isabella232 / fscodec

F# Event-Union Contract Encoding with versioning tolerant converters supporting System.Text.Json and Newtonsoft.Json

FsCodec

Defines a minimal interface for serialization and deserialization of events for event-sourcing systems on .NET. Provides implementation packages for writing simple yet versionable Event Contract definitions in F# using ubiquitous serializers.

Typically used in applications leveraging Equinox and/or Propulsion, but also applicable to defining DTOs for other purposes such as Web APIs.

Components

The components within this repository are delivered as multi-targeted Nuget packages supporting netstandard2.1 (F# 4.5+) profiles.

• FsCodec Defines interfaces with trivial implementation helpers.
  • No dependencies.
  • FsCodec.IEventCodec: defines a base interface for serializers.
  • FsCodec.Codec: enables plugging in a serializer and/or Union Encoder of your choice (typically this is used to supply a pair encode and tryDecode functions)
  • FsCodec.StreamName: strongly-typed wrapper for a Stream Name, together with factory functions and active patterns for parsing same
• FsCodec.Box: See FsCodec.Box.Codec; IEventCodec<obj> implementation that provides a null encode/decode step in order to enable decoupling of serialization/deserialization concerns from the encoding aspect, typically used together with Equinox.MemoryStore
  • depends on FsCodec, TypeShape >= 10
• FsCodec.NewtonsoftJson: As described in a scheme for the serializing Events modelled as an F# Discriminated Union, enabled tagging of F# Discriminated Union cases in a versionable manner with low-dependencies using TypeShape's UnionContractEncoder
  • Uses the ubiquitous Newtonsoft.Json library to serialize the event bodies.
  • Provides relevant Converters for common non-primitive types prevalent in F#
  • depends on FsCodec.Box, Newtonsoft.Json >= 11.0.2, Microsoft.IO.RecyclableMemoryStream >= 2.2.0, System.Buffers >= 4.5.1
• FsCodec.SystemTextJson: See #38: drop in replacement that allows one to retarget from Newtonsoft.Json to the .NET Core >= v 3.0 default serializer: System.Text.Json, solely by changing the referenced namespace.
  • depends on FsCodec.Box, System.Text.Json >= 6.0.1,

Features: FsCodec

The purpose of the FsCodec package is to provide a minimal interface on which libraries such as Equinox and Propulsion can depend on in order that they can avoid forcing a specific serialization mechanism.

• FsCodec.IEventData represents a single event and/or related metadata in raw form (i.e. still as a UTF8 string etc, not yet bound to a specific Event Type)
• FsCodec.ITimelineEvent represents a single stored event and/or related metadata in raw form (i.e. still as a UTF8 string etc, not yet bound to a specific Event Type). Inherits IEventData, adding Index and IsUnfold in order to represent the position on the timeline that the event logically occupies.
• FsCodec.IEventCodec presents Encode : 'Context option * 'Event -> IEventData and TryDecode : ITimelineEvent -> 'Event option methods that can be used in low level application code to generate IEventDatas or decode ITimelineEvents based on a contract defined by 'Union
• FsCodec.Codec.Create implements IEventCodec in terms of supplied encode : 'Event -> string * byte array and tryDecode : string * byte array -> 'Event option functions (other overloads are available for advanced cases)
• FsCodec.Core.EventData.Create is a low level helper to create an IEventData directly for purposes such as tests etc.
• FsCodec.Core.TimelineEvent.Create is a low level helper to create an ITimelineEvent directly for purposes such as tests etc.

Features: FsCodec.(Newtonsoft|SystemText)Json

Common API

The concrete implementations implement common type/member/function signatures and behavior that offer consistent behavior using either Newtonsoft.Json or System.Text.Json, emphasizing the following qualities:

• avoid non-straightforward encodings:
  • tuples don't magically become arrays
  • union bodies don't become arrays of mixed types like they do OOTB in JSON.NET (they become JSON Objects with named fields via UnionEncoder, or string values via TypeSafeEnumConverter)
• don't surprise .NET developers used to JSON.NET or System.Text.Json
• having an opinionated core set of behaviors, but don't conflict with the standard extensibility mechanisms afforded by the underlying serializer (one should be able to search up and apply answers from StackOverflow to questions regarding corner cases)
• maintain a minimal but well formed set of built in converters that are implemented per supported serializer - e.g., choices like not supporting F# list types (although System.Text.Json v >= 6 does now provide such support)

Codec

FsCodec.NewtonsoftJson/SystemTextJson.Codec provides an implementation of IEventCodec as described in a scheme for the serializing Events modelled as an F# Discriminated Union. This yields a clean yet versionable way of managing the roundtripping events based on a contract inferred from an F# Discriminated Union Type using Newtonsoft.Json >= 11.0.2 / System.Text.Json to serialize the bodies.

Converters: Newtonsoft.Json.Converters / System.Text.Json.Serialization.JsonConverters

Explicit vs Implicit

While it's alluded to in the recommendations, it's worth calling out that the converters in FsCodec (aside from obvious exceptions like the Option and Record ones) are intended to be used by tagging the type with a JsonConverterAttribute rather than by inclusion in the global converters list of the underlying serializer.

The key effect of this is that any non-trivial mapping will manifest as the application of the relevant attribute on the type or property in question. This also aligns well with the notion of cordoning off a module Events as described in Equinox's module Aggregate documentation: types that participate in an Event union are defined and namespaced together (including any snapshot serialization contracts).

This set might be all you need ...

