# ADR 065: Custom Event Indexing - [ADR 065: Custom Event Indexing](#adr-065-custom-event-indexing) - [Changelog](#changelog) - [Status](#status) - [Context](#context) - [Alternative Approaches](#alternative-approaches) - [Decision](#decision) - [Detailed Design](#detailed-design) - [EventSink](#eventsink) - [Supported Sinks](#supported-sinks) - [`KVEventSink`](#kveventsink) - [`PSQLEventSink`](#psqleventsink) - [Configuration](#configuration) - [Future Improvements](#future-improvements) - [Consequences](#consequences) - [Positive](#positive) - [Negative](#negative) - [Neutral](#neutral) - [References](#references) ## Changelog - April 1, 2021: Initial Draft (@alexanderbez) ## Status Accepted ## Context Currently, Tendermint Core supports block and transaction event indexing through the `tx_index.indexer` configuration. Events are captured in transactions and are indexed via a `TxIndexer` type. Events are captured in blocks, specifically from `BeginBlock` and `EndBlock` application responses, and are indexed via a `BlockIndexer` type. Both of these types are managed by a single `IndexerService` which is responsible for consuming events and sending those events off to be indexed by the respective type. In addition to indexing, Tendermint Core also supports the ability to query for both indexed transaction and block events via Tendermint's RPC layer. The ability to query for these indexed events facilitates a great multitude of upstream client and application capabilities, e.g. block explorers, IBC relayers, and auxiliary data availability and indexing services. Currently, Tendermint only supports indexing via a `kv` indexer, which is supported by an underlying embedded key/value store database. The `kv` indexer implements its own indexing and query mechanisms. While the former is somewhat trivial, providing a rich and flexible query layer is not as trivial and has caused many issues and UX concerns for upstream clients and applications. The fragile nature of the proprietary `kv` query engine and the potential performance and scaling issues that arise when a large number of consumers are introduced, motivate the need for a more robust and flexible indexing and query solution. ## Alternative Approaches With regards to alternative approaches to a more robust solution, the only serious contender that was considered was to transition to using [SQLite](https://www.sqlite.org/index.html). While the approach would work, it locks us into a specific query language and storage layer, so in some ways it's only a bit better than our current approach. In addition, the implementation would require the introduction of CGO into the Tendermint Core stack, whereas right now CGO is only introduced depending on the database used. ## Decision We will adopt a similar approach to that of the Cosmos SDK's `KVStore` state listening described in [ADR-038](https://github.com/cosmos/cosmos-sdk/blob/master/docs/architecture/adr-038-state-listening.md). Namely, we will perform the following: - Introduce a new interface, `EventSink`, that all data sinks must implement. - Augment the existing `tx_index.indexer` configuration to now accept a series of one or more indexer types, i.e sinks. - Combine the current `TxIndexer` and `BlockIndexer` into a single `KVEventSink` that implements the `EventSink` interface. - Introduce an additional `EventSink` that is backed by [PostgreSQL](https://www.postgresql.org/). - Implement the necessary schemas to support both block and transaction event indexing. - Update `IndexerService` to use a series of `EventSinks`. - Proxy queries to the relevant sink's native query layer. - Update all relevant RPC methods. ## Detailed Design ### EventSink We introduce the `EventSink` interface type that all supported sinks must implement. The interface is defined as follows: ```go type EventSink interface { IndexBlockEvents(types.EventDataNewBlockHeader) error IndexTxEvents(*abci.TxResult) error SearchBlockEvents(context.Context, *query.Query) ([]int64, error) SearchTxEvents(context.Context, *query.Query) ([]*abci.TxResult, error) GetTxByHash([]byte) (*abci.TxResult, error) HasBlock(int64) (bool, error) } ``` The `IndexerService` will accept a list of one or more `EventSink` types. During the `OnStart` method it will call the appropriate APIs on each `EventSink` to index both block and transaction events. ### Supported Sinks We will initially support two `EventSink` types out of the box. #### `KVEventSink` This type of `EventSink` is a combination of the `TxIndexer` and `BlockIndexer` indexers, both of which are backed by a single embedded key/value database. A bulk of the existing business logic will remain the same, but the existing APIs mapped to the new `EventSink` API. Both types will be removed in favor of a single `KVEventSink` type. The `KVEventSink` will be the only `EventSink` enabled by default, so from a UX perspective, operators should not notice a difference apart from a configuration change. We omit `EventSink` implementation details as it should be fairly straightforward to map the existing business logic to the new APIs. #### `PSQLEventSink` This type of `EventSink` indexes block and transaction events into a [PostgreSQL](https://www.postgresql.org/). database. We define and automatically migrate the following schema when the `IndexerService` starts. ```sql -- Table Definition ---------------------------------------------- CREATE TYPE IF NOT EXISTS block_event_type AS ENUM ('begin_block', 'end_block'); CREATE TABLE IF NOT EXISTS block_events ( id SERIAL PRIMARY KEY, key VARCHAR NOT NULL, value VARCHAR NOT NULL, height INTEGER NOT NULL, type block_event_type ); CREATE TABLE IF NOT EXISTS tx_results { id