S3Guard: Consistency and Metadata Caching for S3A

1. Choose the Database

A core concept of S3Guard is that the directory listing data of the object store, the metadata is replicated in a higher-performance, consistent, database. In S3Guard, this database is called The Metadata Store

By default, S3Guard is not enabled.

The Metadata Store to use in production is bonded to Amazon’s DynamoDB database service. The following setting will enable this Metadata Store:

<property>
    <name>fs.s3a.metadatastore.impl</name>
    <value>org.apache.hadoop.fs.s3a.s3guard.DynamoDBMetadataStore</value>
</property>

Note that the NullMetadataStore store can be explicitly requested if desired. This offers no metadata storage, and effectively disables S3Guard.

<property>
    <name>fs.s3a.metadatastore.impl</name>
    <value>org.apache.hadoop.fs.s3a.s3guard.NullMetadataStore</value>
</property>

2. Configure S3Guard Settings

More settings will may be added in the future. Currently the only Metadata Store-independent setting, besides the implementation class above, is the allow authoritative flag.

The authoritative expression in S3Guard is present in two different layers, for two different reasons:

• Authoritative S3Guard

  • S3Guard can be set as authoritative, which means that an S3A client will avoid round-trips to S3 when getting directory listings if there is a fully cached version of the directory stored in metadata store.
  • This mode can be set as a configuration property fs.s3a.metadatastore.authoritative
  • All interactions with the S3 bucket(s) must be through S3A clients sharing the same metadata store.
  • This is independent from which metadata store implementation is used.

• Authoritative directory listings (isAuthoritative bit)

  • Tells if the stored directory listing metadata is complete.
  • This is set by the FileSystem client (e.g. s3a) via the DirListingMetadata class (org.apache.hadoop.fs.s3a.s3guard.DirListingMetadata). (The MetadataStore only knows what the FS client tells it.)
  • If set to TRUE, we know that the directory listing (DirListingMetadata) is full, and complete.
  • If set to FALSE the listing may not be complete.
  • Metadata store may persist the isAuthoritative bit on the metadata store.
  • Currently org.apache.hadoop.fs.s3a.s3guard.LocalMetadataStore and org.apache.hadoop.fs.s3a.s3guard.DynamoDBMetadataStore implementation supports authoritative bit.

More on Authoritative S3Guard:

• It is not treating the MetadataStore (e.g. dynamodb) as the source of truth in general.
• It is the ability to short-circuit S3 list objects and serve listings from the MetadataStore in some circumstances.
• For S3A to skip S3’s list objects on some path, and serve it directly from the MetadataStore, the following things must all be true:
  1. The MetadataStore implementation persists the bit DirListingMetadata.isAuthorititative set when calling MetadataStore#put (DirListingMetadata)
  2. The S3A client is configured to allow metadatastore to be authoritative source of a directory listing (fs.s3a.metadatastore.authoritative=true).
  3. The MetadataStore has a full listing for path stored in it. This only happens if the FS client (s3a) explicitly has stored a full directory listing with DirListingMetadata.isAuthorititative=true before the said listing request happens.

This configuration only enables authoritative mode in the client layer. It is recommended that you leave the default setting here:

<property>
    <name>fs.s3a.metadatastore.authoritative</name>
    <value>false</value>
</property>

Note that a MetadataStore MAY persist this bit. (Not MUST). Setting this to true is currently an experimental feature.

Note that if this is set to true, it may exacerbate or persist existing race conditions around multiple concurrent modifications and listings of a given directory tree.

In particular: If the Metadata Store is declared as authoritative, all interactions with the S3 bucket(s) must be through S3A clients sharing the same Metadata Store

3. Configure the Metadata Store.

Here are the DynamoDBMetadataStore settings. Other Metadata Store implementations will have their own configuration parameters.

4. Name Your Table

First, choose the name of the table you wish to use for the S3Guard metadata storage in your DynamoDB instance. If you leave it unset/empty, a separate table will be created for each S3 bucket you access, and that bucket’s name will be used for the name of the DynamoDB table. For example, this sets the table name to my-ddb-table-name

<property>
  <name>fs.s3a.s3guard.ddb.table</name>
  <value>my-ddb-table-name</value>
  <description>
    The DynamoDB table name to operate. Without this property, the respective
    S3 bucket names will be used.
  </description>
</property>

It is good to share a table across multiple buckets for multiple reasons.

1. You are billed for the I/O capacity allocated to the table, even when the table is not used. Sharing capacity can reduce costs.

