Event Emission Support for Compact Smart Contracts (Phase 1)

MPS	0005
Status	Proposed
Category	Standards

Note: This is a legacy MPS that pre-dates the defined process; the components within may not conform to the MPS template.

Event Emission Support for Compact Smart Contracts- Phase 1

Summary

This proposal adds first-class event support to the Compact smart contract language, enabling contracts to emit structured, typed events during execution. Events are captured by the midnight-ledger and exposed via the Midnight Indexer GraphQL API, enabling event-driven DApp development, historical activity queries, and real-time notifications.

Phase 1 delivers public events — the event, emit, and public keywords — using the existing Log opcode and ContractLog infrastructure. No protocol changes are required.

Not part of this MPS: Phase 2 (future work) extends the model with private events — proven blocks for commitment binding and encrypt_for for encrypted delivery — enabling privacy-preserving event patterns. Phase 2 is scoped in Part 2 of this document.

Problem Statement

DApp developers building on Midnight currently have no mechanism to:

• Track historical contract activity (token transfers, swaps, votes)
• Implement event-driven user interfaces with real-time notifications
• Build indexers, analytics tools, or block explorers for custom contracts
• Provide audit trails for governance or compliance

Motivation

The Inefficiency of State Polling

Currently, DApps must poll the contract’s state tree to detect changes. This is structurally inefficient:

• High Bandwidth Cost: Monitoring a contract for 24 hours via polling consumes ~112 MB of bandwidth vs ~200kb on an event-based approach.
• Transient Data Loss: Polling misses state changes that occur within a block execution

The Scanning Problem in Private Systems

Fully encrypted events without metadata force wallets to download and trial-decrypt every event on chain. Phase 2 solves this with Topic-Based Filtering achieving 99%+ scanning reduction.

Phased Scope

Phase 1 — Public Events (MVP)

Typed event definitions, emit statements, public plaintext payloads, GraphQL indexing, fee metering.

Requirements

Compact Events (Language + Compiler)

Developers can define typed event structures and emit them from circuit logic with compile-time validation.

• Phase 1: event keyword, emit statement, compile-time type validation, TypeScript event descriptor generation. Events are public (plaintext).
• Not Part of this MPS: Phase 2: @private field annotation, @topic annotation for filtering, client SDK encryption/decryption, @zkp ZK circuit integration.

Event Indexing & Query API

Emitted events are ingested, stored, and queryable without special-casing event types.

• Phase 1: Ingest ContractLog events into dedicated storage. GraphQL query and subscription API filtered by contract address, event type, and block range. Raw + decoded payloads persisted.
• Not part of this MPS: Phase 2: Topics column for private event filtering. Wallet-specific decryption with session isolation. RelevantContractEvent table for per-wallet relevance.

Fee Metering

Prevent spam and align cost with resource usage.

• Base fee: Fixed cost per emit
• Data fee: Variable cost proportional to payload size
• Encryption fee (Phase 2): Additional cost for encrypted fields and topic generation