Response headers

Response headers that might provide additional information

In addition to standard response headers, several custom headers are also returned. Not all of them have high relevance; some are present as telemetry data. The informative headers are described below.

Important response headers

X-U4-RemainingLimit

Contains the maximum number of calls that can be made in the current minute (sliding window) time-frame with the current subscription claim given by the X-U4-SubscriptionClaim header.

X-U4-QuotaType

As described in the limits section, APIs have separate inbound and outbound quotas (per 5 minutes and per 24 hours). This header provides information about which quota was affected by this call: INBOUND or OUTBOUND.

Retry-After

This is a standard response header, but due to its importance, we mention it here as well. When a response with status code 429 is returned, this header provides information regarding the number of seconds one should wait before the next attempt to make a call. The same information is also available in the response body, but this header can be used for applying automatic retry mechanisms in the caller code. Example: Retry-After: 32.

The following C# example demonstrates how to set up automatic retry behaviour based on the Retry-After header:

// filepath: Example retry policy implementation
using Microsoft.Extensions.DependencyInjection;
using Polly;
using Polly.Extensions.Http;
using System.Net.Http.Headers;

var builder = WebApplication.CreateBuilder(args);

builder.Services.AddHttpClient("RetryClient")
    .AddPolicyHandler(GetRetryPolicy());

static IAsyncPolicy<HttpResponseMessage> GetRetryPolicy()
{
    return HttpPolicyExtensions
        .HandleTransientHttpError()
        .OrResult(msg => msg.StatusCode == System.Net.HttpStatusCode.TooManyRequests || 
                        msg.StatusCode == System.Net.HttpStatusCode.ServiceUnavailable)
        .WaitAndRetryAsync(3, (retryAttempt, response, context) =>
        {
            var randomBuffer = TimeSpan.FromMilliseconds(500 + (Random.Shared.NextDouble() * 500)); // 500ms to 1000ms

            if (response.Result?.Headers?.RetryAfter != null)
            {
                var retryAfter = response.Result.Headers.RetryAfter;
                if (retryAfter.Delta.HasValue)
                {
                    return retryAfter.Delta.Value + randomBuffer;
                }
                else if (retryAfter.Date.HasValue)
                {
                    var delay = retryAfter.Date.Value - DateTimeOffset.UtcNow + randomBuffer;
                    return delay > TimeSpan.Zero ? delay : randomBuffer;
                }
            }
            return TimeSpan.FromSeconds(2); // fallback
        });
}

ERPx