Getting Blob Metadata When Using Azure Functions Blob Storage Triggers

12 February 2019 (0)

(This article refers to Azure Functions V2)

Basic Blob Metadata

There are a few basic pieces of metadata that are often useful.

The following code show a simple example of a blob-triggered Azure Function:

[FunctionName("BlobMetadataExample")]
public static void Run(
    [BlobTrigger("decline-letters/{name}")]Stream myBlob, 
    string name, 
    ILogger log)
{
    log.LogInformation($"Name: {name} Size: {myBlob.Length} Bytes");
}

With the preceding code, if we add a blob called “declineletterrequest.data” to the “decline-letters” container, the function will be triggered with the output: “Name: declineletterrequest.data Size: 50 Bytes”.

Notice that the string name parameter has been automatically populated with the full name of the blob that triggered the function execution.

If you want to get the blob name and blob extension separately you could write the following:

[FunctionName("BlobMetadataExample")]
public static void Run(
    [BlobTrigger("decline-letters/{blobname}.{blobextension}")]Stream myBlob,
    string blobName,
    string blobExtension,
    ILogger log)
{
    log.LogInformation($"Name: {blobName} Extension: {blobExtension} Size: {myBlob.Length} Bytes");
}

If the preceding function executes we get the output: “Name: declineletterrequest Extension: data Size: 50 Bytes”.

In addition to being able to use this simple blob metadata in code, you can also use the elements of the triggering blob name in other bindings:

[FunctionName("BlobMetadataExample")]
public static void Run(
        [BlobTrigger("decline-letters/{blobname}.{blobextension}")]Stream myBlob,
        string blobName,
        string blobExtension,
        [Queue("output-queue-{blobextension}")] out string message,
        ILogger log)
{
    log.LogInformation($"Name: {blobName} Extension: {blobExtension} Size: {myBlob.Length} Bytes");

    message = "Hello world";
}

In the preceding code, the output queue that is written to is dependent on the extension of the triggering blob. If the triggering blob name was “declineletterrequest.bankofmars” then a message will be written to the queue “output-queue-bankofmars” or if the input blob was called “output-queue-bankofvenus” then a message would be written to the “output-queue-bankofvenus”.

You can also do a similar thing by binding an input blob binding to the contents of a triggering queue message.

Advanced Metadata

There are a number of additional metadata items that you can get by simply adding the correct method arguments with the correct names:

[FunctionName("BlobMetadataExample")]
public static void Run(
        [BlobTrigger("decline-letters/{blobname}.{blobextension}")]Stream myBlob,
        string blobName,
        string blobExtension,
        string blobTrigger, // full path to triggering blob
        Uri uri, // blob primary location
        IDictionary<string, string> metaData, // user-defined blob metadata
        BlobProperties properties, // blob system properties, e.g. LastModified
        ILogger log)
{
    log.LogInformation($@"
blobName      {blobName}
blobExtension {blobExtension}
blobTrigger   {blobTrigger}
uri           {uri}
metaData      {metaData.Count}
properties    {properties.Created}");
}

Executing the preceding code will give the following output:

blobName      declineletterrequest
blobExtension data
blobTrigger   decline-letters/declineletterrequest.data
uri           http://127.0.0.1:10000/devstoreaccount1/decline-letters/declineletterrequest.data
metaData      0
properties    12/02/2019 2:15:53 AM +00:00

The BlobProperties give you access to a host of information such as ETag, DeletedTime, ContentEncoding, etc.

You can use this additional metadata in further binding expressions, the following example shows how to bind a blob output name to the ETag of the original triggering blob:

[FunctionName("BlobMetadataExample")]
public static void Run(
[BlobTrigger("decline-letters/{blobname}.{blobextension}")]Stream myBlob,
string blobName,
BlobProperties properties,
[Blob("decline-letters/{properties.ETag}")] out string message,
ILogger log)
{
    message = "Hello world";
}

The preceding code would create an output blob with a name such as “0x8D6909193F68C10”.

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