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Monitoring and Consuming Events

The Casper platform uses event streaming to signal state changes in smart contracts and nodes. Using Casper's client-side SDKs, dApps actively listening for emitted events can consume these events and perform actions based on event data.

Each Casper node streams events through the SSE (Server Sent Event) server via the port specified as the event_stream_server.address in the node's config.toml. This port is by default 9999 for nodes on Testnet and Mainnet.

Events are divided into three categories and streamed on their respective endpoints:

  • Deploy events - Associated with Deploys on a node. Currently, only a DeployAccepted event is emitted. The URL to consume deploy-related events on Mainnet and Testnet is http://<HOST>:9999/events/deploys/.
  • Finality Signature events - Emitted when a block has been finalized and cannot be altered. The URL to consume finality signature events on Mainnet and Testnet is http://<HOST>:9999/events/sigs/.
  • Main events - All other events fall under this type, including: BlockAdded, DeployProcessed, DeployExpired, Fault, Step, and Shutdown events. The URL to consume these events on Mainnet and Testnet is http://<HOST>:9999/events/main/.
note

An ApiVersion event is always emitted when a new client connects to a node's SSE server, informing the client of the node's software version.

Listening to the Event Stream​

Applications can listen for such events for a specific account during a particular era, containing certain data. Then, they can parse the data and discard what they do not need. To consume the event stream, set up an event listener in your dApp using the following code:

const { EventStream, EventName } = require("casper-js-sdk");

const es = new EventStream("http://NODE_ADDRESS:9999/events/" + CHANNEL);
es.start();
es.subscribe(EventName.EVENT_NAME, eventHandler);

const eventHandler = (event) => {
    console.log(event);
};

You can find node addresses of active online peers to replace NODE_ADDRESS, by navigating to cspr.live for Mainnet and testnet.cspr.live for Testnet.

Replace EVENT_NAME with one of the event types listed below.

Replace CHANNEL with one of the following event streams:

  • main for ApiVersion, BlockAdded, DeployExpired, DeployProcessed, Fault, or Step events.
  • deploys for DeployAccepted events.
  • sigs for FinalitySignature events.

Event Types​

ApiVersion​

The ApiVersion event is always the first event emitted when a new client connects to a node's SSE server. It specifies the protocol version of a node on the Casper platform. The following example contains the JSON representation of the ApiVersion event structure.

data:{"ApiVersion":"1.0.0"}

BlockAdded​

A BlockAdded event is emitted when a new block is added to the blockchain and stored locally in the node.

Expand to view output
{
    "BlockAdded": {
        "block_hash": "62ddf902e9b6988b978413e2a9a2c6c95f8e1ddf452afd8e8a68f0ac22bf391a",
        "block": {
            "hash": "62ddf105e9b6988b378413e2a9a2c6c95f8e1ddf458afd8e8268f0ac72bfe91a",
            "header": {
                "parent_hash": "ed11ac2117edb9c5b26cf0cde318a807fd68e76206855a70429012ef16b557f5",
                "state_root_hash": "3c1ad31757ae40f934de4825a818274e0c246d304c661daf656e22b65174ad66",
                "body_hash": "eb2344f37193395bbc83587e498bc12ad5f0019055abcfa4c3b989d382a7969a",
                "random_bit": true,
                "accumulated_seed": "b8b671530f2221c8fdf201083f43c51e215e2f6ffcbe2d63238a2779eb177922",
                "era_end": null,
                "timestamp": "2023-01-01T09:55:25.312Z",
                "era_id": 8426,
                "height": 1566677,
                "protocol_version": "1.4.13"
            },
            "body": {
                "proposer": "010e5669b0f0545e2b32bc66363b9d3d4390fca56bf52305f1411b7fa12ca311c7",
                "deploy_hashes": [],
                "transfer_hashes": []
            },
            "proofs": []
        }
    }
}
  • block_hash - The cryptographic hash that identifies a block.
  • block - The JSON representation of the block.
  • proposer - The validator selected to propose the block.

DeployAccepted​

DeployAccepted events are emitted when a node on the network receives a deploy.

