Understanding the lifetime of Varnish cached objects: TTL, grace, and keep

In the context of caching, the terms TTL (Time to Live), grace, and keep are often associated with controlling the lifetime of cached objects. These parameters are commonly used in caching systems like Varnish to manage how long an object should be considered valid and under what conditions it can still be served from the cache. Let’s explore each term:

  1. Time to Live (TTL):
    • Definition: TTL is the duration for which a cached object is considered fresh and can be served from the cache without checking the origin server.
    • Usage: When a request is made, the caching system checks if the object is still within its TTL. If yes, it serves the cached version; otherwise, it fetches a fresh copy from the origin server.
  2. Grace:
    • Definition: Grace is an extension of the TTL concept. It represents the additional time during which a cached object can be served even after its TTL has expired, while the caching system attempts to fetch a fresh copy from the origin server.
    • Usage: If a cached object’s TTL has expired but it is still within the grace period, the caching system may serve it while initiating a background request to the origin server to refresh the content.
  3. Keep:
    • Definition: Keep is another extension of the TTL concept, specifying the maximum time a cached object can be retained in the cache, irrespective of whether the TTL has expired.
    • Usage: Keep allows the caching system to retain objects in the cache for a longer duration, even if the TTL has expired. It is useful in scenarios where you want to keep certain objects in the cache for a fixed period, regardless of their freshness.

In summary:

  • TTL: Specifies how long an object remains valid in the cache.
  • Grace: Represents the additional time during which an expired object can still be served while a background fetch is attempted.
  • Keep: Defines the maximum duration an object can be retained in the cache, regardless of its TTL.

These parameters provide flexibility in balancing the need for serving fresh content and minimizing the load on the origin server by intelligently managing cached objects. Configuring TTL, grace, and keep values requires consideration of the specific requirements and characteristics of the cached content.

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