Things I would have liked to know about Azure Search

Since I changed job and joined the wonderful William Demant, I have occasion to use Microsoft Azure daily at work. Recently I worked on a web application for documents administration (PDF, Word, Excel, etc.), featuring files upload, download, administration of metadata, search of documents by metadata and contents.

The application consists of following elements:

  1. ASP.NET Core web application, hosted in an application service plan
  2. Front end SPA, implemented using React, with ES6 and Babel.
  3. Azure Storage, to store application data in its Table storage, and files in its Blob storage
  4. Azure Search service, to index documents and offer full-text search feature
  5. Application Insights to collect application telemetries
  6. Integration with Azure Active Directory (AAD)

As part of our automated deployments, most of these resources are provisioned using Azure Resource Manager (ARM) templates. I decided to share my experience with Azure Search, since a few things made me scratch my head.

To use an Azure Search service, three kinds of components can be configured:

  • Index: holds the configuration of which properties must be indexed, how they must be indexed, scoring profiles (i.e. sorting criteria of search results), allowed origins for HTTP access control (CORS), and keeps indexed entries. It can be used alone, if populated programmatically.
  • Data Source: a source of data that need to be indexed. For example, it can be a SQL table, a Table Storage, a Blob Storage.
  • Indexer: creates index entries, reading from a data source. It is associated with a single data source and a single index, and holds configuration of failure handling, scheduling, fields mapping and eventual transformations from source data to indexed values (an example is described below).

In our setup, an index is populated by an indexer, which reads from the Blob storage, indexing files metadata and contents, like described in this article:

Azure Search is a great service, but here I focus mainly on the things I didn’t like, and that required more time than I was expecting.

Limited ARM support

Azure Search Indexer, Index and Data Source cannot be configured using ARM templates. Only Azure Search service itself can be configured in ARM templates, its required components need to be configured using the Azure Search REST api. As things stand at present, only the smallest part of Azure Search configuration can be handled through ARM templates. For continuous delivery and automated deployments, I wrote PowerShell scripts to do web requests to the REST api. Stating the obvious, had it been possible though ARM templates, it would have taken a much smaller amount of time, but that’s how it is.

Useful links:

This recommendation can be voted here: Allow create, update, delete of Data Sources, Indexes, and Indexers with Service through ARM templates.

Another small disappointment about ARM support…

In our solution, the Azure Search index is queried by web requests directly from clients: CORS are configured, in the index configuration, to allow requests from the HTTP origin of web application. In this kind of setup, a so called query key is used to authorize the use of search service from clients. In other words, this query key is stored in web application configuration and served to clients, so they can make direct web requests to search service api. When a new Azure Search service is provisioned, it comes with two administrative keys and a single query key. Also in this case, ARM templates don’t help, since query keys cannot be returned as output parameters; while it is possible to return administrative keys.

A solution for automated deployments is to use the Microsoft.Search/searchServices/createQueryKey command through PowerShell to generate a query key (or obtain one generated previously), then put it in the application configuration.

  # Creating a query key in PowerShell:
  $queryKey = (Invoke-AzureRmResourceAction `
    -ResourceType "Microsoft.Search/searchServices/createQueryKey" `
    -ResourceGroupName $resourceGroupName `
    -ResourceName $serviceName `
    -ApiVersion 2015-08-19 `
    -Action $description `
    -Force `

As a side note, and to point out the asymmetry, an Azure Search administrative key can be obtained in ARM templates using this function:

[listAdminKeys(variables('searchServiceFullName'), '2015-08-19').PrimaryKey]

Non-standard handling of Base64 URL encoded strings

When indexing files metadata in Blob storage, and desiring to support non-ASCII characters (who doesn’t!?), it’s necessary to use URL safe Base64 encoded strings, and configure the indexer fields mappings to use “base64decode” function, like described here:

This is because metadata in Blob storage does not support non-ASCII characters. As a side note, I suspect that this limitation is only caused by how the REST api of Blob storage has been designed, and not due to a real technical limitation of the database used by Blob storage. Let me explain: Blob storage REST api is designed to handle blob metadata using HTTP headers; and HTTP headers only support ASCII characters. Had the api been designed to handle metadata coming inside the request body, together with file bytes array (nothing forbids to do so!), there would be no problem using UTF8 for metadata, whatsoever. If I am right, this should be regarded as a design mistake of Blob storage REST api, causing complexity that would be otherwise unneeded.

Getting back to the main topic, the Azure Search indexer is breaking if one uses standard Base64 URL safe strings¹, it only works with non-standard strings that were returned classically by System.Web.HttpServerUtility.UrlTokenEncode method. Such strings are non-standard in this way, as their trailing padding characters ‘=’ are replaced with a digit indicating the number of = signs that were removed. By the standard, these trailing padding characters are optional and can be omitted when encoding.

For example:

  • standard of “Hello World” is “SGVsbG8gV29ybGQ=”, but this method returns: “SGVsbG8gV29ybGQ1
  • standard for “Lorem Ipsu” is “TG9yZW0gSXBzdQ==”, but this method returns: “TG9yZW0gSXBzdQ2

When using standard strings (with or without optional padding characters), indexing breaks with this error message: “Error applying mapping function ‘base64Decode’ to field ‘NAME’: Array cannot be null.\r\nParameter name: bytes”. The Azure Search Indexer works only when using these non-standard strings. This is clearly a bug: either in documentation not mentioning the caveat, or in Azure Search base64decode function. In my opinion, Azure Search base64decode method should be fixed to support standard strings (with or without padding characters).

I found this problem because of common scenarios:

In both cases, I needed to write wrapper functions that return non-standard strings. This last thing made me waste half day of work, hopefully I will help somebody to save time with this post.



Eugene Shvets, Microsoft engineer working on Azure Search, kindly commented in Twitter to let me know about the possibility to vote for better ARM support for Azure Search and that handling of Base64 strings is currently being fixed (1.). A few days ago I wrote to submit this information, but I didn’t know it was taken into consideration, that’s why I described it in my post. Molto bene! :)

Written on August 3, 2017

Roberto Prevato

Italian graphic designer, applications architect, DevOps, web applications specialist, artist wannabe.
Metal head fond of philosophy and arts.