Expand to view output
{
    "DeployAccepted": {
        "hash": "db84ba229ea37716230ac9874f66c0f12b9731d8d42f28060e481ef3d7263ead",
        "header": {
            "account": "012481699f9231e36ecf002675cd7186b48e6a735d10ec1b30f587ca716937752c",
            "timestamp": "2023-01-01T20:22:45.383Z",
            "ttl": "30m",
            "gas_price": 1,
            "body_hash": "8a377b07a01ac23905b2e416ff388508301feffbb9bdf275c59f87be1e9d0de5",
            "dependencies": [],
            "chain_name": "casper-test"
        },
        "payment": {
            "ModuleBytes": {
                "module_bytes": "",
                "args": [
                    [
                        "amount",
                        {
                            "cl_type": "U512",
                            "bytes": "040008af2f",
                            "parsed": "800000000"
                        }
                    ]
                ]
            }
        },
        "session": {
            "StoredContractByHash": {
                "hash": "1040f40d06f0355a80149befc4b5d1f203231231d66c4903688e178c36066539",
                "entry_point": "test_entry_point",
                "args": [
                    [
                        "cost",
                        {
                            "cl_type": "U512",
                            "bytes": "0500c817a804",
                            "parsed": "20000000000"
                        }
                    ]
                ]
            }
        },
        "approvals": [
            {
                "signer": "012481699f9231e36ecf002675cd7186b48e6a735d10ec1b30f587ca716937752c",
                "signature": "01d81d4dc9504a356c23d3c161b87b39b1708cd282b59d3e44d9b999e787643ab495f168475bed8dc48d1056605e06c8ba74d96c69ae5b506c4312be8871c0c701"
            }
        ]
    }
}
  • hash - The blake2b hash of the Deploy.
  • account - The hexadecimal-encoded public key of the account submitting the Deploy.
  • body_hash - The blake2b hash of the Deploy body.
  • payment - Gas payment information.
  • session - The session logic defining the Deploy's functionality.
  • approvals - The signer's hexadecimal-encoded public key and signature.

For details on custom serializations, check the Serialization Standard. Also, the Types page defines the terms used in the event stream output.

DeployProcessed​

A DeployProcessed event is emitted when a given Deploy has been executed.

Expand to view output
{
    "DeployProcessed": {
        "deploy_hash": "0f33be8f56ff23d7d503a9804675472e043830a6c17e6141dce717b4f0973c7d",
        "account": "0201cbff12155b6ae1e99d571c01d56e9e1ba0def6719a6f06bc3e4a08f30a887444",
        "timestamp": "2023-01-01T10:07:00.401Z",
        "ttl": "30m",
        "dependencies": [],
        "block_hash": "509b754648168a73e6ab67e64d4a783cf580d6fc0c7c0ec560c6650f717841e0",
        "execution_result": {
            "Success": {
                "effect": {
                    "operations": [],
                    "transforms": [
                        {
                            "key": "account-hash-a8261377ef9cf8e7411d6858801c71e28c9322e66355586549c75ab24cdd73f2",
                            "transform": "Identity"
                        }
                    ]
                },
                "transfers": ["transfer-3389144d15238240f48f5966f2dc299b6b20eb19c13d834409b4d28fc50fa909"],
                "cost": "100000000"
            }
        }
    }
}
  • deploy_hash - The cryptographic hash of a Deploy.
  • account - The hexadecimal-encoded public key of the account submitting the Deploy.
  • timestamp - A timestamp type representing a concrete moment in time.
  • dependencies - A list of Deploy hashes.
  • block_hash - A cryptographic hash identifying a Block.
  • execution_result - The execution status of the Deploy, which is either Success or Failure.

DeployExpired​

A DeployExpired event is emitted when the Deploy is no longer valid for processing or being added to a block due to its time to live (TTL) having expired.

Expand to view output
{
    "DeployExpired": {
        "deploy_hash": "7ecf22fc284526d6db16fbf455f489e0a9cbf782234131c010cf3078fb9be353"
    }
}

Fault​

The Fault event is emitted if there is a validator error.

Expand the below section to view the Fault event details:
{
    "Fault": {
        "era_id": 4591448806312642600,
        "public_key": "013da85eb06279da42e28530e1116be04bfd2aa25ed8d63401ebff4d9153a609a9",
        "timestamp": "2023-01-01T01:26:58.364Z"
    }
}
  • era_id - A period of time during which the validator set does not change.
  • public_key - The hexadecimal-encoded public key of the validator that caused the fault.
  • timestamp - A timestamp representing the moment the validator faulted.

FinalitySignature​

This event indicates validators have signed the final approvals and further alterations to the block will not be allowed. Refer to the consensus reached and block finality sections to learn more about finality signatures.

Expand to view output
{
    "FinalitySignature": {
        "block_hash": "eceed827e11f7969a7d3fe91d6fa4ce9749dd79d9f3ea26474fe2014db90e98d",
        "era_id": 8419,
        "signature": "0117087ef4b9a786e5a0ea8f198050e9de93dd94f87469b8124c346aeae5f36ad9adf80f670ee9c5887263267ed32cf932dce9b370353c596d59f91fbd57a1a205",
        "public_key": "01c375b425a36de25dc325c9182861679db2f634abcacd9ae2ee27b84ba62ac1f7"
    }
}
  • block_hash - A cryptographic hash identifying a Block.
  • era_id - A period of time during which the validator set does not change.
  • signature - Serialized bytes representing the validator's signature.
  • public_key - The hexadecimal-encoded public key of the validator.

