## Preparing transformers to work as a plugin for composeAndExecute
To plug in an external transformer we need to:
1. Create a package that exports a variable `TransformerInitializer`, `StorageTransformerInitializer`, or `ContractTransformerInitializer` that are of type [TransformerInitializer](../staging/libraries/shared/transformer/event_transformer.go#L33)
or [StorageTransformerInitializer](../../staging/libraries/shared/transformer/storage_transformer.go#L31),
or [ContractTransformerInitializer](../../staging/libraries/shared/transformer/contract_transformer.go#L31), respectively
2. Design the transformers to work in the context of their [event](../staging/libraries/shared/watcher/event_watcher.go#L83),
* Do not `goose fix` the transformer migrations, this is to ensure they are always ran after the core vulcanizedb migrations which are kept in their fixed form
* Specify migration locations for each transformer in the config with the `exporter.transformer.migrations` fields
* If the base vDB migrations occupy this path as well, they need to be in their `goose fix`ed form
as they are [here](../../staging/db/migrations)
To update a plugin repository with changes to the core vulcanizedb repository, replace the vulcanizedb vendored in the plugin repo (`plugin_repo/vendor/github.com/vulcanize/vulcanizedb`)
with the newly updated version
* The entire vendor lib within the vendored vulcanizedb needs to be deleted (`plugin_repo/vendor/github.com/vulcanize/vulcanizedb/vendor`)
* These complications arise due to this [conflict](https://github.com/golang/go/issues/20481) between `dep` and Go plugins
## Configuration
A .toml config file is specified when executing the command:
The config provides information for composing a set of transformers:
```toml
[database]
name = "vulcanize_public"
hostname = "localhost"
user = "vulcanize"
password = "vulcanize"
port = 5432
[client]
ipcPath = "/Users/user/Library/Ethereum/geth.ipc"
[exporter]
home = "github.com/vulcanize/vulcanizedb"
name = "exampleTransformerExporter"
save = false
transformerNames = [
"transformer1",
"transformer2",
"transformer3",
"transformer4",
]
[exporter.transformer1]
path = "path/to/transformer1"
type = "eth_event"
repository = "github.com/account/repo"
migrations = "db/migrations"
rank = "0"
[exporter.transformer2]
path = "path/to/transformer2"
type = "eth_contract"
repository = "github.com/account/repo"
migrations = "db/migrations"
rank = "0"
[exporter.transformer3]
path = "path/to/transformer3"
type = "eth_event"
repository = "github.com/account/repo"
migrations = "db/migrations"
rank = "0"
[exporter.transformer4]
path = "path/to/transformer4"
type = "eth_storage"
repository = "github.com/account2/repo2"
migrations = "to/db/migrations"
rank = "1"
```
-`home` is the name of the package you are building the plugin for, in most cases this is github.com/vulcanize/vulcanizedb
-`name` is the name used for the plugin files (.so and .go)
-`save` indicates whether or not the user wants to save the .go file instead of removing it after .so compilation. Sometimes useful for debugging/trouble-shooting purposes.
-`transformerNames` is the list of the names of the transformers we are composing together, so we know how to access their submaps in the exporter map
-`exporter.<transformerName>`s are the sub-mappings containing config info for the transformers
-`repository` is the path for the repository which contains the transformer and its `TransformerInitializer`
-`path` is the relative path from `repository` to the transformer's `TransformerInitializer` directory (initializer package).
- Transformer repositories need to be cloned into the user's $GOPATH (`go get`)
-`type` is the type of the transformer; indicating which type of watcher it works with (for now, there are only two options: `eth_event` and `eth_storage`)
-`eth_storage` indicates the transformer works with the [storage watcher](../../staging/libraries/shared/watcher/storage_watcher.go)
that fetches state and storage diffs from an ETH node (instead of, for example, from IPFS)
-`eth_event` indicates the transformer works with the [event watcher](../../staging/libraries/shared/watcher/event_watcher.go)
that fetches event logs from an ETH node
-`eth_contract` indicates the transformer works with the [contract watcher](../staging/libraries/shared/watcher/contract_watcher.go)
that is made to work with [contract_watcher pkg](../../staging/pkg/contract_watcher)
based transformers which work with either a light or full sync vDB to watch events and poll public methods ([example1](https://github.com/vulcanize/account_transformers/tree/master/transformers/account/light), [example2](https://github.com/vulcanize/ens_transformers/tree/working/transformers/domain_records))
-`migrations` is the relative path from `repository` to the db migrations directory for the transformer
-`rank` determines the order that migrations are ran, with lower ranked migrations running first
- this is to help isolate any potential conflicts between transformer migrations
- start at "0"
- use strings
- don't leave gaps
- transformers with identical migrations/migration paths should share the same rank
- Note: If any of the imported transformers need additional config variables those need to be included as well
This information is used to write and build a Go plugin which exports the configured transformers.
These transformers are loaded onto their specified watchers and executed.
Transformers of different types can be run together in the same command using a single config file or in separate instances using different config files
The general structure of a plugin .go file, and what we would see built with the above config is shown below
