Module: Air18n::ChunkCache

Defined in:
lib/air18n/chunk_cache.rb

Constant Summary collapse

MAX_CHUNK_SIZE =

Define constants and cache keys.

We divide things into chunks of 900000 bytes to accomodate memcaches with 1mb max value size.

900000
CHECKSUM_CACHE_KEY = 
"%s_0_nchunks+checksum"
CHUNK_CACHE_KEY = 
"%s_%d"

Class Method Summary collapse

Class Method Details

.get(cache, key) ⇒ Object

This returns nil if the chunks are corrupt or missing.



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# File 'lib/air18n/chunk_cache.rb', line 38

def get(cache, key)
  buffer = ""
  chunk = cache.read(CHECKSUM_CACHE_KEY % key)
  if chunk
    num_chunks, checksum = JSON.parse(chunk)
  else
    LoggingHelper.error "ChunkCache get: no checksum stored for #{key}"
    return nil
  end

  for i in 1..num_chunks
    chunk = cache.read(CHUNK_CACHE_KEY % [key, i])

    if chunk.nil?
      # Return nil if any chunk is missing.
      LoggingHelper.error "ChunkCache get: chunk #{CHUNK_CACHE_KEY % [key, i]} is missing"
      return nil
    end

    buffer << chunk
  end

  if Zlib::crc32(buffer) != checksum
    LoggingHelper.error "ChunkCache get: invalid checksum for #{key}"
    return nil
  else
    return JSON.parse(buffer)[0]
  end
end

.set(cache, key, value, expires_in) ⇒ Object 



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# File 'lib/air18n/chunk_cache.rb', line 19

def set(cache, key, value, expires_in)
  # We put the value in an array because plain strings are not valid JSON.
  value_json = [value].to_json

  # We use divmod to avoid the edge condition where the floating point math
  # result is 5.000001 and the ceil would result in 6.
  quotient, modulus = value_json.bytesize.divmod(MAX_CHUNK_SIZE)
  num_chunks = modulus > 0 ? quotient + 1 : quotient
  cache.write(CHECKSUM_CACHE_KEY % key, [num_chunks, Zlib::crc32(value_json)].to_json)

  1.upto(num_chunks) do |i|
    chunk = value_json.byteslice((i-1)*MAX_CHUNK_SIZE, MAX_CHUNK_SIZE)
    if cache.write(CHUNK_CACHE_KEY % [key, i], chunk, :expires_in => expires_in) == false
      LoggingHelper.error "ChunkCache set: failed to write chunk #{CHUNK_CACHE_KEY % [key, i]} (#{chunk.size} chars /  #{chunk.bytesize} bytes)"
    end
  end
end