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EvmBatchProcessor follows the common squid processor architecture, in which data processing happens within the batch handler, a function repeatedly called on batches of on-chain data. The function takes a single argument called “batch context”. Its structure follows the common batch context layout, with ctx.blocks being an array of BlockData objects containing the data to be processed, aligned at the block level. For EvmBatchProcessor the BlockData interface is defined as follows: 
export type BlockData<F extends FieldSelection = {}> = {
  header: BlockHeader<F>
  transactions: Transaction<F>[]
  logs: Log<F>[]
  traces: Trace<F>[]
  stateDiffs: StateDiff<F>[]
}
F here is the type of the argument of the setFields() processor method. BlockData.header contains the block header data. The rest of the fields are iterables containing four kinds of blockchain data. The canonical ordering within each iterable depends on the data kind:
  • transactions and logs are ordered in the same way as they are within blocks;
  • stateDiffs follow the order of transactions that gave rise to them;
  • traces are ordered in a deterministic but otherwise unspecified way.
The exact fields available in each data item type are inferred from the setFields() call argument. They are documented on the field selection page:

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Example

The handler below simply outputs all the log items emitted by the contract 0x2E645469f354BB4F5c8a05B3b30A929361cf77eC in real time: 
import { TypeormDatabase } from '@subsquid/typeorm-store'
import { EvmBatchProcessor } from '@subsquid/evm-processor'

const CONTRACT_ADDRESS = '0x2E645469f354BB4F5c8a05B3b30A929361cf77eC'.toLowerCase()

const processor = new EvmBatchProcessor()
  .setGateway('https://v2.archive.subsquid.io/network/ethereum-mainnet')
  .setRpcEndpoint('<my_eth_rpc_url>')
  .setFinalityConfirmation(75)
  .setBlockRange({ from: 17000000 })
  .addLog({
    address: [CONTRACT_ADDRESS]
  })
  .setFields({ // could be omitted: this call does not change the defaults
    log: {
      topics: true,
      data: true
    }
  })

processor.run(new TypeormDatabase(), async (ctx) => {
  for (let c of ctx.blocks) {
    for (let log of c.logs) {
      if (log.address === CONTRACT_ADDRESS) {
        ctx.log.info(log, `Log:`)
      }
    }
  }
})
One can experiment with the setFields() argument and see how the output changes. For more elaborate examples, check EVM Examples.