Specification

The Transfer Hook interface specification includes two optional instructions and one required one.

Each instruction of the Transfer Hook interface uses a specific 8-byte discriminator at the start of its instruction data.

Instruction: Execute

The Execute instruction is required by any program who wishes to implement the interface, and this is the instruction in which custom transfer functionality will live.

• Discriminator: First 8 bytes of the hash of the string literal
  "spl-transfer-hook-interface:execute"
• Data:
  • amount: u64 - The transfer amount
• Accounts:
  • 1 []: Source token account
  • 2 []: Mint
  • 3 []: Destination token account
  • 4 []: Source token account authority
  • 5 []: Validation account
  • n number of additional accounts, written into the validation account

The validation account is a key piece of the Transfer Hook interface, and is covered in more detail in the next section. In short, it's an account whose data stores configurations that can be deserialized to determine which additional accounts are required by the transfer hook program.

The next two instructions of the interface deal with these configurations.

(Optional) Instruction: InitializeExtraAccountMetaList

This instruction does exactly what the name implies: it initializes the validation account to store a list of extra required AccountMeta configurations for the Execute instruction.

• Discriminator: First 8 bytes of the hash of the string literal
  "spl-transfer-hook-interface:initialize-extra-account-metas"
• Data:
  • extra_account_metas: Vec<ExtraAccountMeta> - A list of extra account configurations to be written into the validation account
• Accounts:
  • 1 [writable]: Validation account
  • 2 []: Mint
  • 3 [signer]: Mint authority
  • 4 []: System program

(Optional) Instruction: UpdateExtraAccountMetaList

The UpdateExtraAccountMetaList instruction allows an on-chain program to update its list of required accounts for Execute. By implementing this instruction, developers can make updates to their list of required extra accounts stored in the validation account.

• Discriminator: First 8 bytes of the hash of the string literal "spl-transfer-hook-interface:update-extra-account-metas"
• Data:
  • extra_account_metas: Vec<ExtraAccountMeta> - A list of extra account configurations to be written into the validation account
• Accounts:
  • 1 [writable]: Validation account
  • 2 []: Mint
  • 3 [signer]: Mint authority