Pulsar 的 层级存储 功能允许将旧的积压数据卸载到长期存储，这样就可以腾出 BookKeeper 空间并降低储存成本。 本文档会手把手教你在你的 Pulsar 集群中使用层级存储。

Tiered storage currently uses Apache Jclouds to supports Amazon S3 and Google Cloud Storage(GCS for short) for long term storage. With Jclouds, it is easy to add support for more cloud storage providers in the future.

何时使用层级存储？

Tiered storage should be used when you have a topic for which you want to keep a very long backlog for a long time. For example, if you have a topic containing user actions which you use to train your recommendation systems, you may want to keep that data for a long time, so that if you change your recommendation algorithm you can rerun it against your full user history.

卸载机制

A topic in Pulsar is backed by a log, known as a managed ledger. This log is composed of an ordered list of segments. Pulsar only every writes to the final segment of the log. All previous segments are sealed. The data within the segment is immutable. This is known as a segment oriented architecture.

Tiered storage

The Tiered Storage offloading mechanism takes advantage of this segment oriented architecture. When offloading is requested, the segments of the log are copied, one-by-one, to tiered storage. All segments of the log, apart from the segment currently being written to can be offloaded.

On the broker, the administrator must configure the bucket and credentials for the cloud storage service. The configured bucket must exist before attempting to offload. If it does not exist, the offload operation will fail.

Pulsar uses multi-part objects to upload the segment data. It is possible that a broker could crash while uploading the data. We recommend you add a life cycle rule your bucket to expire incomplete multi-part upload after a day or two to avoid getting charged for incomplete uploads.

配置卸载驱动

Offloading is configured in broker.conf.

At a minimum, the administrator must configure the driver, the bucket and the authenticating credentials. There is also some other knobs to configure, like the bucket region, the max block size in backed storage, etc.

Currently we support driver of types:

驱动程序名称没有区分大小写。 有第三个驱动类型 s3, 它与 aws-s3完全相同, 虽然需要你使用 s3ManagedLedgerOffloadServiceEndpoint 来指定一个端点 url。 如果使用 S3 兼容的数据存储而不是 AWS，这会非常有用。

  1. managedLedgerOffloadDriver=aws-s3

“aws-s3”驱动程序配置

Bucket 与地区

Buckets are the basic containers that hold your data. Everything that you store in Cloud Storage must be contained in a bucket. You can use buckets to organize your data and control access to your data, but unlike directories and folders, you cannot nest buckets.

  1. s3ManagedLedgerOffloadBucket=pulsar-topic-offload

Bucket Region is the region where bucket located. Bucket Region is not a required but a recommended configuration. If it is not configured, It will use the default region.

With AWS S3, the default region is US East (N. Virginia). Page AWS Regions and Endpoints contains more information.

  1. s3ManagedLedgerOffloadRegion=eu-west-3

使用 AWS 验证身份

To be able to access AWS S3, you need to authenticate with AWS S3. Pulsar does not provide any direct means of configuring authentication for AWS S3, but relies on the mechanisms supported by the DefaultAWSCredentialsProviderChain.

Once you have created a set of credentials in the AWS IAM console, they can be configured in a number of ways.

  1. Set the environment variables AWS_ACCESS_KEY_ID and AWS_SECRET_ACCESS_KEY in conf/pulsar_env.sh.
  1. export AWS_ACCESS_KEY_ID=ABC123456789
  2. export AWS_SECRET_ACCESS_KEY=ded7db27a4558e2ea8bbf0bf37ae0e8521618f366c

“export” is important so that the variables are made available in the environment of spawned processes.

  1. Add the Java system properties aws.accessKeyId and aws.secretKey to PULSAR_EXTRA_OPTS in conf/pulsar_env.sh.
  1. PULSAR_EXTRA_OPTS="${PULSAR_EXTRA_OPTS} ${PULSAR_MEM} ${PULSAR_GC} -Daws.accessKeyId=ABC123456789 -Daws.secretKey=ded7db27a4558e2ea8bbf0bf37ae0e8521618f366c -Dio.netty.leakDetectionLevel=disabled -Dio.netty.recycler.maxCapacity.default=1000 -Dio.netty.recycler.linkCapacity=1024"
  1. Set the access credentials in ~/.aws/credentials.
  1. [default]
  2. aws_access_key_id=ABC123456789
  3. aws_secret_access_key=ded7db27a4558e2ea8bbf0bf37ae0e8521618f366c

If you are running in EC2 you can also use instance profile credentials, provided through the EC2 metadata service, but that is out of scope for this cookbook.

The broker must be rebooted for credentials specified in pulsar_env to take effect.

配置块读/写的大小

Pulsar also provides some knobs to configure the size of requests sent to AWS S3.