SERIAL PRIMARY KEY, tx_result BYTEA NOT NULL } CREATE TABLE IF NOT EXISTS tx_events ( id SERIAL PRIMARY KEY, key VARCHAR NOT NULL, value VARCHAR NOT NULL, height INTEGER NOT NULL, hash VARCHAR NOT NULL, FOREIGN KEY (tx_result_id) REFERENCES tx_results(id) ON DELETE CASCADE ); -- Indices ------------------------------------------------------- CREATE INDEX idx_block_events_key_value ON block_events(key, value); CREATE INDEX idx_tx_events_key_value ON tx_events(key, value); CREATE INDEX idx_tx_events_hash ON tx_events(hash); ``` The `PSQLEventSink` will implement the `EventSink` interface as follows (some details omitted for brevity): ```go func NewPSQLEventSink(connStr string) (*PSQLEventSink, error) { db, err := sql.Open("postgres", connStr) if err != nil { return nil, err } // ... } func (es *PSQLEventSink) IndexBlockEvents(h types.EventDataNewBlockHeader) error { sqlStmt := sq.Insert("block_events").Columns("key", "value", "height", "type") // index the reserved block height index sqlStmt = sqlStmt.Values(types.BlockHeightKey, h.Header.Height, h.Header.Height, "") for _, event := range h.ResultBeginBlock.Events { // only index events with a non-empty type if len(event.Type) == 0 { continue } for _, attr := range event.Attributes { if len(attr.Key) == 0 { continue } // index iff the event specified index:true and it's not a reserved event compositeKey := fmt.Sprintf("%s.%s", event.Type, string(attr.Key)) if compositeKey == types.BlockHeightKey { return fmt.Errorf("event type and attribute key \"%s\" is reserved; please use a different key", compositeKey) } if attr.GetIndex() { sqlStmt = sqlStmt.Values(compositeKey, string(attr.Value), h.Header.Height, BlockEventTypeBeginBlock) } } } // index end_block events... // execute sqlStmt db query... } func (es *PSQLEventSink) IndexTxEvents(txr *abci.TxResult) error { sqlStmtEvents := sq.Insert("tx_events").Columns("key", "value", "height", "hash", "tx_result_id") sqlStmtTxResult := sq.Insert("tx_results").Columns("tx_result") // store the tx result txBz, err := proto.Marshal(txr) if err != nil { return err } sqlStmtTxResult = sqlStmtTxResult.Values(txBz) // execute sqlStmtTxResult db query... // index the reserved height and hash indices hash := types.Tx(txr.Tx).Hash() sqlStmtEvents = sqlStmtEvents.Values(types.TxHashKey, hash, txr.Height, hash, txrID) sqlStmtEvents = sqlStmtEvents.Values(types.TxHeightKey, txr.Height, txr.Height, hash, txrID) for _, event := range result.Result.Events { // only index events with a non-empty type if len(event.Type) == 0 { continue } for _, attr := range event.Attributes { if len(attr.Key) == 0 { continue } // index if `index: true` is set compositeTag := fmt.Sprintf("%s.%s", event.Type, string(attr.Key)) // ensure event does not conflict with a reserved prefix key if compositeTag == types.TxHashKey || compositeTag == types.TxHeightKey { return fmt.Errorf("event type and attribute key \"%s\" is reserved; please use a different key", compositeTag) } if attr.GetIndex() { sqlStmtEvents = sqlStmtEvents.Values(compositeKey, string(attr.Value), txr.Height, hash, txrID) } } } // execute sqlStmtEvents db query... } func (es *PSQLEventSink) SearchBlockEvents(ctx context.Context, q *query.Query) ([]int64, error) { sqlStmt = sq.Select("height").Distinct().From("block_events").Where(q.String()) // execute sqlStmt db query and scan into integer rows... } func (es *PSQLEventSink) SearchTxEvents(ctx context.Context, q *query.Query) ([]*abci.TxResult, error) { sqlStmt = sq.Select("tx_result_id").Distinct().From("tx_events").Where(q.String()) // execute sqlStmt db query and scan into integer rows... // query tx_results records and scan into binary slice rows... // decode each row into a TxResult... } ``` ### Configuration The current `tx_index.indexer` configuration would be changed to accept a list of supported `EventSink` types instead of a single value. Example: ```toml [tx_index] indexer = [ "kv", "psql" ] ``` If the `indexer` list contains the `null` indexer, then no indexers will be used regardless of what other values may exist. Additional configuration parameters might be required depending on what event sinks are supplied to `tx_index.indexer`. The `psql` will require an additional connection configuration. ```toml [tx_index] indexer = [ "kv", "psql" ] pqsql_conn = "postgresql://:@:/?" ``` Any invalid or misconfigured `tx_index` configuration should yield an error as early as possible. ## Future Improvements Although not technically required to maintain feature parity with the current existing Tendermint indexer, it would be beneficial for operators to have a method of performing a "re-index". Specifically, Tendermint operators could invoke an RPC method that allows the Tendermint node to perform a re-indexing of all block and transaction events between two given heights, H1 and H2, so long as the block store contains the blocks and transaction results for all the heights specified in a given range. ## Consequences ### Positive - A more robust and flexible indexing and query engine for indexing and search block and transaction events. - The ability to not have to support a custom indexing and query engine beyond the legacy `kv` type. - The ability to offload/proxy indexing and querying to the underling sink. - Scalability and reliability that essentially comes "for free" from the underlying sink, if it supports it. ### Negative - The need to support multiple and potentially a growing set of custom `EventSink` types. ### Neutral ## References - [Cosmos SDK ADR-038](https://github.com/cosmos/cosmos-sdk/blob/master/docs/architecture/adr-038-state-listening.md) - [PostgreSQL](https://www.postgresql.org/) - [SQLite](https://www.sqlite.org/index.html)