go-ethereum/docs/interacting-with-geth/rpc/pubsub.md

title	description
Real-time Events	How to subscribe to events using JSON-RPC notifications.

Geth v1.4 and later support publish / subscribe using JSON-RPC notifications. This allows clients to wait for events instead of polling for them.

It works by subscribing to particular events. The node will return a subscription id. For each event that matches the subscription a notification with relevant data is send together with the subscription id.

Example:

// create subscription
{"id": 1, "method": "eth_subscribe", "params": ["newHeads"]}

returns

{"jsonrpc":"2.0","id":1,"result":"0xcd0c3e8af590364c09d0fa6a1210faf5"}
// incoming notifications
{"jsonrpc":"2.0","method":"eth_subscription","params":{"subscription":"0xcd0c3e8af590364c09d0fa6a1210faf5","result":{"difficulty":"0xd9263f42a87",<...>, "uncles":[]}}}
{"jsonrpc":"2.0","method":"eth_subscription","params":{"subscription":"0xcd0c3e8af590364c09d0fa6a1210faf5","result":{"difficulty":"0xd90b1a7ad02", <...>, "uncles":["0x80aacd1ea4c9da32efd8c2cc9ab38f8f70578fcd46a1a4ed73f82f3e0957f936"]}}}

to cancel the subscription:

// cancel subscription
{"id": 1, "method": "eth_unsubscribe", "params": ["0xcd0c3e8af590364c09d0fa6a1210faf5"]}
{"jsonrpc":"2.0","id":1,"result":true}

Considerations

1. Notifications are sent for current events and not for past events. For use cases that cannot afford to miss any notifications, subscriptions are probably not the best option.
2. Subscriptions require a full duplex connection. Geth offers such connections in the form of WebSocket and IPC (enabled by default).
3. Subscriptions are coupled to a connection. If the connection is closed all subscriptions that are created over this connection are removed.
4. Notifications are stored in an internal buffer and sent from this buffer to the client. If the client is unable to keep up and the number of buffered notifications reaches a limit (currently 10k) the connection is closed. Keep in mind that subscribing to some events can cause a flood of notifications, e.g. listening for all logs/blocks when the node starts to synchronize.

Create subscription

Subscriptions are created with a regular RPC call with eth_subscribe as method and the subscription name as first parameter. If successful it returns the subscription id.

Parameters

1. Subscription name
2. Optional arguments

Example

{"id": 1, "method": "eth_subscribe", "params": ["newHeads"]}
{"id": 1, "jsonrpc": "2.0", "result": "0x9cef478923ff08bf67fde6c64013158d"}

Cancel subscription

Subscriptions are cancelled with a regular RPC call with eth_unsubscribe as method and the subscription id as first parameter. It returns a bool indicating if the subscription was cancelled successful.

Parameters

1. subscription id

Example

{"id": 1, "method": "eth_unsubscribe", "params": ["0x9cef478923ff08bf67fde6c64013158d"]}
{"jsonrpc":"2.0","id":1,"result":true}

Supported Subscriptions

newHeads

Fires a notification each time a new header is appended to the chain, including chain reorganizations. Users can use the bloom filter to determine if the block contains logs that are interested to them. Note that if geth receives multiple blocks simultaneously, e.g. catching up after being out of sync, only the last block is emitted.

In case of a chain reorganization the subscription will emit the last header in the new chain. Therefore the subscription can emit multiple headers on the same height.

Example

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

returns

{"jsonrpc":"2.0","id":2,"result":"0x9ce59a13059e417087c02d3236a0b1cc"}

{
  "jsonrpc": "2.0",
  "method": "eth_subscription",
  "params": {
    "result": {
      "difficulty": "0x15d9223a23aa",
      "extraData": "0xd983010305844765746887676f312e342e328777696e646f7773",
      "gasLimit": "0x47e7c4",
      "gasUsed": "0x38658",
      "logsBloom": "0x00000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000",
      "miner": "0xf8b483dba2c3b7176a3da549ad41a48bb3121069",
      "nonce": "0x084149998194cc5f",
      "number": "0x1348c9",
      "parentHash": "0x7736fab79e05dc611604d22470dadad26f56fe494421b5b333de816ce1f25701",
      "receiptRoot": "0x2fab35823ad00c7bb388595cb46652fe7886e00660a01e867824d3dceb1c8d36",
      "sha3Uncles": "0x1dcc4de8dec75d7aab85b567b6ccd41ad312451b948a7413f0a142fd40d49347",
      "stateRoot": "0xb3346685172db67de536d8765c43c31009d0eb3bd9c501c9be3229203f15f378",
      "timestamp": "0x56ffeff8",
      "transactionsRoot": "0x0167ffa60e3ebc0b080cdb95f7c0087dd6c0e61413140e39d94d3468d7c9689f"
    },
    "subscription": "0x9ce59a13059e417087c02d3236a0b1cc"
  }
}

logs

Returns logs that are included in new imported blocks and match the given filter criteria.

In case of a chain reorganization previous sent logs that are on the old chain will be resent with the removed property set to true. Logs from transactions that ended up in the new chain are emitted. Therefore a subscription can emit logs for the same transaction multiple times.

Parameters

1. object with the following (optional) fields
   • address, either an address or an array of addresses. Only logs that are created from these addresses are returned (optional)
   • topics, only logs which match the specified topics (optional)

Example

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

returns

{"jsonrpc":"2.0","id":2,"result":"0x4a8a4c0517381924f9838102c5a4dcb7"}

{"jsonrpc":"2.0","method":"eth_subscription","params": {"subscription":"0x4a8a4c0517381924f9838102c5a4dcb7","result":{"address":"0x8320fe7702b96808f7bbc0d4a888ed1468216cfd","blockHash":"0x61cdb2a09ab99abf791d474f20c2ea89bf8de2923a2d42bb49944c8c993cbf04","blockNumber":"0x29e87","data":"0x00000000000000000000000000000000000000000000000000000000000000010000000000000000000000000000000000000000000000000000000000000003","logIndex":"0x0","topics":["0xd78a0cb8bb633d06981248b816e7bd33c2a35a6089241d099fa519e361cab902"],"transactionHash":"0xe044554a0a55067caafd07f8020ab9f2af60bdfe337e395ecd84b4877a3d1ab4","transactionIndex":"0x0"}}}

newPendingTransactions

Returns the hash for all transactions that are added to the pending state and are signed with a key that is available in the node.

When a transaction that was previously part of the canonical chain isn't part of the new canonical chain after a reorganization its again emitted.

Parameters

none

Example

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

returns

{"jsonrpc":"2.0","id":2,"result":"0xc3b33aa549fb9a60e95d21862596617c"}
{
  "jsonrpc":"2.0",
  "method":"eth_subscription",
  "params":{
    "subscription":"0xc3b33aa549fb9a60e95d21862596617c",
    "result":"0xd6fdc5cc41a9959e922f30cb772a9aef46f4daea279307bc5f7024edc4ccd7fa"
  }
}

syncing

Indicates when the node starts or stops synchronizing. The result can either be a boolean indicating that the synchronization has started (true), finished (false) or an object with various progress indicators.

Parameters

none

Example

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

{"jsonrpc":"2.0","id":2,"result":"0xe2ffeb2703bcf602d42922385829ce96"}
{"subscription":"0xe2ffeb2703bcf602d42922385829ce96","result":{"syncing":true,"status":{"startingBlock":674427,"currentBlock":67400,"highestBlock":674432,"pulledStates":0,"knownStates":0}}}}