---
order: 6
---

# Indexing Transactions

Tendermint allows you to index transactions and blocks and later query or
subscribe to their results. Transactions are indexed by `TxResult.Events` and
blocks are indexed by `Response(Begin|End)Block.Events`. However, transactions
are also indexed by a primary key which includes the transaction hash and maps
to and stores the corresponding `TxResult`. Blocks are indexed by a primary key
which includes the block height and maps to and stores the block height, i.e.
the block itself is never stored.

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:

```json
"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:

```toml
##### 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
and height and blocks by their height.

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

## Default Indexes

The Tendermint tx and block event indexer indexes a few select reserved events
by default.

### Transactions

The following indexes are indexed by default:

- `tx.height`
- `tx.hash`

### Blocks

The following indexes are indexed by default:

- `block.height`

## 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:

```go
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}
}
```

If the indexer is not `null`, the transaction will be indexed. Each event is
indexed using a composite key in the form of `{eventType}.{eventAttribute}={eventValue}`,
e.g. `transfer.sender=bob`.

## Querying Transactions Events

You can query for a paginated set of transaction by their events by calling the
`/tx_search` RPC endpoint:

```bash
curl "localhost:26657/tx_search?query=\"message.sender='cosmos1...'\"&prove=true"
```

Check out [API docs](https://docs.tendermint.com/master/rpc/#/Info/tx_search)
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.

```json
{
  "jsonrpc": "2.0",
  "method": "subscribe",
  "id": "0",
  "params": {
    "query": "message.sender='cosmos1...'"
  }
}
```

Check out [API docs](https://docs.tendermint.com/master/rpc/#subscribe) for more information
on query syntax and other options.

## Querying Blocks Events

You can query for a paginated set of blocks by their events by calling the
`/block_search` RPC endpoint:

```bash
curl "localhost:26657/block_search?query=\"block.height > 10 AND val_set.num_changed > 0\""
```

Check out [API docs](https://docs.tendermint.com/master/rpc/#/Info/block_search)
for more information on query syntax and other options.