F# Event store for Azure Cosmos DB and Azure Table Storage
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F# Event store library for Azure Cosmos DB & Azure Table Storage


  • Support for Azure Cosmos DB
  • Support for Azure Table Storage
  • Optimistic concurrency
  • ACID compliant
  • Storage agnostic F# API
  • Simple Stream querying


First install NuGet package NuGet

Install-Package CosmoStore

or using Paket

nuget CosmoStore

Event store

Event store (defined as F# record) is by design storage agnostic which means that no matter if you use Cosmos DB or Table Storage, the API is the same.

type EventStore = {
    AppendEvent : string -> ExpectedPosition -> EventWrite -> Task<EventRead>
    AppendEvents : string -> ExpectedPosition -> EventWrite list -> Task<EventRead list>
    GetEvent : string -> int64 -> Task<EventRead>
    GetEvents : string -> EventsReadRange -> Task<EventRead list>
    GetStreams : StreamsReadFilter -> Task<Stream list>

Each function on record is explained in separate chapter.

Initializing Event store for Azure Cosmos DB

Cosmos DB Event store has own configuration type that follows some specifics of database like Request units throughput or collection capacity (fixed, unlimited).

type Capacity =
    | Fixed
    | Unlimited

type Configuration = {
    DatabaseName : string
    ServiceEndpoint : Uri
    AuthKey : string
    Capacity: Capacity
    Throughput: int

Note: If you don't know these terms check official documentation

Configuration can be created "manually" or use default setup (fixed collection with 400 RU/s)

open CosmoStore

let cosmosDbUrl = Uri "https://mycosmosdburl" // check Keys section on Azure portal
let cosmosAuthKey = "VeryPrivateKeyValue==" // check Keys section on Azure portal
let myConfig = CosmosDb.Configuration.CreateDefault cosmosDbUrl cosmosAuthKey

let eventStore = myConfig |> CosmosDb.EventStore.getEventStore

Initializing Event store for Azure Table Storage

Configuration for Table Storage is much easier since we need only account name and authentication key.

type StorageAccount =
    | Cloud of accountName:string * authKey:string
    | LocalEmulator

type Configuration = {
    DatabaseName : string
    Account : StorageAccount

As for Cosmos DB, you can easily create default configuration.

open CosmoStore

let storageAccountName = "myStoreageAccountName" // check Keys section on Azure portal
let storageAuthKey = "VeryPrivateKeyValue==" // check Keys section on Azure portal
let myConfig = TableStorage.Configuration.CreateDefault storageAccountName storageAuthKey

let eventStore = myConfig |> TableStorage.EventStore.getEventStore

Writing Events to Stream

Events are data structures you want to write (append) to some "shelf" also known as Stream. Event for writing is defined as this type:

type EventWrite = {
    Id : Guid
    CorrelationId : Guid
    Name : string
    Data : JToken
    Metadata : JToken option

Note: Newtonsoft.Json library (.NET industry standard) JToken is used as default type for data and metadata.

When writing Events to some Stream, you usually expect them to be written at some position hence you must specify optimistic concurrency strategy. For this purpose the type ExpectedPosition exists:

type ExpectedPosition =
    | Any // we don't care
    | NoStream // no event must exist in that stream
    | Exact of int64 // exact position of next event

There are two defined functions to write Event to Stream. AppendEvent for writing single Event and AppendEvents to write more Events.

let expected = ExpectedPosition.NoStream // we are expecting brand new stream
let eventToWrite = ... // get new event to be written
let streamId = "MyAmazingStream"

// writing first event
eventToWrite |> eventStore.AppendEvent streamId expected 

let moreEventsToWrite = ... // get list of another events
let newExpected = ExpectedPosition.Exact 2L // we are expecting next event to be on 2nd position

// writing another N events
moreEventsToWrite |> eventStore.AppendEvents streamId newExpected

If everything goes well, you will get back list (in Task) of written events (type EventRead - explained in next chapter).

Reading Events from Stream

When reading back Events from Stream, you'll a little bit more information than you wrote:

type EventRead = {
    Id : Guid
    CorrelationId : Guid
    StreamId : string
    Position: int64
    Name : string
    Data : JToken
    Metadata : JToken option
    CreatedUtc : DateTime

You have two options how to read back stored Events. You can read single Event by Position using GetEvent function:

// return 2nd Event from Stream
let singleEvent = 2L |> eventStore.GetEvent "MyAmazingStream"

Or read list of Events using GetEvents function. For such reading you need to specify the range:

// return 1st-2nd Event from Stream
let firstTwoEvents = EventsReadRange.PositionRange(1,2) |> eventStore.GetEvents "MyAmazingStream"

// return all events
let allEvents = EventsReadRange.AllEvents |> eventStore.GetEvents "MyAmazingStream"

To fully understand what are the possibilities have a look at EventsReadRange definition:

type EventsReadRange =
    | AllEvents
    | FromPosition of int64
    | ToPosition of int64
    | PositionRange of fromPosition:int64 * toPosition:int64

Reading Streams from Event store

Each Stream has own metadata:

type Stream = {
    Id : string
    LastPosition : int64
    LastUpdatedUtc: DateTime

Streams can be queried using GetStreams function. The querying works similar way as filtering Events by range, but here you can query Streams by string Id:

let allAmazingStream = StreamsReadFilter.StartsWith("MyAmazing") |> eventStore.GetStreams
let allStreams = StreamsReadFilter.AllStream |> eventStore.GetStreams

The complete possibilities are defined by StreamsReadFilter type:

type StreamsReadFilter =
    | AllStreams
    | StarsWith of string
    | EndsWith of string
    | Contains of string

Known issues (Azure Table Storage only)

Azure Table Storage currently allows only 100 operations (appends) in one batch. CosmoStore reserves one operation per batch for Stream metadata, so if you want to append more than 99 events to single Stream, you will get InvalidOperationException.