While this may not seem like a sufficiently large set of converters for a large app, it should be mentioned that the serializer-neutral escape hatch represented by JsonIsomorphism has resulted in this set alone proving sufficient for two major subsystems of a large e-commerce software suite. See recommendations for further expansion on this (TL;DR it does mean ruling out using some type constructs directly in event and/or binding contracts and using Anti Corruption Layer and/or event versioning techniques.

... but don't forget FSharp.SystemTextJson

The role and intention of the converters in the box in FsCodec.SystemTextJson and/or FsCodec.NewtonsoftJson has always been to be minimal but provide escape hatches; short lived shims absolutely fit within this remit. For example, with regard to System.Text.Json, over time the shimming provided has been adjusted in alignment with the STJ implementation:

• System.Text.Json v4 did not even support F# records that are not marked [<CLIMutable>] out-of-the-box (it was similarly spartan wrt C# types, requiring a default constructor on classes). This library previously provided a shim for that.
• Version 5 added support for records.
• Version 6 added support for F# options, lists, Set and Map out of the box. This enabled the removal of the JsonOptionConverter that once lived here.
• There is an open issue on the System.Text.Json repo wrt supporting F# Unions. UnionConverter and TypeSafeEnumConverter provide for round-tripping of the most common usages of F# discriminated union types in two canonical formats that are known to have good versioning properties, rendering in formats that are known to be interoperable with other ecosystems, i.e. there are clean ways of generating and consuming in the same way in e.g. on the JVM and JavaScript.

It's worth calling out explicitly that there are no plans to extend the representations FsCodec.SystemTextJson can handle in any significant way over time (the advice for FsCodec.NewtonsoftJson has always been to avoid stuff outside of records, options and arrays) - if you have specific exotic corner cases and determine you need something more specifically tailored, the Converters abstraction affords you ability to mix and match as necessary for specific applications.

The single most complete set of System.Text.Json Converters is the FSharp.SystemTextJson library; it provides a much broader, well tested set of converters with a broader remit than what FsCodec is trying to succinctly address as its sweet spot. In general, there should be a smooth path to transition from using FsCodec to that as and when needed

Core converters

The respective concrete Codec packages include relevant Converter/JsonConverter in order to facilitate interoperable and versionable renderings:

• TypeSafeEnumConverter represents discriminated union (whose cases are all nullary), as a string in a trustworthy manner (Newtonsoft.Json.Converters.StringEnumConverter permits values outside the declared values) 🙏 @amjjd

• UnionConverter represents F# discriminated unions as a single JSON object with both the tag value and the body content as named fields directly within 🙏 @amjdd; System.Text.Json reimplementation 🙏 @NickDarvey

  NOTE: The encoding differs from that provided by Newtonsoft.Json's default converter: Newtonsoft.Json.Converters.DiscriminatedUnionConverter, which encodes the fields as an array without names, which has some pros, but many obvious cons

  NOTE System.Text.Json, even in v 6.0 does not support F# unions out of the box. It's not intended to extend the representations FsCodec.SystemTextJson can handle in any significant way over time - if you have specific requirements, the powerful and complete FSharp.SystemTextJson library is likely your best option in this space.

Custom converter base classes

• JsonIsomorphism - allows one to cleanly map a type's internal representation to something that the underlying serializer and converters can already cleanly handle 🙏 @EirikTsarpalis
• JsonPickler - removes boilerplate from simple converters, used in implementation of JsonIsomorphism 🙏 @EirikTsarpalis

Newtonsoft.Json-specific low level converters

• OptionConverter represents F#'s Option<'t> as a value or null; included in the standard Options.Create profile.
• VerbatimUtf8JsonConverter captures/renders known valid UTF8 JSON data into a byte array without decomposing it into an object model (not typically relevant for application level code, used in Equinox.Cosmos versions prior to 3.0).

System.Text.Json-specific low level converters

• UnionOrTypeSafeEnumConverterFactory: Global converter that can apply TypeSafeEnumConverter to all Discriminated Unions that do not have cases with values, and UnionConverter to ones that have values. See this System.Text.Json issue for background information as to the reasoning behind and tradeoffs involved in applying such a policy.

FsCodec.NewtonsoftJson.Options

FsCodec.NewtonsoftJson.Options provides a clean syntax for building a Newtonsoft.Json.JsonSerializerSettings with which to define a serialization contract profile for interoperability purposes. Methods:

• CreateDefault: as per Newtonsoft.Json defaults with the following override:
  • DateTimeZoneHandling = DateTimeZoneHandling.Utc (default is RoundtripKind)
  • no custom IContractResolver (one is expected to use camelCase field names within records, for which this does not matter)
• Create: as CreateDefault with the following difference:
  • adds an OptionConverter (see Converters, above)
• Default: Default settings; same as calling Create() produces

FsCodec.SystemTextJson.Options

FsCodec.SystemTextJson.Options provides a clean syntax for building a System.Text.Json.Serialization.JsonSerializerOptions as per FsCodec.NewtonsoftJson.Options, above. Methods:

• CreateDefault: configures the settings equivalent to new JsonSerializerSettings() or JsonSerializerSettings.Default, without overrides of any kind (see Create, below for the relevant differences)
• Create: as CreateDefault with the following difference:
  • By default, inhibits the HTML-safe escaping that System.Text.Json provides as a default by overriding Encoder with System.Text.Encodings.Web.JavaScriptEncoder.UnsafeRelaxedJsonEscaping
  • (camelCase = true): opts into camel case conversion for PascalCased properties and Dictionary keys
  • (autoTypeSafeEnumToJsonString = true): triggers usage of TypeSafeEnumConverter for any F# Discriminated Unions that only contain nullary cases. See AutoUnionTests.fs for examples
  • (autoUnionToJsonObject = true): triggers usage of a UnionConverter to round-trip F# Discriminated Unions (with at least a single case that has a body) as JSON Object structures. See AutoUnionTests.fs for examples
• Default: Default settings; same as calling Create() produces (same intent as JsonSerializerOptions.Default)

