F# wrapper API for AWS DynamoDB
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FSharp.AWS.DynamoDB an F# wrapper over the standard Amazon.DynamoDB library which allows you to represent table items using F# records and perform updates, queries and scans using F# quotation expressions.

The API draws heavily on the corresponding FSharp.Azure.Storage wrapper for Azure table storage.

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Install-Package FSharp.AWS.DynamoDB


Table items can be represented using F# records:

open FSharp.AWS.DynamoDB

type WorkItemInfo =
		ProcessId : int64
		WorkItemId : int64

		Name : string
		UUID : Guid
		Dependencies : Set<string>
		Started : DateTimeOffset option

We can now perfom table operations on DynamoDB like so

open Amazon.DynamoDBv2

let client : IAmazonDynamoDB = ``your DynamoDB client instance``
let table = TableContext.Create<WorkItemInfo>(client, tableName = "workItems", createIfNotExists = true)

let workItem = { ProcessId = 0L ; WorkItemId = 1L ; Name = "Test" ; UUID = guid() ; Dependencies = set ["mscorlib"] ; Started = None }

let key : TableKey = table.PutItem(workItem)
let workItem' = table.GetItem(key)

Queries and scans can be performed using quoted predicates

let qResults = table.Query(keyCondition = <@ fun r -> r.ProcessId = 0 @>, 
                            filterCondition = <@ fun r -> r.Name = "test" @>)
let sResults = table.Scan <@ fun r -> r.Started.Value >= DateTimeOffset.Now - TimeSpan.FromMinutes 1.  @>

Values can be updated using quoted update expressions

let updated = table.UpdateItem(<@ fun r -> { r with Started = Some DateTimeOffset.Now } @>, 
                                preCondition = <@ fun r -> r.DateTimeOffset = None @>)

Or they can be updated using the UpdateOp DSL which is closer to the underlying DynamoDB API

let updated = table.UpdateItem <@ fun r -> SET r.Name "newName" &&& ADD r.Dependencies ["MBrace.Core.dll"] @>

Supported Field Types

FSharp.AWS.DynamoDB supports the following field types:

  • Numerical types, enumerations and strings.
  • Array, Nullable, Guid, DateTimeOffset and TimeSpan.
  • F# lists
  • F# sets with elements of type number, string or byte[].
  • F# maps with key of type string.
  • F# records and unions (recursive types not supported).

Supported methods in Query Expressions

Query expressions support the following F# methods in their predicates:

  • Array.length, List.length, Set.count and Map.Count.
  • String.StartsWith and String.Contains.
  • Set.contains and Map.containsKey.
  • Array.isEmpty and List.isEmpty.
  • Option.isSome, Option.isNone, Option.Value and Option.get.
  • fst and snd for tuple records.

Supported methods in Update Expressions

Update expressions support the following F# value constructors:

  • (+) and (-) in numerical and set types.
  • Array.append and List.append (or @).
  • List consing (::).
  • defaultArg on optional fields.
  • Set.add and Set.remove.
  • Map.add and Map.remove.
  • Option.Value and Option.get.
  • fst and snd for tuple records.

Example: Creating an atomic counter

type private CounterEntry = { [<HashKey>]Id : Guid ; Value : int64 }

type Counter private (table : TableContext<CounterEntry>, key : TableKey) =
    member __.Value = table.GetItem(key).Value
    member __.Incr() = 
        let updated = table.UpdateItem(key, <@ fun e -> { e with Value = e.Value + 1L } @>)

    static member Create(client : IAmazonDynamoDB, table : string) =
        let table = TableContext.Create<CounterEntry>(client, table, createIfNotExists = true)
        let entry = { Id = Guid.NewGuid() ; Value = 0L }
        let key = table.PutItem entry
        new Counter(table, key)

Projection Expressions

Projection expressions can be used to fetch a subset of table attributes, which can be useful when performing large queries:

table.QueryProjected(<@ fun r -> r.HashKey = "Foo" @>, <@ fun r -> r.HashKey, r.Values.Nested.[0] @>)

which returns a tuple of specified attributes. Tuples can be of any arity and must contain non-conflicting document paths.

Secondary Indices

Global Secondary Indices can be defined using the GlobalSecondaryHashKey and GlobalSecondaryRangeKey attributes:

type Record =
        [<HashKey>] HashKey : string
        [<GlobalSecondaryHashKey(indexName = "Index")>]GSIH : string
        [<GlobalSecondaryRangeKey(indexName = "Index")>]GSIR : string

Queries can now be performed on the GSIH and GSIR fields as if they were regular hashkey and rangekey attributes. Global secondary indices are created using the same provisioned throughput as the primary keys.

Local Secondary Indices can be defined using the LocalSecondaryIndex attribute:

type Record =
        [<HashKey>] HashKey : string
        [<RangeKey>] RangeKey : Guid
        [<LocalSecondaryIndex>] LSI : double

Queries can now be performed using LSI as a secondary RangeKey.

NB: Due to API restrictions, secondary indices in the scope of FSharp.AWS.DynamoDB always project all table attributes which can incur additional costs from Amazon.

Notes on value representation

Due to restrictions of DynamoDB, it may sometimes be the case that objects are not persisted faithfully. For example, consider the following record definition:

type Record = 
        HashKey : Guid

        Optional : int option option
        Lists : int list list
let item = { HashKey = Guid.NewGuid() ; Optional = Some None ; Lists = [[1;2];[];[3;4]] }
let key = table.PutItem item

Subsequently recovering the given key will result in the following value:

> table.GetItem key
val it : Record = {HashKey = 8d4f0678-6def-4bc9-a0ff-577a53c1337c;
                   Optional = None;
                   Lists = [[1;2]; [3;4]];}

Precomputing DynamoDB Expressions

It is possible to precompute a DynamoDB expression as follows:

let precomputedConditional = table.Template.PrecomputeConditionalExpr <@ fun w -> w.Name <> "test" && w.Dependencies.Contains "mscorlib" @>

This precomputed conditional can now be used in place of the original expression in the FSharp.AWS.DynamoDB API:

let results = table.Scan precomputedConditional

FSharp.AWS.DynamoDB also supports precomputation of parametric expressions:

let startedBefore = table.Template.PrecomputeConditionalExpr <@ fun time w -> w.StartTime.Value <= time @>
table.Scan(startedBefore (DateTimeOffset.Now - TimeSpan.FromDays 1.))

Building & Running Tests

Depending on your platform, you can build and run tests running build.bat or build.cmd. To successfully run unit tests, you need to have credentials set to your default profile in your local credentials store. Alternative, you could set the following environment variables:

export AWS_REGION="eu-central-1"                   # defaults to eu-central-1
export AWS_CREDENTIAL_STORE_PROFILE=<profile name> # uses "default" if unset
export AWS_ACCESS_KEY_ID=<your access key>         # your account's access key
export AWS_SECRET_ACCESS_KEY=<your secret key>     # your account's secret key

Build Status

  • Windows/.NET Build status
  • Linux/Mono 4.2 Build Status


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