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    A Mongo-based Cache Plugin for Play

    on October 23, 2014

    A quick engineering-related post: we built a cache plugin for Play that uses capped collections in Mongo. It's available on Github if you'd like to use it.

    Motivation

    Domino's web frontend is built in Scala, on Play — and, for now, somewhat begrudgingly, hosted on Heroku.

    We have multiple dynos running, and we want each dyno to access the same cache instance. So a cache that lives in memory of the web process, or on disk of the machine, isn't going to work.

    We initially tried using Memcached via Memcachier but we were getting intermittent timeouts that we could never resolve, even with help from the Memcachier folks.

    Solution

    Capped Collections in Mongo have characteristics that work pretty well for a cache: fast writes and FIFO eviction. Our plugin augments that with an optional expiration time for each cached item.

    Instructions for using this yourself are in the readme file in the project.

    Caveats

    This is not meant for insane throughput. We are not doing anything at "web scale" and I doubt this solution would hold up under massive load. This is a fairly basic solution that's probably good enough for many use cases — not something engineered to be industrial strength.

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