ERC-721 Transfers

This guide is part of a series of tutorials on how you can export transfer data of different ERC standards into your datawarehouse. Here we will be focusing on ERC-721 Transfers, visit the following two other guides for other types of Transfers:

• ERC-20 Transfers
• ERC-1155 Transfers

In other guides we focus on the ELT use-case; here we will cover more advanced topics in transforming our decoded datasets and some of the functionality of the Flink SQL backend.

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What you’ll need

1. A basic understanding of the Mirror product’s more basic ETL use case.
2. A basic understanding of SQL, though we use the syntax and functionality of Flink v1.17.
3. A destination sink to write your data to.

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Preface

In this example we’ll provide a YAML file that includes the transforms needed to sync token transfers to your database.

We need to differentate ERC-721 (NFT) transfers from ERC-20 token transfers since they have the same event signature in decoded data: Transfer(address,address,uint256)

In this example, we have included a number of columns that you may or may not need, the main columns needed for most purposes are: id, address (if you are syncing multiple contract addresses), sender, recipient, token_id, and value.

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Pipeline YAML

There is one transform in this configuration and we’ll go through each one to explain how they work. If you copy and use this configuration file, make sure to update:

1. Your secretName. If you already created a secret, you can find it via the CLI command goldsky secret list.
2. The schema and table you want the data written to, by default it writes to mirror.transfers.
3. Optionally, the contract address or addresses you want to sync.

sources:
  - referenceName: ethereum.decoded_logs
    version: 1.0.0
    type: dataset
    startAt: earliest
    description: Decoded logs for events emitted from contracts. Contains the
      decoded event signature and event parameters, contract address, data,
      topics, and metadata for the block and transaction.
transforms:
  - type: sql
    referenceName: ethereum_721_transfers
    primaryKey: id
    description: ERC721 Transfers
    sql: >-
      SELECT
              lower(address) AS contract_address,
              lower(event_params[1]) AS sender,
              lower(event_params[2]) AS recipient,
              COALESCE(TRY_CAST(event_params[3] AS NUMERIC), -999) AS token_id,
              raw_log.block_number       AS block_number,
              raw_log.block_hash         AS block_hash,
              raw_log.log_index          AS log_index,
              raw_log.transaction_hash   AS transaction_hash,
              raw_log.transaction_index  AS transaction_index,
              id
              FROM ethereum.decoded_logs WHERE raw_log.topics LIKE '0xddf252ad1be2c89b69c2b068fc378daa952ba7f163c4a11628f55a4df523b3ef%'
              AND SPLIT_INDEX(raw_log.topics, ',', 3) IS NOT NULL
sinks:
  - type: postgres
    table: ecr721_transfers
    schema: mirror
    secretName: <YOUR SECRET>
    description: Postgres sink for 721 NFT transfers
    referenceName: ethereum_721_sink
    sourceStreamName: ethereum_721_transfers

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ERC-721 (NFT) Transfers

The main thing we need to do here is to make sure we’re pulling the sender, recipient, and token_id from the event_params array, and only getting NFT transfers rather than other transfers that may share the same event signature. This isn’t usually a problem when filtering for a specific contract address, but can become one when looking at all contract addresses, or contracts that may implement multiple types of tokens.

We’ll start at the top.

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Contract Address

SELECT lower(address) AS contract_address,

We use the lower function here to lower-case the address to make using this data simpler downstream, we also rename the column to contract_address to make it more explicit.

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Sender

lower(event_params[1]) AS sender,

Here we continue to lower-case values for consistency. In this case we’re using the first element of the event_params array (using a 1-based index), and renaming it to sender. Each event parameter maps to an argument to the event_signature, for ERC-721, they are the sender, recipient, and token_id respectively.

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Recipient

lower(event_params[2]) AS recipient,

Like the previous column, we’re pulling the second element in the event_params array and renaming it to recipient.

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Token ID

COALESCE(TRY_CAST(event_params[3] AS NUMERIC), -999) AS token_id,

Here we introduce a few more SQL functions, we’ll start from the inside and work our way out.

