes a call into the get or put function of that canister and parses the result.

Motoko

import Debug "mo:base/Debug";
import Array "mo:base/Array";
import Cycles "mo:base/ExperimentalCycles";
import Buckets "Buckets";

actor Map {

  let n = 4; // number of buckets

  // divide initial balance amongst self and buckets
  let cycleShare = Cycles.balance() / (n + 1);

  type Key = Nat;
  type Value = Text;

  type Bucket = Buckets.Bucket;

  let buckets : [var ?Bucket] = Array.init(n, null);

  public func getUpdate(k : Key) : async ?Value {
    switch (buckets[k % n]) {
      case null null;
      case (?bucket) await bucket.get(k);
    };
  };

  public composite query func get(k : Key) : async ?Value {
    switch (buckets[k % n]) {
      case null null;
      case (?bucket) await bucket.get(k);
    };
  };

  public func put(k : Key, v : Value) : async () {
    let i = k % n;
    let bucket = switch (buckets[i]) {
      case null {
        // provision next send, i.e. Bucket(n, i), with cycles
        Cycles.add(cycleShare);
        let b = await Buckets.Bucket(n, i); // dynamically install a new Bucket
        buckets[i] := ?b;
        b;
      };
      case (?bucket) bucket;
    };
    await bucket.put(k, v);
  };

  public func test() : async () {
    var i = 0;
    while (i < 16) {
      let t = debug_show(i);
      assert (null == (await getUpdate(i)));
      Debug.print("putting: " # debug_show(i, t));
      await Map.put(i, t);
      assert (?t == (await getUpdate(i)));
      i += 1;
    };
  };

};

Rust

use ic_cdk::api::call::{call};
use ic_cdk::api::management_canister::main::{CreateCanisterArgument, create_canister, InstallCodeArgument, install_code, CanisterInstallMode};
use ic_cdk::api::management_canister::provisional::CanisterSettings;
use ic_cdk_macros::{query, update};
use candid::Principal;

use std::sync::Arc;
use std::sync::RwLock;

const NUM_PARTITIONS: usize = 5;

// Inline wasm binary of data partition canister
pub const WASM: &[u8] =
    include_bytes!("../../target/wasm32-unknown-unknown/release/data_partition.wasm");

thread_local! {
    // A list of canister IDs for data partitions
    static CANISTER_IDS: Arc<RwLock<Vec<Principal>>> = Arc::new(RwLock::new(vec![]));
}

#[update]
async fn put(key: u128, value: u128) -> Option<u128> {

    // Create partitions if they don't exist yet
    if CANISTER_IDS.with(|canister_ids| {
        let canister_ids = canister_ids.read().unwrap();
        canister_ids.len() == 0
    }) {
        for _ in 0..NUM_PARTITIONS {
            create_data_partition_canister_from_wasm().await;
        }
    }

    let canister_id = get_partition_for_key(key);
    ic_cdk::println!("Put in frontend for key={} .. using backend={}", key, canister_id.to_text());
    match call(canister_id, "put", (key, value), ).await {
        Ok(r) => {
            let (res,): (Option<u128>,) = r;
            res
        },
        Err(_) => None,
    }
}

#[query(composite = true)]
async fn get(key: u128) -> Option<u128> {
    let canister_id = get_partition_for_key(key);
    ic_cdk::println!("Get in frontend for key={} .. using backend={}", key, canister_id.to_text());
    match call(canister_id, "get", (key, ), ).await {
        Ok(r) => {
            let (res,): (Option<u128>,) = r;
            res
        },
        Err(_) => None,
    }
}

#[update]
async fn get_update(key: u128) -> Option<u128> {
    let canister_id = get_partition_for_key(key);
    ic_cdk::println!("Get as update in frontend for key={} .. using backend={}", key, canister_id.to_text());
    match call(canister_id, "get", (key, ), ).await {
        Ok(r) => {
            let (res,): (Option<u128>,) = r;
            res
        },
        Err(_) => None,
    }
}

fn get_partition_for_key(key: u128) -> Principal {
    let canister_id = CANISTER_IDS.with(|canister_ids| {
        let canister_ids = canister_ids.read().unwrap();
        canister_ids[lookup(key).0 as usize]
    });
    canister_id
}

#[query(composite = true)]
fn lookup(key: u128) -> (u128, String) {
    let r = key % NUM_PARTITIONS as u128;
    (r, CANISTER_IDS.with(|canister_ids| {
        let canister_ids = canister_ids.read().unwrap();
        canister_ids[r as usize].to_text()
    }))
}

async fn create_data_partition_canister_from_wasm() {
    let create_args = CreateCanisterArgument {
        settings: Some(CanisterSettings {
            controllers: Some(vec![ic_cdk::id()]),
            compute_allocation: Some(0.into()),
            memory_allocation: Some(0.into()),
            freezing_threshold: Some(0.into()),
        })
    };

    let canister_record = create_canister(create_args).await.unwrap();
    let canister_id = canister_record.0.canister_id;

    ic_cdk::println!("Created canister {}", canister_id);

    let install_args = InstallCodeArgument {
        mode: CanisterInstallMode::Install,
        canister_id,
        wasm_module: WASM.to_vec(),
        arg: vec![],
    };

    install_code(install_args).await.unwrap();

    CANISTER_IDS.with(|canister_ids| {
        let mut canister_ids = canister_ids.write().unwrap();
        canister_ids.push(canister_id);
    });
}

TypeScript

Learn more about Azle.

Python

Learn more about Kybra.

Resources

The following example canisters demonstrate how to use composite queries:

• Motoko example
• Rust example

Feedback and suggestions can be contributed on the forum here: https://forum.dfinity.org/t/proposal-composite-queries/15979