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 usingEventHubConsumerClient 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 anAzure subscription, anEvent Hubs Namespace to use this package, and aStorage 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 enablecompilerOptions.allowSyntheticDefaultImports in your tsconfig.json. Note that if you have enabledcompilerOptions.esModuleInterop,allowSyntheticDefaultImports is enabled by default. SeeTypeScript'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: Applications that support load balancing consist of one or more instances ofEventHubConsumerClient which have been configured to consume events from the same Event Hub and consumer groupand the sameCheckpointStore.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 andoccurs 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 positionin 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.

  ABlobCheckpointStoreis a class that implements key methods required by the EventHubConsumerClient to balance load and update checkpoints.

Examples

• Create a CheckpointStore using Azure Blob Storage
• Checkpoint events using Azure Blob storage

Create aCheckpointStore using Azure Blob Storage

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

import{ContainerClient}from"@azure/storage-blob",import{BlobCheckpointStore}from"@azure/eventhubs-checkpointstore-blob"constcontainerClient=newContainerClient("storage-connection-string","container-name");if(!containerClient.exists()){awaitcontainerClient.create();// This can be skipped if the container already exists}constcheckpointStore=newBlobCheckpointStore(containerClient);

Checkpoint events using Azure Blob storage

To checkpoint events received using Azure Blob Storage, you will need to pass an objectthat is compatible with theSubscriptionEventHandlersinterface along with code to call theupdateCheckpoint() method.

In this example,SubscriptionHandlers implementsSubscriptionEventHandlers and also handles checkpointing.

import{ContainerClient}from"@azure/storage-blob";import{BlobCheckpointStore}from"@azure/eventhubs-checkpointstore-blob";import{EventHubConsumerClient}from"@azure/event-hubs";constconsumerGroup="consumer-group-name";constconnectionString="event-hub-connectionstring";constcontainerClient=newContainerClient("storage-connection-string","container-name");if(!(awaitcontainerClient.exists())){awaitcontainerClient.create();// This can be skipped if the container already exists}constcheckpointStore=newBlobCheckpointStore(containerClient);classSubscriptionHandlers{asyncprocessEvents(event,context){// custom logic for processing events goes here// Checkpointing will allow your service to restart and pick// up from where it left off.//// You'll want to balance how often you checkpoint with the// performance of your underlying checkpoint store.awaitcontext.updateCheckpoint(event);}asyncprocessError(err,context){// handle any errors that occur during the course of// this subscriptionconsole.log(`Errors in subscription:${err}`);}}constconsumerClient=newEventHubConsumerClient(consumerGroup,connectionString,checkpointStore);constsubscription=consumerClient.subscribe(newSubscriptionHandlers());// events will now flow into the handlers defined above// to stop the subscription:subscription.close();

Troubleshooting

Enable logs

You can set theAZURE_LOG_LEVEL environment variable to one of the following values to enable logging tostderr:

• verbose
• info
• warning
• error

You can also set the log level programatically by importing the@azure/logger package and calling thesetLogLevel function with one of the log level values.

When setting a log level either programatically or via theAZURE_LOG_LEVEL environment variable,any logs that are written using a log level equal to or less than the one you choose will be emitted.For example, when you set the log level toinfo, the logs that are written for levelswarning anderror are also emitted.This SDK follows the Azure SDK for TypeScriptguidelineswhen determining which level to log to.

You can alternatively set theDEBUG environment variable to get logs when using this library.This can be useful if you also want to emit logs from the dependenciesrhea-promise andrhea as well.

Note: AZURE_LOG_LEVEL, if set, takes precedence over DEBUG.Do not specify anyazure libraries via DEBUG when also specifyingAZURE_LOG_LEVEL or calling setLogLevel.

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

• Getting only info level debug logs from the Eventhubs Checkpointstore Blob.

export DEBUG=azure:eventhubs-checkpointstore-blob:info

Logging to a file

• Enable logging as shown above and then run your test script as follows:

  • Logging statements from your test script go toout.log and logging statements from the sdk go todebug.log.

    node your-test-script.js> out.log2>debug.log

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

    node your-test-script.js>out.log2>&1

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

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

Next Steps

Please take a look at thesamplesdirectory for detailed example.

Contributing

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