Load Balancer¶
Ocelot can load balance across available downstream services for each Route. This means you can scale your downstream services and Ocelot can use them effectively.
The types of load balancer available are:
LeastConnection tracks which services are dealing with requests and sends new requests to service with least existing requests. The algorithm state is not distributed across a cluster of Ocelot’s.
RoundRobin loops through available services and sends requests. The algorithm state is not distributed across a cluster of Ocelot’s.
NoLoadBalancer takes the first available service from config or service discovery.
CookieStickySessions uses a cookie to stick all requests to a specific server. More info below.
You must choose in your configuration which load balancer to use.
Configuration¶
The following shows how to set up multiple downstream services for a Route using ocelot.json and then select the LeastConnection load balancer.
This is the simplest way to get load balancing set up.
{
"UpstreamPathTemplate": "/posts/{postId}",
"UpstreamHttpMethod": [ "Put", "Delete" ],
"DownstreamPathTemplate": "/api/posts/{postId}",
"DownstreamScheme": "https",
"DownstreamHostAndPorts": [
{ "Host": "10.0.1.10", "Port": 5000 },
{ "Host": "10.0.1.11", "Port": 5000 }
],
"LoadBalancerOptions": {
"Type": "LeastConnection"
}
}
Service Discovery¶
The following shows how to set up a Route using service discovery then select the LeastConnection load balancer.
{
// ...
"ServiceName": "product",
"LoadBalancerOptions": {
"Type": "LeastConnection"
}
}
When this is set up Ocelot will lookup the downstream host and port from the service discover provider and load balance requests across any available services. If you add and remove services from the service discovery provider (Consul) then Ocelot should respect this and stop calling services that have been removed and start calling services that have been added.
Custom Load Balancers¶
David Lievrouw implemented a way to provide Ocelot with custom load balancer in PR 1155 (his issue 961).
In order to create and use a custom load balancer you can do the following.
Below we setup a basic load balancing config and not the Type is CustomLoadBalancer which is the name of a class we will setup to do load balancing.
{
"UpstreamPathTemplate": "/posts/{postId}",
"UpstreamHttpMethod": [ "Put", "Delete" ],
"DownstreamPathTemplate": "/api/posts/{postId}",
"DownstreamScheme": "https",
"DownstreamHostAndPorts": [
{ "Host": "10.0.1.10", "Port": 5000 },
{ "Host": "10.0.1.11", "Port": 5000 }
],
"LoadBalancerOptions": {
"Type": "CustomLoadBalancer"
}
}
Then you need to create a class that implements the ILoadBalancer interface. Below is a simple round robin example:
public class CustomLoadBalancer : ILoadBalancer
{
private readonly Func<Task<List<Service>>> _services;
private readonly object _lock = new object();
private int _last;
public CustomLoadBalancer(Func<Task<List<Service>>> services)
{
_services = services;
}
public async Task<Response<ServiceHostAndPort>> Lease(HttpContext httpContext)
{
var services = await _services?.Invoke();
lock (_lock)
{
if (_last >= services.Count)
_last = 0;
var next = services[_last++];
return new OkResponse<ServiceHostAndPort>(next.HostAndPort);
}
}
public void Release(ServiceHostAndPort hostAndPort) { }
}
Finally, you need to register this class with Ocelot.
We have used the most complex example below to show all of the data / types that can be passed into the factory that creates load balancers.
Func<IServiceProvider, DownstreamRoute, IServiceDiscoveryProvider, CustomLoadBalancer> loadBalancerFactoryFunc =
(serviceProvider, Route, serviceDiscoveryProvider) => new CustomLoadBalancer(serviceDiscoveryProvider.Get);
services.AddOcelot()
.AddCustomLoadBalancer(loadBalancerFactoryFunc);
However, there is a much simpler example that will work the same:
services.AddOcelot()
.AddCustomLoadBalancer<CustomLoadBalancer>();
There are numerous extension methods to add a custom load balancer and the interface is as follows:
IOcelotBuilder AddCustomLoadBalancer<T>()
where T : ILoadBalancer, new();
IOcelotBuilder AddCustomLoadBalancer<T>(Func<T> loadBalancerFactoryFunc)
where T : ILoadBalancer;
IOcelotBuilder AddCustomLoadBalancer<T>(Func<IServiceProvider, T> loadBalancerFactoryFunc)
where T : ILoadBalancer;
IOcelotBuilder AddCustomLoadBalancer<T>(Func<DownstreamRoute, IServiceDiscoveryProvider, T> loadBalancerFactoryFunc)
where T : ILoadBalancer;
IOcelotBuilder AddCustomLoadBalancer<T>(Func<IServiceProvider, DownstreamRoute, IServiceDiscoveryProvider, T> loadBalancerFactoryFunc)
where T : ILoadBalancer;
When you enable custom load balancers Ocelot looks up your load balancer by its class name when it decides if it should do load balancing. If it finds a match, it will use your load balaner to load balance. If Ocelot cannot match the load balancer type in your configuration with the name of registered load balancer class then you will receive a HTTP 500 Internal Server Error. If your load balancer factory throw an exception when Ocelot calls it, you will receive a HTTP 500 Internal Server Error.
Remember, if you specify no load balancer in your config, Ocelot will not try and load balance.