> ## Documentation Index
> Fetch the complete documentation index at: https://optimism-373f39ad-feat-swap-documentation.mintlify.site/llms.txt
> Use this file to discover all available pages before exploring further.

# OP-Supervisor

> Learn the basics of OP-Supervisor.

<Info>OP Stack interop is in active development. Some features may be experimental.</Info>
OP-Supervisor is a service that verifies cross-chain messages and manages interoperability between chains in the OP Stack. Anyone is able to run OP-Supervisor but only chain operators and full-node operators are expected to run OP-Supervisor. The main information it contains about other blockchains is:

* Log entries, which could be [initiating messages](/op-stack/interop/explainer#how-messages-get-from-one-chain-to-the-other) for cross-domain messages.
* Blockchain heads, which are the latest blocks at various levels of confidence and safety:
  * Unsafe (the latest block available through the gossip protocol)
  * Local-safe (the latest block written to L1)
  * Cross-safe (the latest block written to L1, and for which all the dependencies are written to L1)
  * Finalized (the latest block written to L1, and that L1 block is safe from reorgs)

```mermaid theme={null}

graph LR

  classDef chain fill:#FFE
  classDef transparent fill:none, stroke:none
  
  subgraph chain1[OP Stack chain #1]
    node1[OP Node]
    super1[OP-Supervisor]
    geth1[Execution Engine]
    node1<-->super1--->geth1<-->node1
  end
  subgraph X[ ]
    chain2[OP Stack chain #2]
    chain3[OP Stack chain #3]
    l1node[L1 Consensus Layer]
  end

  chain2-->|log events|super1
  chain3-->|log events|super1
  l1node-->|block status|super1

  class chain1,chain2,chain3 chain
  class X transparent
```

To do this, OP-Supervisor has to have RPC access to all the chains in the dependency set (all those that can contain an initiating message whose destination is this blockchain).

## How other components use OP-Supervisor

* The execution client (typically `op-geth`) queries `op-supervisor` during block-building to verify if a message is sufficiently safe to include.
  To do this, the execution client looks at every executing message and  queries `op-supervisor` to see if there is a corresponding initiating message.

* `op-node` queries cross-chain safety information and coordinates safety updates between OP stack nodes and `op-supervisor`. It uses the API provided by `op-supervisor` to:
  * Retrieve the unsafe, local-safe, cross-safe, and finalized heads for other chains.
  * Update the unsafe, local-safe, and finalized heads for its own chain.
  * Attempt to promote blocks in its own chain to cross-safe status.
  * Attempt to finalize L2 blocks based on L1 finality.

### API

Here are the most important API methods exposed by OP-Supervisor.
For a full listing of API names, see the [source code](https://github.com/ethereum-optimism/optimism/blob/develop/op-service/sources/supervisor_client.go).

| Method(s)                                 | Description                                                                           |
| ----------------------------------------- | ------------------------------------------------------------------------------------- |
| `UnsafeView` and `SafeView`               | Return the Local and Cross heads for their respective levels                          |
| `DerivedFrom`                             | OP Nodes use to check the L1 source of the Supervisor (needed for Safe Head tracking) |
| `UpdateLocalSafe` and `UpdateLocalUnsafe` | Tell the Supervisor when the Node's heads change                                      |
| `Finalized`                               | Returns the Finalized Head                                                            |
| `UpdateFinalizedL1`                       | Signals to the Supervisor new finality signals                                        |
| `CheckMessage`                            | Checks logs in the DB directly in tests                                               |

## Next steps

* Learn [how ETH can move across chains](/op-stack/interop/superchain-eth-bridge)
* For more info about how OP Stack interoperability works under the hood, [check out the specs](https://specs.optimism.io/interop/overview.html?utm_source=op-docs\&utm_medium=docs).