Flattening Paged Responses with IAsyncEnumerables

I’ve recently found myself in a situation where I’m calling APIs that return a collection of items in pages. Especially when calling OData APIs (looking at you Microsoft Graph). Usually these come in the form of a response model that wraps the array of items and a continuation token, or a next link. Something like this:

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{
    "values": [...],
    "nextLink": "someUrl"
}

we can represent that easily with a Generic class:

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public class PagedResponse<TResult> where TResult : class
{
    public IList<TResult> Values { get; set; }

    public string NextLink { get; set; }
}

When I’m building APIs that need to iterate across the entire list, I have to do some logic to loop over the values, check if there’s a nextLink and then repeat the process again. Now, I’m not a huge fan of repeating code everywhere, so I think it would be nice if I could move that logic somewhere nice and get the entire list. I could build a method that looks like this:

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Task<IEnumerable<TResult>> GetEntireList<TResult>(Func<string, Task<PagedResponse<TResult>>> getPage);

The downside to doing this is that I need to download the entire collection into memory before I can start iterating. This is terrible if I only end up wanting to work with a subset of the items. Especially if the full collection could be arbitrarily large (trying to load a few GB into memory could be problematic). What would be nicer is if I could return an IEnumerable that was also awaitable.

This is the exact scenario where IAsyncEnumerable saves the day. Introduced in C# 8, IAsyncEnumerable is both an IEnumerable and awaitable. Now I can make a method like:

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IAsyncEnumerable<TResult> GetEntireList<TResult>(Func<string, Task<PagedResponse<TResult>>> getPage);

Now all we have to do is build a loop to fetch a page, return the elements from that page, then loop back around and do it again. The implementation for this is actually super clean:

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public static async IAsyncEnumerable<TResult> GetEntireList<TResult>(Func<string, Task<PagedResponse<TResult>>> getPage) {
    string nextLink = null;

    do {
        var currentPage = await getPage(nextLink);

        // Stop if the page is missing
        if (currentPage == null) {
            yield break;
        }

        nextLink = currentPage.NextLink;

        foreach (var item in currentPage.Values) {
            yield return item;
        }
    } while (!string.IsNullOrEmpty(nextLink));
}

Let’s go over what all of that is doing.

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public static async IAsyncEnumerable<TResult> GetEntireList<TResult>(Func<string, Task<PagedResponse<TResult>>> getPage) {

For the purposes of this example, the getPage delegate will be a function that takes in a nextLink and returns an awaitable Task containing the next PagedResponse. This allows the caller of this method to used whatever source they want to fetch the pages, as long as that source returns a PagedResponse. We’ll also assume that a null value passed in means we want the first page. We could also define the delegate to take in an int and then tell it which page number we want to use next. This is nice for situations where instead of getting a Next Link, we get a flag indicating more pages.

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    string nextLink = null;

    do
    {
        // ...
    } while (!string.IsNullOrEmpty(nextLink));

Next up, we’re defining a variable we’ll use to build our loop. Because we’re using a nextLink to determine if we have pages, we need to keep track of the current nextLink. We’ll use a do-while loop to ensure the loop always runs at least once. After the first run, we want to loop until there isn’t a nextLink.

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        var currentPage = await getPage(nextLink);

        // Stop if the page is missing
        if (currentPage == null) {
            yield break;
        }

Inside the loop, the first thing we’re going to do is load the next page from the getPage delegate. If the delegate returns null, we’ll treat that as the end of the pages and call yield break to indicate we’re done returning elements. Otherwise we’ll keep going.

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        nextLink = currentPage.NextLink;

        foreach (var item in currentPage.Values) {
            yield return item;
        }

Now that we’ve loaded the page, we’ll store the current page’s NextLink in our variable. Then we will iterate over all the elements in the Values collection. We’ll use a foreach to iterate over every item and yield return to return that item.

That’s it, we’re done with the implementation. Obviously this is a very light implementation and lacks things like error handling and a bunch of edge cases or other handy features, but it shows the power of IAsyncEnumerable. Now let’s see how we could consume it:

We’ll use the System.Linq.Async package from dotnet/reactive to give us the traditional Linq expressions that we’re used to:

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using System.Linq.Async;

Now we can do some really powerful things:

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var results = GetEntireList((nextLink) => dataService.GetItemsPage<Member>(nextLink))
                .Where(member => member.IsPending)
                .Select(member => new { member.Id, member.Name });

await foreach (var member in results) {
    // Some business logic on the ID and Names
}

As you can see, we can use the standard Where and Select statements (brought in from System.Linq.Async) to process our IAsyncEnumerable just like an IEnumerable.

Hopefully you found this post informative and as always if you see any errors or issues, please reach out to me on twitter.

Flattening Paged Responses with IAsyncEnumerables

References

See Also: