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transformers: format documentation

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pkg/transformers/DOCUMENTATION.md
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The main goal of creating a transformer is to fetch specific log events from Ethereum, convert/decode them into usable data and then persist them to VulcanizeDB. For Maker there are two main types of log events that we're tracking: custom events that are defined in the contract solidity code, and LogNote events which utilize the [DSNote library](https://github.com/dapphub/ds-note). The transformer process for each of these different log types is the same, except for the converting process, as denoted below.
# Transformers
To illustrate how to create a custom log event transformer we'll use the Kick event defined in [flop.sol](https://github.com/makerdao/dss/blob/master/src/flop.sol) as an example.
## Architecture
1. Get an example raw log event either from mainnet (if the contract has already been deployed), from the Kovan testnet, or by deploying the contract to a local chain and emitting the event manually. We will use the example log event to test drive converting the log to a database model.
Transformers fetch logs from Ethereum, convert/decode them into usable data, and then persist them in postgres.
A transformer consists of:
- A fetcher -> Fetches raw logs from the blockchain encoded as go datatypes
- A converter -> Converts this raw data into a representation suitable for consumption in the API
- A repository -> Abstracts the database
For Maker, vulcanize will be run in `lightSync` mode, so it will store all headers, and then fetchers make RPC calls to pull th
## Event Types
For Maker there are two main types of log events that we're tracking:
1. Custom events that are defined in the contract solidity code.
1. LogNote events which utilize the [DSNote library](https://github.com/dapphub/ds-note).
The transformer process for each of these different log types is the same, except for the converting process, as denoted below.
## Creating a Transformer
**Fetching Logs**
1. Generate an example raw log event, by either:
- Pulling the log directly from the Kovan deployment ([constants.go](https://github.com/8thlight/maker-vulcanizedb/blob/master/pkg/transformers/shared/constants.go)).
- Deploying the contract to a local chain and emiting the event manually.
1. Fetch the logs from the chain based on the example event's topic zero:
1. Fetch the logs from the chain based on log event's topic zero.
- The topic zero is based on the keccak-256 hash of the log event's method signature. These are located in `pkg/transformers/shared/constants.go`.
- Most transformers use `shared.LogFetcher` to fetch all logs that match the given topic zero for that log event.
- Since there are multiple price feed contract address that all use the same `LogValue` event, we have a special implementation of a fetcher specifically for price feeds that can query using all of the contract addresses at once, thus only needing to make one call to the blockchain.
1. Convert the raw log into a database model.
- **Converting most custom events** (such as FlopKick)
**Coverting logs**
- **Converting most custom events** (such as FlopKick)
1. Convert the raw log into a Go struct.
- We've been using [go-ethereum's abigen tool](https://github.com/ethereum/go-ethereum/tree/master/cmd/abigen) to get the contract's ABI, and a Go struct that represents the event log. We will unpack the raw logs into this struct.
- To use abigen: `abigen --sol flip.sol --pkg flip --out {/path/to/output_file}`
@ -30,14 +59,15 @@ To illustrate how to create a custom log event transformer we'll use the Kick ev
```
- Using go-ethereum's `contract.UnpackLog` method we can unpack the raw log into the FlopperKick struct (which we're referring to as the `entity`).
- See the `ToEntity` method in `pkg/transformers/flop_kick/converter`.
- The unpack method will not add the `Raw` or `TransactionIndex` values to the entity struct - both of these values are accessible from the raw types.Log.
1. Convert the entity into a database model. See the `ToModel` method in `pkg/transformers/flop_kick/converter`.
- **Converting Price Feed custom events**
- **Converting Price Feed custom events**
- Price Feed contracts use the [LogNote event](https://github.com/makerdao/medianizer/blob/master/src/medianizer.sol#L23)
- The LogNote event takes in the value of the price feed as it's sole argument, and does not index it. This means that this value can be taken directly from the log's data, and then properly converted using the `price_feeds.Convert` method (located in the model.go file).
- Since this conversion from raw log to model includes less fields than some others, we've chosen to convert it directly to the database model, skipping the `ToEntity` step.
- **Converting LogNote events** (such as tend)
- **Converting LogNote events** (such as tend)
- Since LogNote events are a generic structure, they depend on the method signature of the method that is calling them. For example, the `tend` method is called on the [flip.sol contract](https://github.com/makerdao/dss/blob/master/src/flip.sol#L117), and it's method signature looks like this: `tend(uint,uint,uint)`.