Serdes

FsCodec.SystemTextJson/NewtonsoftJson.Serdes provides light wrappers over (JsonConvert|JsonSerializer).(Des|S)erialize(Object)? based on an explicitly supplied serialization profile created by Options.Create (above), or using Options.Default. This enables one to smoothly switch between System.Text.Json vs Newtonsoft.Json serializers with minimal application code changes, while also ensuring consistent and correct options get applied in each case. Methods:

• Serialize<T>: serializes an object per its type using the settings defined in Options.Create
• Deserialize<T>: deserializes an object per its type using the settings defined in Options.Create
• Options: Allows one to access the JsonSerializerSettings/JsonSerializerOptions used by this instance.

Usage of Converters with ASP.NET Core

ASP.NET Core's out-of-the-box behavior is to use System.Text.Json. One can explicitly opt to use the more ubiquitous Newtonsoft.Json via the Microsoft.AspNetCore.Mvc.NewtonsoftJson package's AddNewtonsoftJson by adjusting one's .AddMvc).

If you follow the policies covered in the rest of the documentation here, your DTO types (and/or types in your module Events that you surface while you are scaffolding and/or hacking without an anti-corruption layer) will fall into one of two classifications:

1. Types that have an associated Converter explicitly annotated (e.g., DU types bear an associated UnionConverter, TypeSafeEnumConverter or JsonIsomorphism-based custom converter, custom types follow the conventions or define a JsonIsomorphism-based converter)
2. Types that require a global converter to be registered. While it may seem that the second set is open-ended and potentially vast, experience teaches that you want to keep it minimal.. This boils down to:

• records arrays and all other good choices for types Just Work already
• Nullable<MyType>: Handled out of the box by both NSJ and STJ - requires no converters, provides excellent interop with other CLR languages. Would recommend.
• MyType option: Covered by the global OptionConverter for Newtonsoft, handled intrinsically by System.Text.Json versions >= 6 (see below for a clean way to add them to the default MVC view rendering configuration). Note that while this works well with ASP.NET Core, it may be problematic if you share contracts (yes, not saying you should) or rely on things like Swashbuckle which will need to be aware of the types when they reflect over them.

The bottom line is that using exotic types in DTOs is something to think very hard about before descending into. The next sections are thus only relevant if you decide to add that extra complexity to your system...

ASP.NET Core with Newtonsoft.Json

Hence the following represents the recommended default policy:-

/// Define a Serdes instance with a given policy somewhere (globally if you need to do explicit JSON generation) 
let serdes = Serdes Options.Default

services.AddMvc(fun options -> ...
).AddNewtonsoftJson(fun options ->
    // Borrow the Converters from the Options the Serdes is holding
    serdes.Options.Converters |> Seq.iter options.SerializerSettings.Converters.Add
    // OR, in the trivial case: Options.Default.Converters |> Seq.iter options.SerializerSettings.Converters.Add
) |> ignore	        

This adds all the converters used by the serdes serialization/deserialization policy (currently only FsCodec.NewtonsoftJson.OptionConverter) into the equivalent managed by ASP.NET.

ASP.NET Core with System.Text.Json

The equivalent for the native System.Text.Json, as v6, thanks to the great work of the .NET team, is presently a no-op.

The following illustrates how opt into autoTypeSafeEnumToJsonString and/or autoUnionToJsonObject modes for the rendering of View Models by ASP.NET:

// Default behavior throws an exception if you attempt to serialize a DU or TypeSafeEnum without an explicit JsonConverterAttribute
// let serdes = FsCodec.SystemTextJson.Options.Default |> FsCodec.SystemTextJson.Serdes

// If you use autoTypeSafeEnumToJsonString = true or autoUnionToJsonObject = true, serdes.Serialize / Deserialize applies the relevant converters
let serdes = FsCodec.SystemTextJson.Options.Create(autoTypeSafeEnumToJsonString = true, autoUnionToJsonObject = true) |> FsCodec.SystemTextJson.Serdes

services.AddMvc(fun options -> ...
).AddJsonOptions(fun options ->
    // Register the converters (actually a single JsonConverterFactory) from the Options passed to the `serdes` above
    serdes.Options.Converters |> Seq.iter options.JsonSerializerOptions.Converters.Add
) |> ignore

Examples: FsCodec.(Newtonsoft|SystemText)Json

There's a test playground in tests/FsCodec.NewtonsoftJson.Tests/Examples.fsx. It's highly recommended to experiment with conversions using FSI. (Also, PRs adding examples are much appreciated...)

There's an equivalent of that for FsCodec.SystemTextJson: tests/FsCodec.SystemTextJson.Tests/Examples.fsx.

