389 lines
14 KiB
Markdown
389 lines
14 KiB
Markdown
# Watching Contract Events
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One approach VulcanizeDB takes to caching and indexing smart contracts is to watch contract events emitted in receipt logs.
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With a light synced vDB we can watch events by iterating over headers retrieved from the synced `headers` table and using these headers to
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fetch and verify relevant event logs from a full Ethereum node, keeping track of which headers we have checked for which events
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with our `checked_headers` table.
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## Assumptions
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This approach assumes you are running a vDB light sync which is itself run against a light Ethereum node,
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this approach also assumes there is a full node available.
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Looking forward, we will be building fetchers that enable sourcing data from IPFS instead of an ETH node.
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## Shared Code
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VulcanizeDB has shared code built out for building and plugging in event transformers
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### [Event Watcher (light sync)](https://github.com/vulcanize/maker-vulcanizedb/blob/staging/libraries/shared/watcher/event_watcher.go)
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The event watcher is responsible for continuously fetching and delegating chunks of logs and their associated header to the appropriate transformers.
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Using the `compose` or `composeAndExecute` command, event watchers can be loaded with plugin event transformers and execute over them.
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### [Event Transformer](https://github.com/vulcanize/maker-vulcanizedb/blob/staging/libraries/shared/transformer/event_transformer.go)
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The event transformer is responsible for converting event logs into more useful data objects and storing them in Postgres.
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The event transformer is composed of converter and repository interfaces and a config struct:
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```go
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type EventTransformer struct {
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Config transformer.EventTransformerConfig
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Converter Converter
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Repository Repository
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}
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```
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The event transformer executes over provided event logs at a given header.
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In this process, the converter unpacks these logs into entities and then converts these entities
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to their final db models. These models are then written to the Postgres db by the repository.
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```go
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func (transformer Transformer) Execute(logs []types.Log, header core.Header, recheckHeaders constants.TransformerExecution) error {
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transformerName := transformer.Config.TransformerName
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config := transformer.Config
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if len(logs) < 1 {
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err := transformer.Repository.MarkHeaderChecked(header.Id)
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if err != nil {
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log.Printf("Error marking header as checked in %v: %v", transformerName, err)
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return err
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}
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return nil
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}
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entities, err := transformer.Converter.ToEntities(config.ContractAbi, logs)
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if err != nil {
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log.Printf("Error converting logs to entities in %v: %v", transformerName, err)
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return err
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}
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models, err := transformer.Converter.ToModels(entities)
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if err != nil {
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log.Printf("Error converting entities to models in %v: %v", transformerName, err)
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return err
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}
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err = transformer.Repository.Create(header.Id, models)
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if err != nil {
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log.Printf("Error persisting %v record: %v", transformerName, err)
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return err
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}
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return nil
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}
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```
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## Custom Code
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In order to watch events at a smart contract, for those events the developer must create:
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1. Config - struct to hold configuration information (contract address, starting block, event name and signature).
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1. Entity - struct to unpack the event log into.
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1. Model - struct representing the final data model we want to write to Postgres.
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1. Converter - an interface which can unpack event logs into our entities and convert those entities to our models.
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1. Repository - an interface to write our models to Postgres.
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1. EventTransformerInitializer - a public variable which exports our configured transformer to be loaded as part of a plugin.
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1. DB migrations - migrations to generate the Postgres schema, tables, views, function, etc that are needed to store and interface with the transformed data models.
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The example event we will use looks like:
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```
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event ExampleEvent(bytes32 indexed arg1, address indexed arg2, bytes32 arg3, uint256 arg4, uint256 arg5);
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```
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### Config
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The config holds configuration variables for the event transformer, including a name for the transformer, the contract address
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it is working at, the contract's ABI, the topic (e.g. event signature; topic0) that it is filtering for, and starting
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and ending block numbers.
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```go
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type EventTransformerConfig struct {
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TransformerName string
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ContractAddresses []string
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ContractAbi string
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Topic string
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StartingBlockNumber int64
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EndingBlockNumber int64 // Set -1 for indefinite transformer
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}
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```
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### Entity
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Entity field names for event arguments need to be exported and match the argument's name and type. LogIndex,
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TransactionIndex, and the Raw log are retained in order to link the data to it's source for downstream validation.
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```go
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type ExampleEntity struct {
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Arg1 common.Hash
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Arg2 common.Address
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Arg3 common.Hash
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Arg4 *big.Int
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Arg5 *big.Int
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LogIndex uint
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TransactionIndex uint
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Raw types.Log
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}
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```
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### Model
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Model fields are not constrained by the event log structure.
