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How to Implement CQRS with Separate Read and Write Models in Python
Implements Command Query Responsibility Segregation (CQRS) by splitting data into separate write and read models with dedicated repositories, using dataclasses for structure.
from dataclasses import dataclass, field
from typing import List, Dict, Optional
@dataclass
class OrderWriteModel:
order_id: int
customer: str
items: List[str] = field(default_factory=list)
def add_item(self, item: str) -> None:
self.items.append(item)
@dataclass
class OrderReadModel:
…How to Implement a Simple MVVM Binding Mock in Python
A minimal Python implementation of the MVVM pattern, mocking data binding so views auto-update when the view model changes.
class BindingMock:
def __init__(self, view_model):
self.view_model = view_model
self.subscribers = []
def bind(self, property_name, callback):
self.subscribers.append((property_name, callback))
def set(self, property_name, value):
setattr(self.view_model, property_name, va…How to Structure a Three-Tier Layered Architecture in Python
A mock three-tier architecture with presentation, business, and data layers that process a user request from input to response.
class PresentationLayer:
def __init__(self, business_layer):
self.business = business_layer
def handle_request(self, user_id):
print(f"[Presentation] Received request for user {user_id}")
data = self.business.process_user(user_id)
print(f"[Presentation] Response: {data}")
…Inbox pattern consumer dedupe mock in Python
Implements a mock inbox consumer that deduplicates incoming messages by ID, with automatic eviction of old seen IDs to prevent unbounded memory growth.
import json
from collections import deque
from dataclasses import dataclass, field
from hashlib import sha256
from typing import Any
@dataclass
class InboxConsumer:
max_seen: int = 1000
seen_ids: set = field(default_factory=set)
seen_history: deque = field(default_factory=deque)
def _mark_seen(self,…Object Pool Pattern for Database Connections in Python
Implements a reusable connection pool with acquire/release and context manager support, mocking database connections with idle reuse and exhaustion handling.
import time
from contextlib import contextmanager
from collections import deque
class ConnectionPool:
def __init__(self, size=3, max_idle=5):
self._idle = deque(maxlen=max_idle)
self._active = set()
self.size = size
def _create(self):
return {"created_at": time.time(), "queri…How to Build Cursor Pagination with Next and Prev Tokens in Python
A minimal cursor pagination implementation that returns next and previous cursor tokens for navigating a dataset.
from pprint import pprint
def make_cursor(page):
return f"page:{page:04d}"
def parse_cursor(cursor):
_, page = cursor.split(":", 1)
return int(page)
def paginate(all_items, page_size, cursor=None):
start = parse_cursor(cursor) if cursor else 0
end = start + page_size
items = all_items[sta…How to Parse Multipart Form Data in Python
Parse multipart/form-data uploads using the Python standard library's cgi module to extract both regular fields and file uploads.
import cgi
from io import BytesIO
def parse_multipart_form(headers, body_bytes):
content_type = headers.get("Content-Type", "")
content_length = int(headers.get("Content-Length", len(body_bytes)))
# Create a file-like object from bytes for cgi.FieldStorage
body_file = BytesIO(body_bytes)
…How to Aggregate Periodic Snapshot Data in Python
Generates mock snapshot data and groups values into periods to compute average aggregates with Python's standard library.
import random
from collections import defaultdict
def snapshot_aggregate(n=10, period=3):
data = defaultdict(list)
for i in range(n):
key = f"item_{i % period}"
data[key].append(random.randint(1, 100))
return dict(data)
def aggregate_periodic(snapshots, period=3):
result = {}
for …How to Encode and Decode Avro Data in Python (Roundtrip)
Serialize a Python dict to Avro binary bytes and decode it back using the fastavro-compatible avro library.
import io
import json
from avro.schema import parse
from avro.io import DatumWriter, DatumReader, BinaryEncoder, BinaryDecoder
def avro_roundtrip(schema_json, data):
schema = parse(json.dumps(schema_json))
bytes_writer = io.BytesIO()
encoder = BinaryEncoder(bytes_writer)
writer = DatumWriter(schema)
…How to Implement an Outbox Table Poll Publisher in Python
This code simulates an outbox pattern with a class that polls for pending records and publishes them as JSON messages, removing only those that are due.
import time
import json
from dataclasses import dataclass, asdict
from datetime import datetime, timedelta
@dataclass
class OutboxRecord:
id: int
topic: str
payload: dict
created_at: datetime
class OutboxPollPublisher:
def __init__(self, poll_interval_seconds=1):
self.poll_interval = poll…How to mock a CQRS projector read model update in Python
Build a CQRS projector class that maintains denormalized read models by applying domain events in a mock order-processing service.
from dataclasses import dataclass, field
from typing import Dict, List, Optional
@dataclass
class OrderReadModel:
order_id: str
customer_name: str
total: float
status: str = "pending"
items: List[Dict] = field(default_factory=list)
def apply_event(self, event_type: str, payload: Dict) -> Non…Implement the Transactional Outbox Pattern with SQLite in Python
A Python implementation of the transactional outbox pattern using SQLite, ensuring atomic writes of order data and outbox events in a single transaction while supporting reliable message publishing and consumption.
