Updated on March 1, 2020

RPC Pub/Sub over WebSockets

Introduction

Subscribe to events by using either RPC Pub/Sub over WebSockets or filters over HTTP.

Use RPC Pub/Sub over WebSockets to wait for events instead of polling for them. For example, a Dapp can subscribe to logs and receive notification when a specific event occurs.

Methods specific to RPC Pub/Sub are:

  • eth_subscribe - create a subscription for specific events
  • eth_unsubscribe - cancel a subscription.

Important

Unlike other Hyperledger Besu API methods, you cannot call the RPC Pub/Sub methods over HTTP. Use the --rpc-ws-enabled option to enable the WebSockets JSON-RPC service.

Using RPC Pub/Sub

WebSockets supports the RPC Pub/Sub API.

To create subscriptions, use eth_subscribe. Once subscribed, the API publishes notifications using eth_subscription.

Note

eth_subscription publishes notifications; you do not need to call eth_subscription.

Subscriptions couple with connections. If a connection is closes, this removes all subscriptions created over the connection.

Subscription ID

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

Example

For example, to create a synchronizing subscription:

{"id": 1, "method": "eth_subscribe", "params": ["syncing"]}

The result includes the subscription ID of "0x1":

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

The notifications also include the subscription ID of "0x1":

{"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

To notify you about each block added to the blockchain, use the newHeads parameter with eth_subscribe.

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, the notifications include whole transaction objects, Otherwise, the notifications include transaction hashes.

Example

To subscribe to new header notifications:

{"id": 1, "method": "eth_subscribe", "params": ["newHeads", {"includeTransactions": true}]}

Example result:

{"jsonrpc":"2.0","id":2,"result":"0x1"}

Example notification without the {"includeTransactions": true} parameter included:

{
  "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

To notify you about logs included in new blocks, use the logs parameter with eth_subscribe. 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:

 {"id": 1, "method": "eth_subscribe", "params": ["logs",{}]}

To subscribe to logs for a specific address and topic:

{"id": 1, "method": "eth_subscribe", "params": ["logs", {"address": "0x8320fe7702b96808f7bbc0d4a888ed1468216cfd", "topics": ["0xd78a0cb8bb633d06981248b816e7bd33c2a35a6089241d099fa519e361cab902"]}]}

Example result:

{"jsonrpc":"2.0","id":1,"result":"0x2"}

Example notification:

{
  "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

To notify you about pending transactions added to the transaction pool for the node, use the newPendingTransactions parameter with eth_subscribe.

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, Besu resubmits transactions for inclusion in the new canonical chain. This means the subscription can publish notifications for the same pending transaction more than once.

Transaction Hashes

To subscribe to pending transaction notifications and receive transaction hashes only:

{"id": 1, "method": "eth_subscribe", "params": ["newPendingTransactions", {"includeTransactions":false}]}

Example result:

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

Example notification:

{
  "jsonrpc":"2.0",
  "method":"eth_subscription",
  "params":{
    "subscription":"0x1",
    "result":"0x5705bc8bf875ff03e98adb98489428835892dc6ba6a6b139fee1becbc26db0b8"
  }
}

Transaction Details

To subscribe to pending transaction notifications and receive transaction details:

{"id": 1, "method": "eth_subscribe", "params": ["newPendingTransactions", {"includeTransactions":true}]}

Example result:

{"jsonrpc":"2.0","id":1,"result":"0x2"}

Example notification:

{
  "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

To notify you about transactions dropped from the transaction pool for the node, use the droppedPendingTransactions parameter with eth_subscribe.

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

Dropped transactions can be re-added to the transaction pool from a variety of sources. 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:

{"id": 1, "method": "eth_subscribe", "params": ["droppedPendingTransactions"]}

Example result:

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

Example notification:

{
  "jsonrpc":"2.0",
  "method":"eth_subscription",
  "params":{
    "subscription":"0x1",
    "result":"0xf57d6a90a7fb30880cfbdf6b432b487d0e94a3b55b34dc4b45e3b0b237ecab4c"
  }
}

Synchronizing

To notify you about synchronization progress, use the syncing parameter with eth_subscribe.

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:

{"id": 1, "method": "eth_subscribe", "params": ["syncing"]}

Example result:

{"jsonrpc":"2.0","id":1,"result":"0x4"}

Example notification while synchronizing:

{
  "jsonrpc":"2.0",
  "method":"eth_subscription",
  "params":{
    "subscription":"0x4",
    "result":{
      "startingBlock":"0x0",
      "currentBlock":"0x3e80",
      "highestBlock":"0x67b93c"
     }
  }
}

Example notification when synchronized with chain head:

{
  "jsonrpc":"2.0",
  "method":"eth_subscription",
  "params":{
    "subscription":"0x4",
    "result":false
  }
}

Unsubscribing

To cancel a subscription, use the subscription ID with eth_unsubscribe. Only the connection that created a subscription can unsubscribe from it.

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

Example

To unsubscribe from a subsciption with subscription ID of 0x1:

{"id": 1, "method": "eth_unsubscribe", "params": ["0x1"]}

Example result:

{"jsonrpc":"2.0","id":1,"result":true}