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Scylla Alternator for DynamoDB users

Scylla supports the DynamoDB API (this feature is codenamed "Alternator"). Our goal is to support any application written for Amazon DynamoDB. Nevertheless, there are a few differences between DynamoDB and Scylla, and and a few DynamoDB features that have not yet been implemented in Scylla. The purpose of this document is to inform users of these differences.

Provisioning

The most obvious difference between DynamoDB and Scylla is that while DynamoDB is a shared cloud service, Scylla is a dedicated service running on your private cluster. Whereas DynamoDB allows you to "provision" the number of requests per second you'll need - or at an extra cost not even provision that - Scylla requires you to provision your cluster. You need to reason about the number and size of your nodes - not the throughput.

When creating a table, the BillingMode and ProvisionedThroughput options are ignored by Scylla.

Load balancing

DynamoDB applications specify a single "endpoint" address, e.g., dynamodb.us-east-1.amazonaws.com. Amazon's cloud service distributes request for this single URL to many different backend nodes. Such a load-balancing setup is not included inside Alternator. You should either set one up, or configure the client library to do the load balancing itself. Instructions for doing this can be found in: https://github.com/scylladb/alternator-load-balancing/

Write isolation policies

Scylla was designed to optimize the performance of pure write operations - writes which do not need to read the the previous value of the item. In CQL, writes which do need the previous value of the item must explicitly use the slower LWT ("LightWeight Transaction") feature to be correctly isolated from each other. It is not allowed to mix LWT and non-LWT writes to the same item.

In contrast, in the DynamoDB API every write operation may need the previous value of the item. So without making further assumptions, Alternator would need to use the slower LWT for all writes - to correctly isolate concurrent writes. However, if Alternator is told that a certain workload does not have any read-modify-write operations, it can do all writes with the faster non-LWT write. Furthermore, if Alternator is told that a certain workload does have do both write-only and read-modify-write, but to different items, it could use LWT only for the read-modify-write operations.

Therefore, Alternator must be explicitly configured to tell it which of the above assumptions it may make on the write workload. This configuration is mandatory, and described in the "Write isolation policies" section of alternator.md. One of the options, always_use_lwt, is always safe, but the other options result in significantly better write performance and should be considered when the workload involves pure writes (e.g., ingestion of new data) or if pure writes and read-modify-writes go to distinct items.

Authorization

Alternator implements the same signature protocol as DynamoDB and the rest of AWS. Clients use, as usual, an access key ID and a secret access key to prove their identity and the authenticity of their request. Alternator can then validate the authenticity and authorization of each request using a known list of authorized key pairs.

In the current implementation, the user stores the list of allowed key pairs in the system_auth.roles table: The access key ID is the role column, and the secret key is the salted_hash, i.e., the secret key can be found by SELECT salted_hash from system_auth.roles WHERE role = ID;.

By default, authorization is not enforced at all. It can be turned on by providing an entry in Scylla configuration: alternator_enforce_authorization: true

Although Alternator implements DynamoDB's authentication, including the possibility of listing multiple allowed key pairs, there is currently no implementation of access control. All authenticated key pairs currently get full access to the entire database. This is in contrast with DynamoDB which supports fine-grain access controls via "IAM policies" - which give each authenticated user access to a different subset of the tables, different allowed operations, and even different permissions for individual items. All of this is not yet implemented in Alternator.

Metrics

Scylla has an advanced and extensive monitoring framework for inspecting and graphing hundreds of different metrics of Scylla's usage and performance. Scylla's monitoring stack, based on Grafana and Prometheus, is described in https://docs.scylladb.com/operating-scylla/monitoring/. This monitoring stack is different from DynamoDB's offering - but Scylla's is significantly more powerful and gives the user better insights on the internals of the database and its performance.

Unimplemented API features

In general, every DynamoDB API feature available in Amazon DynamoDB should behave the same in Alternator. However, there are a few features which we have not implemented yet. Unimplemented features return an error when used, so they should be easy to detect. Here is a list of these unimplemented features:

  • Currently in Alternator, a GSI (Global Secondary Index) can only be added to a table at table creation time. Unlike DynamoDB which also allows adding a GSI (but not an LSI) to an existing table using an UpdateTable operation. scylladb#5022

  • GSI (Global Secondary Index) and LSI (Local Secondary Index) may be configured to project only a subset of the base-table attributes to the index. This option is not yet respected by Alternator - all attributes are projected. This wastes some disk space when it is not needed. scylladb#5036

  • DynamoDB's TTL (per-item expiration) feature is not supported. Note that this is a different feature from Scylla's feature with the same name. scylladb#5060

  • DynamoDB's new multi-item transaction feature (TransactWriteItems, TransactGetItems) is not supported. Note that the older single-item conditional updates feature are fully supported. scylladb#5064

  • The "Select" option of Scan and Query operations, which allows to only read parts of items or to just count them, is not yet supported. scylladb#5058

  • The ScanIndexForward=false of the Query operation - to read a partition in reverse order - is currently implemented inefficiently, and limited by default to partitions of size 100 MB. scylladb#7586

  • Alternator does not yet support the DynamoDB API calls that control which table is available in which data center (DC): CreateGlobalTable, UpdateGlobalTable, DescribeGlobalTable, ListGlobalTables, UpdateGlobalTableSettings, DescribeGlobalTableSettings, and UpdateTable. Currently, all Alternator tables are created as global tables and can be accessed from all the DCs existing at the time of the table's creation. If a DC is added after a table is created, the table won't be visible from the new DC and changing that requires a CQL "ALTER TABLE" statement to modify the table's replication strategy. scylladb#5062

  • The DynamoDB Streams API for capturing change is supported, but still considered experimental so needs to be enabled explicitly with the --experimental-features=alternator-streams configuration option. Alternator streams also differ in some respects from DynamoDB Streams:

    • The number of separate "shards" in Alternator's streams is significantly larger than is typical on DynamoDB.
    • While in DynamoDB data usually appears in the stream less than a second after it was written, in Alternator Streams there is currently a 10 second delay by default. scylladb#6929
    • Some events are represented differently in Alternator Streams. For example, a single PutItem is represented by a REMOVE + MODIFY event, instead of just a single MODIFY or INSERT. scylladb#6930 scylladb#6918

  • The on-demand backup APIs are not supported: CreateBackup, DescribeBackup, DeleteBackup, ListBackups, RestoreTableFromBackup. For now, users can use Scylla's existing backup solutions such as snapshots or Scylla Manager. scylladb#5063

  • Continuous backup (the ability to restore any point in time) is also not supported: UpdateContinuousBackups, DescribeContinuousBackups, RestoreTableToPoinInTime

  • DynamoDB's encryption-at-rest settings are not supported. The Encryption- at-rest feature is available in Scylla Enterprise, but needs to be enabled and configured separately, not through the DynamoDB API.

  • No support for throughput accounting or capping. As mentioned above, the BillingMode option is ignored by Alternator, and if a provisioned throughput is specified, it is ignored. Requests which are asked to return the amount of provisioned throughput used by the request do not return it in Alternator. scylladb#5068

  • DAX (DynamoDB Accelerator), an in-memory cache for DynamoDB, is not available in for Alternator. Anyway, it should not be necessary - Scylla's internal cache is already rather advanced and there is no need to place another cache in front of the it. We wrote more about this here: https://www.scylladb.com/2017/07/31/database-caches-not-good/

  • The DescribeTable is missing information about creation data and size estimates, and also part of the information about indexes enabled on the table. scylladb#5013 scylladb#5026 scylladb#7550 scylladb#7551