DynamoDB

AWS docs: https://docs.aws.amazon.com/amazondynamodb/latest/APIReference/Welcome.html

DynamoDB accepts AWS JSON 1.0 and Smithy RPC v2 CBOR. JSON operations are identified by the X-Amz-Target header (e.g. DynamoDB_20120810.PutItem); CBOR operations use /service/DynamoDB/operation/<Operation> with Smithy-Protocol: rpc-v2-cbor.

All data types are supported in the request/response format. The emulator stores items in their DynamoDB JSON wire format internally to avoid serialisation round-trip issues.

---

Tables are region-scoped

A DynamoDB table is a regional resource, and Overcast models it as one. A table created in us-east-1 is invisible from eu-west-1: ListTables does not report it, DescribeTable answers ResourceNotFoundException, and the same table name can exist independently in both regions with entirely separate items, GSI index entries, ItemCount and stream records. Deleting the table in one region leaves the same-named table in the other untouched.

The region comes from the request, exactly as on AWS — the SigV4 credential scope, a regional endpoint hostname, or OVERCAST_DEFAULT_REGION when the request names none. A create in one region and a list in another therefore correctly disagree; that is the emulated behaviour, not a bug.

Upgrading an existing database. Before this, item rows, GSI index entries and stream records were keyed by table name alone, so same-named tables in different regions shared them. A startup migration rewrites existing rows to the region their table was created in, which is already recorded on disk, so single-region data carries over untouched. If the same table name existed in two regions, those regions genuinely shared one set of rows and nothing on disk says which write came from where: the rows are assigned to the alphabetically first of those regions and the others start out empty.

---

Known limitations

• GSI consistency: real DynamoDB GSIs are eventually consistent; the emulator is immediately consistent — items are visible in GSI queries the instant they are written. Asking for a strongly consistent read on a GSI (ConsistentRead=true with a GSI IndexName) is still rejected with a ValidationException, exactly as AWS does, so code written against the emulator cannot come to depend on a read mode AWS has no way to serve.
• TTL expiry is not enforced in real-time. Items with expired TTL are removed by a background sweeper (runs hourly), not lazily on read.
• PartiQL (ExecuteStatement, ExecuteTransaction, BatchExecuteStatement) is explicitly out of scope for v1.
• Every other modeled DynamoDB operation — global tables, backups, exports and imports, resource policies, contributor insights, PartiQL — answers 501 Not Implemented with x-emulator-unsupported: true, in DynamoDB’s own AWS JSON 1.0 error envelope. Only an X-Amz-Target naming no AWS operation at all gets 400 UnknownOperationException. The endpoint tables below name the global-table operations explicitly; the rest follow the same rule without being listed one by one.

Summary

Category	✅ Supported	❌ Unsupported
Table management	7	1
Item operations	6
Query & scan	2
Transactions	2
Tags	3
Streams interoperability	1
Global tables		6

---

Endpoints

Table management

Operation	Status	Notes	AWS Docs
CreateTable	✅ Supported	Includes GSI/LSI definitions; an omitted BillingMode defaults to PROVISIONED, which requires ProvisionedThroughput on the table and on every GSI, while PAY_PER_REQUEST rejects it	docs
DeleteTable	✅ Supported		docs
DescribeTable	✅ Supported		docs
ListTables	✅ Supported	Region-scoped — lists only tables in the request’s region; Limit (default/max 100) and ExclusiveStartTableName honored; LastEvaluatedTableName echoed when more tables remain	docs
UpdateTable	✅ Supported	BillingMode, ProvisionedThroughput, GSI create/delete/update-throughput, AttributeDefinitions, StreamSpecification	docs
DescribeTimeToLive	✅ Supported		docs
UpdateTimeToLive	✅ Supported	TTL-based item expiry; sweeper deletes expired items hourly	docs
RestoreTableFromBackup	❌ Unsupported		docs

Item operations

Operation	Status	Notes	AWS Docs
PutItem	✅ Supported	Includes ConditionExpression, ReturnValues (ALL_OLD); records AWS/DynamoDB CloudWatch metrics SuccessfulRequestLatency, ConsumedWriteCapacityUnits, UserErrors/SystemErrors (service-metrics-platform.md phase 2)	docs
GetItem	✅ Supported	Includes ProjectionExpression; records SuccessfulRequestLatency, ConsumedReadCapacityUnits, UserErrors/SystemErrors	docs
UpdateItem	✅ Supported	SET/REMOVE/ADD/DELETE clauses; all ReturnValues variants; upsert; records SuccessfulRequestLatency, ConsumedWriteCapacityUnits, UserErrors/SystemErrors	docs
DeleteItem	✅ Supported	ConditionExpression, ReturnValues (ALL_OLD); records SuccessfulRequestLatency, ConsumedWriteCapacityUnits, UserErrors/SystemErrors	docs
BatchGetItem	✅ Supported	Up to 100 items across tables; records SuccessfulRequestLatency/ConsumedReadCapacityUnits per table touched	docs
BatchWriteItem	✅ Supported	Up to 25 put/delete operations; records SuccessfulRequestLatency/ConsumedWriteCapacityUnits per table touched	docs

Query & scan

Operation	Status	Notes	AWS Docs
Query	✅ Supported	KeyConditionExpression, FilterExpression, Limit (applied before FilterExpression per AWS semantics), ExclusiveStartKey/LastEvaluatedKey pagination, ScanIndexForward, Select=COUNT; ConsistentRead=true on a GSI is rejected as AWS does; records SuccessfulRequestLatency and ConsumedReadCapacityUnits (with GlobalSecondaryIndexName when IndexName names a real GSI)	docs
Scan	✅ Supported	FilterExpression, Limit (applied before FilterExpression per AWS semantics), ExclusiveStartKey/LastEvaluatedKey pagination, parallel scan (Segment/TotalSegments), Select=COUNT; ConsistentRead=true on a GSI is rejected as AWS does; records SuccessfulRequestLatency and ConsumedReadCapacityUnits (with GlobalSecondaryIndexName when IndexName names a real GSI)	docs

Transactions

Operation	Status	Notes	AWS Docs
TransactGetItems	✅ Supported	Up to 100 items across tables; records SuccessfulRequestLatency/ConsumedReadCapacityUnits per table (2x weighted, matching AWS’s transactional-read capacity accounting)	docs
TransactWriteItems	✅ Supported	Put, Update, Delete, ConditionCheck; all-or-nothing; records SuccessfulRequestLatency/ConsumedWriteCapacityUnits per table (2x weighted, matching AWS’s transactional-write capacity accounting)	docs

Tags

Operation	Status	Notes	AWS Docs
TagResource	✅ Supported	Merges tags; max 50; validates key/value lengths	docs
ListTagsOfResource	✅ Supported	Returns tags as Key/Value array	docs
UntagResource	✅ Supported	Removes specified keys; idempotent on missing keys	docs

Streams interoperability

Operation	Status	Notes	AWS Docs
GetShardIterator	✅ Supported	TRIM_HORIZON, LATEST, AT/AFTER_SEQUENCE_NUMBER	docs

Global tables

Operation	Status	Notes	AWS Docs
CreateGlobalTable	❌ Unsupported	Overcast emulates a single region	docs
DescribeGlobalTable	❌ Unsupported	Overcast emulates a single region	docs
DescribeGlobalTableSettings	❌ Unsupported	Overcast emulates a single region	docs
ListGlobalTables	❌ Unsupported	Overcast emulates a single region	docs
UpdateGlobalTable	❌ Unsupported	Overcast emulates a single region	docs
UpdateGlobalTableSettings	❌ Unsupported	Overcast emulates a single region	docs