Add Core Cache API Support

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#53 opened on Feb 20, 2026 0 comments 0 reactions 0 assignees View on GitHub

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Assessment

Difficulty: 5/5
Estimated time: Over a week
Newbie friendliness: 35/100

Issue type: Feature
Clarity: Mostly clear
Activity status: Stale
Tech stack: python
Domain: api, backend

Research direction

Start with stubs/wit_world/imports/cache.py and compare the proposed wrapper with the existing KV Store streaming integration. Verify hostcall invocation, option and state-flag mapping, error handling, context-manager transactions, and IOBase-compatible body streaming; done means wrapper-focused tests cover these behaviors without testing Viceroy cache persistence.

Written by the indexing model from the issue text.

Description

Overview

Add support for Fastly's Core Cache API, providing advanced caching capabilities with request collapsing, stale-while-revalidate, and streaming.

WIT Interface

interface cache {
  use types.{error};
  use http-body.{body};
  use http-req.{request};
  use async-io.{pollable as pending-entry};

  resource entry {
    lookup: static func(key: list<u8>, options: lookup-options) -> result<entry, error>;
    transaction-lookup: static func(key: list<u8>, options: lookup-options) -> result<entry, error>;
    transaction-lookup-async: static func(key: list<u8>, options: lookup-options) -> result<pending-entry, error>;
    transaction-insert: func(options: write-options) -> result<body, error>;
    transaction-insert-and-stream-back: func(options: write-options) -> result<tuple<body, entry>, error>;
    transaction-update: func(options: write-options) -> result<_, error>;
    get-state: func() -> result<lookup-state, error>;
    get-user-metadata: func(max-len: u64) -> result<option<list<u8>>, error>;
    get-body: func(options: get-body-options) -> result<body, error>;
    get-length: func() -> result<option<object-length>, error>;
    get-max-age-ns: func() -> result<option<duration-ns>, error>;
    get-stale-while-revalidate-ns: func() -> result<option<duration-ns>, error>;
    get-age-ns: func() -> result<option<duration-ns>, error>;
    get-hits: func() -> result<option<cache-hit-count>, error>;
    transaction-cancel: func() -> result<_, error>;
  }

  resource replace-entry {
    replace: static func(key: list<u8>, options: replace-options) -> result<replace-entry, error>;
    get-age-ns: func() -> result<option<duration-ns>, error>;
    get-body: func(options: get-body-options) -> result<option<body>, error>;
    get-hits: func() -> result<option<cache-hit-count>, error>;
    get-length: func() -> result<option<object-length>, error>;
    get-max-age-ns: func() -> result<option<duration-ns>, error>;
    get-stale-while-revalidate-ns: func() -> result<option<duration-ns>, error>;
    get-state: func() -> result<option<lookup-state>, error>;
    get-user-metadata: func(max-len: u64) -> result<option<list<u8>>, error>;
  }

  type object-length = u64;
  type duration-ns = u64;
  type cache-hit-count = u64;

  resource extra-lookup-options {
    constructor();
  }

  record lookup-options {
    request-headers: option<borrow<request>>,
    always-use-requested-range: bool,
    extra: option<borrow<extra-lookup-options>>,
  }

  resource extra-write-options {
    constructor();
  }

  record write-options {
    max-age-ns: duration-ns,
    request-headers: option<borrow<request>>,
    vary-rule: option<string>,
    initial-age-ns: option<duration-ns>,
    stale-while-revalidate-ns: option<duration-ns>,
    surrogate-keys: option<string>,
    length: option<object-length>,
    user-metadata: option<list<u8>>,
    edge-max-age-ns: option<duration-ns>,
    sensitive-data: bool,
    extra: option<borrow<extra-write-options>>,
  }

  resource extra-get-body-options;

  record get-body-options {
    %from: option<u64>,
    to: option<u64>,
    extra: option<borrow<extra-get-body-options>>,
  }

  flags lookup-state {
    found,
    usable,
    stale,
    must-insert-or-update,
    usable-if-error,
  }

  resource extra-replace-options {
    constructor();
  }

  enum replace-strategy {
    immediate,
    immediate-force-miss,
    wait,
  }

  record replace-options {
    request-headers: option<borrow<request>>,
    replace-strategy: option<replace-strategy>,
    always-use-requested-range: bool,
    extra: option<borrow<extra-replace-options>>,
  }

  insert: func(key: list<u8>, options: write-options) -> result<body, error>;
  await-entry: func(handle: pending-entry) -> result<entry, error>;
  close-pending-entry: func(handle: pending-entry) -> result<_, error>;
  close-entry: func(handle: entry) -> result<_, error>;
  replace-insert: func(handle: replace-entry, options: write-options) -> result<body, error>;
  close-replace-entry: func(handle: replace-entry) -> result<_, error>;
}

WIT bindings: stubs/wit_world/imports/cache.py

API Design

Implement CacheEntry resource wrapper for cache operations
Support lookup and transaction-based caching patterns
Implement LookupOptions and WriteOptions dataclasses
Provide access to cache metadata: age, TTL, hits, state flags
Transaction methods: transaction_lookup(), insert(), update(), cancel()
Entry body should implement io.IOBase for streaming (similar to KV Store)
get_body() returns file-like object that can be read in chunks or passed to stdlib functions

Cross-SDK Comparison: All SDKs provide lookup(), insert(), transaction_lookup() with similar semantics. Rust uses strong typing for state/options enums, Go uses simple structs, JS uses async/Promise-based API. Python should support both blocking operations and context managers for transactions.

Viceroy Testing

Viceroy's cache implementation is limited/stubbed. Testing focus should be on:

Verifying hostcall invocations work correctly
Testing SDK wrapper logic (option parsing, state flag handling, error mapping)
Ensuring body streaming integration functions properly

Do NOT:

Try to verify actual cache persistence behavior
Create elaborate tests around Viceroy's stubbed functionality
Test cache hit/miss logic (this is host behavior, not SDK code)

The goal is to verify our Python wrapper code works, not to test Viceroy's cache implementation.

Reference

Fastly Core Cache Documentation

Dominant language: Python
Stars: 5
Forks: 1
PR merge metrics: No merged PRs in 30d

Contributor guide

Open the contributing guide

First steps

Read the whole issue, then the project's contributing guide.
Comment on the issue to say you are picking it up — it saves two people doing the same work.
Fork the repository and make your change on a branch.
Open a pull request that references the issue number.

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