PLOG Tuesday evening

Laurence Rowe: Cache invalidation

Sharing experience from a non-Plone project. Cache invalidation is always hard. Pages may contain snippets from other pages, from other content items. It is hard to invalidate all pages that may contain such a snippet when updating this item.

ZODB caches the objects. Zope stores the transactions. A transaction has a list of objects that are changed. Other versions of the same object can then be removed from the cache.

I wondered if I could do this for caching pages in Varnish or in Elasticsearch. I tried this in the experimental.depends package. The first simple way kind-of worked. But new items that would need to end up on the page and were never there before, did not get inserted.

I looked at the Darcs version control system, which does similar things.

A folder contents page that shows the first five items, needs to take all items into account for possible inclusion.

I have an indexing process, asynchronous, that updates the info for anything that needs to be invalidated. It leads to a materialized view of the data, allowing faceted search across a deep join.

I think this principle could be extended to Plone. It would allow indexing to a page cache like varnish, or a search cache like Elasticsearch.

Varnish helps making often visited pages fast, it does not help with the long tail of pages that are hardly ever visited.