import sqlite3
from dataclasses import dataclass
from datetime import datetime, timezone
import json
@dataclass
class Order:
order_id: str
amount: float
status: str
class TransactionalOutbox:
def __init__(self, db_path=":memory:"):
self.conn = sqlite3.connect(db_path)
self._create_tab…Cache Penetration Null Object Mock in Python
Implement a cache that stores a null marker on misses to prevent repeated database hits, reducing cache penetration.
import time
from collections import defaultdict
from typing import Any, Optional
class Cache:
def __init__(self):
self.store: dict[str, Any] = {}
self.ttl: dict[str, float] = {}
self.null_marker = object()
def get(self, key: str, ttl: int = 60, fallback:
Any = None) -> An…Consistent Hashing Cache Shard in Python
A minimal consistent hashing ring with virtual nodes that distributes cache keys across shards and minimizes re-mapping when a node is removed.
import hashlib
import bisect
class ConsistentHashRing:
def __init__(self, nodes=None, replicas=3):
self.replicas = replicas
self.ring = {}
self.sorted_keys = []
if nodes:
for node in nodes:
self.add_node(node)
def _hash(self, key):
return i…How to Cache Data in Redis with Python
Build a simple Redis cache wrapper that stores and retrieves JSON data with automatic TTL and serialization.
import redis
import json
import time
class Cache:
def __init__(self, host="localhost", port=6379, db=0, default_ttl=60):
self.client = redis.Redis(host=host, port=port, db=db)
self.default_ttl = default_ttl
def get(self, key):
value = self.client.get(key)
if value is None:
…How to Implement a Write-Through Cache in Python with a Mock Database
A thread-safe write-through cache that updates both cache and mock database atomically, computing values only after a successful write to the database.
import threading
import time
import random
class WriteThroughCache:
def __init__(self):
self.cache = {}
self.db = {}
self.lock = threading.Lock()
def write(self, key, value):
with self.lock:
# Simulate slow database write
time.sleep(random.uniform(0.01…How to Implement an LFU Cache in Python
Implement a Least Frequently Used (LFU) cache with frequency tracking dictionaries to evict the least accessed items when capacity is reached.
class LFUCache:
def __init__(self, capacity: int):
self.capacity = capacity
self.data = {}
self.freq = {}
self.min_freq = 0
def get(self, key: int) -> int:
if key not in self.data:
return -1
self._increment_freq(key)
return self.data[key]
…How to Validate and Cache Data with Redis in Python
A beginner-friendly helper that validates email, phone, and age data and caches validated entries in Redis for 5 minutes.
import redis
import json
from functools import wraps
class DataValidator:
def __init__(self, host="localhost", port=6379, db=0):
self.cache = redis.Redis(host=host, port=port, db=db)
self.validators = {
"email": lambda v: "@" in v and "." in v.split("@")[-1],
"phone": lambd…How to Implement a Circuit Breaker in Python
A Python dataclass that provides circuit breaker logic with closed, open, and half-open states to fail fast on repeated errors.
from dataclasses import dataclass
from datetime import datetime, timedelta
import time
@dataclass
class CircuitBreaker:
failure_threshold: int = 3
timeout_seconds: float = 5.0
failures: int = 0
state: str = "closed"
last_failure: datetime = None
def call(self, func):
if self.state ==…Export Metrics with OTLP Mock in Python
Simulates system metric collection and exports them as an OTLP-like JSON payload using only Python's standard library.
from dataclasses import dataclass, asdict
import json
import random
import time
@dataclass
class Metric:
name: str
value: float
timestamp: int
unit: str = "1"
def collect_system_metrics() -> list[Metric]:
"""Mock metric collection for OTLP export simulation."""
now = int(time.time())
re…Summary Quantile Mock Sketch in Python
Build a memory-efficient sketch that stores sorted bins of data points to answer approximate quantile queries like median without keeping all values in memory.
import random
import statistics
from collections import Counter
class SummaryQuantileSketch:
"""
A simple sketch that stores a fixed-size summary of data (min, max, deciles)
using sorted bins, then answers approximate quantile queries.
"""
def __init__(self, bins=10):
self.bins = bins
…CQRS with Separate Read and Write Repositories in Python
Implement CQRS in Python with separate write and read repositories, using commands for mutations and frozen DTOs for queries.
from dataclasses import dataclass
from typing import Dict, List, Optional
# --- Write side: commands mutate state ---
@dataclass
class CreateUserCommand:
id: int
name: str
class UserWriteRepository:
def __init__(self) -> None:
self._store: Dict[int, Dict[str, object]] = {}
def create(self,…How to Build an Anti-Corruption Layer in Python
Translate messy legacy system data into a clean domain model using an anti-corruption layer in Python.
class MockLegacySystem:
"""Simulates a legacy system with messy data formats."""
def get_user_data(self):
# Legacy format: fields are abbreviated and types are inconsistent
return {
"usr_id": "USR-123",
"usr_nm": "john_doe",
"email_addrs": "John.Doe@example.c…How to Mock a Choreography Saga in Python
Simulate a choreography-based saga with event envelopes, status tracking, and compensating actions to model distributed transactions.
import json
from dataclasses import dataclass, asdict
from typing import List, Optional
from enum import Enum
class SagaStatus(Enum):
PENDING = "PENDING"
COMPLETING = "COMPLETING"
COMPLETED = "COMPLETED"
FAILED = "FAILED"
@dataclass
class EventEnvelope:
event_type: str
order_id: str
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