batching-caching

What this skill does

# Batching and Caching in Effect

## Overview

Effect provides automatic optimization for API calls:

- **Batching** - Combine multiple requests into single API calls
- **Caching** - Avoid redundant requests with smart caching
- **Deduplication** - Prevent duplicate concurrent requests

This solves the N+1 query problem automatically.

## The Problem: N+1 Queries

```typescript
const program = Effect.gen(function* () {
  const todos = yield* getTodos();

  const owners = yield* Effect.forEach(todos, (todo) => getUserById(todo.ownerId), { concurrency: "unbounded" });
});
```

Effect's batching transforms this into optimized batch calls.

## Request-Based Batching

### Step 1: Define Request Types

```typescript
import { Request } from "effect";

// Define request shape
interface GetUserById extends Request.Request<User, UserNotFound> {
  readonly _tag: "GetUserById";
  readonly id: number;
}

// Create tagged constructor
const GetUserById = Request.tagged<GetUserById>("GetUserById");
```

### Step 2: Create Resolver

```typescript
import { RequestResolver, Effect } from "effect";

// Batched resolver - handles multiple requests at once
const GetUserByIdResolver = RequestResolver.makeBatched((requests: ReadonlyArray<GetUserById>) =>
  Effect.gen(function* () {
    // Single batch API call
    const users = yield* Effect.tryPromise(() =>
      fetch("/api/users/batch", {
        method: "POST",
        body: JSON.stringify({ ids: requests.map((r) => r.id) }),
      }).then((res) => res.json()),
    );

    // Complete each request with its result
    yield* Effect.forEach(requests, (request, index) => Request.completeEffect(request, Effect.succeed(users[index])));
  }),
);
```

### Step 3: Define Query

```typescript
const getUserById = (id: number) => Effect.request(GetUserById({ id }), GetUserByIdResolver);
```

### Step 4: Use with Automatic Batching

```typescript
const program = Effect.gen(function* () {
  const todos = yield* getTodos();

  const owners = yield* Effect.forEach(todos, (todo) => getUserById(todo.ownerId), { concurrency: "unbounded" });
});
```

## Resolver Types

### Standard Resolver (No Batching)

```typescript
const SingleUserResolver = RequestResolver.fromEffect((request: GetUserById) =>
  Effect.tryPromise(() => fetch(`/api/users/${request.id}`).then((r) => r.json())),
);
```

### Batched Resolver

```typescript
const BatchedUserResolver = RequestResolver.makeBatched((requests: ReadonlyArray<GetUserById>) =>
  // Handle all requests in one call
  batchFetch(requests),
);
```

### Resolver with Context

```typescript
const UserResolverWithContext = RequestResolver.makeBatched((requests: ReadonlyArray<GetUserById>) =>
  Effect.gen(function* () {
    // Access services from context
    const httpClient = yield* HttpClient;
    const logger = yield* Logger;

    yield* logger.info(`Batching ${requests.length} user requests`);

    return yield* httpClient.post("/api/users/batch", {
      ids: requests.map((r) => r.id),
    });
  }),
);

// Provide context to resolver
const ContextualResolver = UserResolverWithContext.pipe(RequestResolver.provideContext(context));
```

## Caching

### Effect.cached - Memoize Effect Result

```typescript
import { Effect } from "effect";

const fetchConfig = Effect.promise(() => fetch("/api/config").then((r) => r.json()));

const cachedConfig = yield * Effect.cached(fetchConfig);

const config1 = yield * cachedConfig;
const config2 = yield * cachedConfig;
```

### Effect.cachedWithTTL - Time-Based Expiry

```typescript
const cachedUser = yield * Effect.cachedWithTTL(fetchCurrentUser, "5 minutes");

const user1 = yield * cachedUser;
yield * Effect.sleep("6 minutes");
const user2 = yield * cachedUser;
```

### Effect.cachedInvalidateWithTTL - Manual Invalidation

```typescript
const [cachedUser, invalidate] = yield * Effect.cachedInvalidateWithTTL(fetchCurrentUser, "5 minutes");

const user = yield * cachedUser;
yield * invalidate;
const freshUser = yield * cachedUser;
```

## Cache Service

For more control, use the Cache service:

```typescript
import { Cache } from "effect";

const program = Effect.gen(function* () {
  const cache = yield* Cache.make({
    capacity: 100,
    timeToLive: "10 minutes",
    lookup: (userId: string) => fetchUser(userId),
  });

  const user1 = yield* cache.get("user-1");
  const user2 = yield* cache.get("user-1");

  const isCached = yield* cache.contains("user-1");

  yield* cache.invalidate("user-1");

  const stats = yield* cache.cacheStats;
});
```

## Request Caching

Requests are automatically cached within a query context:

```typescript
const program = Effect.gen(function* () {
  const user1 = yield* getUserById(1);
  const user2 = yield* getUserById(1);

  const user3 = yield* getUserById(2);
});
```

### Disabling Request Caching

```typescript
const noCaching = getUserById(1).pipe(Effect.withRequestCaching(false));
```

### Custom Cache for Requests

```typescript
const customCache =
  yield *
  Request.makeCache({
    capacity: 1000,
    timeToLive: "30 minutes",
  });

const program = getUserById(1).pipe(Effect.withRequestCache(customCache));
```

## Disabling Batching

```typescript
const noBatching = program.pipe(Effect.withRequestBatching(false));
```

## Complete Example

```typescript
import { Effect, Request, RequestResolver, Schema } from "effect";

// Error types
class UserNotFound extends Schema.TaggedError<UserNotFound>()("UserNotFound", { id: Schema.Number }) {}

// Request type
interface GetUserById extends Request.Request<User, UserNotFound> {
  readonly _tag: "GetUserById";
  readonly id: number;
}
const GetUserById = Request.tagged<GetUserById>("GetUserById");

// Batched resolver
const UserResolver = RequestResolver.makeBatched((requests: ReadonlyArray<GetUserById>) =>
  Effect.gen(function* () {
    const ids = requests.map((r) => r.id);

    const response = yield* Effect.tryPromise({
      try: () =>
        fetch("/api/users/batch", {
          method: "POST",
          headers: { "Content-Type": "application/json" },
          body: JSON.stringify({ ids }),
        }).then((r) => r.json() as Promise<User[]>),
      catch: () => new Error("Batch fetch failed"),
    });

    yield* Effect.forEach(requests, (request, index) => {
      const user = response[index];
      return user
        ? Request.completeEffect(request, Effect.succeed(user))
        : Request.completeEffect(request, Effect.fail(new UserNotFound({ id: request.id })));
    });
  }),
);

// Query function
const getUserById = (id: number) => Effect.request(GetUserById({ id }), UserResolver);

// Usage - automatically batched
const program = Effect.gen(function* () {
  const todos = yield* getTodos();

  const owners = yield* Effect.forEach(todos, (todo) => getUserById(todo.ownerId), { concurrency: "unbounded" });

  return owners;
});
```

## Best Practices

1. **Use batching for N+1 scenarios** - Especially with databases/APIs
2. **Cache expensive computations** - Use Effect.cached
3. **Set appropriate TTLs** - Balance freshness vs performance
4. **Use Request deduplication** - Automatic with Effect.request
5. **Batch at API boundaries** - Group related requests

## Additional Resources

For comprehensive batching and caching documentation, consult `${CLAUDE_PLUGIN_ROOT}/references/llms-full.txt`.

Search for these sections:

- "Batching" for complete batching guide
- "Caching" for caching patterns
- "Cache" for Cache service
- "Caching Effects" for Effect.cached patterns