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This allows us to rename our fields, decode or convert our log values into more useful types, and perform operations
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with or on the values before persisting the data to Postgres.
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```go
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type ExampleModel struct {
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EventHash string
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UserAddress string
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FractionSkimmed string
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Surplus string
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Deficit string
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FinalPosition string
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LogIndex uint
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TransactionIndex uint
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Raw types.Log
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}
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```
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### Converter
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The converter needs to satisfy the interface. One for unpacking logs into the custom defined entities, and
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another for converting those entities to their final db models.
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```go
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type Converter interface {
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ToEntities(contractAbi string, ethLog []types.Log) ([]interface{}, error)
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ToModels([]interface{}) ([]interface{}, error)
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}
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```
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For the example event, this might look like:
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```go
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type ExampleConverter struct{}
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func (ExampleConverter) ToEntities(contractAbi string, ethLogs []types.Log) ([]interface{}, error) {
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var entities []interface{}
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for _, ethLog := range ethLogs {
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entity := &ExampleEntity{}
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address := ethLog.Address
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abi, err := geth.ParseAbi(contractAbi)
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if err != nil {
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return nil, err
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}
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contract := bind.NewBoundContract(address, abi, nil, nil, nil)
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err = contract.UnpackLog(entity, "ExampleEvent", ethLog)
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if err != nil {
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return nil, err
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}
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entity.Raw = ethLog
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entity.LogIndex = ethLog.Index
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entity.TransactionIndex = ethLog.TxIndex
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entities = append(entities, *entity)
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}
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return entities, nil
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}
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func (converter ExampleConverter) ToModels(entities []interface{}) ([]interface{}, error) {
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var models []interface{}
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for _, entity := range entities {
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entity, ok := entity.(ExampleModel)
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if !ok {
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return nil, fmt.Errorf("entity of type %T, not %T", entity, ExampleModel{})
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}
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fractionSkimmed, err := hexutil.DecodeBig(entity.Arg3.Hex())
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if err != nil {
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reuturn nil, err
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}
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position := new(big.Int)
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position.Sub(entity.Arg4, entity.Arg5)
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finalPosition := new(big.Int)
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if preTaxPosition.Sign() < 0 {
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finalPosition = position
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} else {
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skim := new(big.Int)
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skim.Div(position, fractionSkimmed)
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finalPosition = position.Sub(position, skim)
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}
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rawLog, err := json.Marshal(entity.Raw)
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if err != nil {
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return nil, err
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}
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model := ExampleModel{
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EventHash: entity.Arg1.Hex(),
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UserAddress: entity.Arg2.Hex(),
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FractionSkimmed: fractionSkimmed.String(),
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Surplus: entity.Arg4.String(),
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Deficit: entity.Arg5.String(),
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FinalPosition: finalPosition,
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LogIndex: entity.LogIndex,
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TransactionIndex: entity.TransactionIndex,
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Raw: rawLog,
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}
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models = append(models, model)
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}
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return models, nil
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}
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```
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Notice that in this example we have a bytes32 argument in the event that needs to be decoded to an integer before it can be worked with
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to produce our hypothetical `FinalPosition` field. This is to highlight the fact that contracts can and sometimes do encode the
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data types we want to work with into raw bytes. Writing custom transformers with these converters allows us to account for this.
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### Repository
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The repository needs to satisfy the interface and use the `Create` method to write the model to Postgres.
