How to Build an In-Memory Service Registry Mock in Python

A simple in-memory ServiceRegistry class to register, retrieve, list, and unregister microservice endpoints or configs using a dict, with KeyError guards.

Easy Python 3.9+ Aug 9, 2026 Microservices patterns 14 views 0 copies

service-registry microservices in-memory dict mock

Python code

32 lines

Python 3.9+

class ServiceRegistry:
    def __init__(self):
        self._services = {}

    def register(self, name, service):
        self._services[name] = service

    def unregister(self, name):
        if name not in self._services:
            raise KeyError(f"Service '{name}' not found")
        del self._services[name]

    def get(self, name):
        if name not in self._services:
            raise KeyError(f"Service '{name}' not found")
        return self._services[name]

    def list_services(self):
        return sorted(self._services.keys())


if __name__ == "__main__":
    registry = ServiceRegistry()
    registry.register("database", {"host": "localhost", "port": 5432})
    registry.register("cache", {"host": "localhost", "port": 6379})

    print("Registered services:", registry.list_services())
    print("Database config:", registry.get("database"))
    print("Cache config:", registry.get("cache"))

    registry.unregister("cache")
    print("After unregistering cache:", registry.list_services())

Output

stdout

Registered services: ['database', 'cache']
Database config: {'host': 'localhost', 'port': 5432}
Cache config: {'host': 'localhost', 'port': 6379}
After unregistering cache: ['database']

How it works

The ServiceRegistry class wraps an internal dict _services that stores services by name. register simply adds a new key-value pair, while get and unregister use membership checks (in) to raise a descriptive KeyError if the service doesn't exist. list_services uses sorted() on the dict keys to return a deterministic, alphabetically ordered list. Because the registry is in-memory, it's ideal for mocking in tests or for single-process prototypes, not for distributed production use.

Common mistakes

Forgetting to check existence before `unregister`, which silently removes nothing or throws a raw KeyError.
Using `self._services.keys()` directly in `list_services` without sorting, leading to non-deterministic output order.
Sharing a single registry across threads or processes without locks, causing race conditions.
Storing mutable service objects and unintentionally modifying them from outside the registry.

Variations

Use a `defaultdict` if you want unregistered lookups to return a default value instead of raising.
Add a `threading.RLock` to make the registry thread-safe.
Return a copy of the service (e.g., `copy.deepcopy`) to prevent external mutation.

Real-world use cases

Unit test a microservice that discovers dependencies via a lightweight, in-memory registry substitute.
Prototype service discovery config in a monolith before moving to a real service mesh or DNS-based registry.
In a CLI tool, maintain a registry of pluggable commands registered at startup from different modules.

How to Build an In-Memory Service Registry Mock in Python

Python code

Output

How it works

Common mistakes

Variations

Real-world use cases

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Python code

Output

How it works

Common mistakes

Variations

Real-world use cases

More from Microservices patterns

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