1-watch-single-output

Example 1 - Watching a single output

In this example, we want to check when a particular transaction output was spent on chain.

Let's consider the following output:

transaction-id: 60eb97c0b6f83c433dbecd371ff8e14b697b1b0c87377dd80421dddd59baeaae
output-index: 0

This output was created in slot 4725297, so to find out when it was spent, we will:

• Start synchronizing just before that point, so we can index this output and look at every block after.
• Match only this particular output, so we don't need to needlessly index more than necessary.

We'll thus consider the following point:

absolute slot number: 4725274
block header hash: 13c1f69913564b37c5c9241bba77eb5b9af63bb2d8d930fc4c36219383209c1c

Note

Figuring out which points to synchronize from can be a little tricky. It is possible to run Kupo just for the sake of indexing chain points; Kupo then provides a useful interface for querying ancestor points in a friendly fashion:

See: GET /checkpoints/{slot-no}

Put together, we can start our Kupo instance as follows:

kupo \
  --node-socket /path/to/cardano-node/node.socket \
  --node-config /path/to/cardano-node/config.json \
  --workdir example-1.db \
  --since "4725274.13c1f69913564b37c5c9241bba77eb5b9af63bb2d8d930fc4c36219383209c1c" \
  --match "0@60eb97c0b6f83c433dbecd371ff8e14b697b1b0c87377dd80421dddd59baeaae"

Note that, if you're running this example through demeter.run, you only need to report the --since and --match options; the rest is handled for you.

Warning

Do not specify (or enable in Demeter) --prune-utxo; since we want to know when the output is spent, we need Kupo to keep the spent output indexed and not remove it!

From there, we can poll the Kupo server's API for our output, until the field spent_at is no longer null.

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You can run the example using yarn via:

yarn example-1