ActiveSupport::Cache::MemoryStore does not count the length of keys as it computes its size. This can lead to much more memory than expected being consumed, especially in the case where the keys are big (URLs, IP addresses, etc.) and the values are small, e.g., integers or booleans. In the example below a cache of size 100 bytes adds two entries…

gistfile1.txt

2.0.0p247 :034 > cache = ActiveSupport::Cache.lookup_store :memory_store, size: 100
 => <#ActiveSupport::Cache::MemoryStore entries=0, size=0, options={:size=>100}>
2.0.0p247 :035 > key_prefix = '*' * 50
 => "**************************************************"
2.0.0p247 :036 > cache.fetch("#{key_prefix}-1") { 0 }
 => 0
2.0.0p247 :037 > cache.fetch("#{key_prefix}-2") { 0 }
 => 0
2.0.0p247 :038 > cache
 => <#ActiveSupport::Cache::MemoryStore entries=2, size=8, options={:size=>100}>