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This library wraps an Azure Blob Storage container which stores objects in JSON format.

Creating an instance of DataContainer

DataContainer is a wrapper over Azure Blob Storage container which stores only objects in JSON format. All the objects that will be stored will be validated against the schema that is provided at the creation time of the container.

Create a DataContainer an options object, described below, then call its async init method before doing anything else.

let {DataContainer} = require('azure-blob-storage');

let container = new DataContainer({
  // Azure connection details for use with SAS from
  account:           '...',                  // Azure storage account name
  container:         'AzureContainerName',   // Azure container name
  // TaskCluster credentials
  credentials: {
    clientId:        '...',                  // TaskCluster clientId
    accessToken:     '...',                  // TaskCluster accessToken
  accessLevel:       'read-write',           // The access level of the container: read-only/read-write (optional)
  authBaseUrl:       '...',                  // baseUrl for auth (optional)
  schema:            '...',                  // JSON schema object
  schemaVersion:     1,                      // JSON schema version. (optional)
                                             // The default value is 1.

  // Max number of update blob request retries
  updateRetries:              10,
  // Multiplier for computation of retry delay: 2 ^ retry * delayFactor
  updateDelayFactor:          100,

  // Randomization factor added as:
  // delay = delay * random([1 - randomizationFactor; 1 + randomizationFactor])
  updateRandomizationFactor:  0.25,

  // Maximum retry delay in ms (defaults to 30 seconds)
  updateMaxDelay:             30 * 1000,
await container.init();

Using the options format provided above a shared-access-signature will be fetched from To fetch the shared-access-signature the following scope is required:

In case you have the Azure credentials, the options are:

   // Azure credentials
   credentials: {
     accountName: '...',         // Azure account name
     accountKey: '...',          // Azure account key

DataContainer operations

  • init() (async) - This method must be called after construction and before any other methods.
    let container = new DataContainer({ /* ... */ });
    await container.init();
  • ensureContainer() This method will ensure that the underlying Azure container actually exists. This is an idempotent operation, and is called automatically by init, so there is never any need to call this method.
    await container.ensureContainer();
  • removeContainer() Deletes the underlying Azure container. This method will not work if you are authenticated with SAS. Note that when the container is deleted, a container with the same name cannot be created for at least 30 seconds.
    await container.removeContainer();
  • listBlobs(options) Returns a paginated list of blobs contained by the underlying container.
    let blob = await container.listBlobs({
      prefix: 'state',
      maxResults: 1000,
  • scanDataBlockBlob(handler, options) Executes the provided function on each data block blob from the container, while handling pagination.
    let handler = async (blob) => {
        await blob.modify((content) => {
          content.version += 1;
    let options = {
      prefix: 'state',
    await container.scanDataBlockBlob(handler, options);
  • createDataBlockBlob(options, content) Creates an instance of DataBlockBlob. Using this instance of blob, a JSON file can be stored in Azure storage. The content will be validated against the schema defined at the container level.

This is equivalent to creating a new DataBlockBlob instance with the given options (see below), then calling its create method. This will unconditionally overwrite any existing blob with the same name.

    let options = {
        name: 'state-blob',
        cacheContent: true,
    let content = {
      value: 30,
    let dataBlob = await container.createDataBlockBlob(options, content);
  • createAppendDataBlob(options, content) Creates an instance of AppendDataBlob. Each object appended must be in JSON format and must match the schema defined at container level. Updating and deleting the existing content is not supported.