Examples of using Serdes to define a contract

In a contract assembly used as a way to supply types as part of a client library, one way of encapsulating the conversion rules that need to be applied is as follows:

Simple contracts that tag all types or fields necessitating Converters directly and only records and options

The minimal code needed to define helpers to consistently roundtrip where one only uses simple types is to simply state" _Please use FsCodec.NewtonsoftJson.Serdes to encode/decode JSON payloads correctly. However, an alternate approach is to employ the convention of providing a pair of helper methods alongside the type :-

module Contract =
    type Item = { value : string option }
    /// Options to be used within this contract (opinionated ones compared to just using JsonSerializer.Serialize / Deserialize)
    let private serdes = FsCodec.SystemTextJson.Serdes FsCodec.SystemTextJson.Default
    // implies default settings from Options.Create(), i.e., includes UnsafeRelaxedJsonEscaping
    let serialize (x : Item) : string = serdes.Serialize x
    // implies default settings from Options.Create()
    let deserialize (json : string) = serdes.Deserialize json

More advanced case necessitating a custom converter

While it's hard to justify the wrapping in the previous case, this illustrates how one can employ the same basic layout yet override a setting (register a necessary custom Newtonsoft.Json.Converter type):

module Contract =
    type Item = { Value : string option; other : TypeThatRequiresMyCustomConverter }
    /// Options to be used within this contract - note the Pascal Cased Value property compared to the previous record definition
    let private serdes = FsCodec.SystemTextJson.Options.Create(converters = [| MyCustomConverter() |], camelCase = true) |> FsCodec.SystemTextJson.Serdes
    let serialize (x : Item) = serdes.Serialize x
    let deserialize (json : string) : Item = serdes.Deserialize json

Encoding and conversion of F# types

Recommended round-trippable constructs

Newtonsoft.Json, thanks to its broad usage throughout .NET systems has well known (with some idiosyncratic quirks) behaviors for most common types one might use for C# DTOs.

Normal primitive F#/.NET such as bool, byte, int16, int, int64, float32 (Single), float (Double), decimal work as expected.

The default settings for FsCodec applies Json.NET's default behavior, which is to render fields that have a null or null-equivalent value with the value null. This behavior can be overridden via Options(ignoreNulls = true), which will cause such JSON fields to be omitted.

The recommendations here apply particularly to Event Contracts - the data in your store will inevitably outlast your code, so being conservative in the complexity of one's encoding scheme is paramount. Explicit is better than Implicit.

Type kind	TL;DR	Notes	Example input	Example output
't[]	As per C#	Don't forget to handle null	[ 1; 2; 3]	[1,2,3]
DateTimeOffset	Roundtrips cleanly	The default Options.Create requests RoundtripKind	DateTimeOffset.Now	"2019-09-04T20:30:37.272403+01:00"
Nullable<'t>	As per C#; Nullable() -> null, Nullable x -> x	OOTB Json.NET and STJ roundtrip cleanly. Works with Options.CreateDefault(). Worth considering if your contract does not involve many option types	Nullable 14	14
't option	Some null,None -> null, Some x -> x with the converter Options.Create() adds	OOTB Json.NET does not roundtrip option types cleanly; Options.Create wires in an OptionConverter by default in FsCodec.NewtonsoftJson NOTE Some null will produce null, but deserialize as None - i.e., it's not round-trippable	Some 14	14
string	As per C#; need to handle null	One can use a string option to map null and Some null to None	"Abc"	"Abc"
types with unit of measure	Works well (doesnt encode the unit)	Unit of measure tags are only known to the compiler; Json.NET does not process the tags and treats it as the underlying primitive type	54<g>	54
FSharp.UMX tagged string, DateTimeOffset	Works well	FSharp.UMX enables one to type-tag string and DateTimeOffset values using the units of measure compiler feature, which Json.NET will render as if they were unadorned	SkuId.parse "54-321"	"000-054-321"
records	Just work	For System.Text.Json v 4.x, usage of [<CLIMutable>] or a custom JsonRecordConverter was once required	{| a = 1; b = Some "x" |}	"{"a":1,"b":"x"}"
Nullary unions (Enum-like DU's without bodies)	Tag type with TypeSafeEnumConverter	Works well - guarantees a valid mapping, as opposed to using a System.Enum and StringEnumConverter, which can map invalid values and/or silently map to 0 etc	State.NotFound	"NotFound"
Discriminated Unions (where one or more cases has a body)	Tag type with UnionConverter	This format can be readily consumed in Java, JavaScript and Swift. Nonetheless, exhaust all other avenues before considering encoding a union in JSON. The "case" label id can be overridden.	Decision.Accepted { result = "54" }	{"case": "Accepted","result":"54"}

Unsupported types and/or constructs

The mechanisms in the previous section have proven themselves sufficient for diverse systems inside and outside Jet. Here, we summarize some problematic constructs, with suggestions for alternate approaches to apply in preference.

Type kind	TL;DR	Example input	Example output	Notes
't list	Don't use; use 't[]	[ 1; 2; 3]	[1,2,3]	While the happy path works, null or missing field maps to a null object rather than [] [which is completely wrong from an F# perspective]. (System.Text.Json v >= 6 does now handle them correctly, but arrays are still the preferred representation, and there is no plan at present to have FsCodec.NewtonsoftJson provide support for it in the name of interoperability)
DateTime	Don't use; use DateTimeOffset			Round-tripping can be messy, wrong or lossy; DateTimeOffset covers same use cases
Guid or FSharp.UMX tagged Guid	don't use; wrap as a reference type and use a JsonIsomorphism, or represent as a tagged string	Guid.NewGuid()	"ba7024c7-6795-413f-9f11-d3b7b1a1fe7a"	If you wrap the value in a type, you can have that roundtrip with a specific format via a Converter implemented as a JsonIsomorphism. Alternately, represent in your contract as a FSharp.UMX tagged-string.
maps/Dictionary etc.	avoid; prefer arrays			As per C#; not always the best option for many reasons, both on the producer and consumer side. Json.NET has support for various maps with various idiosyncracies typically best covered by Stack Overflow, but often a list of records is clearer For System.Text.Json, use an IDictionary<'K, 'V> or Dictionary<'K, 'V>
tuples	Don't use; use records	(1,2)	{"Item1":1,"Item2":2}	While converters are out there, using tuples in contracts of any kind is simply Not A Good Idea

