What is IDistributedCache, and how does it differ from IMemoryCache in a multi-instance deployment?

IDistributedCache is an abstraction over an external, shared cache store (like Redis or SQL Server) that all instances of a horizontally-scaled application read from and write to consistently, unlike IMemoryCache where each instance maintains its own separate, unsynchronized in-process cache. This ensures cache coherency across multiple servers/pods -- a value cached by one instance is immediately visible to all others -- at the cost of network latency per cache operation compared to in-memory access.

builder.Services.AddStackExchangeRedisCache(options => {
    options.Configuration = "localhost:6379";
});

public class ProductService {
    private readonly IDistributedCache _cache;
    public async Task<Product?> GetProductAsync(int id) {
        var cached = await _cache.GetStringAsync($"product-{id}");
        if (cached != null) return JsonSerializer.Deserialize<Product>(cached);
        // ... fetch and cache
    }
}