Fragment caching for expensive JSON serialization

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                  json.array! @posts do |post|
  json.cache! ['v1', post], expires_in: 1.hour do
    json.id post.id
    json.title post.title
    json.excerpt post.excerpt
    json.published_at post.published_at
    json.author do
      json.id post.author.id
      json.name post.author.name
      json.avatar_url post.author.avatar_url
    end
    json.comments_count post.comments_count
    json.likes_count post.likes_count
  end
end

Serializing complex ActiveRecord objects to JSON can consume significant CPU time, especially when rendering collections with nested associations. Fragment caching stores rendered JSON fragments in Redis keyed by a cache key that includes the record's cache_key_with_version, which automatically invalidates when the record updates. This approach delivers massive performance gains for read-heavy endpoints—instead of serializing hundreds of records on every request, I serve cached fragments and only serialize changed records. The cache key must include all dependencies, so I use touch: true on associations to invalidate parent caches when children change. For endpoints with heavy traffic, this can reduce response times from seconds to milliseconds.

Alex Kumar

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Fragment caching for expensive JSON serialization

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