You're seeing just the callback handle_begin, go back to DBConnection module for more information.
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handle_begin(opts, state)

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handle_begin(opts :: Keyword.t(), state :: any()) ::
  {:ok, result(), new_state :: any()}
  | {status(), new_state :: any()}
  | {:disconnect, Exception.t(), new_state :: any()}

Handle the beginning of a transaction.

Return {:ok, result, state} to continue, {status, state} to notify caller that the transaction can not begin due to the transaction status status, {:error, exception, state} (deprecated) to error without beginning the transaction, or {:disconnect, exception, state} to error and disconnect.

A callback implementation should only return status if it can determine the database's transaction status without side effect.

This callback is called in the client process.