2. You can share the “provision burden” across the buckets. That is, rather than allocating for the peak load on a single bucket, you can allocate for the peak load across all the buckets, which is likely to be significantly lower.

3. It’s easier to measure and tune the load requirements and cost of S3Guard, because there is only one table to review and configure in the AWS management console.

When wouldn’t you want to share a table?

1. When you do explicitly want to provision I/O capacity to a specific bucket and table, isolated from others.

2. When you are using separate billing for specific buckets allocated to specific projects.

3. When different users/roles have different access rights to different buckets. As S3Guard requires all users to have R/W access to the table, all users will be able to list the metadata in all buckets, even those to which they lack read access.

5. Locate your Table

You may also wish to specify the region to use for DynamoDB. If a region is not configured, S3A will assume that it is in the same region as the S3 bucket. A list of regions for the DynamoDB service can be found in Amazon’s documentation. In this example, to use the US West 2 region:

<property>
  <name>fs.s3a.s3guard.ddb.region</name>
  <value>us-west-2</value>
</property>

When working with S3Guard-managed buckets from EC2 VMs running in AWS infrastructure, using a local DynamoDB region ensures the lowest latency and highest reliability, as well as avoiding all long-haul network charges. The S3Guard tables, and indeed, the S3 buckets, should all be in the same region as the VMs.

6. Optional: Create your Table

Next, you can choose whether or not the table will be automatically created (if it doesn’t already exist). If you want this feature, set the fs.s3a.s3guard.ddb.table.create option to true.

<property>
  <name>fs.s3a.s3guard.ddb.table.create</name>
  <value>true</value>
  <description>
    If true, the S3A client will create the table if it does not already exist.
  </description>
</property>

7. If creating a table: Set your DynamoDB IO Capacity

Next, you need to set the DynamoDB read and write throughput requirements you expect to need for your cluster. Setting higher values will cost you more money. Note that these settings only affect table creation when fs.s3a.s3guard.ddb.table.create is enabled. To change the throughput for an existing table, use the AWS console or CLI tool.

For more details on DynamoDB capacity units, see the AWS page on Capacity Unit Calculations.

The charges are incurred per hour for the life of the table, even when the table and the underlying S3 buckets are not being used.

There are also charges incurred for data storage and for data IO outside of the region of the DynamoDB instance. S3Guard only stores metadata in DynamoDB: path names and summary details of objects —the actual data is stored in S3, so billed at S3 rates.

<property>
  <name>fs.s3a.s3guard.ddb.table.capacity.read</name>
  <value>500</value>
  <description>
    Provisioned throughput requirements for read operations in terms of capacity
    units for the DynamoDB table.  This config value will only be used when
    creating a new DynamoDB table, though later you can manually provision by
    increasing or decreasing read capacity as needed for existing tables.
    See DynamoDB documents for more information.
  </description>
</property>

<property>
  <name>fs.s3a.s3guard.ddb.table.capacity.write</name>
  <value>100</value>
  <description>
    Provisioned throughput requirements for write operations in terms of
    capacity units for the DynamoDB table.  Refer to related config
    fs.s3a.s3guard.ddb.table.capacity.read before usage.
  </description>
</property>

Attempting to perform more IO than the capacity requested throttles the IO, and may result in operations failing. Larger IO capacities cost more. We recommending using small read and write capacities when initially experimenting with S3Guard.

Create a table: s3guard init

hadoop s3guard init -meta URI ( -region REGION | s3a://BUCKET )

Creates and initializes an empty metadata store.

A DynamoDB metadata store can be initialized with additional parameters pertaining to Provisioned Throughput:

[-write PROVISIONED_WRITES] [-read PROVISIONED_READS]

Tag argument can be added with a key=value list of tags. The table for the metadata store will be created with these tags in DynamoDB.

[-tag key=value;]

Example 1

hadoop s3guard init -meta dynamodb://ireland-team -write 5 -read 10 s3a://ireland-1

Creates a table “ireland-team” with a capacity of 5 for writes, 10 for reads, in the same location as the bucket “ireland-1”.

Example 2

hadoop s3guard init -meta dynamodb://ireland-team -region eu-west-1

Creates a table “ireland-team” in the region “eu-west-1.amazonaws.com”

Example 3

hadoop s3guard init -meta dynamodb://ireland-team -tag tag1=first;tag2=second;

Creates a table “ireland-team” with tags “first” and “second”.

Import a bucket: s3guard import

hadoop s3guard import [-meta URI] s3a://BUCKET

Pre-populates a metadata store according to the current contents of an S3 bucket. If the -meta option is omitted, the binding information is taken from the core-site.xml configuration.