Custom converters using JsonIsomorphism

JsonIsomorphism enables one to express the Reading and Writeing of the JSON for a type in terms of another type. As alluded to above, rendering and parsing of Guid values can be expressed succinctly in this manner. The following Converter, when applied to a field, will render it without dashes in the rendered form:

type GuidConverter() =
    inherit JsonIsomorphism<Guid, string>()
    override _.Pickle g = g.ToString "N"
    override _.UnPickle g = Guid.Parse g

TypeSafeEnumConverter basic usage

[<JsonConverter(typeof<TypeSafeEnumConverter>)>]
type Outcome = Joy | Pain | Misery

type Message = { name: string option; outcome: Outcome }

let value = { name = Some null; outcome = Joy}
ser value
// {"name":null,"outcome":"Joy"}

des<Message> """{"name":null,"outcome":"Joy"}"""
// val it : Message = {name = None; outcome = Joy;}

By design, we throw when a value is unknown. Often this is the correct design. If, and only if, your software can do something useful with catch-all case, see the technique in OutcomeWithOther (below)

des<Message> """{"name":null,"outcome":"Discomfort"}"""
// throws System.Collections.Generic.KeyNotFoundException: Could not find case 'Discomfort' for type 'FSI_0012+Outcome'

TypeSafeEnum fallback converters using JsonIsomorphism

While, in general, one wants to version contracts such that invalid values simply don't arise, in some cases you want to explicitly handle out of range values. Here we implement a converter as a JsonIsomorphism to achieve such a mapping

[<JsonConverter(typeof<OutcomeWithCatchAllConverter>)>]
type OutcomeWithOther = Joy | Pain | Misery | Other
and OutcomeWithCatchAllConverter() =
    inherit JsonIsomorphism<OutcomeWithOther, string>()
    override _.Pickle v =
        TypeSafeEnum.toString v
    override _.UnPickle json =
        json
        |> TypeSafeEnum.tryParse<OutcomeWithOther>
        |> Option.defaultValue Other

type Message2 = { name: string option; outcome: OutcomeWithOther }

Because the type is tagged with a Converter attribute, valid values continue to be converted correctly:

let value2 = { name = Some null; outcome = Joy}
ser value2
// {"name":null,"outcome":"Joy"}

des<Message2> """{"name":null,"outcome":"Joy"}"""
// val it : Message = {name = None; outcome = Joy;}

More importantly, the formerly invalid value now gets mapped to our fallback value (Other) as intended.

des<Message2> """{"name":null,"outcome":"Discomfort"}"""
// val it : Message = {name = None; outcome = Other;}

Features: IEventCodec

See tests/FsCodec.NewtonsoftJson.Tests/Examples.fsx for a worked example suitable for playing with in F# interactive based on the following tutorial

FsCodec.IEventCodec

/// Defines a contract interpreter that encodes and/or decodes events representing the known set of events borne by a stream category
type IEventCodec<'Event, 'Format, 'Context> =
    /// Encodes a <c>'Event</c> instance into a <c>'Format</c> representation
    abstract Encode : context: 'Context option * value: 'Event -> IEventData<'Format>
    /// Decodes a formatted representation into a <c>'Event<c> instance. Does not throw exception on undefined <c>EventType</c>s
    abstract TryDecode : encoded: ITimelineEvent<'Format> -> 'Event option

IEventCodec represents a standard contract for the encoding and decoding of events used in event sourcing and event based notification scenarios:

• encoding pending/tentative "source of truth" events ('Facts') in Event Sourced systems (including encoding ones on the way to the store that are not yet accepted on a Timeline) - (see FsCodec.IEventData)
• decoding event records from an Event Store in a programming model, which involves mapping from the source event together with contextual information (see FsCodec.ITimelineEvent) such as:
  • The event type, which signifies the event that has taken place (if you're familiar with ADTs, this maps to the Discriminator in a Discriminated Union)
  • the core event data (often encoded as JSON, protobufs etc), the schema for which typically varies by event type
  • event metadata (contextual information optionally stored alongside the event)
  • the Timestamp at which the event was generated
  • the Index representing the position of this event within the sequence of events on the timeline represented by the stream from which one is hydrating the event
  • Correlation/causation identifiers for the activity that triggered the event
• routing and filtering of events for the purpose of managing projections, notification or reactions to events. Such events may either emanate directly from an Event Store's timeline as in the preceding cases, or represent versioned summary events

FsCodec.IEventData

Pending and timeline Events share the following common contract:

