README.md

Azure Event Hubs Checkpoint Store library for Javascript using Storage Blobs

An Azure Blob storage based solution to store checkpoints and to aid in load balancing when using EventProcessor from the @azure/event-hubs library

Source code | Package (npm) | API Reference Documentation | Samples

Getting started

Install the package

Install the Azure Event Hubs Checkpoint Store Blob library using npm

npm install @azure/eventhubs-checkpointstore-blob

Prerequisites: You must have an Azure subscription, an Event Hubs Namespace to use this package, and a Storage account

If you are using this package in a Node.js application, then use Node.js 8.x or higher.

Configure Typescript

TypeScript users need to have Node type definitions installed:

npm install @types/node

You also need to enable compilerOptions.allowSyntheticDefaultImports in your tsconfig.json. Note that if you have enabled compilerOptions.esModuleInterop, allowSyntheticDefaultImports is enabled by default. See TypeScript's compiler options handbook for more information.

Key concepts

• Scale: Create multiple consumers, with each consumer taking ownership of reading from a few Event Hubs partitions.

• Load balance: Event Processor based application consists of one or more instances of EventProcessor which have been configured to consume events from the same Event Hub and consumer group. They balance the workload across different instances by distributing the partitions to be processed among themselves.

• Checkpointing: It is a process by which readers mark or commit their position within a partition event sequence. Checkpointing is the responsibility of the consumer and occurs on a per-partition basis within a consumer group. This responsibility means that for each consumer group, each partition reader must keep track of its current position in the event stream, and can inform the service when it considers the data stream complete.

  If a reader disconnects from a partition, when it reconnects it begins reading at the checkpoint that was previously submitted by the last reader of that partition in that consumer group. When the reader connects, it passes the offset to the event hub to specify the location at which to start reading. In this way, you can use checkpointing to both mark events as "complete" by downstream applications, and to provide resiliency if a failover between readers running on different machines occurs. It is possible to return to older data by specifying a lower offset from this checkpointing process. Through this mechanism, checkpointing enables both failover resiliency and event stream replay.

  A BlobPartitionManager is a class that implements key methods required by the Event Processor to balance load and update checkpoints.

Examples

• Create a Partition Manager using Azure Blob Storage
• Checkpoint events using Azure Blob storage

Create a Partition Manager using Azure Blob Storage

Use the below code snippet to create a Partition Manager. You will need to provide the connection string to your storage account.

import { ContainerClient } from "@azure/storage-blob",
import { BlobPartitionManager } from "@azure/eventhubs-checkpointstore-blob"

const containerClient = new ContainerClient("storage-connection-string", "container-name");
await containerClient.create(); // This can be skipped if the container already exists

const partitionManager =  new BlobPartitionManager(containerClient);

Checkpoint events using Azure Blob storage

To checkpoint events received using an EventProcessor to Azure Blob Storage, you will need to pass an instance of the Partition Manager to the Event Processor along with the code to call the updateCheckpoint() method.

In this example, we will use a SamplePartitionProcessor that extends the PartitionProcessor class in order to checkpoint the last event in the batch.

import { ContainerClient } from "@azure/storage-blob",
import { BlobPartitionManager } from "@azure/eventhubs-checkpointstore-blob"
import { EventHubClient, PartitionProcessor } from "@azure/event-hubs"

const eventhubClient = new EventHubClient("event-hub-connectionstring")
const containerClient = new ContainerClient("storage-connection-string", "container-name");
await containerClient.create(); // This can be skipped if the container already exists
const partitionManager =  new BlobPartitionManager(containerClient);

SamplePartitionProcessor extends PartitionProcessor  {
  async processEvents(events) {
    if (events.length) {
     /* custom logic for processing events, then checkpoint*/
      await this.updateCheckpoint(events[events.length - 1]);
    } 
  }
}

const eventProcessor = new EventProcessor(
    EventHubClient.defaultConsumerGroupName,
    client,
    SamplePartitionProcessor,
    partitionManager);
eventProcessor.start();

Troubleshooting

Enable logs

You can set the following environment variable to get the debug logs when using this library.

• Getting debug logs from the Eventhubs Checkpointstore Blob

export DEBUG=azure:eventhubs-checkpointstore-blob*

• If you are interested only in errors, then you can set the DEBUG environment variable as follows:

export DEBUG=azure:event-hubs:error,azure:eventhubs-checkpointstore-blob:error

Logging to a file

• Set the DEBUG environment variable as shown above and then run your test script as follows:

  • Logging statements from your test script go to out.log and logging statements from the sdk go to debug.log.

    node your-test-script.js > out.log 2>debug.log

  • Logging statements from your test script and the sdk go to the same file out.log by redirecting stderr to stdout (&1), and then redirect stdout to a file:

    node your-test-script.js >out.log 2>&1

  • Logging statements from your test script and the sdk go to the same file out.log.

      node your-test-script.js &> out.log

Next Steps

Please take a look at the samples directory for detailed example.

Contributing

If you'd like to contribute to this library, please read the contributing guide to learn more about how to build and test the code.