Request Aggregation

Ocelot allow’s you to specify Aggregate ReRoutes that compose multiple normal ReRoutes and map their responses into one object. This is usual where you have a client that is making multiple requests to a server where it could just be one. This feature allows you to start implementing back end for a front end type architecture with Ocelot.

This feature was requested as part of Issue 79 and further improvements were made as part of Issue 298.

In order to set this up you must do something like the following in your ocelot.json. Here we have specified two normal ReRoutes and each one has a Key property. We then specify an Aggregate that composes the two ReRoutes using their keys in the ReRouteKeys list and says then we have the UpstreamPathTemplate which works like a normal ReRoute. Obviously you cannot have duplicate UpstreamPathTemplates between ReRoutes and Aggregates. You can use all of Ocelot’s normal ReRoute options apart from RequestIdKey (explained in gotchas below).

Advanced register your own Aggregators

Ocelot started with just the basic request aggregation and since then we have added a more advanced method that let’s the user take in the responses from the downstream services and then aggregate them into a response object.

The ocelot.json setup is pretty much the same as the basic aggregation approach apart from you need to add an Aggregator property like below.

{
    "ReRoutes": [
        {
            "DownstreamPathTemplate": "/",
            "UpstreamPathTemplate": "/laura",
            "UpstreamHttpMethod": [
                "Get"
            ],
            "DownstreamScheme": "http",
            "DownstreamHostAndPorts": [
                {
                    "Host": "localhost",
                    "Port": 51881
                }
            ],
            "Key": "Laura"
        },
        {
            "DownstreamPathTemplate": "/",
            "UpstreamPathTemplate": "/tom",
            "UpstreamHttpMethod": [
                "Get"
            ],
            "DownstreamScheme": "http",
            "DownstreamHostAndPorts": [
                {
                    "Host": "localhost",
                    "Port": 51882
                }
            ],
            "Key": "Tom"
        }
    ],
    "Aggregates": [
        {
            "ReRouteKeys": [
                "Tom",
                "Laura"
            ],
            "UpstreamPathTemplate": "/",
            "Aggregator": "FakeDefinedAggregator"
        }
    ]
}

Here we have added an aggregator called FakeDefinedAggregator. Ocelot is going to look for this aggregator when it tries to aggregate this ReRoute.

In order to make the aggregator available we must add the FakeDefinedAggregator to the OcelotBuilder like below.

services
    .AddOcelot()
    .AddSingletonDefinedAggregator<FakeDefinedAggregator>();

Now when Ocelot tries to aggregate the ReRoute above it will find the FakeDefinedAggregator in the container and use it to aggregate the ReRoute. Because the FakeDefinedAggregator is registered in the container you can add any dependencies it needs into the container like below.

services.AddSingleton<FooDependency>();

services
    .AddOcelot()
    .AddSingletonDefinedAggregator<FooAggregator>();

In this example FooAggregator takes a dependency on FooDependency and it will be resolved by the container.

In addition to this Ocelot lets you add transient aggregators like below.

services
    .AddOcelot()
    .AddTransientDefinedAggregator<FakeDefinedAggregator>();

In order to make an Aggregator you must implement this interface.

public interface IDefinedAggregator
{
    Task<DownstreamResponse> Aggregate(List<DownstreamResponse> responses);
}

With this feature you can pretty much do whatever you want because DownstreamResponse contains Content, Headers and Status Code. We can add extra things if needed just raise an issue on GitHub. Please note if the HttpClient throws an exception when making a request to a ReRoute in the aggregate then you will not get a DownstreamResponse for it but you would for any that succeed. If it does throw an exception this will be logged.

Basic expecting JSON from Downstream Services

{
    "ReRoutes": [
        {
            "DownstreamPathTemplate": "/",
            "UpstreamPathTemplate": "/laura",
            "UpstreamHttpMethod": [
                "Get"
            ],
            "DownstreamScheme": "http",
            "DownstreamHostAndPorts": [
                {
                    "Host": "localhost",
                    "Port": 51881
                }
            ],
            "Key": "Laura"
        },
        {
            "DownstreamPathTemplate": "/",
            "UpstreamPathTemplate": "/tom",
            "UpstreamHttpMethod": [
                "Get"
            ],
            "DownstreamScheme": "http",
            "DownstreamHostAndPorts": [
                {
                    "Host": "localhost",
                    "Port": 51882
                }
            ],
            "Key": "Tom"
        }
    ],
    "Aggregates": [
        {
            "ReRouteKeys": [
                "Tom",
                "Laura"
            ],
            "UpstreamPathTemplate": "/"
        }
    ]
}

You can also set UpstreamHost and ReRouteIsCaseSensitive in the Aggregate configuration. These behave the same as any other ReRoutes.

If the ReRoute /tom returned a body of {“Age”: 19} and /laura returned {“Age”: 25} the the response after aggregation would be as follows.

{"Tom":{"Age": 19},"Laura":{"Age": 25}}

At the moment the aggregation is very simple. Ocelot just gets the response from your downstream service and sticks it into a json dictionary as above. With the ReRoute key being the key of the dictionary and the value the response body from your downstream service. You can see that the object is just JSON without any pretty spaces etc.

All headers will be lost from the downstream services response.

Ocelot will always return content type application/json with an aggregate request.

If you downstream services return a 404 the aggregate will just return nothing for that downstream service. It will not change the aggregate response into a 404 even if all the downstreams return a 404.

Gotcha’s / Further info

You cannot use ReRoutes with specific RequestIdKeys as this would be crazy complicated to track.

Aggregation only supports the GET HTTP Verb.