Don't be afraid to store it twice

In most cases we try to follow DRY approach - Don't Repeat Yourself. This simple yet powerful statement, which - if executed correctly - has the power of changing meh projects to good ones, doesn't work well in all scenarios and use cases. As one once said "One size doesn't fit all", you shouldn't always follow each pattern or approach blindly. You can easily get burnt.

Problem

Imagine following scenario(or rather requirement):

We have following concepts in the application: Company, Group and User. User can have access to multiple Companies(from which one is his main). Group is a logical thing, which exists within a Company

The question is - how to store data in Table Storage, so we can easily query both all Groups within a Company and a single Group. 

Let's skip User for a second and consider following implementation:

/
[FunctionName("GroupList")]
public static Task<HttpResponseMessage> GroupList(
	[HttpTrigger(AuthorizationLevel.Anonymous, "get", Route = "group")] HttpRequestMessage req,
	[Table(TableName, "AzureWebJobsStorage")] IQueryable<GroupEntity> groups,
	[Table(Company.TableName, "AzureWebJobsStorage")] IQueryable<Company.CompanyEntity> companies,
	TraceWriter log)
{
	var groupsQuery = groups.Take(100).ToList().Select(_ => new
	{
		Id = _.RowKey,
		Name = _.Name,
		Description = _.Description,
		// ReSharper disable once ReplaceWithSingleCallToFirst
		CompanyName = companies.Where(company => company.RowKey == _.CompanyId.ToString()).FirstOrDefault()?.Name
	}).ToList();

	var response = req.CreateResponse(HttpStatusCode.OK,
		groupsQuery);

	return Task.FromResult(response);
}

[FunctionName("GroupGet")]
public static Task<HttpResponseMessage> GroupGet(
	[HttpTrigger(AuthorizationLevel.Anonymous, "get", Route = "group/{id}")] HttpRequestMessage req,
	[Table(TableName, "{partitionKey}", "{id}", Connection = "AzureWebJobsStorage")] GroupEntity group,
	TraceWriter log)
{
	var response = req.CreateResponse(HttpStatusCode.OK, group);
	return Task.FromResult(response);
}

public class GroupEntity : TableEntity
{
	public GroupEntity(Guid companyId)
	{
		PartitionKey = companyId.ToString();
		RowKey = Guid.NewGuid().ToString();
	}

	public string Name { get; set; }
	public Guid CompanyId { get; set; }
	public string Description { get; set; }
}

All right, initially it all looks fine. If we want to get a list of User specific Groups, we'll just add some context(e.g. store all user's companies somewhere and just use their identifiers as a parameter in a query). Now let's request a single Group. We have a Group identifier, we can get the whole row from a database...

NO WE CAN'T!

Since GroupEntity's PK is equal to Company's identifier, we'd have to make one query per each company a user has access to. Not very smart, not very clean. What to do? Change PK in GroupEntity to a generic one? We'll lost the possibility to make fast queries for all groups within a company. Make a combined identifier and user it as a PK? We still have to perform multiple queries. Go for SQL and perform proper JOINs? This is definitely a possibility - but we don't need other features of a relative database. Is it a dead end? 

Solution

One thing in Azure Storage is really cheap - it's the storage itself. How can we remodel our tables so we can improve both performance and lower transactions amount? Well, we can store our data twice!

Before you start throwing tomatoes at me, consider following example:

/
[FunctionName("GroupCreate")]
public static async Task<HttpResponseMessage> GroupCreate(
	[HttpTrigger(AuthorizationLevel.Anonymous, "post", Route = "group")] HttpRequestMessage req,
	[Table(TableName, "AzureWebJobsStorage")] IAsyncCollector<GroupEntity> groups,
	TraceWriter log)
{
	var data = JsonConvert.DeserializeObject<GroupEntity>(await req.Content.ReadAsStringAsync());

	var group = new GroupEntity(data.CompanyId)
	{
		Name = data.Name,
		CompanyId = data.CompanyId,
		Description = data.Description
	};

	await groups.AddAsync(group);
	await groups.AddAsync(new GroupEntity
	{
		RowKey = group.RowKey,
		Name = data.Name,
		CompanyId = data.CompanyId,
		Description = data.Description
	});

	var response = req.CreateResponse(HttpStatusCode.OK, group);
	return response;
}

Here we're storing a row twice but with slight changes. The first insert stores data with no changes to the previous version. The second one is the crucial one - it performs one change, which is changing PK to a generic one named "group". Thanks to this solution we can have two almost the same rows, one for being displayed as a part of a list, one for storing all info regarding a row. 

Now you may ask - how it secures a row from being displayed if a User doesn't have access to a given Company? That's why we're storing company identifier along with a row in CompanyId column. This is much quickier and cleaner solution than performing several requests to Table Storage - we can cache the data locally and just check whether identifiers match.

Summary

Modeling Table Storage is both challenging and rewarding - you can easily hit performance problems if tables are not designed carefully, on the other hand wise design allow you to really push the limit. Those redesigns are important also because of one more thing - they save time. And in a cloud time = money. Make sure you pay only as much as needed.

Don't "admin" me!

This was somehow unexpected. I was flawlessly developing my API using Azure Functions and then BANG! - suddenly my functions are in error state. Just like that - no stack trace, no detailed explanation. I took me some time to realize, that they all have one thing in common - they share the admin part. Apparently Functions have some problems when you're using HttpTrigger with a custom route containg admin word. What is the reason?

Check the codebase

Let's take a quick look at the codebase of Functions. Finding a place, which causes the error is pretty simple:

/
internal static void ValidateHttpFunction(string functionName, HttpTriggerAttribute httpTrigger, bool isProxy = false)
{
	if (string.IsNullOrWhiteSpace(httpTrigger.Route) && !isProxy)
	{
		// if no explicit route is provided, default to the function name
		httpTrigger.Route = functionName;
	}

	// disallow custom routes in our own reserved route space
	string httpRoute = httpTrigger.Route.Trim('/').ToLowerInvariant();
	if (httpRoute.StartsWith("admin"))
	{
		throw new InvalidOperationException("The specified route conflicts with one or more built in routes.");
	}
}

As you can see, all routes which start with admin are disallowed. While this is perfectly fine, I couldn't find any mention in the documentation, which would clarify this. 

When in doubt always try to check the source. Some answers will be quite difficult to obtain, but you'll end up with a much better overall understanding of the library. The rest will be rather straightforward, nothing you can't handle, isn't it?