An enterprise-grade Java-based, Apache 2.0 licensed Ethereum client https://wiki.hyperledger.org/display/besu

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description: Using RPC Pub/Sub with WebSockets

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, the RPC Pub/Sub methods cannot be called over HTTP. Use the --rpc-ws-enabled option to enable the WebSockets JSON-RPC service.

Using RPC Pub/Sub

The RPC Pub/Sub API is supported on WebSockets.

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
Logs
Pending transactions
Dropped transactions
Synchronizng

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. The second parameter is optional. If specified, whole transaction objects 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 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.

If a chain reorganization occurs, the subscription publishes notifications for logs from the old chain with the removed property in the 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.

!!!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 of the pending transactions.

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 To subscribe to pending transaction notifications:

```json
{"id": 1, "method": "eth_subscribe", "params": ["newPendingTransactions"]}
```

Example result:
```json
{"jsonrpc":"2.0","id":1,"result":"0x1"}
```


Example notification: 
```json
{
  "jsonrpc":"2.0",
  "method":"eth_subscription",
  "params":{
    "subscription":"0x1",
    "result":"0x5705bc8bf875ff03e98adb98489428835892dc6ba6a6b139fee1becbc26db0b8"
  }
}
```

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 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}
```

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