Event Store Design

# event-store-design — templates and worked examples

## Templates

### Template 1: PostgreSQL Event Store Schema

```sql
-- 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

```python
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,