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Alternator: DynamoDB API in Scylla

Introduction

Alternator is a Scylla feature adding compatibility with Amazon DynamoDB(TM). DynamoDB’s API uses JSON-encoded requests and responses which are sent over an HTTP or HTTPS transport. It is described in detail on Amazon’s site: https://docs.aws.amazon.com/amazondynamodb/latest/APIReference/

Our goal is that any application written to use Amazon DynamoDB could be run, unmodified, against Scylla with Alternator enabled. Alternator’s compatibility with DynamoDB is fairly complete, but users should be aware of some differences and some of unimplemented features. The extent of Alternator’s compatibility with DynamoDB is described in the Scylla Alternator for DynamoDB users document, which we updated as the work on Alternator progresses and compatibility continues to improve.

Running Alternator

By default, Scylla does not listen for DynamoDB API requests. To enable such requests, you must set the alternator-port configuration option (via command line or YAML) to the port on which you wish to listen for DynamoDB API requests.

For example., “--alternator-port=8000” on the command line will run Alternator on port 8000.

By default, Scylla listens on this port on all network interfaces. To listen only on a specific interface, pass also an “alternator-address” option.

In addition to (or instead of) serving HTTP requests on alternator-port, Scylla can accept DynamoDB API requests over HTTPS (encrypted), on the port specified by alternator-https-port. As usual for HTTPS servers, the operator must specify certificate and key files. By default these should be placed in /etc/scylla/scylla.crt and /etc/scylla/scylla.key, but these default locations can overridden by specifying --alternator-encryption-options keyfile="..." and --alternator-encryption-options certificate="...".

As we explain below in the “Write isolation policies”, Alternator has four different choices for the implementation of writes, each with different advantages. You should consider which of the options makes more sense for your intended use case, and use the “--alternator-write-isolation” option to choose one. There is currently no default for this option: Trying to run Scylla with Alternator enabled without passing this option will result in an error asking you to set it.

DynamoDB applications specify a single “endpoint” address, e.g., dynamodb.us-east-1.amazonaws.com. Behind the scenes, a DNS server and/or load balancers distribute the connections to many different backend nodes. Alternator does not provide such a load-balancing setup, so you should either set one up, or set up the client library to do the load balancing itself. Instructions, code and examples for doing this can be found in: https://github.com/scylladb/alternator-load-balancing/

Alternator-specific API

Write isolation policies

DynamoDB API update requests may involve a read before the write - e.g., a conditional update or an update based on the old value of an attribute. The read and the write should be treated as a single transaction - protected (isolated) from other parallel writes to the same item.

Alternator could do this isolation by using Scylla’s LWT (lightweight transactions) for every write operation, but this significantly slows down writes, and not necessary for workloads which don’t use read-modify-write (RMW) updates.

So Alternator supports four write isolation policies, which can be chosen on a per-table basis and may make sense for certain workloads as explained below.

A default write isolation policy must be chosen using the --alternator-write-isolation configuration option. Additionally, the write isolation policy for a specific table can be overridden by tagging the table (at CreateTable time, or any time later with TagResource) with the key system:write_isolation, and one of the following values:

  • a, always, or always_use_lwt - This mode performs every write operation - even those that do not need a read before the write - as a lightweight transaction.

    This is the slowest choice, but also the only choice guaranteed to work correctly for every workload.

  • f, forbid, or forbid_rmw - This mode forbids write requests which need a read before the write. An attempt to use such statements (e.g., UpdateItem with a ConditionExpression) will result in an error. In this mode, the remaining write requests which are allowed - pure writes without a read - are performed using standard Scylla writes, not LWT, so they are significantly faster than they would have been in the always_use_lwt, but their isolation is still correct.

    This mode is the fastest mode which is still guaranteed to be always safe. However, it is not useful for workloads that do need read-modify- write requests on this table - which this mode forbids.

  • o, or only_rmw_uses_lwt - This mode uses LWT only for updates that require read-modify-write, and does normal quorum writes for write-only updates.

