tendermint/docs/app-dev/indexing-transactions.md

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Indexing Transactions

Tendermint allows you to index transactions and later query or subscribe to their results.

Events can be used to index transactions and blocks according to what happened during their execution. Note that the set of events returned for a block from BeginBlock and EndBlock are merged. In case both methods return the same type, only the key-value pairs defined in EndBlock are used.

Each event contains a type and a list of attributes, which are key-value pairs denoting something about what happened during the method's execution. For more details on Events, see the [ABCI]https://github.com/tendermint/spec/blob/master/spec/abci/abci.md#events) documentation.

An Event has a composite key associated with it. A compositeKey is constructed by its type and key separated by a dot.

For example:

"jack": [
  "account.number": 100
]

would be equal to the composite key of jack.account.number.

Let's take a look at the [tx_index] config section:

##### transactions indexer configuration options #####
[tx_index]

# What indexer to use for transactions
#
# Options:
#   1) "null"
#   2) "kv" (default) - the simplest possible indexer, backed by key-value storage (defaults to levelDB; see DBBackend).
indexer = "kv"

By default, Tendermint will index all transactions by their respective hashes using an embedded simple indexer. Note, we are planning to add more options in the future (e.g., PostgreSQL indexer).

You can turn off indexing completely by setting tx_index to null.

Adding Events

Applications are free to define which events to index. Tendermint does not expose functionality to define which events to index and which to ignore. In your application's DeliverTx method, add the Events field with pairs of UTF-8 encoded strings (e.g. "transfer.sender": "Bob", "transfer.recipient": "Alice", "transfer.balance": "100").

Example:

func (app *KVStoreApplication) DeliverTx(req types.RequestDeliverTx) types.Result {
    //...
    events := []abci.Event{
        {
            Type: "transfer",
            Attributes: []abci.EventAttribute{
                {Key: []byte("sender"), Value: []byte("Bob"), Index: true},
                {Key: []byte("recipient"), Value: []byte("Alice"), Index: true},
                {Key: []byte("balance"), Value: []byte("100"), Index: true},
                {Key: []byte("note"), Value: []byte("nothing"), Index: true},
            },
        },
    }
    return types.ResponseDeliverTx{Code: code.CodeTypeOK, Events: events}
}

The transaction will be indexed (if the indexer is not null) with a certain attribute if the attribute's Index field is set to true. In the above example, all attributes will be used.

Querying Transactions

You can query the transaction results by calling /tx_search RPC endpoint:

curl "localhost:26657/tx_search?query=\"account.name='igor'\"&prove=true"

Check out API docs for more information on query syntax and other options.

Subscribing to Transactions

Clients can subscribe to transactions with the given tags via WebSocket by providing a query to /subscribe RPC endpoint.

{
  "jsonrpc": "2.0",
  "method": "subscribe",
  "id": "0",
  "params": {
    "query": "account.name='igor'"
  }
}

Check out API docs for more information on query syntax and other options.