description: Using RPC Pub/Sub with WebSockets
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# RPC Pub/Sub over WebSockets

## Introduction 

Use RPC Pub/Sub over WebSockets to wait for events instead of polling for them. For example, a Dapp can 
subscribe to logs to be notified when a specific event has occurred.

Methods specific to RPC Pub/Sub are: 

* `eth_subscribe` - create a subscription for specific events.
* `eth_unsubscribe` - cancel a subscription.

!!!important
    Unlike other [Pantheon API methods](../Reference/Pantheon-API-Methods.md), 
    the RPC Pub/Sub methods cannot be called over HTTP. Use the [`--rpc-ws-enabled`](../Reference/Pantheon-CLI-Syntax.md#rpc-ws-enabled) option to enable
    the WebSockets JSON-RPC service. 
    
### Using RPC Pub/Sub

The RPC Pub/Sub API is supported on [WebSockets](../Pantheon-API/Using-JSON-RPC-API.md#http-and-websocket-requests). 

Use `eth_subscribe` to create subscriptions. Once subscribed, notifications are published by the API using `eth_subscription`. 

!!!note 
    Notifications are published by `eth_subscription`; you do not need to call `eth_subscription`. 

Subscriptions are coupled to a connection. If the connection is closed, all subscriptions created over this connection are removed.

### Subscription ID

`eth_subscribe` returns a subscription ID for each subscription created. Notifications include the subscription ID. 


!!!example
    For example, to create a synchonizing subscription
  
    ```json
    {"id": 1, "method": "eth_subscribe", "params": ["syncing"]}
    ```
    The result includes the subscription ID of `"0x1"`: 
    ```json
    {"jsonrpc":"2.0","id":1,"result":"0x1"}
    ```
    The notifications also include the subscription ID of `"0x1"`: 
    ```json
    {"jsonrpc":"2.0","method":"eth_subscription","params":{"subscription":"0x1","result":{"startingBlock":"0x0","currentBlock":"0x50","highestBlock":"0x343c19"}}}
    ```

### Notifications when Synchronizing

Subscribing to some events (for example, logs) can cause a flood of notifications while the node is synchronizing.

## Subscribing

Use `eth_subscribe` to create subscriptions for the following event types:

* [New headers](#new-headers)
* [Logs](#logs)
* [Pending transactions](#pending-transactions)
* [Dropped transactions](#dropped-transactions)
* [Synchronizng](#synchronizing) 

### New Headers

Use the `newHeads` parameter with `eth_subscribe` to be notified each time a block is added to the blockchain.  

If a chain reorganization occurs, the subscription publishes notifications for blocks in the new chain. 
This means the subscription can publish notifications for multiple blocks at the same height on the blockchain.

The new headers notification returns [block objects](../Reference/Pantheon-API-Objects.md#block-object). The 
second parameter is optional.  If specified, whole [transaction objects](../Reference/Pantheon-API-Objects.md#transaction-object) 
are included in the notifications. Otherwise, the transaction hashes are included. 

!!!example
    To subscribe to new header notifications:
    
    ```json
    {"id": 1, "method": "eth_subscribe", "params": ["newHeads", {"includeTransactions": true}]}
    ```
    
    Example result:
    ```json
    {"jsonrpc":"2.0","id":2,"result":"0x1"}
    ```
    
    Example notification without the `{"includeTransactions": true}` parameter included:
    ```json
    {
      "jsonrpc": "2.0",
      "method": "eth_subscription",
      "params":{
        "subscription":"0x1",
        "result": {
          "number":"0x40c22",
          "hash":"0x16af2ee1672203c7ac13ff280822008be0f38e1e5bdc675760015ae3192c0e3a",
          "parentHash":"0x1fcf5dadfaf2ab4d985eb05d40eaa23605b0db25d736610c4b87173bfe438f91",
          "nonce":"0x0000000000000000",
          "sha3Uncles":"0x1dcc4de8dec75d7aab85b567b6ccd41ad312451b948a7413f0a142fd40d49347",
          "logsBloom":"0x00008000000000080000000000000000000000000000000000000000000000000000000000000000000000000010000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000010000000000000000000000000000000000000040000000000000000000000000000000000000000001000000000000000000000040000000000000000000000000000000000000400000000010000000000000000100000000000020000000000000000000000000000000000010000000000000000000000000000000000000000000",
          "transactionsRoot":"0x5b2e3c1a49352f1ca9fb5dfe74b7ffbbb6d70e23a12693444e26058d8a8e6296",
          "stateRoot":"0xbe8d3bc58bd982421a3ea8b66753404502df0f464ae78a17661d157c406dd38b",
          "receiptsRoot":"0x81b175ec1f4d44fbbd6ba08f1bd3950663b307b7cb35751c067b535cc0b58f12",
          "miner":"0x0000000000000000000000000000000000000000",
          "difficulty":"0x1",
          "totalDifficulty":"0x7c16e",
          "extraData":"0xd783010600846765746887676f312e372e33856c696e757800000000000000002160f780bb1f61eda045c67cdb1297ba37d8349df8035533cb0cf82a7e45f23f3d72bbec125a9f499b3eb110b7d1918d466cb2ede90b38296cfe2aaf452c513f00",
          "size":"0x3a1",
          "gasLimit":"0x47e7c4",
          "gasUsed":"0x11ac3a",
          "timestamp":"0x592afc24",
          "uncles":[],
          "transactions":["0x419c69d21b14e2e8f911def22bb6d0156c876c0e1c61067de836713043364d6c","0x70a5b2cb2cee6e0b199232a1757fc2a9d6053a4691a7afef8508fd88aeeec703","0x4b3035f1d32339fe1a4f88147dc197a0fe5bbd63d3b9dec2dad96a3b46e4fddd"],
          },
      }
    }
    ```
    