This is equivalent to creating a new AppendDataBlob instance with the given options (see below), then calling its create and (if content is provided) append methods.

    let options = {
        name: 'auth-log',
    let content = {
      user: 'test',
    let appendBlob = await container.createAppendDataBlob(options, content);
  • load(blobName, cacheContent) This method returns an instance of DataBlockBlob or AppendDataBlob that was previously created in Azure storage. It makes sense to set the cacheContent to true only for DataBlockBlob, because AppendDataBlob blobs do not keep the content in their instance. It will throw an error if the blob does not exist.
    let blob = await container.load(blob, false);
  • remove(blob, ignoreIfNotExists) Remove a blob from Azure storage without loading it. Set the ignoreIfNotExists to true to ignore the error that is thrown in case the blob does not exist. Returns true, if the blob was deleted. It makes sense to read the return value only if ignoreIfNotExists is set.
    await container.remove('state-blob', true);

Schema Versions

Each blob has an associated schema version, and all schema versions are stored in the blob storage alongside the blobs containing user data. The version declared to the constructor defines the "current" version, but blobs may exist that use older versions.

When a blob is loaded, it is validated against the schema with which it was stored.

When a blob is written (via create, modify, or append), it is validated against the current schema. Thus operations that modify an existing blob are responsible for detecting and "upgrading" any old data structures.

DataBlockBlob and AppendDataBlob

DataBlockBlob is a wrapper over an Azure block blob which stores a JSON data which is conform with schema defined at container level.

AppendDataBlob is a wrapper over an Azure append blob. This type is optimized for fast append operations and all writes happen at the end of the blob. Updating and deleting the existing content is not supported. This type of blob can be used for e.g. logging or auditing.

The constructor of the blob takes the following options:

let {DataBlockBlob, AppendDataBlob} = require('azure-blob-storage');
   name:                '...',        // The name of the blob (required)
   container:           '...',        // An instance of DataContainer (required)
   contentEncoding:     '...',        // The content encoding of the blob
   contentLanguage:     '...',        // The content language of the blob
   cacheControl:        '...',        // The cache control of the blob
   contentDisposition:  '...',        // The content disposition of the blob
   cacheContent:        true|false,   // This can be set true in order to keep a reference of the blob content.
                                      // Default value is false

The options cacheContent can be set to true only for DataBlockBlob because, AppendDataBlob does not support the caching of its content.

Note that the createDataBlockBlob and createAppendDataBlob methods of DataContainer provide shortcuts to calling these constructors.

DataBlockBlob operations

  • create(content, options) Creates the blob in Azure storage having the specified content which will be validated against container schema. The options, if given are passed to putBlob.
    let content = {
      value: 40,
    let options = {
      ifMatch: 'abcd',
    let content = await dataBlob.create(content, options);

To conditionally create a blob, use ifNoneMatch: '*' and catch the BlobAlreadyExists error:

try {
  await dataBlob.create(content, {ifNoneMatch: '*'});
} catch (e) {
  if (e.code !== 'BlobAlreadyExists') {
    throw e;
  console.log('blob already exists, not overwriting..');
  • load() This method returns the content of the underlying blob. After the content is loaded, it is validated and also cached, if the cacheContent was set.
    let content = await dataBlob.load();
  • modify(modifier, options) This method modifies the content of the blob. The modifier is a function that will be called with a clone of the blob content as first argument and it should apply the changes to the instance of the object passed as argument. The options, if given, are passed to putBlob, with type and ifMatch used to achieve atomicity.
    let modifier = (data) => {
      data.value = 'new value';
    let options = {
      ifUnmodifiedSince: new Date(2017, 1, 1),
    await dataBlob.modify(modifier, options);

This method uses ETags to ensure that modifications are atomic: if some other process writes to the blob while modifier is executing, modify will automatically fetch the updated blob and call modifier again, retrying several times.

Note that the modifier function must be synchronous.

AppendDataBlob operations

  • create(options) Creates the blob in Azure storage without initial content. The options, if given are passed to putBlob.
    await logBlob.create();
  • append(content, options) Appends a JSON content that must be conform to container schema.
    let content = {
      user: 'test2',
    await logBlob.append(content);
  • load() Load the content of the underlying blob.
    let content = await logBlob.load();


To test this library in development, copy user-config-example.yml to user-config.yml and fill in the necessary fields. You will need an Azure acocunt to test this library.