1. event_params[3] is the third element of the event_params array, and for ERC-721 this is the token ID. Although not covered in this example, since ERC-20 shares the same signature, this element represents a token balance rather than token ID if you’re decoding ERC-20 transfers.
2. TRY_CAST(event_params[3] AS NUMERIC) is casting the string element event_params[3] to NUMERIC - token IDs can be as large as an unsigned 256 bit integer, so make sure your database can handle that, if not, you can cast it to a different data type that your sink can handle. We use TRY_CAST because it will prevent the pipeline from failing in case the cast fails returning a NULL value instead.
3. COALESCE(TRY_CAST(event_params[3] AS NUMERIC), -999): COALESCE can take an arbitrary number of arguments and returns the first non-NULL value. Since TRY_CAST can return a NULL we’re returning -999 in case it does. This isn’t strictly necessary but is useful to do in case you want to find offending values that were unable to be cast.

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Block Metadata

raw_log.block_number       AS block_number,
raw_log.block_hash         AS block_hash,
raw_log.log_index          AS log_index,
raw_log.transaction_hash   AS transaction_hash,
raw_log.transaction_index  AS transaction_index,

These columns aren’t necessarily needed for this example, but they’re included so you’re aware of them. A complete list of block metadata columns is available here. They are often useful for looking up the transfers in another tool, such as a block explorer like Etherscan.

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ID Primary Key

id

This is the Goldsky provided id, it is a string composed of the dataset name, block hash, and log index, which is unique per event, here’s an example: decoded_log_0x60eaf5a2ab37c73cf1f3bbd32fc17f2709953192b530d75aadc521111f476d6c_18.

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Topic Filter and Length Check

AND raw_log.topics LIKE '0xddf252ad1be2c89b69c2b068fc378daa952ba7f163c4a11628f55a4df523b3ef%'
AND SPLIT_INDEX(raw_log.topics, ',', 3) IS NOT NULL

As mentioned in the preface, ERC-721 transfers share the same event_signature as ERC-20 transfers. What differentiates ERC-721 transfers from ERC-20 transfers are the number of topics associated with the event. ERC-721 transfers have four topics, and ERC-20 transfers have three. If you want to get into the nitty gritty you may enjoy the Solidity developer documentation for events, but for now know that in Mirror, raw_log.topics is a comma separated string. Each value in the string is a hash. The first is the hash of the event_signature and event_params, in our case Transfer(address,address,uint256) for ERC-721, which is hashed to 0xddf252ad1be2c89b69c2b068fc378daa952ba7f163c4a11628f55a4df523b3ef as seen in our WHERE clause.

We use LIKE to only consider the first signature, with a % at the end, which acts as a wildcard.

We could also use a filter such as event_signature = 'Transfer(address,address,uint256)', but we wanted to introduce the idea of topics as they can be a useful filter for some older contracts that may not completely follow the specification for ERC-721 contracts.

SPLIT_INDEX splits the first argument by the second argument, and then extracts the 0-indexed argument, in this case 3 which would be the fourth element. Here’s an example topic string to consider:

0xddf252ad1be2c89b69c2b068fc378daa952ba7f163c4a11628f55a4df523b3ef,0x000000000000000000000000441e1e47a6fa2dbfd3cd9b54291e9ab3a58d7975,0x00000000000000000000000097d2e8eeb59e521f10c2e7716eac3dd805ea9a46,0x0000000000000000000000000000000000000000000000000000000000043321

We check that the fourth element after splitting is NOT NULL to make sure this is an NFT transfer. An ERC-20 transfer would only have three elements when the topics are split, so SPLIT_INDEX would return NULL.

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Deploying the pipeline

To deploy this pipeline and start sinking ERC-721 transfer data into your database simply execute:

goldsky pipeline create <pipeline_name> --definition-path <yaml_file>

Can't find what you're looking for? Reach out to us at support@goldsky.com for help.