    Example notification with the `{"includeTransactions": true}` parameter included: 
        ```json
        {
          "jsonrpc": "2.0",
          "method": "eth_subscription",
          "params":{
            "subscription":"0x1",
            "result": {
               ....
               "transactions":[
                 {
                   "blockHash":"0xa30ee4d7c271ae5150aec494131c5f1f34089c7aa8fb58bd8bb916a55275bb90",
                   "blockNumber":"0x63",
                   "from":"0xfe3b557e8fb62b89f4916b721be55ceb828dbd73",
                   "gas":"0x5208",
                   "gasPrice":"0x3b9aca00",
                   "hash":"0x11f66c3e96a92e3c14c1c33ad77381221bf8b58a887b4fed6aee456fc6f39b24",
                   "input":"0x",
                   "nonce":"0x1",
                   "to":"0x627306090abab3a6e1400e9345bc60c78a8bef57",
                   "transactionIndex":"0x0",
                   "value":"0x56bc75e2d63100000",
                   "v":"0xfe8",
                   "r":"0x4b57d179c74885ef5f9326fd000665ea7fae44095c1e2016a2817fc671beb8cc",
                   "s":"0x7ec060b115746dda392777df07ae1feacc0b83b3646f0a3de9a5fc3615af9bb8",
                 }
               ],
            },
          }
        }
        ```

### Logs

Use the `logs` parameter with `eth_subscribe` to be notified of [logs](../Using-Pantheon/Events-and-Logs.md) included in new blocks. You can 
specify a filter object to receive notifications only for logs matching your filter.   

Logs subscriptions have an filter object parameter with the following fields:

  - `address` - (optional) Either an address or an array of addresses. Returns only logs created from these addresses.
  - `topics` - (optional) Returns only logs that match the [specified topics](../Using-Pantheon/Events-and-Logs.md#topic-filters).

If a chain reorganization occurs, the subscription publishes notifications for logs from the old chain 
with the `removed` property in the [log object](../Reference/Pantheon-API-Objects.md#log-object) set to `true`. 
This means the subscription can publish notifications for multiple logs for the same transaction.

The logs subscription returns [log objects](../Reference/Pantheon-API-Objects.md#log-object). 

!!!example
    To subscribe to all logs notifications:
    ```json
     {"id": 1, "method": "eth_subscribe", "params": ["logs",{}]}
    ```
    To subscribe to logs for a specific address and topic: 
    ```json
    {"id": 1, "method": "eth_subscribe", "params": ["logs", {"address": "0x8320fe7702b96808f7bbc0d4a888ed1468216cfd", "topics": ["0xd78a0cb8bb633d06981248b816e7bd33c2a35a6089241d099fa519e361cab902"]}]}
    ```
    
    Example result:
    ```json
    {"jsonrpc":"2.0","id":1,"result":"0x2"}
    ```
    Example notification: 
    ```json
    {
      "jsonrpc":"2.0",
      "method":"eth_subscription",
      "params":{
        "subscription":"0x2",
        "result":{
          "logIndex":"0x0",
          "removed":false,
          "blockNumber":"0x2174",
          "blockHash":"0x7bc83837534aa13df55ff7db77784b1d1ba666d4c4bdd223cae9fe09c7c37eba",
          "transactionHash":"0x942179373e413824c6bc7045e92295aff91b679215446549b4aeb084da46495b",
          "transactionIndex":"0x0",
          "address":"0x9b8397f1b0fecd3a1a40cdd5e8221fa461898517",
          "data":"0x",
          "topics":["0x199cd93e851e4c78c437891155e2112093f8f15394aa89dab09e38d6ca072787","0x0000000000000000000000000000000000000000000000000000000000000005"]
         }
       }
    }
    ```

### Pending Transactions

Use the `newPendingTransactions` parameter with `eth_subscribe` to be notified of pending transactions 
added to the transaction pool for the node. 

The pending transactions subscription returns the transaction hashes or transaction details of the pending transactions. 
If the `includeTransactions` parameter is not included, the default is transaction hashes only. 

