A web-caching policy emulator.
Switch branches/tags
Nothing to show
Clone or download
Fetching latest commit…
Cannot retrieve the latest commit at this time.
Type Name Latest commit message Commit time
Failed to load latest commit information.


Edgecast Caching Emulator

A web-caching policy emulator.

standard-readme compliant

The Edgecast Caching Emulator (ECE) is a tool for comparing the impacts of arbitrary caching policies on cache performance for fixed workloads. In particular, it is intended to consume log data from production caching servers and provide insight into how the cache would behave, both in terms of traditional cache metrics (hit rate, eviction rate, etc.) and more tangential metrics (disk writes, origin pulls, etc.).

Table of Contents


This project was developed to allow for the easy implementation and comparison of arbitrary caching policies. Doing this in an isolated environment allows one to avoid having to implement each potential policy in production, which can be both time consuming and risky. Furthermore, the system is log-driven, and is designed to be driven by production traffic traces. Since it uses these production workloads, we dubbed this system an emulator, rather than a simulator.

The Edgecast Caching Emulator includes the following features.

  • Cache Admission
  • N-hit caching: items are cached on the n-th request
  • Probabilistic caching: items are cached based on a pre-defined probability value
  • Cache Eviction
  • FIFO: Items are purged in first-in-first-out order
  • Regular LRU: items are purged in a least recently used/requested fashion
  • Size-based LRU: during each purge, the largest sized item from a list of X least recently used items is deleted

The emulator provides data from various metrics to show the performance of such techniques over time, including hit and byte-hit ratios (overall and per customer), cache reads and writes, and reads from origin


To install the ECE, first clone the repo:

git clone https://github.com/EdgeCast/edgecast_caching_emulator.git

Simply hop into the directory and run make:

cd edgecast_caching_emulator


The Makefile will use clang++ by default, but you can override the default with another compiler of your choice:

CPP=g++ make


The ECE comes with a ready made set of cache admission (probabilistic, second-hit caching) and eviction (LRU, FIFO, S4LRU) policies. To run a ready made example script will perform an example using Second-Hit Caching and LRU:

./run_em.sh <log directory> 2hc_lru

Where <log directory> is a directory containing logs of the appropriate format. Sample logs are provided in the directory input_request_sequence.

Output will be placed in out/<timestamp>.

A script is included that provides a simple plot of the hit rates, along with examples of how to parse the output format. Please note, this script requires matplotlib and numpy - they are not required for the emulator itself. Run this script with the following command:

python analysis/parse.py out/<timestamp>/


  • Input to the emulator is a list of individual asset request entries. Each entry is a new line, with the following columns:
  1. Timestamp
  2. Size
  3. Destination port (e.g. 80 or 443) - unused column
  4. Cache status code (e.g.: TCP_HIT/200, TCP_HIT/206 TCP_MISS/200, TCP_PARTIAL_HIT/200, etc.)
  5. Bytes out (bytes served from the requested file, could be smaller in case of range requests)
  6. URL (asset requested)
  • Output is stored in a .dat file (e.g. out/29_Nov_17-18_54/lru_2hc.dat).

Below is an example of the dat file entry: Emulator_periodic_reporting 1503625504 32720333373821 213587 | ghr 0.623736 0.998646 0.720994 0.999307 | cache 0.623736 0.998646 133222 80365 32676017633906 44315739915 63820417982 42267494 0 44315739915 : 2hc_r ot 0.252592 : lru 229997792359 0.0311111

In the above sample output log entry, each line is an aggregate of 15 minutes of requests processed. Each line begins with emulator_periodic_reporting key word. This line can then be parsed by any plotting script to generate figures. In this repo one such script is analysis/parse.py.

Below is the definition of each column in the output line above.

  1. emulator_periodic_reporting - static keyword
  2. 1503625504 - timestamp of the output (once very 15 mins)
  3. 32720333373821 - Total traffic seen in the last timestamp (in bytes)
  4. 213587 - Total numer of URLs/files seen in last chunk
  5. | - delimiter (below we have global stats)
  6. ghr - global stats (hit rate and byte-hit rate)
  7. 0.623736 - total hit rate
  8. 0.998646 - total byte hit rate
  9. 0.720994 - total hit rate - perfect cache (admission on first hit, no eviction)
  10. 0.999307 - total byte hit rate - perfect cache
  11. | delimiter (below we have cache specific stats; followed by two colons (:))
  12. cache - has cache specific stats (next 10 columns, i.e. 13 through 22.) Each new cache/storage tier we add begins with keyword 'cache'. The output here is ordered how they were tied together in the emulator. Legend for the next 10 columns can be found here: analysis/parse.py#L79
  13. : - delimiter (below have the cache admission specific stats)
  14. 2hc_rot - cache admission scheme name
  15. 0.252592 - bloom filter (used for keeping track of hit count) fill rate
  16. : - delimiter (below we have the cache eviction specific stats)
  17. lru - cache eviction scheme name
  18. 229997792359 - size of all content on disk
  19. 0.0311111 - oldest file age
  • Sample input logs can be found in directory: input_request_sequence
  • Sample emulator output can be found in directory: out

Adding Additional Experiments

Additional experiments can be written and placed in the src/ directory. The idea is similar to NS-3, where individual experiments can be written as stand alone main() functions that instantiate an emulator object, add caches, and then consume stdin.

The run_em.sh script can then be called with a list of binaries, i.e.

./run_sum.sh <log directory> lru_2hc my_experimet1 my_experiment2

Adding Policies

Additional policies can be added to lib/ (and headers to include/, as needed) and can inherit from the appropriate class in cache_policy.cc (i.e. admission or eviction). cache_policy.cc defines two classes that provide the necessary framework for creating a cache policy. In particular, mimicking the definitions of the following components would be a good place to start:

  • size admission (include/size_admission.h and lib/size_admission.cc) for admission policies.
  • fifo (include/fifo.h and lib/fifo.cc) for eviction policies.

New policies that implement custom periodic_output functions will also need new parsing functions; these can be implemented at the top of parse.py.


Please refer to the contributing.md file for information about how to get involved. We welcome issues, questions, and pull requests. Pull Requests are welcome. We are particularly interested in implementations of additional caching policies.


This project is licensed under the terms of the Apache 2.0 open source license. Please refer to LICENSE for the full terms.