event-store-design

Event Store Design

Comprehensive guide to designing event stores for event-sourced applications.

When to Use This Skill

• Designing event sourcing infrastructure
• Choosing between event store technologies
• Implementing custom event stores
• Optimizing event storage and retrieval
• Setting up event store schemas
• Planning for event store scaling

Core Concepts

1. Event Store Architecture

┌─────────────────────────────────────────────────────┐
│                    Event Store                       │
├─────────────────────────────────────────────────────┤
│  ┌─────────────┐  ┌─────────────┐  ┌─────────────┐ │
│  │   Stream 1   │  │   Stream 2   │  │   Stream 3   │ │
│  │ (Aggregate)  │  │ (Aggregate)  │  │ (Aggregate)  │ │
│  ├─────────────┤  ├─────────────┤  ├─────────────┤ │
│  │ Event 1     │  │ Event 1     │  │ Event 1     │ │
│  │ Event 2     │  │ Event 2     │  │ Event 2     │ │
│  │ Event 3     │  │ ...         │  │ Event 3     │ │
│  │ ...         │  │             │  │ Event 4     │ │
│  └─────────────┘  └─────────────┘  └─────────────┘ │
├─────────────────────────────────────────────────────┤
│  Global Position: 1 → 2 → 3 → 4 → 5 → 6 → ...     │
└─────────────────────────────────────────────────────┘

2. Event Store Requirements

Requirement	Description
Append-only	Events are immutable, only appends
Ordered	Per-stream and global ordering
Versioned	Optimistic concurrency control
Subscriptions	Real-time event notifications
Idempotent	Handle duplicate writes safely

Technology Comparison

Technology	Best For	Limitations
EventStoreDB	Pure event sourcing	Single-purpose
PostgreSQL	Existing Postgres stack	Manual implementation
Kafka	High-throughput streaming	Not ideal for per-stream queries
DynamoDB	Serverless, AWS-native	Query limitations
Marten	.NET ecosystems	.NET specific

Templates

Template 1: PostgreSQL Event Store Schema

-- Events table
CREATE TABLE events (
    id UUID PRIMARY KEY DEFAULT gen_random_uuid(),
    stream_id VARCHAR(255) NOT NULL,
    stream_type VARCHAR(255) NOT NULL,
    event_type VARCHAR(255) NOT NULL,
    event_data JSONB NOT NULL,
    metadata JSONB DEFAULT '{}',
    version BIGINT NOT NULL,
    global_position BIGSERIAL,
    created_at TIMESTAMPTZ DEFAULT NOW(),

    CONSTRAINT unique_stream_version UNIQUE (stream_id, version)
);

-- Index for stream queries
CREATE INDEX idx_events_stream_id ON events(stream_id, version);

-- Index for global subscription
CREATE INDEX idx_events_global_position ON events(global_position);

-- Index for event type queries
CREATE INDEX idx_events_event_type ON events(event_type);

-- Index for time-based queries
CREATE INDEX idx_events_created_at ON events(created_at);

-- Snapshots table
CREATE TABLE snapshots (
    stream_id VARCHAR(255) PRIMARY KEY,
    stream_type VARCHAR(255) NOT NULL,
    snapshot_data JSONB NOT NULL,
    version BIGINT NOT NULL,
    created_at TIMESTAMPTZ DEFAULT NOW()
);

-- Subscriptions checkpoint table
CREATE TABLE subscription_checkpoints (
    subscription_id VARCHAR(255) PRIMARY KEY,
    last_position BIGINT NOT NULL DEFAULT 0,
    updated_at TIMESTAMPTZ DEFAULT NOW()
);

Template 2: Python Event Store Implementation

from dataclasses import dataclass, field
from datetime import datetime
from typing import Any, Optional, List
from uuid import UUID, uuid4
import json
import asyncpg

