Hashmod

This program will take a hostname, IP address, or other unique host identifier, to distribute an enabled / disabled flag across a cluster without coordination, using consistent hashing.

Use Case

The motiviation behind this project was to provide a stateless way to enable New Relic on a portion of the nodes in a cluster of web servers in order to reduce costs, but still have metrics on some of the servers. It was also an excuse to study a topic of interest.

By installing this binary in a Docker container and setting an environment variable based on the output in a container startup script, I was able to disable the New Relic agent on a portion of the servers, without requiring a centralized service.

Examples

# Use hostname (default) (use hostname, divide cluster into 2 buckets)
$ ./hashmod
false

# Use hostname, with options (use hostname, divide cluster into 4 buckets)
$ ./hashmod -input hostname -buckets 4
false

# Use input from stdin to test behavior
echo -e "abc.example.com\ndef.example.com" | ./hashmod_export -input stdin -buckets 2 -hashalg xxhash64
2018/12/09 23:45:47 index: 0
2018/12/09 23:45:47 line: abc.example.com
2018/12/09 23:45:47 enabled: false
2018/12/09 23:45:47 ----------
2018/12/09 23:45:47 index: 1
2018/12/09 23:45:47 line: def.example.com
2018/12/09 23:45:47 enabled: true
2018/12/09 23:45:47 ----------
2018/12/09 23:45:47 buckets: 2
2018/12/09 23:45:47 enabledCount: 1
2018/12/09 23:45:47 totalCount: 2
2018/12/09 23:45:47 percent: 50.00%

# Test against a list of 100 random IP addresses
./hashmod_export -input stdin -hashalg xxhash64 -buckets 2 < ips_random_100.txt
2018/12/10 00:03:58 index: 0
2018/12/10 00:03:58 line: 46.212.162.110
2018/12/10 00:03:58 enabled: true
2018/12/10 00:03:58 ----------
...
2018/12/10 00:05:59 index: 99
2018/12/10 00:05:59 line: 208.185.157.13
2018/12/10 00:05:59 enabled: true
2018/12/10 00:05:59 ----------
2018/12/10 00:05:59 buckets: 2
2018/12/10 00:05:59 enabledCount: 55
2018/12/10 00:05:59 totalCount: 100
2018/12/10 00:05:59 percent: 55.00%