SetHook

The SetHook transaction allows users to install, update, delete, or perform other operations on hooks in Xahau.

[Source]

(Added by the [Hooks amendment][].)

Example

{
    "TransactionType": "SetHook",
    "Account": "rWYkbWkCeg8dP6rXALnjgZSjjLyih5NXm",
    "Flags": 0,
    "Hooks": [
        {
            "Hook": {
                "HookHash": "610F33B8EBF7EC795F822A454FB852156AEFE50BE0CB8326338A81CD74801864",
                "CreateCode": "697066733A2F2F4445414442454546697066733A2F2F44454144424545467878",
                "HookGrants": [],
                "HookNamespace": "0000000000000000000000000000000000000000000000000000000000000000",
                "HookParameters": [],
                "HookOn": "0000000000000000000000000000000000000000000000000000000000000000",
                "HookApiVersion": 0,
                "Flags": 0
            }
        }
    ]
}

Field	JSON Type	[Internal Type][]	Description
Account	String	AccountID	The address of the account that will own the hook.
Hooks	Array	Array	The array of hooks to be set.

Special Transaction Cost

The SetHook transaction has a standard transaction cost, which is the minimum transaction cost required for all transactions.

Error Cases

Besides errors that can occur for all transactions, SetHook transactions can result in the following transaction result codes:

Error Code	Description
tecDUPLICATE	Occurs if a hook with the same hash already exists.
tecDIR_FULL	Occurs if the owner's directory is full and cannot accommodate the new hook.
terNO_ACCOUNT	Occurs if the sending account does not exist.
terNO_HOOK	Occurs if no hook exists with the specified hash.
temDISABLED	Occurs if the Hooks Amendment is not enabled.
temMALFORMED	Occurs if the transaction is malformed.

SetHook Operations

There are six possible operations: No Operation, Create, Update, Delete, Install and Namespace Delete

Each operation is specified by the inclusion or omission of certain HookSet Object fields. This might seem confusing at first but by working through a few examples the reader should find it intuitive; Essentially HookSet operations are a type of diff between a specific Hook's defaults, existing and newly specified fields.

Achieving each type of operation is explained in a subsection below.

No Operation

Occurs when:

• The HookSet Object is empty

Behaviour:

• No change of any kind is made.

Example:

JSON

{ 
    Account: "r4GDFMLGJUKMjNhhycgt2d5LXCdXzCYPoc",
    TransactionType: "SetHook",
    Fee: "2000000",
    Hooks:
    [        
        {                        
            Hook: {}
        }
    ]
}

Create Operation

Occurs when:

All of the following conditions are met:

• The Corresponding Hook does not exist orFLAG_OVERRIDE is specified.

• CreateCode field is specified and is not blank and contains the valid web assembly bytecode for a valid Hook.

• No instance of the same web assembly bytecode already exists on the XRPL. (If it does and all other requirements are met then interpret as an Install Operation — see below.)

Behaviour:

• A reference counted HookDefinition object is created on the XRPL containing the fields in the HookSet Object, with all specified fields (Namespace, Parameters, HookOn) becoming defaults (but not Grants.)

• A Hooks array is created on the executing account, if it doesn't already exist. (This is the structure that contains the Corresponding Hooks.)

• A Hook object is created at the Corresponding Hook position if one does not already exist.

• The Hook object points at the HookDefinition.

• The Hook object contains no fields except HookHash which points at the created HookDefinition.

• If hsfNSDELETE flag is specified then any HookState entires in the destination namespace are deleted if they currently exist.

Example:

JSON

{ 
    Account: "r4GDFMLGJUKMjNhhycgt2d5LXCdXzCYPoc",
    TransactionType: "SetHook",
    Fee: "2000000",
    Hooks:
    [        
        {                        
            Hook: {                
                CreateCode: fs.readFileSync('accept.wasm').toString('hex').toUpperCase(),
                HookOn: '0000000000000000',
                HookNamespace: addr.codec.sha256('accept').toString('hex').toUpperCase(),
                HookApiVersion: 0
            }
        }
    ]
}

Install Operation

Occurs when:

All of the following conditions are met:

• The Corresponding Hook does not exist orFLAG_OVERRIDE is specified.

• HookHash field is specified and is not blank and contains the hash of a Hook that already exists as a HookDefinition on the ledger or CreateCode field is specified and is not blank and contains the valid web assembly bytecode for a valid hook that already exists on the ledger as a HookDefinition.

Behaviour:

• The reference count of the HookDefinition object is incremented.

• A Hooks array is created on the executing account, if it doesn't already exist. (This is the structure that contains the Corresponding Hooks.)

• A Hook object is created at the Corresponding Hook position if one does not already exist.

• The Hook object points at the HookDefinition.

• The Hook object contains all the fields in the HookSet Object, except and unless:

• A field or key-pair within a field is identical to the Hook Defaults set on the HookDefinition, in which case it is omitted due to defaults.

• If hsfNSDELETE flag is specified then any HookState entires in the destination namespace are deleted if they currently exist.

Example:

JSON

{ 
    Account: "r4GDFMLGJUKMjNhhycgt2d5LXCdXzCYPoc",
    TransactionType: "SetHook",
    Fee: "2000000",
    Hooks:
    [        
        {                        
            Hook: {                
                HookHash: "A5663784D04ED1B4408C6B97193464D27C9C3334AAF8BBB4FA5EB8E557FC4A2C",
                HookOn: '0000000000000000',
                HookNamespace: addr.codec.sha256('accept').toString('hex').toUpperCase(),
            }
        }
    ]
}

Update Operation

Occurs when:

All of the following conditions are met:

• The Corresponding Hook exists.

• HookHash is absent.

