DynamoDB in short

The terms of DynamoDB that the guide uses, each with where DynaSharp meets it. The examples are on the shop. The DynamoDB developer guide of AWS has the whole picture.

Item

A record in a table, up to 400 KB, found by its primary key: the partition key, or the partition key and the sort key together. An entity is a C# type that maps to an item, see Entities and keys.

Attribute

A named value of an item: a string, number, binary, boolean, null, list, map or set. Items of one table need not share their attributes, which is how several types live in one table, told apart by the discriminator _type. Attrs holds a typed handle per attribute, see Keys and Attrs.

Partition key

The part of the primary key that decides where an item is stored. Items with the same partition key form a partition, and a Get or a Query always names one. Order.Keys.Pk.Of("alice") builds it, see Keys and Attrs.

Sort key

The second part of the primary key. A partition keeps its items in the order of their sort keys, and a Query reads all of them or a range, such as those that begin with ORDER#. See Query.

Scan

A read of the whole table, charged for every item it reads, whatever a filter leaves. It suits a tool, not a hot path, see Scan.

Global secondary index (GSI)

A second key on other attributes, so the same items can be found another way, such as orders by status. It can be added to a table later, and its reads are eventually consistent. [GlobalIndexKey] declares one, see Indexes and Adding an index.

Local secondary index (LSI)

Another sort key for the same partition key, such as the orders of a customer by total. It can only be created with the table, and it allows strongly consistent reads. [LocalIndexKey] declares one, see Local indexes.

Sparse index

An index holds only the items that have its key attributes, so an item without them stays out of it. That makes an index of the few, such as shipped orders by tracking number. See Sparse indexes.

Projection

Two things with one name. The projection of an index says which attributes it copies, and CreateTable() copies all of them, see Creating a table. The projection of a read says which attributes come back, which is what Project(...) asks for, see Projections.

Fan-out

A query over several partitions at once, whose results are merged into one list in the order of the sort key. DynamoDB has no such operation, so DynaSharp runs a Query per partition, see One list over many partitions.

Consumed capacity

What a request costs, in read and write units: a strongly consistent read of up to 4 KB is one read unit, an eventually consistent one half of it, and a write of up to 1 KB is one write unit. On-demand billing charges for them, and ReturnCapacity() makes a command report them, see GetItem.

Conditional write

A write with a condition that DynamoDB checks in the same atomic step, such as "only if no item has this key". When the condition does not hold, nothing is written and the call throws ConditionalCheckFailedException. See Conditions.

Transaction

Up to 100 writes, or up to 100 reads, as one step: all of them or none, at twice the capacity. See TransactWriteItems.

Time to live (TTL)

A number attribute that holds a time in seconds since 1970. Once that time has passed, DynamoDB deletes the item, some time later and at no cost. Cart.ExpiresAt uses EpochSecondsConverter for it, and its [TimeToLive] makes CreateTable() turn time to live on, see Time to live.

Stream

The change log of a table: each write to an item becomes a record, kept for 24 hours, in shards that split and close over time. A consumer such as a Lambda function reads it, and DynamoDbStreamsClient.ReadStreamAsync reads it in one process. See Streams.

IRSA

IAM roles for service accounts: a pod on EKS gets a role through a token file, which the client trades for credentials at STS. See Use IRSA on EKS.