Can i use chash as a load balancing algorithm? after test i find this seems not distribute requests equally.

[Cong Wu]

here is my nginx.conf

gist https://gist.github.com/woodgear/d4de35b30be303900233ccf00bf2b554

```ningx.conf

events {
    worker_connections  1024;
}

http {
    error_log /var/log/nginx/error.log info;
    access_log /var/log/nginx/access.log;

    lua_package_path '/usr/local/lib/lua/?.lua;;';
    lua_package_cpath '/usr/local/lib/lua/?.so;;';

    client_max_body_size           0;
    server {
        listen 81;

        location /ip-test {
             content_by_lua_block {
                 local resty_chash = require "resty.chash"
                 local servers={
                 }

              servers["10.244.1.10:80"]=25
              servers["10.244.1.11:80"]=25
              servers["10.244.1.12:80"]=25
              servers["10.244.1.13:80"]=25

                 local keys={
                    "10.244.0.17",
                    "10.244.0.16",
                    "10.244.0.20",
                    "10.244.0.14",
                    "10.244.0.19",
                    "10.244.0.24",
                    "10.244.0.21",
                    "10.244.0.28",
                    "10.244.0.25",
                    "10.244.0.3",
                    "10.244.0.23",
                    "10.244.0.27",
                    "10.244.0.2",
                    "10.244.0.22",
                    "10.244.0.29",
                    "10.244.0.26",
                    "10.244.0.7",
                    "10.244.0.15",
                    "10.244.0.13",
                    "10.244.0.61"
                 }

                 local counter={}
                 local chash = resty_chash:new(servers)
                 for i,key in ipairs(keys) do
                     local backend, idx = chash:find(key)
                     ngx.log(ngx.NOTICE,"key "..key.. " backend " ..backend.. "idx " ..idx)
                     if counter[backend] ==nil then
                        counter[backend]=1
                     else
                       counter[backend]=counter[backend]+1
                     end
                 end

                 for b,c in pairs(counter) do
                      ngx.log(ngx.NOTICE," backend " ..b.. " count " ..c)
                 end

                 ngx.log(ngx.NOTICE," len " ..#keys)
                 ngx.say("ok")
             }
        }
    }
}

```

the result is like

```

backend 10.244.1.11:80 count 3

backend 10.244.1.12:80 count 9

backend 10.244.1.13:80 count 3

backend 10.244.1.10:80 count 5

```

should i expect this count will be always ~5

[Piotr Przybylski]

The count will be more or less the same given varied enough input - this algorithm was made to be fast and work for diverse data.

If it fails to work for your use case then you may implement your own consistent hashing, using consistent hashing ring or HRW algorithm [1]. Sounds complicated but when you look at the code example in Wikipedia it's really easy. And you can change underlying hash function to one that works well with your inputs.

[1] https://en.wikipedia.org/wiki/Rendezvous_hashing

[DeJiang Zhu]

Hi,

The number of keys is too small. you can try more keys.

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