If a chain reorganization occurs, transactions are resubmitted to be included in the new canonical chain. 
This means the subscription can publish notifications for the same pending transaction more than once.

!!!example "Transaction Hashes"
    To subscribe to pending transaction notifications and receive transaction hashes only:
    
    ```json
    {"id": 1, "method": "eth_subscribe", "params": ["newPendingTransactions", {"includeTransactions":false}]}
    ```
    
    Example result:
    ```json
    {"jsonrpc":"2.0","id":1,"result":"0x1"}
    ```
    
    
    Example notification: 
    ```json
    {
      "jsonrpc":"2.0",
      "method":"eth_subscription",
      "params":{
        "subscription":"0x1",
        "result":"0x5705bc8bf875ff03e98adb98489428835892dc6ba6a6b139fee1becbc26db0b8"
      }
    }
    ```

!!!example "Transaction Details"
    To subscribe to pending transaction notifications and receive transaction details:
    
    ```json
    {"id": 1, "method": "eth_subscribe", "params": ["newPendingTransactions", {"includeTransactions":true}]}
    ```
        
    Example result:
    ```json
    {"jsonrpc":"2.0","id":1,"result":"0x2"}
    ```    
        
    Example notification: 
    ```json
    {
      "jsonrpc":"2.0",
      "method":"eth_subscription",
      "params":{
        "subscription":"0x2",
        "result":{
          "from":"0xfe3b557e8fb62b89f4916b721be55ceb828dbd73",
          "gas":"0x5208",
          "gasPrice":"0x2540be400",
          "hash":"0x7a4185f40ee93cb27eb132f301d0a5414c1f871051f166fc8804c376aab3ffec",
          "input":"0x",
          "nonce":"0x13",
          "to":"0x9d8f8572f345e1ae53db1dfa4a7fce49b467bd7f",
          "value":"0x8ac7230489e80000",
          "v":"0xfe7",
          "r":"0xdd9013c67469d2fe79afdc61777c55bdced33c90fa6f9b83d8f9b7e445085123",
          "s":"0x45823a1ab22ae9c83876ea435dc5ecc4fe3a83c1bfbc340a5f57df2f5a474fa5"
        }
      }
    }
    ```
    
### Dropped Transactions

Use the `droppedPendingTransactions` parameter with `eth_subscribe` to be notified of transactions 
dropped from the transaction pool for the node. 

The dropped transactions subscription returns the transaction hashes of the dropped transactions. 

Transactions can be re-added to the transaction pool from a variety of sources after being dropped. For example, 
receiving a previously dropped transaction from a peer. As a result it's possible to receive multiple dropped 
transaction notifications for the same transaction.

!!!example
    To subscribe to dropped transaction notifications:
    
    ```json
    {"id": 1, "method": "eth_subscribe", "params": ["droppedPendingTransactions"]}
    ```
    
    Example result:
    ```json
    {"jsonrpc":"2.0","id":1,"result":"0x1"}
    ```
    
    
    Example notification: 
    ```json
    {
      "jsonrpc":"2.0",
      "method":"eth_subscription",
      "params":{
        "subscription":"0x1",
        "result":"0xf57d6a90a7fb30880cfbdf6b432b487d0e94a3b55b34dc4b45e3b0b237ecab4c"
      }
    }
    ```

### Synchronizing

Use the `syncing` parameter with `eth_subscribe` to be notified about synchronization progress.

When behind the chain head, the synchronizing subscription returns an object indicating the synchronization 
progress. When fully synchronized, returns false. 

!!!example
    To subscribe to synchronizing notifications:
    ```json
    {"id": 1, "method": "eth_subscribe", "params": ["syncing"]}
    ```
    
    Example result:
    ```json
    {"jsonrpc":"2.0","id":1,"result":"0x4"}
    ```
    
    Example notification while synchronizing: 
    
    ```json
    {
      "jsonrpc":"2.0",
      "method":"eth_subscription",
      "params":{
        "subscription":"0x4",
        "result":{
          "startingBlock":"0x0",
          "currentBlock":"0x3e80",
          "highestBlock":"0x67b93c"
         }
      }
    }
    ```
    
    Example notification when synchronized with chain head:
   
    ```json
    {
      "jsonrpc":"2.0",
      "method":"eth_subscription",
      "params":{
        "subscription":"0x4",
        "result":false
      }
    }
    ```
   
## Unsubscribing

Use the [subscription ID](#subscription-id) with `eth_unsubscribe` to cancel a subscription. Only the 
connection that created a subscription can unsubscribe from it. 

`eth_unsubscribe` returns `true` if subscription succuessfully unsubscribed; otherwise, an error is returned. 

!!!example
    To unsubscribe from a subsciption with subscription ID of `0x1`:
    ```json
    {"id": 1, "method": "eth_unsubscribe", "params": ["0x1"]}
    ```
    
    Example result: 
    ```json
    {"jsonrpc":"2.0","id":1,"result":true}
    ```