The oracle is deployed on specified validator nodes (only nodes whose private keys correspond to addresses specified in the smart contracts) in the network. It connects to two chains via a Remote Procedure Call (RPC) and is responsible for:
A watcher listens for a certain event and creates proper jobs in the queue. These jobs contain the transaction data (without the nonce) and the transaction hash for the related event. The watcher runs on a given frequency, keeping track of the last processed block.
If the watcher observes that the transaction data cannot be prepared, which generally means that the corresponding method of the bridge contract cannot be invoked, it inspects the contract state to identify the potential reason for failure and records this in the logs.
There are three Watchers:
- **Signature Request Watcher**: Listens to `UserRequestForSignature` events on the Home network.
- **Collected Signatures Watcher**: Listens to `CollectedSignatures` events on the Home network.
- **Affirmation Request Watcher**: Depends on the bridge mode.
A sender subscribes to the queue and keeps track of the nonce. It takes jobs from the queue, extracts transaction data, adds the proper nonce, and sends it to the network.
There are two Senders:
- **Home Sender**: Sends a transaction to the `Home` network.
- **Foreign Sender**: Sends a transaction to the `Foreign` network.
### RabbitMQ
[RabbitMQ](https://www.rabbitmq.com/) is used to transmit jobs from watchers to senders.
### Redis DB
Redis is used to store the number of blocks that were already inspected by watchers, and the NOnce (Number of Operation) which was used previously by the sender to send a transaction.
For more information on the Redis/RabbitMQ requirements, see [#90](/../../issues/90). We also provide [useful commands for development](#useful-commands-for-development).
**Note:** The following steps detail the bridge deployment process for development and testing. For deployment in a production environment we recommend using the [Bridge Deployment Playbooks](../deployment/README.md).
There are two options to run the TokenBridge processes:
1. Docker containers. This requires [Docker](https://docs.docker.com/install/) and [Docker Compose](https://docs.docker.com/compose/install/). If you are on Linux, it's also recommended that you [create a docker group and add your user to it](https://docs.docker.com/install/linux/linux-postinstall/), so that you can use the CLI without sudo.
Redis and RabbitMQ data are placed in `~/bridge_data` directory.
In case you need to reset your bridge or setup a new one (with different configuration) you must delete this directory to prevent old data from being read.
If the bridge does not handle an event properly (i.e. a transaction stalls due to a low gas price), the Redis DB can be rolled back. You must identify which watcher needs to re-run. For example, if the validator signatures were collected but the transaction with signatures was not sent to the Foreign network, the `collected-signatures` watcher must look at the block where the corresponding `CollectedSignatures` event was raised.
Execute this command in the bridge root directory:
| `HOME_MIN_AMOUNT_PER_TX` | Value (in _eth_ or tokens) to be sent in one transaction for the Home network. This should be greater than or equal to the value specified in the `tokenbridge-contracts/deploy/.env` file. The default value in that file is 500000000000000000, which is equivalent to 0.5. |
| `FOREIGN_MIN_AMOUNT_PER_TX` | Value (in _eth_ or tokens) to be sent in one transaction for the Foreign network. This should be greater than or equal to the value specified in the `tokenbridge-contracts/deploy/.env` file. The default value in that file is 500000000000000000, which is equivalent to 0.5. |
