How do I read the datum of an output?

Question

How do I get the datum of an output from cardano-graphql (or another service)? I have attached it using the --tx-out-datum-embed-file flag already when building the tx using cardano-cli.

Thanks!

Answer 1

If you are running a DbSync node you can retrieve the datum and redeemer:

redeemer

A table containing redeemers. A redeemer is provided for all items that are validated by a script.

• Primary Id: id

Column name	Type	Description
id	integer (64)
tx_id	integer (64)	The Tx table index that contains this redeemer.
unit_mem	word63type	The budget in Memory to run a script.
unit_steps	word63type	The budget in Cpu steps to run a script.
fee	lovelace	The budget in fees to run a script. The fees depend on the ExUnits and the current prices.
purpose	scriptpurposetype	What kind pf validation this redeemer is used for. It can be one of 'spend', 'mint', 'cert', 'reward'.
index	uinteger	The index of the redeemer pointer in the transaction.
script_hash	hash28type	The script hash this redeemer is used for.
datum_id	integer (64)

datum

A table containing Plutus Data available in the blockchain, found in redeemers or witnesses

• Primary Id: id

Column name	Type	Description
id	integer (64)
hash	hash32type	The Hash of the Plutus Data
tx_id	integer (64)	The Tx table index for the transaction where this script first became available.
value	jsonb	The actual data in json format

Taken from dbSync:

https://github.com/input-output-hk/cardano-db-sync

It is also possible by just inspecting the blockchain. Even tho only the Datum hash is attached to the eUTXO, the datum value should be in the witness set of the transaction that created such eUTXO, so we should be able to:

a) Get the datum hash from the eUTXO and then.

b) Get the transaction that created such eUTXO and use the hash to retrieve the actual datum data.

Otherwise, if the Datum were not present at all on the chain it would be impossible to re-validate the blockchain from scratch.