tendermint/docs/architecture/adr-065-custom-event-indexing.md

ADR 065: Custom Event Indexing

• ADR 065: Custom Event Indexing
  • Changelog
  • Status
  • Context
  • Alternative Approaches
  • Decision
  • Detailed Design
    • EventSink
    • Supported Sinks
      • KVEventSink
      • PSQLEventSink
    • Configuration
  • Future Improvements
  • Consequences
    • Positive
    • Negative
    • Neutral
  • References

Changelog

• April 1, 2021: Initial Draft (@alexanderbez)
• April 28, 2021: Specify search capabilities are only supported through the KV indexer (@marbar3778)

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.

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.

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.
  • 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:

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. database. We define and automatically migrate the following schema when the IndexerService starts.

The postgres eventsink will not support tx_search and block_search.

-- 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):

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) {
  return nil, errors.New("block search is not supported via the postgres event sink")
}

func (es *PSQLEventSink) SearchTxEvents(ctx context.Context, q *query.Query) ([]*abci.TxResult, error) {
  return nil, errors.New("tx search is not supported via the postgres event sink")
}

Configuration

The current tx_index.indexer configuration would be changed to accept a list of supported EventSink types instead of a single value.

Example:

[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.

[tx_index]

indexer = [
  "kv",
  "psql"
]

pqsql_conn = "postgresql://<user>:<password>@<host>:<port>/<db>?<opts>"

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
• PostgreSQL
• SQLite