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ICP Developer Docs

Onchain Calls

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Call functions on other canisters from your canister code. Start with the query vs update decision: queries are fast (~200ms), free, and run on a single node but aren’t replicated (caller must trust the node); updates go through consensus (~2s), cost cycles, and modify state reliably. Explain when to use each and the trust tradeoffs. Cover canister discovery (env vars, ic_env cookie), error handling (reject codes, trap handling), calling third-party canisters, and the pub/sub pattern. Show Rust (ic_cdk::call) and Motoko (async message sends) patterns for both query and update calls. Explain the 2MB response limit. Link to parallel-calls for composite queries (the advanced query pattern) and to certified-variables for making query responses verifiable.

  • Portal: building-apps/integrations/advanced-calls.mdx, advanced/using-third-party-canisters.mdx, developer-tools/cdks/rust/intercanister.mdx
  • icp-cli: concepts/canister-discovery.md
  • icskills: multi-canister
  • Examples: inter-canister-calls (Rust), pub-sub (Motoko), composite_query (both)
  • concepts/canisters — messaging model
  • guides/canister-calls/candid — interface definitions
  • guides/canister-calls/parallel-calls — concurrent calls and composite queries
  • guides/backends/certified-variables — making query responses verifiable
  • guides/security/inter-canister-calls — async safety