• CreateCode is absent.

• One or more of HookNamespace, HookParameters or HookGrants is present.

General Behaviour:

• The Corresponding Hook is updated in such a way that the desired changes are reflected in the Corresponding Hook.

Specific Behaviour:

If HookNamespace is specified and differs from the Corresponding Hook's Namespace:

• the Corresponding Hook's HookNamespace is updated, and

• if the hsfNSDELETE flag is specified all HookState entires in the old namespace are deleted.

If HookParameters is specified, then for each entry:

• If HookParameterName exists but HookParameterValue is absent and the Corresponding Hook's Parameters (either specifically or via defaults) contains this HookParameterName then the parameter is marked as deleted on the Corresponding Hook.

• If HookParameterName exists and HookParameterValue exists then the Corresponding Hook's Parameters are modified to include the new or updated parameter.

If HookGrants is specified then:

• The Corresponding Hook's HookGrants array is replaced with the array.

Example:

JSON

{ 
    Account: "r4GDFMLGJUKMjNhhycgt2d5LXCdXzCYPoc",
    TransactionType: "SetHook",
    Fee: "2000000",
    Hooks:
    [        
        {                        
            Hook: {
                HookNamespace: addr.codec.sha256('new_accept').toString('hex').toUpperCase(),
            }
        }
    ]
}

Delete Operation

Occurs when:

All of the following conditions are met:

• The Corresponding Hook exists.

• hsfOVERRIDE is specified.

• optionally hsfNSDELETE is also specified.

• HookHash is absent.

• CreateCode is present but empty.

Behaviour:

• The reference count of the HookDefinition object is decremented.

• If the reference count is now zero the HookDefintion is removed from the ledger.

• The Hook object in the Corresponding Hook position is deleted, leaving an empty position.

• If hsfNSDELETE is specified the namespace and all HookState entries are also deleted.

Example:

JSON

{ 
    Account: "r4GDFMLGJUKMjNhhycgt2d5LXCdXzCYPoc",
    TransactionType: "SetHook",
    Fee: "2000000",
    Hooks:
    [        
        {                        
            Hook: {
                CreateCode: "",
                Flags: 1,
            }
        }
    ]
}

Namespace Reset

Occurs when:

All of the following conditions are met:

• flags is present and hsfNSDELETE is set. hsfOVERRIDE can optionally also be specified if the Hook at this position is to be deleted.

• HookNamespace is specified.

• CreateCode is absent.

• HookHash is absent.

• HookGrants, HookParameters, HookOn and HookApiVersion are absent.

Behaviour:

• If the Corresponding Hook exists, it remains, nothing happens to it.

• A subset of HookState objects and the HookState directory for the specified namespace are removed from the ledger, up to the defined limit (512). Further transactions are needed to continue the deletion process until all relevant records are removed. See tesPARTIAL.

Example:

JSON

{ 
    Account: "r4GDFMLGJUKMjNhhycgt2d5LXCdXzCYPoc",
    TransactionType: "SetHook",
    Fee: "2000000",
    Hooks:
    [        
        {                        
            Hook: {
              	HookNamespace: addr.codec.sha256('accept').toString('hex').toUpperCase(),
                Flags: 3,
            }
        }
    ]
}

Hook Fields

The following fields are used in the hook object:

Field	JSON Type	Internal Type	Description
HookHash	String	Hash256	The hash of the hook.
CreateCode	String	Blob	The WebAssembly code for the hook.
HookGrants	Array	Array	The grants associated with the hook.
HookNamespace	String	Hash256	The namespace of the hook.
HookParameters	Array	Array	The parameters of the hook.
HookOn	String	Hash256	The event on which the hook is triggered.
HookApiVersion	Number	UInt16	The API version of the hook.
Flags	Number	UInt32	Additional flags for the hook.

Flags

The Flags field in the hook object specifies additional flags for the hook. The following flags are supported:

Flag Name	Description
hsfOVERRIDE	Allows the hook to be deleted even if it is referenced by other objects.
hsfNSDELETE	Deletes an entire namespace of hooks.
hsfCOLLECT	Collects the hook's associated objects.

Hook Grants

The HookGrants field is an array of objects that specify the grants associated with the hook. Each grant object has the following fields:

Field	JSON Type	Internal Type	Description
HookHash	String	Hash256	The hook to apply the grant to.
Authorize	String	AccountID	The address of the account that is granted access.
Flags	Number	Uint32	Flags

Hook Parameters

The HookParameters field is an array of objects that specify the parameters of the hook. Each parameter object has the following fields:

Field	JSON Type	Internal Type	Description
HookParameterName	String	Blob	The name of the parameter.
HookParameterValue	String	Blob	The value of the parameter.

Hook Executions

When Hooks execute they leave behind information about the status of that execution. This appears in the Originating Transaction metadata as an sfHookExecutions block. This block contains the following fields:

Field	JSON Type	Internal Type	Description
HookAccount	String	AccountID	The account the Hook ran on.
HookEmitCount	Number	UInt16	The total number of Emitted Transactions produced by the Hook.
HookExecutionIndex	Number	UInt16	The SHA512H of the Hook at the time it was executed.
HookHash	String	Hash256	The value of the parameter.
HookInstructionCount	String	UInt64	The total number of webassembly instructions that were executed when the Hook ran.
HookResult	Number	UInt8	Hooks can end in three ways: accept, rollback and error. This is not the same as sfHookReturnCode!
HookReturnCode	String	UInt64	The integer returned as the third parameter of accept or rollback.
HookReturnString	String	Blob	The string returned in the first two parameters of accept or rollback, if any.
HookStateChangeCount	Number	UInt16	The number of Hook State changes the Hook made during execution.