Example

hadoop s3guard import s3a://ireland-1

Audit a table: s3guard diff

hadoop s3guard diff [-meta URI] s3a://BUCKET

Lists discrepancies between a metadata store and bucket. Note that depending on how S3Guard is used, certain discrepancies are to be expected.

Example

hadoop s3guard diff s3a://ireland-1

Display information about a bucket, s3guard bucket-info

Prints and optionally checks the s3guard and encryption status of a bucket.

hadoop s3guard bucket-info [ -guarded ] [-unguarded] [-auth] [-nonauth] [-magic] [-encryption ENCRYPTION] s3a://BUCKET

Options

The server side encryption options are not directly related to S3Guard, but it is often convenient to check them at the same time.

Example

hadoop s3guard bucket-info -guarded -magic s3a://ireland-1

List the details of bucket s3a://ireland-1, mandating that it must have S3Guard enabled (“-guarded”) and that support for the magic S3A committer is enabled (“-magic”)

Filesystem s3a://ireland-1
Location: eu-west-1
Filesystem s3a://ireland-1 is using S3Guard with store DynamoDBMetadataStore{region=eu-west-1, tableName=ireland-1}
Authoritative S3Guard: fs.s3a.metadatastore.authoritative=false
Metadata Store Diagnostics:
  ARN=arn:aws:dynamodb:eu-west-1:00000000:table/ireland-1
  description=S3Guard metadata store in DynamoDB
  name=ireland-1
  read-capacity=20
  region=eu-west-1
  retryPolicy=ExponentialBackoffRetry(maxRetries=9, sleepTime=100 MILLISECONDS)
  size=12812
  status=ACTIVE
  table={AttributeDefinitions: [{AttributeName: child,AttributeType: S},
    {AttributeName: parent,AttributeType: S}],TableName: ireland-1,
    KeySchema: [{AttributeName: parent,KeyType: HASH}, {AttributeName: child,KeyType: RANGE}],
    TableStatus: ACTIVE,
    CreationDateTime: Fri Aug 25 19:07:25 BST 2017,
    ProvisionedThroughput: {LastIncreaseDateTime: Tue Aug 29 11:45:18 BST 2017,
    LastDecreaseDateTime: Wed Aug 30 15:37:51 BST 2017,
    NumberOfDecreasesToday: 1,
    ReadCapacityUnits: 20,WriteCapacityUnits: 20},
    TableSizeBytes: 12812,ItemCount: 91,
    TableArn: arn:aws:dynamodb:eu-west-1:00000000:table/ireland-1,}
  write-capacity=20
The "magic" committer is supported

S3A Client
  Endpoint: fs.s3a.endpoint=s3-eu-west-1.amazonaws.com
  Encryption: fs.s3a.server-side-encryption-algorithm=none
  Input seek policy: fs.s3a.experimental.input.fadvise=normal

This listing includes all the information about the table supplied from

hadoop s3guard bucket-info -unguarded -encryption none s3a://landsat-pds

List the S3Guard status of clients of the public landsat-pds bucket, and verifies that the data is neither tracked with S3Guard nor encrypted.

Filesystem s3a://landsat-pdsLocation: us-west-2
Filesystem s3a://landsat-pds is not using S3Guard
Endpoint: fs.s3a.endpoints3.amazonaws.com
Encryption: fs.s3a.server-side-encryption-algorithm=none
Input seek policy: fs.s3a.experimental.input.fadvise=normal

Note that other clients may have a S3Guard table set up to store metadata on this bucket; the checks are all done from the perspective of the configuration settings of the current client.

hadoop s3guard bucket-info -guarded -auth s3a://landsat-pds

Require the bucket to be using S3Guard in authoritative mode. This will normally fail against this specific bucket.

List or Delete Leftover Multipart Uploads: s3guard uploads

Lists or deletes all pending (uncompleted) multipart uploads older than given age.

hadoop s3guard uploads (-list | -abort | -expect <num-uploads>) [-verbose] \
    [-days <days>] [-hours <hours>] [-minutes <minutes>] [-seconds <seconds>] \
    [-force] s3a://bucket/prefix

The command lists or deletes all multipart uploads which are older than the given age, and that match the prefix supplied, if any.

For example, to delete all uncompleted multipart uploads older than two days in the folder at s3a://my-bucket/path/to/stuff, use the following command:

hadoop s3guard uploads -abort -days 2 s3a://my-bucket/path/to/stuff

We recommend running with -list first to confirm the parts shown are those that you wish to delete. Note that the command will prompt you with a “Are you sure?” prompt unless you specify the -force option. This is to safeguard against accidental deletion of data, which is especially risky without a long age parameter as it can affect in-fight uploads.

