# 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
unsubscribe_allRPC methods over websocket.
The event log API, comprising the
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:
Multiple terms can be combined with
AND (case matters), for example to match
the transaction event with a given hash, use the query:
Operands may be strings in single quotes (
'Tx'), numbers (
45), dates, or
The comparison operators include
substring match). In addition, the
EXISTS operator checks for the presence
of an attribute regardless of its value.
Tendermint implicitly defines a string-valued
tm.event attribute for all
event types. Transaction items (type
Tx) are also assigned
(string), giving the hash of the transaction, and and
giving the height of the block containing the transaction. For
NewBlockHeader events, Tendermint defines a
block.height attribute giving
the height of the block.
Additional attributes can be provided by the application as ABCI
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
a query may refer to the names
reward.balance, as in:
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
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
parameters to fetch events earlier in the log.
For example, this request:
will return a result similar to the following:
"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
"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:
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:
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_all) or until the websocket connection is terminated.