/// Common form for either a Domain Event or an Unfolded Event, without any context regarding its place in the timeline of events
type IEventData<'Format> =
    /// The Event Type, used to drive deserialization
    abstract member EventType : string
    /// Event body, as UTF-8 encoded JSON ready to be injected into the Store
    abstract member Data : 'Format
    /// Optional metadata (null, or same as Data, not written if missing)
    abstract member Meta : 'Format
    /// Application-generated identifier used to drive idempotent writes based on deterministic Ids and/or Request Id
    abstract member EventId : System.Guid
    /// The Correlation Id associated with the flow that generated this event. Can be `null`
    abstract member CorrelationId : string
    /// The Causation Id associated with the flow that generated this event. Can be `null`
    abstract member CausationId : string
    /// The Event's Creation Time (as defined by the writer, i.e. in a mirror, this is intended to reflect the original time)
    /// <remarks>- For EventStore, this value is not honored when writing; the server applies an authoritative timestamp when accepting the write.</remarks>
    abstract member Timestamp : System.DateTimeOffset

FsCodec.ITimelineEvent

Events from a versioned feed and/or being loaded from an Event Store bring additional context beyond the base information in IEventData

/// Represents a Domain Event or Unfold, together with it's 0-based <c>Index</c> in the event sequence
type ITimelineEvent<'Format> =
    inherit IEventData<'Format>
    /// The 0-based index into the event sequence of this Event
    abstract member Index : int64
    /// Application-supplied context related to the origin of this event
    abstract member Context : obj
    /// Indicates this is not a true Domain Event, but actually an Unfolded Event based on the State inferred from the Events up to and including that at <c>Index</c>
    abstract member IsUnfold : bool

Contracts for parsing / routing event records

See a scheme for the serializing Events modelled as an F# Discriminated Union for details of the representation scheme used for the events when using FsCodec.NewtonsoftJson.Codec.Create. We'll use the following example contract for the illustration:

module Events =

    // By convention, each contract defines a 'category' used as the first part of the stream name (e.g. `"Favorites-ClientA"`)
    let [<Literal>] Category = "Favorites"

    type Added = { item : string }
    type Removed = { name: string }
    type Event =
        | Added of Added
        | Removed of Removed
        interface TypeShape.UnionContract.IUnionContract

    let codec = FsCodec.NewtonsoftJson.Codec.Create<Event>()

    // See "logging unmatched events" later in this section for information about this wrapping using an EventCodec helper
    let (|TryDecode|_|) stream = EventCodec.tryDecode codec Serilog.Log.Logger stream

Strongly typed stream ids using FSharp.UMX

The example event stream contract above uses a ClientId type which (while being a string at heart) represents the identifier for a specific entity. We use the FSharp.UMX library that leans on the F# units of measure feature to tag the strings such that they can't be confused with other identifiers - think of it as a type alias on steroids.

open FSharp.UMX

type [<Measure>] clientId
type ClientId = string<clientId>
module ClientId =
    let parse (str : string) : ClientId = % str
    let toString (value : ClientId) : string = % value
    let (|Parse|) = ClientId.parse

Stream naming conventions

The de-facto standard Event Store EventStore.org and its documentation codifies the following convention for the naming of streams:-

{CategoryName}-{Identifier}

Where:

• {CategoryName} represents a high level contract/grouping; all stream names starting with Category- are the same category. Must not contain any - characters.
• - (hyphen/minus) represents the by-convention standard separator between category and identifier
• {Identifier} represents the identity of the Aggregate / Aggregate Root instance for which we're storing the events within this stream. The _ character is used to separate composite ids; see the code.

The StreamName module will reject invalid values by throwing exceptions when fields have erroneously embedded - or _ values.

It's important to apply some consideration in mapping from values in your domain to a StreamName. Domain values might include characters such as - (which may cause issues with EventStoreDb's $by_category projections) and/or arbitrary Unicode chars (which may not work well for other backing stores e.g. if CosmosDB were to restrict the character set that may be used for a Partition Key). You'll also want to ensure it's appropriately cleansed, validated and/or canonicalized to cover SQL Injection and/or XSS concerns. In short, no, you shouldn't just stuff an email address into the {Identifier} portion.

FsCodec.StreamName: presents the following set of helpers that are useful for splitting and filtering Stream Names by Categories and/or Identifiers. Similar helpers would of course make sense in other languages e.g. C#:

// Type aliases for a type-tagged `string`
type [<Measure>] streamName
type StreamName = string<streamName>

module StreamName =

    (* Creators: Building from constituent parts
       Guards against malformed category, aggregateId and/or aggregateIdElements with exceptions *)

    // Recommended way to specify a stream identifier; a category identifier and an aggregate identity
    // category is separated from id by `-`
    let create (category : string) aggregateId : StreamName = ...

    // Composes a StreamName from a category and > 1 name elements.
    // category is separated from the aggregateId by '-'; elements are separated from each other by '_'
    let compose (category : string) (aggregateIdElements : string seq) : StreamName = ...

    // Validates and maps a trusted Stream Name consisting of a Category and an Id separated by a '-` (dash)
    // Throws <code>InvalidArgumentException</code> if it does not adhere to that form
    let parse (rawStreamName : string) : StreamName = ...

    (* Rendering *)


    let toString (streamName : StreamName) : string = UMX.untag streamName

    (* Parsing: Raw Stream name Validation functions/pattern that handle malformed cases without throwing *)

    // Attempts to split a Stream Name in the form {category}-{id} into its two elements.
    // The {id} segment is permitted to include embedded '-' (dash) characters
    let trySplitCategoryAndId (rawStreamName : string) : (string * string) option = ...

    // Attempts to split a Stream Name in the form {category}-{id} into its two elements.
    // The {id} segment is permitted to include embedded '-' (dash) characters
    // Yields <code>NotCategorized</code> if it does not adhere to that form.
    let (|Categorized|NotCategorized|) (rawStreamName : string) : Choice<string*string,unit> = ...