The -expect option is similar to -list, except it is silent by default, and terminates with a success or failure exit code depending on whether or not the supplied number matches the number of uploads found that match the given options (path, age).

Delete a table: s3guard destroy

Deletes a metadata store. With DynamoDB as the store, this means the specific DynamoDB table use to store the metadata.

hadoop s3guard destroy [-meta URI] ( -region REGION | s3a://BUCKET )

This does not delete the bucket, only the S3Guard table which it is bound to.

Examples

hadoop s3guard destroy s3a://ireland-1

Deletes the table which the bucket ireland-1 is configured to use as its MetadataStore.

hadoop s3guard destroy -meta dynamodb://ireland-team -region eu-west-1

Clean up a table, s3guard prune

Delete all file entries in the MetadataStore table whose object “modification time” is older than the specified age.

hadoop s3guard prune [-days DAYS] [-hours HOURS] [-minutes MINUTES]
    [-seconds SECONDS] [-m URI] ( -region REGION | s3a://BUCKET )

A time value of hours, minutes and/or seconds must be supplied.

1. This does not delete the entries in the bucket itself.
2. The modification time is effectively the creation time of the objects in the S3 Bucket.
3. If an S3A URI is supplied, only the entries in the table specified by the URI and older than a specific age are deleted.

Example

hadoop s3guard prune -days 7 s3a://ireland-1

Deletes all entries in the S3Guard table for files older than seven days from the table associated with s3a://ireland-1.

hadoop s3guard prune -days 7 s3a://ireland-1/path_prefix/

Deletes all entries in the S3Guard table for files older than seven days from the table associated with s3a://ireland-1 and with the prefix “path_prefix”

hadoop s3guard prune -hours 1 -minutes 30 -meta dynamodb://ireland-team -region eu-west-1

Delete all entries more than 90 minutes old from the table “ireland-team” in the region “eu-west-1”.

Tune the IO capacity of the DynamoDB Table, s3guard set-capacity

Alter the read and/or write capacity of a s3guard table.

hadoop s3guard set-capacity [--read UNIT] [--write UNIT] ( -region REGION | s3a://BUCKET )

The --read and --write units are those of s3guard init.

Example

hadoop s3guard set-capacity  -read 20 -write 20 s3a://ireland-1

Set the capacity of the table used by bucket s3a://ireland-1 to 20 read and 20 write. (This is a low number, incidentally)

2017-08-30 16:21:26,343 [main] INFO  s3guard.S3GuardTool (S3GuardTool.java:initMetadataStore(229)) - Metadata store DynamoDBMetadataStore{region=eu-west-1, tableName=ireland-1} is initialized.
2017-08-30 16:21:26,344 [main] INFO  s3guard.DynamoDBMetadataStore (DynamoDBMetadataStore.java:updateParameters(1084)) - Current table capacity is read: 25, write: 25
2017-08-30 16:21:26,344 [main] INFO  s3guard.DynamoDBMetadataStore (DynamoDBMetadataStore.java:updateParameters(1086)) - Changing capacity of table to read: 20, write: 20
Metadata Store Diagnostics:
  ARN=arn:aws:dynamodb:eu-west-1:00000000000:table/ireland-1
  description=S3Guard metadata store in DynamoDB
  name=ireland-1
  read-capacity=25
  region=eu-west-1
  retryPolicy=ExponentialBackoffRetry(maxRetries=9, sleepTime=100 MILLISECONDS)
  size=12812
  status=UPDATING
  table={ ... }
  write-capacity=25

After the update, the table status changes to UPDATING; this is a sign that the capacity has been changed.

Repeating the same command will not change the capacity, as both read and write values match that already in use.

2017-08-30 16:24:35,337 [main] INFO  s3guard.DynamoDBMetadataStore (DynamoDBMetadataStore.java:updateParameters(1090)) - Table capacity unchanged at read: 20, write: 20
Metadata Store Diagnostics:
  ARN=arn:aws:dynamodb:eu-west-1:00000000000:table/ireland-1
  description=S3Guard metadata store in DynamoDB
  name=ireland-1
  read-capacity=20
  region=eu-west-1
  retryPolicy=ExponentialBackoffRetry(maxRetries=9, sleepTime=100 MILLISECONDS)
  size=12812
  status=ACTIVE
  table={ ... }
  write-capacity=20

Note: There is a limit to how many times in a 24 hour period the capacity of a bucket can be changed, either through this command or the AWS console.

