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Memflash is a gem which enables storing really long values in the Rails FlashHash without writing them to the session. Instead, it transparently uses Rails.cache, thus enabling the flash in your actions to contain large values, and still fit in a cookie-based session store.

How do I use it?

Memflash is completely transparent -- requiring the gem automatically enhances FlashHash with caching-enabled reads and writes.

By default, any message that is over 300 characters long, will be saved in Rails.cache, and a pointer to it stored in the flash. You can change this anywhere in your app by:

Memflash.threshold = #{length-at-which-writing-to-the-cache-is-trigerred}

How does it work?

Memflash ties into the [] and []= methods of Rails's FlashHash. On writes, if the value being written has a length over Memflash.threshold, Memflash generates a pseudo-random key for it, writes the pseudo-random key and original value to Rails.cache, and stores the original key and pseudo-random key in the flash. Conceptually, when you write flash[:error] = "some message", this is equivalent to:

if "some message".length >=  Memflash.threshold
  # generate a psedudo-random key, memflash_key
  # write memflash_key, "some message" to Rails.cache
  # write :error, memflash_key to the flash
  # write :error, "some message" to the flash, as usual

On the flip side, reading from the flash, flash[:error] is conceptually equivalent to:

if the value for :error stored in the flash is a memflash key
  # read the original (large) value from Rails.cache
  # return the original (large) value
  # return the value stored in the flash, as usual


Authored by Vladimir Andrijevik

Copyright 2013 Zendesk

Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except in compliance with the License. You may obtain a copy of the License at

Unless required by applicable law or agreed to in writing, software distributed under the License is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the License for the specific language governing permissions and limitations under the License.