# Indexing Transactions

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

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" (default)
    #   2) "kv" - the simplest possible indexer, backed by key-value storage (defaults to levelDB; see DBBackend).
    indexer = "kv"

    # Comma-separated list of tags to index (by default the only tag is tx hash)
    #
    # It's recommended to index only a subset of tags due to possible memory
    # bloat. This is, of course, depends on the indexer's DB and the volume of
    # transactions.
    index_tags = ""

    # When set to true, tells indexer to index all tags. Note this may be not
    # desirable (see the comment above). IndexTags has a precedence over
    # IndexAllTags (i.e. when given both, IndexTags will be indexed).
    index_all_tags = false

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).

## Adding tags

In your application's `DeliverTx` method, add the `Tags` field with the
pairs of UTF-8 encoded strings (e.g. "account.owner": "Bob", "balance":
"100.0", "date": "2018-01-02").

Example:

    func (app *KVStoreApplication) DeliverTx(tx []byte) types.Result {
        ...
        tags := []cmn.KVPair{
          {[]byte("account.name"), []byte("igor")},
          {[]byte("account.address"), []byte("0xdeadbeef")},
          {[]byte("tx.amount"), []byte("7")},
        }
        return types.ResponseDeliverTx{Code: code.CodeTypeOK, Tags: tags}
    }

If you want Tendermint to only index transactions by "account.name" tag,
in the config set `tx_index.index_tags="account.name"`. If you to index
all tags, set `index_all_tags=true`

Note, there are a few predefined tags:

-   `tm.event` (event type)
-   `tx.hash` (transaction's hash)
-   `tx.height` (height of the block transaction was committed in)

Tendermint will throw a warning if you try to use any of the above keys.

## 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](https://tendermint.github.io/slate/?shell#txsearch)
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](https://tendermint.github.io/slate/#subscribe) for
more information on query syntax and other options.
