Advanced types of calls
In addition to queries and updates, there are advanced types of calls that can be made to canisters.
Certified queries
Certified queries ensure the integrity of the data being retrieved. They use use certified variables to prove the authenticity of a piece of data that comes along with the query's response. Certified variables can be set via an update call and then read via a query call.
Replicated queries
Replicated queries, also referred to as the "query-as-update" execution model, are query calls executed as updates. The query still discards the state changes, but execution happens on all subnet nodes, and the results go through consensus.
Inter-canister calls
Inter-canister calls are used to make calls between different canisters. An inter-canister call involves two messages: a request from the caller to the callee along with a response that the callee will return to the caller.
One example of inter-canister calls is the parallel calls Motoko example.
Authorized calls
An authorized call on ICP refers to a call to a canister that can only be made by a principal (a user or another canister) that has the necessary permissions to perform the call. For example, only a controller of a canister can call the canister's install_code function.
Authorized calls that return a value of () indicate that the function was successful but may not take an argument or return a meaningful value. If an authorized call is unsuccessful, it will return an error.
Making calls with IDLs
On the protocol level, calls on ICP use blobs to describe arguments and results passed to and returned from canisters. It's typically easier to use an interface description language (IDL) to define a canister's interfaces that can be called by end users or other canisters.
While any IDL can be used for this purpose, most canisters on ICP use Candid, a specialized IDL that is developed for ICP.