- The first four bytes of the Keccak-256 hashed method signature will be located in `topic[0]` on the log.
- The message sender will be in `topic[1]`.
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- The second parameter passed to `tend` will be `topic[3]`.
- Any additional parameters will be in the log's data field.
- More detail is located in the [DSNote repo](https://github.com/dapphub/ds-note).
1. Get all MissingHeaders:
- Headers are inserted into VulcanizeDB as part of the `lightSync` command. Then for each transformer we check each header for matching logs.
- The MissingHeaders method queries the `checked_headers` table to see if the header has been checked for the given log type.
1. Persist the log record to VulcanizeDB.
- Each event log has it's own table in the database, as well as it's own column in the `checked_headers` table.
**Get all MissingHeaders**
- Headers are inserted into VulcanizeDB as part of the `lightSync` command. Then for each transformer we check each header for matching logs.
- The MissingHeaders method queries the `checked_headers` table to see if the header has been checked for the given log type.
**Persist the log record to VulcanizeDB**
- Each event log has it's own table in the database, as well as it's own column in the `checked_headers` table.
- The `checked_headers` table allows us to keep track of which headers have been checked for a given log type.
- To create a new migration file: `./scripts/create_migration create_flop_kick`
- To create a new migration file: `./scripts/create_migration create_flop_kick`
- See `db/migrations/1536942529_create_flop_kick.up.sql`.
- The specific log event tables are all created in the `maker`
schema.
- There is a one-many association between `headers` and the log
event tables. This is so that if a header is removed due to a reorg, the associated log event records are also removed.
- To run the migrations: `make migrate HOST=local_host PORT=5432 NAME=vulcanize_private`
- When a new log record is inserted into VulcanizeDB, we also need to make sure to insert a record into the `checked_headers` table for the given log type.
- We have been using the repository pattern (i.e. wrapping all SQL/ORM invocations in isolated namespaces per table) to interact with the database, see the `Create` method in `pkg/transformers/flop_kick/repository.go`.
1. MarkHeaderChecked:
- There is a chance that a header does not have a log for the given transformer's log type, and in this instance we also want to record that the header has been "checked" so that we don't continue to query that header over and over.
- In the transformer we'll make sure to insert a row for the header indicating that it has been checked for the log type that the transformer is responsible for.
1. Wire each component up in the transformer.
- We use a TransformerInitializer struct for each transformer so that we can inject ethRPC and postgresDB connections as well as configuration data (including the contract address, block range, etc.) into the transformer. The TransformerInitializer interface is defined in `pkg/transformers/shared/transformer.go`.
- See any of `pkg/transformers/flop_kick/transformer.go`
- All of the transformers are then initialized in `pkg/transformers/transformers.go` with their configuration.
- The transformers can be executed by using the `continuousLogSync` command, which can be configured to run specific transformers or all transformers.
- To run the migrations: `make migrate HOST=local_host PORT=5432 NAME=vulcanize_private`
- When a new log record is inserted into VulcanizeDB, we also need to make sure to insert a record into the `checked_headers` table for the given log type.
- We have been using the repository pattern (i.e. wrapping all SQL/ORM invocations in isolated namespaces per table) to interact with the database, see the `Create` method in `pkg/transformers/flop_kick/repository.go`.
**MarkHeaderChecked**
- There is a chance that a header does not have a log for the given transformer's log type, and in this instance we also want to record that the header has been "checked" so that we don't continue to query that header over and over.
- In the transformer we'll make sure to insert a row for the header indicating that it has been checked for the log type that the transformer is responsible for.
**Wire each component up in the transformer**
- We use a TransformerInitializer struct for each transformer so that we can inject ethRPC and postgresDB connections as well as configuration data (including the contract address, block range, etc.) into the transformer. The TransformerInitializer interface is defined in `pkg/transformers/shared/transformer.go`.
- See any of `pkg/transformers/flop_kick/transformer.go`
- All of the transformers are then initialized in `pkg/transformers/transformers.go` with their configuration.
- The transformers can be executed by using the `continuousLogSync` command, which can be configured to run specific transformers or all transformers.
## Useful Documents
[Ethereum Event ABI Specification](https://solidity.readthedocs.io/en/develop/abi-spec.html#events)