Slots and Keylets

Inspect and manipulate on-ledger objects.

Background

The XRP Ledger is contains numerous heterogenous object types which a Hook has read-access to. For example: transactions, accounts, ledgers, and the subcomponents of each of these, to name just a few.

It is very easy to carelessly program a computer to do a lot of needless copy operations when disciplined access to the same underlying data (i.e. through a view) would suffice. The deliberate avoidance of copy operations in programming is referred to as Zero copy in programming.

With Hooks the same principle applies. We want to avoid copying where possible. In particular we want to avoid as much as possible needlessly copying large objects such as whole ledgers, we also want to avoid serializaing and unserializing these where possible.

What are slots?

Slots are part of the Hook API and provide a zero-copy heterogenous access system for on-ledger objects and transactions.

  • Each Hook has access to 255 slots during runtime.
  • Each slot may be empty or may contain a slotted object.
  • The slot API allows traversal into inner objects, and allows these inner objects themselves to be slotted.
  • The slot API allows slotted objects to be dumped to a buffer or otherwise read by the Hook.

The avilable slot APIs are:

Hook API

What it does

slot

Serialize and output a slotted object

slot_clear

Free up a currently occupied slot

slot_count

Count the elements of an array object in a slot

slot_id

Compute the canonical hash of the slotted object and return it

slot_set

Locate an object based on its keylet and place it into a slot

slot_subarray

Index into a slotted array and assign a sub-object to another slot

slot_subfield

Index into a slotted object and assign a sub-object to another slot

slot_type

Retrieve the field code of an object in a slot and, optionally, some other information

slot_float

Parse the STI_AMOUNT in the specified slot and return it as an XFL enclosed number

slot_size

Compute the serialized size of an object in a slot

What are keylets?

Keylets are used to locate (point to) on-ledger objects. In brief they are a hash of identifying information from the object, which is the canonical handle for that object.

Hooks use a serialized 34 byte keylet format which can be derrived using the important util_keylet function. Without this looking up and slotting objects would be generally impossible.

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Tip

The Hook APIs which accept a 34 byte keylet will also generally accept a 32 byte canonical transaction hash.

Example

In the following example a 34 byte keylet for a signers object is used to slot that object.

uint8_t keylet[34];
if (util_keylet(SBUF(keylet), KEYLET_SIGNERS, SBUF(hook_accid), 0, 0, 0, 0) != 34)
    rollback(SBUF("Notary: Internal error, could not generate keylet"), 10);

// then requesting XRPLD slot that keylet into a new slot for us
int64_t slot_no = slot_set(SBUF(keylet), 0);
if (slot_no < 0)
   rollback(SBUF("Notary: Could not set keylet in slot"), 10);