Grace Periods

Grace periods let you serve stale data when the underlying source fails. Not unlike my middle-school lunch when New Jersey ran out of whatever crap they put between two pieces of “bread”.

The Problem

TypeScript

import { QueryCache, at, wild, MemoryAdapter } from '@t87s/core';

const cache = QueryCache({
  schema: at('users', () => wild),
  adapter: new MemoryAdapter(),
  queries: (tags) => ({
    getUser: (id: string) => ({
      tags: [tags.users(id)],
      ttl: '10m',
      fn: () => db.users.findById(id), // What if DB is down?
    }),
  }),
});

Python

from t87s import QueryCache, TagSchema, Wild, cached
from t87s.adapters import AsyncMemoryAdapter

class Tags(TagSchema):
    users: Wild[TagSchema]

class Cache(QueryCache[Tags]):
    @cached(Tags.users(), ttl="10m")
    async def get_user(self, id: str):
        return await db.users.find_by_id(id)  # What if DB is down?

cache = Cache(adapter=AsyncMemoryAdapter())

Without grace: if TTL expires and DB is down, users get an error, then a refund, then a lawsuit.

The Solution

TypeScript

import { QueryCache, at, wild, MemoryAdapter } from '@t87s/core';

const cache = QueryCache({
  schema: at('users', () => wild),
  adapter: new MemoryAdapter(),
  queries: (tags) => ({
    getUser: (id: string) => ({
      tags: [tags.users(id)],
      ttl: '10m',
      grace: '6h', // Serve stale data for up to 6 hours if fn fails
      fn: () => db.users.findById(id),
    }),
  }),
});

Python

from t87s import QueryCache, TagSchema, Wild, cached
from t87s.adapters import AsyncMemoryAdapter

class Tags(TagSchema):
    users: Wild[TagSchema]

class Cache(QueryCache[Tags]):
    @cached(Tags.users(), ttl="10m", grace="6h")  # Serve stale for up to 6 hours if fn fails
    async def get_user(self, id: str):
        return await db.users.find_by_id(id)

cache = Cache(adapter=AsyncMemoryAdapter())

With grace: when TTL expires, t87s tries to refresh. If that fails and we have stale data within the grace period, we hold our nose and serve the stale data while retrying in the background.