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Sequential Proxying in KrakenD API Gateway

Document updated on Oct 15, 2023

Sequential Proxying in KrakenD API Gateway

The best experience consumers can have with KrakenD API is by letting the system fetch all the data from the different backends simultaneously. However, sometimes you need to delay a backend call until you have called a previous service. Although this is not ideal, the sequential proxy allows you to chain backend requests.

Do you really need a sequential proxy?

Chained calls are considered an anti-pattern

Using sequential calls is considered an anti-pattern because when you make a network service dependent on the other, you are increasing the latency, decreasing the performance, and augmenting the error rate.

It is much worse than using aggregation. In aggregation, parallel requests execute simultaneously, whereas, in sequential aggregation, requests are executed one at a time, with each one waiting for the previous request to finish before moving on to the next. If a backend in a sequence fails, the process aborts, and the next backend is never reached, so there are many more chances that your users are left without data.

From an error rate perspective, the nature of sequential proxy performs more deficient: Suppose you have three backends with an error rate of 10% each, then the probability of success separately in each is 90%. But when executing the three of them sequentially, the success rate drops to 73% (because 0.9 * 0.9 * 0.9 = 0.729).

In an aggregation scenario, the probability of having at least one working call is the opposite of the likelihood of having all calls result in errors. So, the chance of all three calls resulting in errors is 0.1% (because 0.1 * 0.1 * 0.1 = 0.001).

The contrast between a 99.9% chance of some data availability and a 73% probability is quite substantial. Isn’t it? That being said, from an architectural point of view, the sequential proxy should be your last resort.

In addition, since KrakenD 2.5, you can add to a sequence multiple unsafe methods (methods different than GET). When you chain several write requests in multiple nodes, you execute a distributed transaction in a flowery disguise, as in a database. But a gateway is not a database, and you don’t have any rollback mechanism if one of your write methods fails, so you can only hope for the best.

Sequential proxy configuration

To enable the sequential proxy, you need to add in the endpoint definition the following configuration:

{
    "endpoint": "/hotels/{id}",
    "extra_config": {
          "proxy": {
              "sequential": true
          }
      }
}

After doing this, the list of backend is executed one by one, and the next call has access to the data returned by the previous in the url_pattern and not anywhere else. The body of the last request is not sent to the next (but you can still access the original body).

When the sequential proxy is enabled, the url_pattern of every backend can use a new variable that references the response of a previous API call. The variable has the following construction:

{resp0_XXXX}

Where 0 is the index of the specific backend you want to access (0 is the first backend), and where XXXX is the attribute name you want to inject from the response of the previous call. You can access nested objects of the response using the dot notation. For example, given a response {"user": { "hash": "abcdef }}, the variable{resp0_user.hash} will contain the value abcdef. You cannot access nested objects inside arrays or collections: responses must be objects.

If the encoding of your backend is string, then you can access its contents using resp0_content.

It does not matter if the {resp0_XXXX} variable is part of the URL or if it is passed as a query string. For instance, the following examples would work:

{
    "url_pattern": "/user/{resp0_user.hash}"
}

And also:

{
    "url_pattern": "/user?hash={resp0_user.hash}"
}

Example

It’s easier to understand with the example of the graph:

Chained call

The user calls the gateway with an URL like /hotel-destinations/{id}, which needs to fetch the hotel information and all its associated destinations. Let’s say the ID they request is 25. The gateway calls a backend /hotels/25 that returns data for the requested hotel, including a destination_id field that is a relationship identifier. The output for GET /hotels/25 is like the following:

{
    "hotel_id": 25,
    "name": "Hotel California",
    "destination_id": 1034
}

KrakenD waits for the backend response and injects the value of destination_id in the URL of the next backend call. In this case, the next call is GET /destinations/1034, and the response is:

{
    "destination_id": 1034,
    "destinations": [
        "LAX",
        "SFO",
        "OAK"
    ]
}

Now KrakenD has both responses from the backends and can merge the data, returning the following aggregated object to the user:

{
    "hotel_id": 25,
    "name": "Hotel California",
    "destination_id": 1034,
    "destinations": [
        "LAX",
        "SFO",
        "OAK"
    ]
}

The configuration needed for this example is:

{
    "endpoint": "/hotel-destinations/{id}",
    "backend": [
        {
            "@comment": "This is the index position 0",
            "host": [
                "https://hotels.api"
            ],
            "url_pattern": "/hotels/{id}"
        },
        {
            "@comment": "This is the index position 1",
            "host": [
                "https://destinations.api"
            ],
            "@comment2": "resp0_ is the response of index position 0",
            "url_pattern": "/destinations/{resp0_destination_id}"
        }
    ],
    "extra_config": {
        "proxy": {
            "sequential": true
        }
    }
}

The key here is the variable {resp0_destination_id} that refers to destination_id for the backend with index 0 (first in the list).

Scarf

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