    (* Splitting: functions/Active patterns for (i.e. generated via `parse`, `create` or `compose`) well-formed Stream Names
       Will throw if presented with malformed strings [generated via alternate means] *)

    // Splits a well-formed Stream Name of the form {category}-{id} into its two elements.
    // Throws <code>InvalidArgumentException</code> if it does not adhere to the well known format (i.e. if it was not produced by `parse`).
    // <remarks>Inverse of <code>create</code>
    let splitCategoryAndId (streamName : StreamName) : string * string = ...
    let (|CategoryAndId|) : StreamName -> (string * string) = splitCategoryAndId

    // Splits a `_`-separated set of id elements (as formed by `compose`) into its (one or more) constituent elements.
    // <remarks>Inverse of what <code>compose</code> does to the subElements
    let (|IdElements|) (aggregateId : string) : string[] = ...

    // Splits a well-formed Stream Name of the form {category}-{id1}_{id2}_{idN} into a pair of category and ids
    // Throws <code>InvalidArgumentException</code> if it does not adhere to the well known format (i.e. if it was not produced by `parse`).
    // <remarks>Inverse of <code>create</code>
    let splitCategoryAndIds (streamName : StreamName) : string * string[] = ...
    let (|CategoryAndIds|) : StreamName -> (string * string[]) = splitCategoryAndIds

Decoding events

Given the following example events from across streams:

let utf8 (s : string) = System.Text.Encoding.UTF8.GetBytes(s)
let events = [
    StreamName.parse "Favorites-ClientA",    Core.TimelineEvent.Create(0L, "Added",     utf8 """{ "item": "a" }""")
    StreamName.parse "Favorites-ClientB",    Core.TimelineEvent.Create(0L, "Added",     utf8 """{ "item": "b" }""")
    StreamName.parse "Favorites-ClientA",    Core.TimelineEvent.Create(1L, "Added",     utf8 """{ "item": "b" }""")
    StreamName.parse "Favorites-ClientB",    Core.TimelineEvent.Create(1L, "Added",     utf8 """{ "item": "a" }""")
    StreamName.parse "Favorites-ClientB",    Core.TimelineEvent.Create(2L, "Removed",   utf8 """{ "item": "a" }""")
    StreamName.create "Favorites" "ClientB", Core.TimelineEvent.Create(3L, "Exported",  utf8 """{ "count": 2 }""")
    StreamName.parse "Misc-x",               Core.TimelineEvent.Create(0L, "Dummy",     utf8 """{ "item": "z" }""")
]

and the helpers defined above, we can route and/or filter them as follows:

let runCodec () =
    for stream, event in events do
        match stream, event with
        | StreamName.Category (Events.Category, ClientId.Parse id), (Events.Decode stream e) ->
            printfn "Client %s, event %A" (ClientId.toString id) e
        | StreamName.Category (cat, id), e ->
            printfn "Unhandled Event: Category %s, Id %s, Index %d, Event: %A " cat id e.Index e.EventType
        | StreamName.Other streamName, _e ->
            failwithf "Invalid Stream Name: %s" streamName

invoking runCodec () yields:

Client ClientA, event Added {item = "a";}
Client ClientB, event Added {item = "b";}
Client ClientA, event Added {item = "b";}
Client ClientB, event Added {item = "a";}
Client ClientB, event Removed {name = null;}
Unhandled Event: Category Favorites, Id ClientB, Index 3, Event: "Exported"
Unhandled Event: Category Misc, Id x, Index 0, Event: "Dummy"

There are two events that we were not able to decode, for varying reasons:

1. "Misc-x", FsCodec.Core.TimelineEvent.Create(0L, "Dummy", utf8 """{ "item": "z" }""") represents an Event that happens to pass through our event processor that we don't want to decode and/or handle - we don't need to define a contract type for
2. "Favorites" "ClientB", FsCodec.Core.TimelineEvent.Create(3L, "Exported", utf8 """{ "count": 2 }""") represents an Event that has recently been defined in the source system, but not yet handled by the processor. In the event of such an unclassified event occurring within a stream contract we want to know when we're not handling a given event. That's trapped above and logged as Unhandled Event: Category Favorites, Id ClientB, Index 3, Event: "Exported".

Note however, that we don't have a clean way to trap the data and log it. See Logging unmatched events for an example of how one might log such unmatched events

Handling introduction of new fields in JSON

The below example demonstrates the addition of a CartId property in a newer version of CreateCart. It's worth noting that deserializing CartV1.CreateCart into CartV2.CreateCart requires CartId to be an optional property or the property will deserialize into null which is an invalid state for the CartV2.CreateCart record in F# (F# types are assumed to never be null).

module CartV1 =
    type CreateCart = { name: string }

    type Events =
        | Created of CreateCart
        interface IUnionContract

module CartV2 =
    type CreateCart = { name: string; cartId: CartId option }
    type Events =
        | Created of CreateCart
        interface IUnionContract

FsCodec.SystemTextJson looks to provide an analogous mechanism. In general, FsCodec is seeking to provide a pragmatic middle way of using NewtonsoftJson or SystemTextJson in F# without completely changing what one might expect to happen when using JSON.NET in order to provide an F# only experience.

The aim is to provide helpers to smooth the way for using reflection based serialization in a way that would not surprise people coming from a C# background and/or in mixed C#/F# codebases.

