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Events

Synopsis

Events are objects that contain information about the execution of the application. They are mainly used by service providers like block explorers and wallet to track the execution of various messages and index transactions.

Pre-requisite Readings

Events​

Events are implemented in the Cosmos SDK as an alias of the ABCI Event type and take the form of: {eventType}.{attributeKey}={attributeValue}.

https://github.com/cosmos/cosmos-sdk/blob/v0.52.0-beta.2/proto/cosmos/streaming/v1/grpc.proto#L49-L53

An Event contains:

  • A type to categorize the Event at a high-level; for example, the Cosmos SDK uses the "message" type to filter Events by Msgs.
  • A list of attributes are key-value pairs that give more information about the categorized Event. For example, for the "message" type, we can filter Events by key-value pairs using message.action={some_action}, message.module={some_module} or message.sender={some_sender}.
tip

To parse the attribute values as strings, make sure to add ' (single quotes) around each attribute value.

Typed Events are Protobuf-defined messages used by the Cosmos SDK for emitting and querying Events. They are defined in a event.proto file, on a per-module basis and are read as proto.Message. Legacy Events are defined on a per-module basis in the module's /types/events.go file. They are triggered from the module's Protobuf Msg service by using the EventManager.

In addition, each module documents its events under in the Events sections of its specs (x/{moduleName}/README.md).

Lastly, Events are returned to the underlying consensus engine in the response of the following ABCI messages:

Examples​

The following examples show how to query Events using the Cosmos SDK.

EventDescription
tx.height=23Query all transactions at height 23
message.action='/cosmos.bank.v1beta1.Msg/Send'Query all transactions containing an x/bank Send Service Msg. Note the 's around the value.
message.module='bank'Query all transactions containing messages from the x/bank module. Note the 's around the value.
create_validator.validator='cosmosval1...'x/staking-specific Event, see x/staking SPEC.

EventManager​

In Cosmos SDK applications, Events are managed by an abstraction called the EventManager. Internally, the EventManager tracks a list of Events for the entire execution flow of FinalizeBlock (i.e. transaction execution, BeginBlock, EndBlock).

https://github.com/cosmos/cosmos-sdk/blob/v0.52.0-beta.2/types/events.go#L18-L25

The EventManager comes with a set of useful methods to manage Events. The method that is used most by module and application developers is EmitTypedEvent or EmitEvent that tracks an Event in the EventManager.

https://github.com/cosmos/cosmos-sdk/blob/v0.52.0-beta.2/types/events.go#L62-L86

Module developers should handle Event emission via the EventManager#EmitTypedEvent or EventManager#EmitEvent in each message Handler and in each BeginBlock/EndBlock handler. The EventManager is accessed via the Context, where Event should be already registered, and emitted like this:

Note: it is preferred to use EmitTypedEvent over EmitEvent as the latter has been deprecated.

Typed events:

https://github.com/cosmos/cosmos-sdk/blob/v0.52.0-beta.2/x/group/keeper/msg_server.go#L94-L96

Legacy events:

ctx.EventManager().EmitEvent(
    sdk.NewEvent(eventType, sdk.NewAttribute(attributeKey, attributeValue)),
)

Where the EventManager is accessed via the Context.

See the Msg services concept doc for a more detailed view on how to typically implement Events and use the EventManager in modules.

Subscribing to Events​

You can use CometBFT's Websocket to subscribe to Events by calling the subscribe RPC method:

{
  "jsonrpc": "2.0",
  "method": "subscribe",
  "id": "0",
  "params": {
    "query": "tm.event='eventCategory' AND eventType.eventAttribute='attributeValue'"
  }
}

The main eventCategory you can subscribe to are:

  • NewBlock: Contains Events triggered during BeginBlock and EndBlock.
  • Tx: Contains Events triggered during DeliverTx (i.e. transaction processing).
  • ValidatorSetUpdates: Contains validator set updates for the block.

These Events are triggered from the state package after a block is committed. You can get the full list of Event categories on the CometBFT Go documentation.

The type and attribute value of the query allow you to filter the specific Event you are looking for. For example, a Mint transaction triggers an Event of type EventMint and has an Id and an Owner as attributes (as defined in the events.proto file of the NFT module).

Subscribing to this Event would be done like so:

{
  "jsonrpc": "2.0",
  "method": "subscribe",
  "id": "0",
  "params": {
    "query": "tm.event='Tx' AND mint.owner='ownerAddress'"
  }
}

where ownerAddress is an address following the AccAddress format.

The same way can be used to subscribe to legacy events.

Default Events​

There are a few events that are automatically emitted for all messages, directly from baseapp.

  • message.action: The name of the message type.
  • message.sender: The address of the message signer.
  • message.module: The name of the module that emitted the message.
tip

The module name is assumed by baseapp to be the second element of the message route: "cosmos.bank.v1beta1.MsgSend" -> "bank". In case a module does not follow the standard message path, (e.g. IBC), it is advised to keep emitting the module name event. Baseapp only emits that event if the module have not already done so.