    1. s3ManagedLedgerOffloadMaxBlockSizeInBytes configures the maximum size of a "part" sent during a multipart upload. This cannot be smaller than 5MB. Default is 64MB.
    1. s3ManagedLedgerOffloadReadBufferSizeInBytes configures the block size for each individual read when reading back data from AWS S3. Default is 1MB.

In both cases, these should not be touched unless you know what you are doing.

“google-cloud-storage” 驱动程序配置

Buckets are the basic containers that hold your data. Everything that you store in Cloud Storage must be contained in a bucket. You can use buckets to organize your data and control access to your data, but unlike directories and folders, you cannot nest buckets.

  1. gcsManagedLedgerOffloadBucket=pulsar-topic-offload

Bucket Region is the region where bucket located. Bucket Region is not a required but a recommended configuration. If it is not configured, It will use the default region.

Regarding GCS, buckets are default created in the us multi-regional location, page Bucket Locations contains more information.

  1. gcsManagedLedgerOffloadRegion=europe-west3

使用 GCS 验证身份

The administrator needs to configure gcsManagedLedgerOffloadServiceAccountKeyFile in broker.conf for the broker to be able to access the GCS service. gcsManagedLedgerOffloadServiceAccountKeyFile is a Json file, containing the GCS credentials of a service account. Service Accounts section of this page contains more information of how to create this key file for authentication. More information about google cloud IAM is available here.

Usually these are the steps to create the authentication file: 1. Open the API Console Credentials page. 2. If it’s not already selected, select the project that you’re creating credentials for. 3. To set up a new service account, click New credentials and then select Service account key. 4. Choose the service account to use for the key. 5. Download the service account’s public/private key as a JSON file that can be loaded by a Google API client library.

  1. gcsManagedLedgerOffloadServiceAccountKeyFile="/Users/hello/Downloads/project-804d5e6a6f33.json"

配置块读/写的大小

Pulsar also provides some knobs to configure the size of requests sent to GCS.

    1. gcsManagedLedgerOffloadMaxBlockSizeInBytes configures the maximum size of a "part" sent during a multipart upload. This cannot be smaller than 5MB. Default is 64MB.
    1. gcsManagedLedgerOffloadReadBufferSizeInBytes configures the block size for each individual read when reading back data from GCS. Default is 1MB.

In both cases, these should not be touched unless you know what you are doing.

配置卸载的自动运行

Namespace policies can be configured to offload data automatically once a threshold is reached. The threshold is based on the size of data that the topic has stored on the pulsar cluster. Once the topic reaches the threshold, an offload operation will be triggered. Setting a negative value to the threshold will disable automatic offloading. Setting the threshold to 0 will cause the broker to offload data as soon as it possiby can.

  1. $ bin/pulsar-admin namespaces set-offload-threshold --size 10M my-tenant/my-namespace

Automatic offload runs when a new segment is added to a topic log. If you set the threshold on a namespace, but few messages are being produced to the topic, offload will not until the current segment is full.

手动触发卸载

Offloading can manually triggered through a REST endpoint on the Pulsar broker. We provide a CLI which will call this rest endpoint for you.

When triggering offload, you must specify the maximum size, in bytes, of backlog which will be retained locally on the bookkeeper. The offload mechanism will offload segments from the start of the topic backlog until this condition is met.

  1. $ bin/pulsar-admin topics offload --size-threshold 10M my-tenant/my-namespace/topic1
  2. Offload triggered for persistent://my-tenant/my-namespace/topic1 for messages before 2:0:-1

The command to triggers an offload will not wait until the offload operation has completed. To check the status of the offload, use offload-status.

  1. $ bin/pulsar-admin topics offload-status my-tenant/my-namespace/topic1
  2. Offload is currently running

To wait for offload to complete, add the -w flag.

  1. $ bin/pulsar-admin topics offload-status -w my-tenant/my-namespace/topic1
  2. Offload was a success

If there is an error offloading, the error will be propagated to the offload-status command.

  1. Error in offload null
  3. Reason: Error offloading: org.apache.bookkeeper.mledger.ManagedLedgerException: java.util.concurrent.CompletionException: com.amazonaws.services.s3.model.AmazonS3Exception: Anonymous users cannot initiate multipart uploads. Please authenticate. (Service: Amazon S3; Status Code: 403; Error Code: AccessDenied; Request ID: 798758DE3F1776DF; S3 Extended Request ID: dhBFz/lZm1oiG/oBEepeNlhrtsDlzoOhocuYMpKihQGXe6EG8puRGOkK6UwqzVrMXTWBxxHcS+g=), S3 Extended Request ID: dhBFz/lZm1oiG/oBEepeNlhrtsDlzoOhocuYMpKihQGXe6EG8puRGOkK6UwqzVrMXTWBxxHcS+g= `

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