Consistent Hash load balancer

This project is a sample proof of concept on using consistent hash as a load balancer.

This uses the awesome projects from consistent-hash from buraksezer/consistent and gofiber to serve endpoints.

Excellent articles / videos about consistent hashing

This project is a simple implementation of a look aside load balancer using consistent hash.

The use case

Our use case is to be able to route a key to a particular node in order to make use of the local cache held in that node. The process that needs to be performed by this node for this particular key must be in order.

As an example: Calculating an account balance

Using Consistent hash, we can direct the request from an account number to a specific node.

Usage

First update the list of cluster members, an example is here

infrastructure:
    nodes:
    - name: localhost:10001
    - name: localhost:10002
    - name: localhost:10003
    - name: localhost:10004
    - name: localhost:10005
    - name: localhost:10006
    - name: localhost:10007
    - name: localhost:10008
    - name: localhost:10009
    - name: localhost:10010

When you boostrap the consistent hash, it will automatically populate the hash ring with these members.

Run the lb

./consistent-demo balancer --config ./hack/config.yaml

Pick the member

curl http://localhost:3000/pick/12345

This will return a json

{"previousNode":"","currentNode":"localhost:10003"}

The previousNode is to help the participating service to determine if the request was previously handled by it. This can be very useful to tell the service that please refresh you local cache, since the previous request was not sent to you.

The client code can now use the currentNode to send its request to.

Adding a node

Say you want to add a node localhost:9999 as a member.

curl --location --request POST 'http://localhost:3000/node/' --form 'node="localhost:9999"'

Removing a node

Say you want to remove the node localhost:9999 from membership. Example, the node was taken down for service.

curl --location --request DELETE 'http://localhost:3000/node/' --form 'node="localhost:9999"'

Why look aside

The look aside load balancer frees the load balancer from proxying the request to the intended service. This can be kept really light and should be able to serve plenty of requests.

Miscellaneous

There is an accompanying simple service which doesn't do much, except print out whether the previousNode is the same as the currentNode

To use it.

./consistent-demo simple --port 10000

Health Check

The consistent hash balancer regularly performs health checks based on the list of members defined in the config.yaml

If it cannot reach the member, it will remove it from the consistent hash. When it comes back online, it will add the node back to the hash.

The health check is dont every 200ms.

Distribution

To get the partition distribution for each node

curl --location --request GET 'http://localhost:3000/list'

Will return you the following:

{
    "localhost:10000": 5,
    "localhost:10001": 5,
    "localhost:10002": 6,
    "localhost:10003": 7,
    "localhost:10005": 6,
    "localhost:10006": 4,
    "localhost:10007": 5,
    "localhost:10008": 2,
    "localhost:10009": 7,
    "localhost:10010": 3
}

balchua/consistent-hash-demo