Subscribing to Events

A Tendermint node emits events about important state transitions during consensus. These events can be queried by clients via the RPC interface (opens new window) on nodes that enable it. The list of supported event types (opens new window) can be found in the tendermint/types Go package.

In Tendermint v0.36 there are two APIs to query events:

• The legacy streaming API, comprising the subscribe, unsubscribe, and unsubscribe_all RPC methods over websocket.

• The event log API, comprising the events RPC method.

The legacy streaming API is deprecated in Tendermint v0.36, and will be removed in Tendermint v0.37. Clients are strongly encouraged to migrate to the new event log API as soon as is practical.

Filter Queries

Event requests take a filter query (opens new window) parameter. A filter query is a string that describes a subset of available event items to return. An empty query matches all events; otherwise a query comprises one or more terms comparing event metadata to target values.

For example, to select new block events, use the term:

Copy tm.event = 'NewBlock'

Multiple terms can be combined with AND (case matters), for example to match the transaction event with a given hash, use the query:

Copy tm.event = 'Tx' AND tx.hash = 'EA7B33F'

Operands may be strings in single quotes ('Tx'), numbers (45), dates, or timestamps.

The comparison operators include =, <, <=, >, >=, and CONTAINS (for substring match). In addition, the EXISTS operator checks for the presence of an attribute regardless of its value.

Attributes

Tendermint implicitly defines a string-valued tm.event attribute for all event types. Transaction items (type Tx) are also assigned tx.hash (string), giving the hash of the transaction, and and tx.height (number) giving the height of the block containing the transaction. For NewBlock and NewBlockHeader events, Tendermint defines a block.height attribute giving the height of the block.

Additional attributes can be provided by the application as ABCI Event records (opens new window) in response to the FinalizeBlock request. The full name of the attribute in the query is formed by combining the type and attribute key with a period.

For example, given the events

Copy []abci.Event{{ Type: "reward", Attributes: []abci.EventAttribute{ {Key: "address", Value: "cosmos1xyz012pdq"}, {Key: "amount", Value: "45.62"}, {Key: "balance", Value: "100.390001"}, }, }}

a query may refer to the names reward.address, reward.amount, and reward.balance, as in:

Copy reward.address EXISTS AND reward.balance > 45

Certain application-specific metadata are also indexed for offline queries. See Indexing transactions for more details.

Event Log API

Starting in Tendermint v0.36, when the rpc.event-log-window-size configuration is enabled, the node maintains maintains a log of all events within this operator-defined time window. This API supersedes the websocket subscription API described below.

Clients can query these events can by long-polling the /events RPC method, which returns the most recent items from the log that match the request parameters (opens new window). Each item returned includes a cursor that marks its location in the log. Cursors can be passed via the before and after parameters to fetch events earlier in the log.

For example, this request:

Copy { "jsonrpc": "2.0", "id": 1, "method": "events", "params": { "filter": { "query": "tm.event = 'Tx' AND app.key = 'applesauce'" }, "maxItems": 1, "after": "" } }

will return a result similar to the following:

Copy { "jsonrpc": "2.0", "id": 1, "result": { "items": [ { "cursor": "16ee3d5e65be53d8-03d5", "event": "Tx", "data": { "type": "tendermint/event/Tx", "value": { "height": 70, "tx": "YXBwbGVzYXVjZT1zeXJ1cA==", "result": { "events": [ { "type": "app", "attributes": [ { "key": "creator", "value": "Cosmoshi Netowoko", "index": true }, { "key": "key", "value": "applesauce", "index": true }, { "key": "index_key", "value": "index is working", "index": true }, { "key": "noindex_key", "value": "index is working", "index": false } ] } ] } } } } ], "more": false, "oldest": "16ee3d4c471c3b00-0001", "newest": "16ee3d5f2e05a4e0-0400" } }

The "items" array gives the matching items (up to the requested "maxResults") in decreasing time order (i.e., newest to oldest). In this case, there is only one result, but if there are additional results that were not returned, the "more" flag will be true. Calling /events again with the same query and "after" set to the cursor of the newest result (in this example, "16ee3d5e65be53d8-03d5") will fetch newer results.

Go clients can use the eventstream (opens new window) package to simplify the use of this method. The eventstream.Stream automatically handles polling for new events, updating the cursor, and reporting any missed events.

Legacy Streaming API

• Note: This API is deprecated in Tendermint v0.36, and will be removed in Tendermint v0.37. New clients and existing use should use the event log API instead. See ADR 075 (opens new window) for more details.

To subscribe to events in the streaming API, you must connect to the node RPC service using a websocket (opens new window). From the command line you can use a tool such as wscat (opens new window), for example:

Copy wscat ws://127.0.0.1:26657/websocket

To subscribe to events, call the subscribe JSON-RPC method method passing in a filter query (opens new window) for the events you wish to receive:

Copy { "jsonrpc": "2.0", "method": "subscribe", "id": 1, "params": { "query": "tm.event='NewBlock'" } }

The subscribe method returns an initial response confirming the subscription, then sends additional JSON-RPC response messages containing the matching events as they are published. The subscription continues until either the client explicitly cancels the subscription (by calling unsubscribe or unsubscribe_all) or until the websocket connection is terminated.