Cancelling Long Running Couchbase N1QL Queries

The recent release of the Couchbase .NET SDK 2.4.0 has added many new features.  There is a minor feature, however, that is worth a mention.  It’s now possible to cancel long-running N1QL queries.

For example, in a web application a user might browse away from the page in impatience.  When they do, you don’t want  the query to keep executing pointlessly.  Instead, you can cancel the query, freeing web server and Couchbase  server resources for other requests.

How To Cancel

Using this new feature is very easy, when executing your query simply supply a CancellationToken.  For web applications, this can be acquired by including a CancellationToken as a parameter on an asynchronous action method.

public async Task<ActionResult> Index(CancellationToken cancellationToken)
{
    var bucket = ClusterHelper.GetBucket("my-bucket");
    var query = new QueryRequest("SELECT * FROM `my-bucket` WHERE type = 'docType' LIMIT 1000");

    var result = await bucket.QueryAsync<Document>(query, cancellationToken);
    if (result.Success) {
        return Json(result.Rows);
    }
}

Compatibility

Note: Documentation on what versions of ASP.NET MVC and Web API support CancellationToken is a bit sparse.  Apparently some versions only use it for timeouts (using  AsyncTimeout), while some versions have support for cancellations from the browser.  There is also a way to add support for browser cancellation using CreateLinkedTokenSource.

The behavior may also depend on the web server you’re using (i.e. IIS versus Kestrel, and Kestrel version). For example, this change to Kestrel appears to cause client disconnects to do a better job of triggering the CancellationTokken. If anyone knows more details about version support, please let me know and I’ll update the post.