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```go
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type Repository interface {
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Create(headerID int64, models []interface{}) error
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MarkHeaderChecked(headerID int64) error
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MissingHeaders(startingBlockNumber, endingBlockNumber int64) ([]core.Header, error)
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RecheckHeaders(startingBlockNumber, endingBlockNUmber int64) ([]core.Header, error)
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SetDB(db *postgres.DB)
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}
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```
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For the example event, this might look like:
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```go
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type ExampleRepository struct {
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db *postgres.DB
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}
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func (repository *ExampleRepository) SetDB(db *postgres.DB) {
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repository.db = db
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}
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func (repository ExampleRepository) Create(headerID int64, models []interface{}) error {
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tx, dBaseErr := repository.db.Beginx()
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if dBaseErr != nil {
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return dBaseErr
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}
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for _, model := range models {
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model, ok := model.(ExampleModel)
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if !ok {
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rollbackErr := tx.Rollback()
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if rollbackErr != nil {
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log.Error("failed to rollback ", rollbackErr)
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}
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return fmt.Errorf("model of type %T, not %T", model, ExampleModel{})
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}
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_, execErr := tx.Exec(
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`INSERT into example_schema.example_event (header_id, event_hash, user_address, fraction_skimmed, surplus, deficit, final_position, log_idx, tx_idx, raw_log)
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VALUES($1, $2, $3, $4, $5, $6, $7, $8, $9, $10)
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ON CONFLICT (header_id, tx_idx, log_idx) DO UPDATE SET event_hash = $2, user_address = $3, fraction_skimmed = $4, surplus = $5, deficit = $6, final_position = $7, raw_log = $10;`,
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headerID, model.EventHash, model.UserAddress, model.FractonSkimmed, model.Surplus, model.Deficit, model.FinalPosition, model.LogIndex, model.TransactionIndex, model.Raw,
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)
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if execErr != nil {
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rollbackErr := tx.Rollback()
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if rollbackErr != nil {
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log.Error("failed to rollback ", rollbackErr)
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}
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return execErr
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}
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}
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checkHeaderErr := repo.MarkHeaderCheckedInTransaction(headerID, tx, "example_event_checked")
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if checkHeaderErr != nil {
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rollbackErr := tx.Rollback()
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if rollbackErr != nil {
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log.Error("failed to rollback ", rollbackErr)
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}
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return checkHeaderErr
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}
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return tx.Commit()
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}
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func (repository ExampleRepository) MarkHeaderChecked(headerID int64) error {
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return repo.MarkHeaderChecked(headerID, repository.db, "example_event_checked")
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}
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func (repository ExampleRepository) MissingHeaders(startingBlockNumber int64, endingBlockNumber int64) ([]core.Header, error) {
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return repo.MissingHeaders(startingBlockNumber, endingBlockNumber, repository.db,"example_event_checked")
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}
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func (repository ExampleRepository) RecheckHeaders(startingBlockNumber int64, endingBlockNumber int64) ([]core.Header, error) {
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return repo.RecheckHeaders(startingBlockNumber, endingBlockNumber, repository.db, "example_event_checked")
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}
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```
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### EventTransformerInitializer
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A transformer initializer variable needs to be exported from somewhere within the transformer repository so that the transformer can be
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loaded as part of a plugin in the `compose` or `composeAndExecute` commands. It is important that this variable is named `EventTransformerInitializer` and
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it must be of `type EventTransformerInitializer func(db *postgres.DB) EventTransformer`.
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```go
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var EventTransformerInitializer transformer.EventTransformerInitializer = factories.Transformer{
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Config: exampleEventConfig,
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Converter: ExampleConverter{},
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Repository: &ExampleRepository{},
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}.NewTransformer
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```
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### DB migrations
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We use `goose` as our migration management tool. Any Go data model that needs to be written to Postgres by the
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repository needs a db migration for the corresponding Postgres data model.
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Each contract or set of transformers being watched should define its own namespace with a db schema:
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```postgresql
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-- +goose Up
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CREATE SCHEMA example_schema;
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-- +goose Down
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DROP SCHEMA example_schema;
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```
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For the example event and its resulting model, the table we write to would look like:
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```postgresql
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-- +goose Up
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CREATE TABLE example_schema.example_event (
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id SERIAL PRIMARY KEY,
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header_id INTEGER NOT NULL REFERENCES headers (id) ON DELETE CASCADE,
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event_hash CHARACTER VARYING(66) NOT NULL,
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user_address CHARACTER VARYING(66) NOT NULL,
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fraction_skimmed NUMERIC NOT NULL,
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surplus NUMERIC NOT NULL,
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deficit NUMERIC NOT NULL,
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final_position NUMERIC NOT NULL,
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tx_idx INTEGER NOT NUll,
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log_idx INTEGER NOT NUll,
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raw_log JSONB,
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UNIQUE (header_id, tx_idx, log_idx)
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);
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ALTER TABLE public.checked_headers
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ADD COLUMN example_event_checked INTEGER NOT NULL DEFAULT 0;
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-- +goose Down
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DROP TABLE maker.bite;
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ALTER TABLE public.checked_headers
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DROP COLUMN bite_checked;
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```
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Notice that we have also added a column to the `checked_headers` table for this event so that we can keep track
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of which headers we have already filtered through for this event.
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## Summary
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To create a transformer for a contract event we need to create entities for unpacking the raw log, models to represent
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the final data structure, a converter to mediate this unpacking and conversion between entities to models, a repository to write
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these models to Postgres, db migrations to accommodate these models in Postgres, and a EventTransformerInitializer to export the
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configured transformer and load it as a plugin to the `compose` or `composeAndExecute` commands as described in the [main readme](https://github.com/vulcanize/maker-vulcanizedb/blob/staging/README.md#composeandexecute-configuration).
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