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Reading the Originating Transaction

Every Hook invocation is triggered by a transaction — the originating transaction, or “otxn” for short. Almost every hook needs to inspect it: who sent it, what type it is, how much it moves, what fields it carries. rshooks exposes this through the api::otxn module, re-exported by the prelude. This page covers dispatching on the transaction’s type, the family of functions for reading its fields, and reading its transaction ID. Hook and originating-transaction parameters (hook_param/otxn_param) are a related but separate mechanism, covered in Hook and Transaction Parameters.

Dispatching on transaction type

otxn_type() returns a TxType, a typed, exhaustive-by-construction enum decoded from the raw tt* code the Hook API returns:

use rshooks::prelude::*;

match otxn_type() {
    TxType::Payment => { /* ... */ }
    TxType::TrustSet => { /* ... */ }
    TxType::Invoke => { /* ... */ }
    other => {
        // `TxType::Unknown(code)` covers any `tt*` code this crate does not
        // (yet) know a name for — forward-compatible with new transaction
        // types without a hard compile error.
        let _ = other;
    }
}

Every known transaction type (ttPAYMENT, ttESCROW_CREATE, ttTRUST_SET, ttHOOK_SET, …) has its own variant; TxType::Unknown(u16) is the catch-all for a code this crate doesn’t model yet. TxType also gives back its raw code via .code() when you need it. Most hooks gate their logic on metadata!’s declared HookOn list already, so otxn_type is typically used for a final sanity check or to branch between a handful of expected types within one hook.

Reading a field: the typed path is the default

rshooks generates a table of typed field constants in the sfield module — one SField<T> per sfXxx code, each carrying the Rust type that field’s value decodes as. sfAccount is an SField<AccountId>, sfSequence an SField<u32>, sfAmount an SField<Amount>. otxn_field_typed reads a field using exactly that pairing — no turbofish, no separate decode step, and no way to accidentally decode the wrong field as the wrong type (the constant itself pins down the return type):

use rshooks::prelude::*;

let sender: Result<AccountId> = otxn_field_typed(sfAccount);
let sequence: Result<u32> = otxn_field_typed(sfSequence);

This is the default way to read any field the generated sfield table gives a value type to. Narrow integers (u8/u16/u32) go through the host’s as-int64 mode; u64 and every fixed-byte type (Hash, AccountId, CurrencyCode) read their exact wire bytes; Amount and Issue are classified by length and come back as AmountBytes/IssueData rather than a single scalar, since their wire encoding is one of two shapes:

use rshooks::prelude::*;

match otxn_field_typed(sfAmount) {
    Ok(AmountBytes::Native(drops)) => {
        // `drops.0` is the raw 8-byte big-endian wire encoding — see
        // "Decoding a raw field" below for why it's `from_be_bytes`, not
        // this crate's `FromBytes` trait.
        let _ = drops;
    }
    Ok(AmountBytes::Iou(_)) => { /* an IOU amount */ }
    Err(_) => { /* field missing or unreadable */ }
}

examples/03_hook-params uses exactly this pattern to reject any non-native Amount:

let drops = match otxn_field_typed(sfAmount) {
    Ok(AmountBytes::Native(n)) => u64::from_be_bytes(n.0) & !NATIVE_AMOUNT_FLAG_BITS,
    Ok(AmountBytes::Iou(_)) | Err(_) => rollback!(
        b"hook-params: unsupported (non-native) Amount",
        HookParamsError::UnsupportedAmount
    ),
};

(The top two bits of a serialized native amount are format flags, not part of the drops value, hence the mask.)

Not every field has a modeled value type — Blob, STObject, STArray, and a handful of others map to Opaque, which supports navigation but no single scalar value(). For those, or when you just want the raw wire bytes, reach for the two escape hatches below.

The raw escapes: otxn_field and otxn_field_exact

otxn_field reads a field into a caller-provided buffer and returns the number of bytes written — the least opinionated option, usable for any field regardless of whether this crate models a typed read for it:

use rshooks::prelude::*;

let mut buf = [0u8; 20];
let written = otxn_field(&mut buf, sfAccount)?;

otxn_field_exact is the fixed-length middle ground: it requires the field to be exactly T’s length (any FixedRead type — a rshooks::types newtype, or a raw [u8; N]), with T inferred from context rather than a turbofish:

use rshooks::prelude::*;

let sender: AccountId = otxn_field_exact(sfAccount)?;
let raw_sequence: [u8; 4] = otxn_field_exact(sfSequence)?;

There’s also otxn_field_u64, the as-int64 escape hatch for a field of at most 8 bytes with the top bit clear — the same convention otxn_field_typed’s narrow-integer impls use internally.

Decoding a raw field: from_be_bytes, not FromBytes

This is the one trap worth calling out explicitly. The bytes otxn_field/otxn_field_exact hand back are Xahau Binary — the protocol’s own big-endian wire format — never this crate’s little-endian FromBytes trait, which is the convention for hook-private data (state and parameter values this crate’s own typed layer wrote, covered in Hook State and Typed Data with Derives). A numeric protocol field read through a raw escape needs an explicit u64::from_be_bytes(...) at the call site — exactly what the hook-params example above does for sfAmount’s native drops value. otxn_field_typed already does this decoding itself for every field it models, so a field with a modeled type never needs the idiom at all.

The transaction ID

otxn_id writes the originating transaction’s hash into a caller buffer; otxn_id_buf is the fixed-size convenience twin that returns a Hash directly:

use rshooks::prelude::*;

let id: Hash = otxn_id_buf(0)?;

flags = 0 prefers the emit-failure transaction ID where applicable; other flag values pass through verbatim to the host.

Burden and generation

For an emitted transaction (one a hook itself created via emit, see Emitting Transactions), otxn_burden and otxn_generation report the emit chain’s burden and depth; for a normal, directly-submitted transaction they read back 1 and 0 respectively.

A worked example: the firewall pattern

examples/05_firewall is a compact illustration of the typed field path end to end — read the sender as an AccountId, compare it against a configured blocklist, and roll back on a match:

#[hook]
fn my_hook() -> i64 {
    let Ok(sender) = otxn_field_typed(sfAccount) else {
        rollback!(
            b"firewall: could not read otxn sender",
            FirewallError::CouldNotReadSender
        )
    };

    let Ok(blocked) = BlockedParam.get_value() else {
        accept!()
    };

    // Avoid `==`, which can compile to an unguarded loop.
    if buf_eq_20(&sender, &blocked) {
        rollback!(b"firewall: blocked account", FirewallError::BlockedAccount);
    }

    accept!()
}

otxn_field_typed(sfAccount) reads back an AccountId directly — no turbofish, no manual length check. Note the comparison: sender == blocked would compile to a byte-compare loop the Hook API’s guard checker would need a guard! for (see Guards and Loops); buf_eq_20 is loop-free by construction (every byte index is a source-level literal) and sidesteps the issue entirely. blocked itself comes from a Hook parameter — see Hook and Transaction Parameters for hook_parameter!.

Nested fields: slots

Everything on this page reads a top-level field of the originating transaction directly. A field nested inside an object or array (an entry in a Memos array, a field of a SignerListSet signer entry, …) needs slot-based access instead — load the transaction into a slot with otxn_slot and navigate from there. See Slots and Ledger Objects for the full slot API.