    The benefit of this mode is that it allows fast write-only updates to some items, while still allowing some slower read-modify-write operations to other items. However, This mode is only safe if the workload does not mix read-modify-write and write-only updates to the same item, concurrently. It cannot verify that this condition is actually honored by the workload.

  • u, unsafe, or unsafe_rmw - This mode performs read-modify-write operations as separate reads and writes, without any isolation guarantees. It is the fastest option, but not safe - it does not correctly isolate read-modify-write updates. This mode is not recommended for any use case, and will likely be removed in the future.

Accessing system tables from Scylla

  • Scylla exposes lots of useful information via its internal system tables, which can be found in system keyspaces: ‘system’, ‘system_auth’, etc. In order to access to these tables via alternator interface, Scan and Query requests can use a special table name: .scylla.alternator.KEYSPACE_NAME.TABLE_NAME which will return results fetched from corresponding Scylla table. This interface can be used only to fetch data from system tables. Attempts to read regular tables via the virtual interface will result in an error. Example: in order to query the contents of Scylla’s system.large_rows, pass TableName=’.scylla.alternator.system.large_rows’ to a Query/Scan request.

Alternator design and implementation

This section provides only a very brief introduction to Alternator’s design. A much more detailed document about the features of the DynamoDB API and how they are, or could be, implemented in Scylla can be found in: https://docs.google.com/document/d/1i4yjF5OSAazAY_-T8CBce9-2ykW4twx_E_Nt2zDoOVs

Almost all of Alternator’s source code (except some initialization code) can be found in the alternator/ subdirectory of Scylla’s source code. Extensive functional tests can be found in the alternator-test/ subdirectory. These tests are written in Python, and can be run against both Alternator and Amazon’s DynamoDB; This allows verifying that Alternator’s behavior matches the one observed on DynamoDB. See alternator-test/README.md for more information about the tests and how to run them.

With Alternator enabled on port 8000 (for example), every Scylla node listens for DynamoDB API requests on this port. These requests, in JSON format over HTTP, are parsed and result in calls to internal Scylla C++ functions - there is no CQL generation or parsing involved. In Scylla terminology, the node receiving the request acts as the the coordinator, and often passes the request on to one or more other nodes - replicas which hold copies of the requested data.

Alternator tables are stored as Scylla tables, each in a separate keyspace. Each keyspace is initialized when the corresponding Alternator table is created (with a CreateTable request). The replication factor (RF) for this keyspace is chosen at that point, depending on the size of the cluster: RF=3 is used on clusters with three or more nodes, and RF=1 is used for smaller clusters. Such smaller clusters are, of course, only recommended for tests because of the risk of data loss.

Each table in Alternator is stored as a Scylla table in a separate keyspace. The DynamoDB key columns (hash and sort key) have known types, and become partition and clustering key columns of the Scylla table. All other attributes may be different for each row, so are stored in one map column in Scylla, and not as separate columns.

DynamoDB supports two consistency levels for reads, “eventual consistency” and “strong consistency”. These two modes are implemented using Scylla’s CL (consistency level) feature: All writes are done using the LOCAL_QUORUM consistency level, then strongly-consistent reads are done with LOCAL_QUORUM, while eventually-consistent reads are with just LOCAL_ONE.

In Scylla (and its inspiration, Cassandra), high write performance is achieved by ensuring that writes do not require reads from disk. The DynamoDB API, however, provides many types of requests that need a read before the write (a.k.a. RMW requests - read-modify-write). For example, a request may copy an existing attribute, increment an attribute, be conditional on some expression involving existing values of attribute, or request that the previous values of attributes be returned. These read-modify-write transactions should be isolated from each other, so by default Alternator implements every write operation using Scylla’s LWT (lightweight transactions). This default can be overridden on a per-table basis, by tagging the table as explained above in the “write isolation policies” section.

DynamoDB allows attributes to be nested - a top-level attribute may be a list or a map, and each of its elements may further be lists or maps, etc. Alternator currently stores the entire content of a top-level attribute as one JSON object. This means that UpdateItem requests which want modify a non-top-level attribute directly (e.g., a.b[3].c) need RMW: Alternator implements such requests by reading the entire top-level attribute a, modifying only a.b[3].c, and then writing back a.