@dataclass
class Event:
    stream_id: str
    event_type: str
    data: dict
    metadata: dict = field(default_factory=dict)
    event_id: UUID = field(default_factory=uuid4)
    version: Optional[int] = None
    global_position: Optional[int] = None
    created_at: datetime = field(default_factory=datetime.utcnow)


class EventStore:
    def __init__(self, pool: asyncpg.Pool):
        self.pool = pool

    async def append_events(
        self,
        stream_id: str,
        stream_type: str,
        events: List[Event],
        expected_version: Optional[int] = None
    ) -> List[Event]:
        """Append events to a stream with optimistic concurrency."""
        async with self.pool.acquire() as conn:
            async with conn.transaction():
                # Check expected version
                if expected_version is not None:
                    current = await conn.fetchval(
                        "SELECT MAX(version) FROM events WHERE stream_id = $1",
                        stream_id
                    )
                    current = current or 0
                    if current != expected_version:
                        raise ConcurrencyError(
                            f"Expected version {expected_version}, got {current}"
                        )

                # Get starting version
                start_version = await conn.fetchval(
                    "SELECT COALESCE(MAX(version), 0) + 1 FROM events WHERE stream_id = $1",
                    stream_id
                )

                # Insert events
                saved_events = []
                for i, event in enumerate(events):
                    event.version = start_version + i
                    row = await conn.fetchrow(
                        """
                        INSERT INTO events (id, stream_id, stream_type, event_type,
                                          event_data, metadata, version, created_at)
                        VALUES ($1, $2, $3, $4, $5, $6, $7, $8)
                        RETURNING global_position
                        """,
                        event.event_id,
                        stream_id,
                        stream_type,
                        event.event_type,
                        json.dumps(event.data),
                        json.dumps(event.metadata),
                        event.version,
                        event.created_at
                    )
                    event.global_position = row['global_position']
                    saved_events.append(event)

                return saved_events

    async def read_stream(
        self,
        stream_id: str,
        from_version: int = 0,
        limit: int = 1000
    ) -> List[Event]:
        """Read events from a stream."""
        async with self.pool.acquire() as conn:
            rows = await conn.fetch(
                """
                SELECT id, stream_id, event_type, event_data, metadata,
                       version, global_position, created_at
                FROM events
                WHERE stream_id = $1 AND version >= $2
                ORDER BY version
                LIMIT $3
                """,
                stream_id, from_version, limit
            )
            return [self._row_to_event(row) for row in rows]

    async def read_all(
        self,
        from_position: int = 0,
        limit: int = 1000
    ) -> List[Event]:
        """Read all events globally."""
        async with self.pool.acquire() as conn:
            rows = await conn.fetch(
                """
                SELECT id, stream_id, event_type, event_data, metadata,
                       version, global_position, created_at
                FROM events
                WHERE global_position > $1
                ORDER BY global_position
                LIMIT $2
                """,
                from_position, limit
            )
            return [self._row_to_event(row) for row in rows]

    async def subscribe(
        self,
        subscription_id: str,
        handler,
        from_position: int = 0,
        batch_size: int = 100
    ):
        """Subscribe to all events from a position."""
        # Get checkpoint
        async with self.pool.acquire() as conn:
            checkpoint = await conn.fetchval(
                """
                SELECT last_position FROM subscription_checkpoints
                WHERE subscription_id = $1
                """,
                subscription_id
            )
            position = checkpoint or from_position

        while True:
            events = await self.read_all(position, batch_size)
            if not events:
                await asyncio.sleep(1)  # Poll interval
                continue

            for event in events:
                await handler(event)
                position = event.global_position

            # Save checkpoint
            async with self.pool.acquire() as conn:
                await conn.execute(
                    """
                    INSERT INTO subscription_checkpoints (subscription_id, last_position)
                    VALUES ($1, $2)
                    ON CONFLICT (subscription_id)
                    DO UPDATE SET last_position = $2, updated_at = NOW()
                    """,
                    subscription_id, position
                )

    def _row_to_event(self, row) -> Event:
        return Event(
            event_id=row['id'],
            stream_id=row['stream_id'],
            event_type=row['event_type'],
            data=json.loads(row['event_data']),
            metadata=json.loads(row['metadata']),
            version=row['version'],
            global_position=row['global_position'],
            created_at=row['created_at']
        )


class ConcurrencyError(Exception):
    """Raised when optimistic concurrency check fails."""
    pass

Template 3: EventStoreDB Usage

from esdbclient import EventStoreDBClient, NewEvent, StreamState
import json

# Connect
client = EventStoreDBClient(uri="esdb://localhost:2113?tls=false")

# Append events
def append_events(stream_name: str, events: list,

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