Wednesday, October 13, 2010

Azure Tables Error Messages

Azure Table Storage (ATS) is a REST-basead data access mechanism, and REST is based on HTTP. So, makes sense that ATS data access errors are returned as HTTP errors. But some translation is needed to understand those errors.

1. DataServiceRequestException and DataServiceClientException exceptions

When a data access error occurs in ATS, the exception returned usually is from DataServiceRequestException class, being the error message “An error ocurred while processing this request”. This exception encapsulates a more specific DataServiceClientException exception, that contains a XML document on the Message property describing the error:

<?xml version="1.0" encoding="utf-8" standalone="yes"?>
    <error xmlns="">
    <message xml:lang="pt-BR">The specified entity already exists.</message>

The value on the <code> element needs to be interpreted considering what the application was doing when the error ocurred. For example, the code InvalidInput is generated if you are trying to save an entity that has a property of an unsupported data type; OR if you are trying to update an entity that has no concurrency control information. Almost the same, right? ;-)

2. Error codes an possible causes

Following are the HTTP error code that we’ve bumped into, and what the application was trying do do when the error ocurred.

Error Code
(value on <code> element)
Possible Causes
ConditionNotMet - Concurrency error. The app read some entity; when trying to modify or delete the entity, it had already been modified by a command from another data context.
EntityAlreadyExists - Primary key violation. The app is trying to insert an entity, but there’s an entity on the table with the same values for PartitionKey and RowKey properties on the entity being inserted.
InternalError - Use of unsupported Linq operators on ATS. (We found this one trying to recover rows that had a string property starting with some value. There’s no string support for like, >, <, and neither string.StartsWith() – the one that generated the error. You have to use string.CompareTo() – :-P)
InvalidInput - Storing an entity that has properties of types not supported by ATS.
- Updating an entity that was attached to the context (TableServiceContext.AttachTo()), but has null on it’s
ETag property.
- Linq queries on a table with no structure defined. We received this error when trying to query a table that had just been created; if you insert some entities, even if you erase all the entities on table later, it seems that the table holds the entity structure and you can use it on Linq queries.
- Linq queries using
anonymous types. The workaround was to make the query return the “whole” entity, and build a second query using anonmymous types on the data returned by ATS.
- Batch save operation (SaveChanges(SaveOptions.Batch)) with 2 or more entities having the same Partion and Row Keys.
ResourceNotFound - Linq query that returns no entitites using equality conditions on PartitionKey and RowKey (where entity.PartitionKey == “…” && entity.RowKey == “…”). (Queries with conditions using other attributes return a 0-sized list)
- Concurrency error. The app read some entity; when trying to modify or delete the entity, it had already been deleted by a command from another data context.
- Linq query on a table that does not exist on ATS.
TableAlreadyExists Found on calls to CloudTableClient.CreateTablesFromModel(). It seems that concurrent calls to this method may end up generating this exception. We placed a call to CreateTablesFromModel() on the data access context static constructor. When we raised the number of role instances to 2, one of them started ok, and the other one generated the exception.
TableBeingDeleted Same as TableAlreadyExists above.
OutOfRangeInput Generated when saving an entity with an uninitialized DateTime property. The minimum allowed value for a DateTime property on ATS is Jan 01, 1601, and an uninitialized DateTime in C# holds Jan 01, year 1. We didn’t detected it on tests against DevStorage because it changes dates before Jan 01, 1753 to Jan 01, 1753, but ATS does not.

3. How to extract the HTTP error code from DataServiceClientException objects

The code is based on an enum that has members named exactly as the error codes returned by ATS:

public enum ATSError
    Unknown  // This one is the "generic" error code 

The following method extracts the error code from the XML returned by ATS, and converts it on the correspondent ATSError enum value:

/// <summary>
/// Translates an ATS exception to one of the ATSErrors values.
/// </summary>
/// <param name="error">Exception returned by ATS.</param>
/// <returns></returns>
public static ATSError ExtractATSError(Exception e)
    // Searches for a DataServiceClientException exception
    while (!(e is System.Data.Services.Client.DataServiceClientException) && (e != null)) 
        e = e.InnerException;
    if (e == null) return ATSError.Unknown;

    // At this point "e" contains a DataServiceClientException object,  
    // and the Message property contains a valid XML document describing the error
    ATSError code;
        XmlDocument xml = new XmlDocument();
        // The XML document uses namespace, so 
        // we have to use a XmlNamespaceManager to represent this namespace
        XmlNamespaceManager namespaceManager = new XmlNamespaceManager(xml.NameTable);
        namespaceManager.AddNamespace("n", "");
        // Retrieves the "<code>" element 
        string s = xml.SelectSingleNode("/n:error/n:code", namespaceManager).InnerText;
        // Converts the element value to the corresponding ATSError value
        code = (ATSError)Enum.Parse(typeof(ATSError), s);
        code = ATSError.Unknown;
    return code;

And that’s it. If you bump into another error code, just add the corresponding member on the ATSError enum, so that the ExtractATSError() method will recognize it.