Object Cache

Size-bounded object cache with pluggable eviction policies.

Overview

VCollab applications frequently need to hold computed or loaded objects in memory with a bounded size budget. When the budget is exceeded, old entries must be evicted according to a configurable policy.

The vcti-cache package provides ObjectCache, a generic, size-bounded cache with a pluggable eviction policy defined by the abstract CachePolicy base class. Implement CachePolicy to control how item sizes are calculated and which items are evicted first.

This package has zero external dependencies.

Installation

From PyPI

pip install vcti-cache

In requirements.txt

vcti-cache>=1.2.0

In pyproject.toml dependencies

dependencies = [
    "vcti-cache>=1.2.0",
]

Quick Start

from vcti.cache import ObjectCache, LruPolicy

# 1. Create a cache with a built-in LRU policy
cache = ObjectCache[str](max_size=100, cache_policy=LruPolicy(get_size=len))

# 2. Add, retrieve, and remove items
cache.add("greeting", "hello")
print(cache.get("greeting"))   # "hello"
print(cache["greeting"])       # "hello"  (bracket syntax)
print("greeting" in cache)     # True
print(len(cache))              # 1
print(cache.current_size)      # 5

# 3. Update (upsert) an existing item
cache.update("greeting", "hi there")
print(cache.get("greeting"))   # "hi there"

# 4. Inspect statistics
print(cache.stats.hits)        # 2  (from the get + bracket access)
print(cache.stats.adds)        # 2  (original add + update)

cache.remove("greeting")
print(cache.get("greeting"))   # None

Five bundled policies are available — FifoPolicy, LruPolicy, LfuPolicy, MruPolicy, and RrPolicy — each accepting a get_size callable. For custom eviction logic, subclass CachePolicy[T] directly (see Extending).

Thread Safety

ObjectCache is not thread-safe. If you share a cache across threads, wrap it with an external lock (e.g. threading.Lock).

Public API

Symbol Kind Description
ObjectCache[T] Class Size-bounded cache with eviction support
CachePolicy[T] ABC Abstract base for eviction strategies
FifoPolicy[T] Policy First-in-first-out eviction
LruPolicy[T] Policy Least-recently-used eviction
LfuPolicy[T] Policy Least-frequently-used eviction
MruPolicy[T] Policy Most-recently-used eviction (scan-resistant)
RrPolicy[T] Policy Random replacement (Bélády-immune)
CacheStats Dataclass Immutable hit/miss/eviction/removal statistics
CacheOverflowError Exception Raised when an item cannot fit

ObjectCache[T]

Method / Property Description
add(key, value) Insert an item; evicts if needed, raises on duplicate key
update(key, value) Insert or replace an item (upsert); evicts if needed
get(key) Retrieve by key or None
cache[key] Retrieve by key or raise KeyError
remove(key) Remove a single item (fires on_remove)
clear() Remove all items
keys() List of current keys
values() List of current values
items() List of (key, value) pairs
iter(cache) Iterate over keys
max_size Maximum capacity in bytes
current_size Total size of cached items (bytes)
get_item_size(key) Cached size of an item (bytes), or None if missing
stats CacheStats instance with hit/miss/eviction counters
policy The active CachePolicy instance
key in cache Membership test
len(cache) Number of cached items

CachePolicy[T] (abstract)

Method Override Description
get_size(item) Required Return item size in bytes (>= 0)
get_eviction_items(space, items) Required Return keys to evict
on_add(key, item) Optional Called after an item is added
on_access(key, item) Optional Called when an item is retrieved
on_evict(key, item) Optional Called when an item is evicted by the policy
on_remove(key, item) Optional Called when an item is explicitly removed by the caller

CacheStats

Field Description
hits Successful get / [] calls
misses get calls returning None / [] raising KeyError
adds Successful add / update insertions
evictions Items removed by the eviction policy
removals Items explicitly removed by remove() / clear() / update() replacement

Bundled Policies

All bundled policies accept get_size: Callable[[T], int] in their constructor.

Policy Eviction Rule Internal Structure
FifoPolicy[T] Oldest insertion first OrderedDict
LruPolicy[T] Least recently accessed OrderedDict (moves on access)
LfuPolicy[T] Lowest access count (ties: oldest) defaultdict(int) + OrderedDict
MruPolicy[T] Most recently accessed OrderedDict (evicts from end)
RrPolicy[T] Random (accepts optional seed) list + random.Random

Start with LruPolicy — it's the best general-purpose default. See Choosing a Policy for a detailed comparison of performance, trade-offs, and when to use each.

Dependencies

None. vcti-cache is pure Python with zero runtime dependencies.

Documentation

If you want to… Read
Get started using the package Quick Start above
Understand the architecture and design decisions docs/design.md
Navigate and understand the source docs/source-guide.md
See practical, real-world usage docs/patterns.md
Write a custom eviction policy docs/extending.md
Look up a specific class or method docs/api.md

Project links

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Credits

Author: Visual Collaboration Technologies Inc.

GitHub Statistics

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License expression

LicenseRef-Proprietary
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Requires

Python <3.15, >=3.12

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