Calculator to find optimal target n-vals for different ring configurations
rebar3 compile
Calculator can either be used as plugin or from Erlang shell. The idea is that you have a number of nodes to your proposal, think VMs in the cloud, but can also be Erlang nodes on your large machine. These nodes can be at different locations, where a location is something that could potentially go down with all the nodes on it.
The computed target n-val provides information about fault resistance against failing nodes, whereas the location n-val provides information about resistance against failing locations.
$ rebar3 shellIn the Erlang shell, use the function
-spec nvals(RingSize :: pos_integer(), NrNodes :: pos_integer(), NrLocations :: pos_integer()) -> ok.Where (for riak) the ring size is a power of 2. Typically number of nodes is at least as large as the number of locations.
Erlang/OTP 26 [erts-14.2.5] [source] [64-bit] [smp:12:12] [ds:12:12:10] [async-threads:1] [jit]
Eshell V14.2.5 (press Ctrl+G to abort, type help(). for help)
1> ring_calculator:nvals(32, 8, 4).
........................................................x................
Best solution:
Target Nval 8
Location val 4
Config [2,2,2,2]
Ring: A1 B1 C1 D1 A2 B2 C2 D2 A1 B1 C1 D1 A2 B2 C2 D2 A1 B1 C1 D1 A2 B2 C2 D2 A1 B1 C1 D1 A2 B2 C2 D2 (0 violations)
ok
2> ring_calculator:nvals(32, 8, 3).
........................................x
Best solution:
Target Nval 8
Location val 2
Config [4,4]
Ring: A1 B1 A2 B2 A3 B3 A4 B4 A1 B1 A2 B2 A3 B3 A4 B4 A1 B1 A2 B2 A3 B3 A4 B4 A1 B1 A2 B2 A3 B3 A4 B4 (0 violations)
ok
With 8 nodes and 4 locations at your proposal, you can get a target n-val of 8 and a location nval of 4. That means that you can configure the system in such a way that you can loose 3 locations or 3 nodes and still have one copy of all data available. Moreover, you have enough additional nodes to act take over the role of the lost nodes.
With 8 nodes and 3 locations at your proposal, you are unlucky and cannot use the third location effectively.
Add the plugin to your rebar config:
{project_plugins, [{ring_calculator, {git, "[email protected]:nhs-riak/ring_calculator.git", {branch, "main"}}}]}.Then just call your plugin directly in an existing application:
$ rebar3 ring_calculatorUsage: rebar3 ring_calculator [-s <ring_size>] [-n <nodes>]
[-l <locations>] [-v <verbose>]
-s, --ring_size size of riak ring (default 64)
-n, --nodes number of available riak nodes
-l, --locations number of physical locations you can run the nodes on
-v, --verbose print computation details
For a riak ring of size 64, we have listed a few solutions for N nodes and L locations:
| nodes | target n-val | locations | location val |
|---|---|---|---|
| 3 | 2 | 1 | 1 |
| 3 | 2 | 2 | 1 |
| 3 | 2 | 3 | 2 |
| 4 | 4 | 2 | 2 |
| 4 | 4 | 3 | 2 |
| 4 | 4 | 4 | 4 |
| 5 | 4 | 3 | 2 |
| 5 | 4 | 4 | 2 |
| 5 | 4 | 5 | 4 |
| 6 | 5 | 3 | 2 |
| 6 | 4 | 4 | 4 |
| 7 | 6 | 3 | 2 |
| 7 | 5 | 4 | 3 |
| 8 | 8 | 2 | 2 |
| 8 | 8 | 3 | 2 |
| 8 | 8 | 4 | 4 |
As you can see from that table, for 5 nodes and 4 locations you only get an location val of 2.
If you want to increase that value, you can either add one more location, or one more node.
This allows you to cost optimize certain solutions.