Miscellaneous Host functions
This page contains all the host API functions that don’t fit into other categories. It covers system time, randomness, and logging.
Get the system time
You can get the agent’s current system time with the sys_time host function, which takes no arguments and returns a result containing a Timestamp. Note: This function is only available to coordinator zomes.
use hdk::prelude::*;
fn get_time() -> ExternResult<Timestamp> {
sys_time()
}
Generate some random bytes
To generate some random bytes, use the random_bytes host function, It takes the number of bytes you want and returns a result containing the bytes, wrapped in a Bytes struct. Note: This function is only available to coordinator zomes.
use hdk::prelude::*;
fn roll_die(sides: u8) -> ExternResult<u8> {
match sides {
0 => Err(wasm_error!("Can't roll a die with zero sides")),
_ => Ok(random_bytes(1)?[0] % sides + 1),
}
}
fn reasonably_random_unique_id() -> ExternResult<Vec<u8>> {
Ok(random_bytes(32)?.into_vec())
}
`random_bytes` uses the host operating system's random number generator
Holochain just uses whatever random number generator the host operating system provides. The bytes are open in memory as they’re passed to the caller, and they’re not provably random or repeatable/seedable. Read the random_bytes Rust documentation to help you decide if this function is appropriate for your use case.
Log things in your zomes
You can emit log messages from your zomes using Rust’s tracing crate. The HDK includes a tracing subscriber that forwards all your tracing calls to the host. This subscriber is active in any public function marked with the #[hdk_extern] macro.
use hdi::prelude::*;
#[hdk_extern]
pub fn foo() -> ExternResult<()> {
trace!("I am a trace message");
debug!("I am a debug message");
info!("I am an info message");
warn!("I am a warning message");
error!("I am an error message");
}
To set the logging level appropriate to your needs, set the WASM_LOG environment variable just like you’d set the RUST_LOG environment variable. The host re-emits the messages to its own tracing subscriber (typically this means outputting them to stdout).
WASM_LOG=info hc-spin workdir/my-app.webhapp
When the foo zome function is called, you’ll see:
[hc-spin] | [hc sandbox]: 2025-03-28T21:17:59.566029Z INFO movies:dnas/movies/zomes/coordinator/movies/src/lib.rs:443 I am an info message
[hc-spin] | [hc sandbox]: 2025-03-28T21:17:59.566102Z WARN movies:dnas/movies/zomes/coordinator/movies/src/lib.rs:444 I am a warning message
[hc-spin] | [hc sandbox]: 2025-03-28T21:17:59.566155Z ERROR movies:dnas/movies/zomes/coordinator/movies/src/lib.rs:445 I am an error message