Step​

The Step event is emitted at the end of every era and contains the execution effects produced by running the auction contract's step function.

Expand to view output:
{
    "Step": {
        "era_id": 1,
        "execution_effect": {
            "operations": [],
            "transforms": [
                {
                    "key": "uref-53df18bf01396fbd1ef3a8757c7bdffc684c407d90f2cfeebff166db1d923613-000",
                    "transform": "Identity"
                },
                {
                    "key": "uref-f268de37fcea55f8fb1abeba8536a1cc041b2aed2691f1cf34aeaaf0ae379aa5-000",
                    "transform": "Identity"
                },
                {
                    "key": "bid-278e5af1ca6cddf5d5438999cb072b47f0d65e1484799f692c3c9c40304be30e",
                    "transform": "Identity"
                },
                {
                    "key": "bid-278e5af1ca6cddf5d5438999cb072b47f0d65e1484799f692c3c9c40304be30e",
                    "transform": {
                        "WriteBid": {
                            "validator_public_key": "0133eaae2821f090ac3ba0eadc0a897742094c0604df72b465c41d4b773298a7b9",
                            "bonding_purse": "uref-136552c255d4d737bf7e43d2be250f9f38691b9fe5d9e34446bff18d6d1cf984-007",
                            "staked_amount": "1000000000000005",
                            "delegation_rate": 5,
                            "vesting_schedule": {
                                "initial_release_timestamp_millis": 1664475057182,
                                "locked_amounts": null
                            },
                            "delegators": {
                                "012a241eaa9fa3bd6ccb0e0aaaf4658538f3540e04e2f58973614a168f2f2f813d": {
                                    "delegator_public_key": "012a241eaa9fa3bd6ccb0e0aaaf4658538f3540e04e2f58973614a168f2f2f813d",
                                    "staked_amount": "51312014671568117976319379",
                                    "bonding_purse": "uref-c5ad00f9e6b2f2631ca647ad188187e63799a278a0a46ca25f6b4da64d556662-007",
                                    "validator_public_key": "0133eaae2821f090ac3ba0eadc0a897742094c0604df72b465c41d4b773298a7b9",
                                    "vesting_schedule": {
                                        "initial_release_timestamp_millis": 1664475057182,
                                        "locked_amounts": null
                                    }
                                }
                            },
                            "inactive": false
                        }
                    }
                }
            ]
        }
    }
}
  • era_id - A period of time during which the validator set does not change.
  • execution_effect - The journal of execution transforms from a single Deploy.
  • operations - Operations performed while executing a Deploy.
  • transform - The actual transformation performed while executing a Deploy.

Shutdown​

The Shutdown event is emitted when the node is about to shut down, usually for an upgrade, causing a termination of the event stream.

Expand the below section to view the Shutdown event details:
"Shutdown"
  • Shutdown - The "Shutdown" text notifies the event listener that a shutdown will occur.

Reacting to Events​

An application may parse each event needed for its use case and respond accordingly. The dApp may act on some events and not others, or it may act upon them all, depending on its use case. Each event type contains additional data that might help in deciding whether or not to take an action. For example, DeployAccepted events contain the account's public key that submitted the deploy, the contract address, and more. This information can help determine how to proceed or whether or not to react.

const eventHandler = (event) => {
    if (event.body.DeployAccepted.header.account == "012481699f9231e36ecf002675cd7186b48e6a735d10ec1b30f587ca716937752c") {
        // Perform an action
    }
};

Unsubscribing from Events​

In many cases, an application may need to unsubscribe after a certain time or may want to unsubscribe from some events but not others. The Casper SDKs provide this ability with the unsubscribe function:

es.unsubscribe(EventName.EVENT_NAME);
  • EVENT_NAME - One of the different event types emitted by a Casper node.

Stopping the Event Stream​

A dApp may cease listening to all events using the stop function:

es.stop();

Replaying the Event Stream​

This command will replay the event stream from an old event onward. Replace the NODE_ADDRESS, CHANNEL, and ID fields with the values of your scenario.

curl -sN http://NODE_ADDRESS:9999/events/CHANNEL?start_from=ID

Example:

curl -sN http://65.21.235.219:9999/events/main?start_from=29267508

Each URL can have a query string added to the form ?start_from=ID, where ID is an integer representing an old event ID. With this query, you can replay the event stream from that old event onward. If you specify an event ID already purged from the cache, the server will replay all the cached events.

note

The server keeps only a limited number of events cached to allow replaying the stream to clients using the ?start_from= query string. The cache size can be set differently on each node using the event_stream_buffer_length value in the config.toml. By default, it is only 5000. The intended use case is to allow a client consuming the event stream that loses its connection to reconnect and catch up with events that were emitted while it was disconnected.

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