tendermint/docs/architecture/adr-021-abci-events.md

ADR 012: ABCI Events

Changelog

• 2018-09-02 Remove ABCI errors component. Update description for events
• 2018-07-12 Initial version

Context

ABCI tags were first described in ADR 002. They are key-value pairs that can be used to index transactions.

Currently, ABCI messages return a list of tags to describe an "event" that took place during the Check/DeliverTx/Begin/EndBlock, where each tag refers to a different property of the event, like the sending and receiving account addresses.

Since there is only one list of tags, recording data for multiple such events in a single Check/DeliverTx/Begin/EndBlock must be done using prefixes in the key space.

Alternatively, groups of tags that constitute an event can be separated by a special tag that denotes a break between the events. This would allow straightforward encoding of multiple events into a single list of tags without prefixing, at the cost of these "special" tags to separate the different events.

TODO: brief description of how the indexing works

Decision

Instead of returning a list of tags, return a list of events, where each event is a list of tags. This way we naturally capture the concept of multiple events happening during a single ABCI message.

TODO: describe impact on indexing and querying

Status

Implemented

Consequences

Positive

• Ability to track distinct events separate from ABCI calls (DeliverTx/BeginBlock/EndBlock)
• More powerful query abilities

Negative

• More complex query syntax
• More complex search implementation

Neutral