Failure Semantics

Operations which modify metadata will make changes to S3 first. If, and only if, those operations succeed, the equivalent changes will be made to the Metadata Store.

These changes to S3 and Metadata Store are not fully-transactional: If the S3 operations succeed, and the subsequent Metadata Store updates fail, the S3 changes will not be rolled back. In this case, an error message will be logged.

Versioning

S3Guard tables are created with a version marker, an entry with the primary key and child entry of ../VERSION; the use of a relative path guarantees that it will not be resolved.

Versioning policy

1. The version number of an S3Guard table will only be incremented when an incompatible change is made to the table structure —that is, the structure has changed so that it is no longer readable by older versions, or because it has added new mandatory fields which older versions do not create.
2. The version number of S3Guard tables will only be changed by incrementing the value.
3. Updated versions of S3Guard MAY continue to support older version tables.
4. If an incompatible change is made such that existing tables are not compatible, then a means shall be provided to update existing tables. For example: an option in the Command Line Interface, or an option to upgrade tables during S3Guard initialization.

Note: this policy does not indicate any intent to upgrade table structures in an incompatible manner. The version marker in tables exists to support such an option if it ever becomes necessary, by ensuring that all S3Guard client can recognise any version mismatch.

Error: S3Guard table lacks version marker.

The table which was intended to be used as a S3guard metadata store does not have any version marker indicating that it is a S3Guard table.

It may be that this is not a S3Guard table.

• Make sure that this is the correct table name.
• Delete the table, so it can be rebuilt.

Error: Database table is from an incompatible S3Guard version

This indicates that the version of S3Guard which created (or possibly updated) the database table is from a different version that that expected by the S3A client.

This error will also include the expected and actual version numbers.

If the expected version is lower than the actual version, then the version of the S3A client library is too old to interact with this S3Guard-managed bucket. Upgrade the application/library.

If the expected version is higher than the actual version, then the table itself will need upgrading.

Error "DynamoDB table TABLE does not exist in region REGION; auto-creation is turned off"

S3Guard could not find the DynamoDB table for the Metadata Store, and it was not configured to create it. Either the table was missing, or the configuration is preventing S3Guard from finding the table.

1. Verify that the value of fs.s3a.s3guard.ddb.table is correct.
2. If the region for an existing table has been set in fs.s3a.s3guard.ddb.region, verify that the value is correct.
3. If the region is not set, verify that the table exists in the same region as the bucket being used.
4. Create the table if necessary.

Error "The level of configured provisioned throughput for the table was exceeded"

org.apache.hadoop.fs.s3a.AWSServiceThrottledException: listFiles on s3a://bucket/10/d1/d2/d3:
com.amazonaws.services.dynamodbv2.model.ProvisionedThroughputExceededException:
The level of configured provisioned throughput for the table was exceeded.
Consider increasing your provisioning level with the UpdateTable API.
(Service: AmazonDynamoDBv2; Status Code: 400;
Error Code: ProvisionedThroughputExceededException;

The IO load of clients of the (shared) DynamoDB table was exceeded.

1. Increase the capacity of the DynamoDB table.
2. Increase the retry count and/or sleep time of S3Guard on throttle events.
3. Enable capacity autoscaling for the table in the AWS console.

Error Max retries exceeded

The I/O load of clients of the (shared) DynamoDB table was exceeded, and the number of attempts to retry the operation exceeded the configured amount.

1. Increase the capacity of the DynamoDB table.
2. Increase the retry count and/or sleep time of S3Guard on throttle events.
3. Enable capacity autoscaling for the table in the AWS console.

Error when running set-capacity: org.apache.hadoop.fs.s3a.AWSServiceThrottledException: ProvisionTable

org.apache.hadoop.fs.s3a.AWSServiceThrottledException: ProvisionTable on s3guard-example:
com.amazonaws.services.dynamodbv2.model.LimitExceededException:
Subscriber limit exceeded: Provisioned throughput decreases are limited within a given UTC day.
After the first 4 decreases, each subsequent decrease in the same UTC day can be performed at most once every 3600 seconds.
Number of decreases today: 6.
Last decrease at Wednesday, July 25, 2018 8:48:14 PM UTC.
Next decrease can be made at Wednesday, July 25, 2018 9:48:14 PM UTC

There’s are limit on how often you can change the capacity of an DynamoDB table; if you call set-capacity too often, it fails. Wait until the after the time indicated and try again.