Adding Matchers to the Event Contract

We can clarify the consuming code a little by adding further helper Active Patterns alongside the event contract :-

module Events =

    // ... (as above)

    // Pattern to determine whether a given {category}-{aggregateId} StreamName represents the stream associated with this Aggregate
    // Yields a strongly typed id from the aggregateId if the Category does match
    let (|StreamName|_|) = function
        | FsCodec.StreamName.CategoryAndId (Category, ClientId.Parse clientId) -> Some clientId
        | _ -> None

    // ... (as above)

    // Yields decoded events and relevant strongly typed ids if the category of the Stream Name is correct
    let (|Match|_|) (streamName, event) =
        match streamName, event with
        | MatchesCategory clientId, TryDecode streamName e -> Some (clientId, e)
        | _ -> None
        
    let (|Decode|) stream = Seq.choose ((|TryDecode|_|) stream)
    let (|Parse|_|) (streamName, span) =
        match streamName, span with
        | Events.MatchesCategory clientId, Decode streamName es -> Some (clientId, es)
        | _ -> None

That boxes off the complex pattern matching close to the contract itself, and lets us match on the events in a handler as follows:

let runCodecCleaner () =
    for stream, event in events do
        match stream, event with
        | Events.Match (clientId, event) ->
            printfn "Client %s, event %A" (ClientId.toString clientId) event
        | StreamName.CategoryAndId (cat, id), e ->
            printfn "Unhandled Event: Category %s, Id %s, Index %d, Event: %A " cat id e.Index e.EventType

Logging unmatched events

The following helper (which uses the Serilog library), can be used to selectively layer on some logging when run with logging upped to Debug level:

module EventCodec =

    // Uses the supplied codec to decode the supplied event record `x` (iff at LogEventLevel.Debug, detail fails to `log` citing the `stream` and content)
    let tryDecode (codec : FsCodec.IEventCodec<_,_,_>) (log : Serilog.ILogger) streamName (x : FsCodec.ITimelineEvent<ReadOnlyMemory<byte>>) =
        match codec.TryDecode x with
        | None ->
            if log.IsEnabled Serilog.Events.LogEventLevel.Debug then
                log.ForContext("event", System.Text.Encoding.UTF8.GetString(x.Data), true)
                    .Debug("Codec {type} Could not decode {eventType} in {stream}", codec.GetType().FullName, x.EventType, streamName)
            None
        | x -> x

Normally, the log.IsEnabled call instantly rules out any need for logging. We can activate this inert logging hook by reconfiguring the logging as follows:

// Switch on debug logging to get detailed information about events that don't match (which has no singificant perf cost when not switched on)
open Serilog
open Serilog.Events
let outputTemplate = "{Message} {Properties}{NewLine}"
Serilog.Log.Logger <-
    LoggerConfiguration()
        .MinimumLevel.Debug()
        .WriteTo.Console(LogEventLevel.Debug, outputTemplate=outputTemplate)
        .CreateLogger()
runCodec ()

This adds the following additional output when triggering runCodec ():-

Codec "<Snipped>" Could not decode "Exported" in "Favorites-ClientB" {event="{ \"count\": 2 }"}

Decoding contextual information

Events arriving from a store (e.g. Equinox etc) or source (e.g. Propulsion) bear contextual information.

Where relevant, a decoding process may want to extract such context alongside mapping the base information.

For example, we may wish to log (or process as part of our domain logic) metadata accompanying an event, while still leaning on the TypeShape UnionEncoder and FsCodec to automate the decoding.

A clean way to wrap such a set of transitions is as follows:

module Reactions =

    type Event = int64 * DateTimeOffset * Events.Event
    let codec =
        let up (raw : FsCodec.ITimelineEvent<ReadOnlyMemory<byte>>, contract : Events.Event) : Event = raw.Index, raw.Timestamp, contract
        let down ((_index, timestamp, event) : Event) = event, None, Some timestamp
        FsCodec.NewtonsoftJson.Codec.Create(up, down)
        
    let (|TryDecode|_|) stream event : Event option = EventCodec.tryDecode codec Serilog.Log.Logger stream event
    let (|Match|_|) (streamName, event) =
        match streamName, event with
        | Events.MatchesCategory clientId, TryDecode streamName event -> Some (clientId, event)
        | _ -> None
        
    let (|Decode|) stream = Seq.choose ((|TryDecode|_|) stream)
    let (|Parse|_|) (streamName, span) =
        match streamName, span with
        | Events.MatchesCategory clientId, Decode streamName es -> Some (clientId, es)
        | _ -> None        

This allows us to tweak the runCodec above as follows to also surface additional contextual information:

let runWithContext () =
    for stream, event in events do
        match stream, event with
        | Reactions.Match (clientId, (index, ts, e)) ->
            printfn "Client %s index %d time %O event %A" (ClientId.toString clientId) index (ts.ToString "u") e
        | FsCodec.StreamName.CategoryAndId (cat, id), e ->
            printfn "Unhandled Event: Category %s, Id %s, Index %d, Event: %A " cat id e.Index e.EventType

which yields the following output:

Client ClientA index 0 time 2020-01-13 09:44:37Z event Added {item = "a";}
Client ClientB index 0 time 2020-01-13 09:44:37Z event Added {item = "b";}
Client ClientA index 1 time 2020-01-13 09:44:37Z event Added {item = "b";}
Client ClientB index 1 time 2020-01-13 09:44:37Z event Added {item = "a";}
Client ClientB index 2 time 2020-01-13 09:44:37Z event Removed {name = null;}
Codec "<Snipped>" Could not decode "Exported" in "Favorites-ClientB" {event="{ \"count\": 2 }"}
Unhandled Event: Category Favorites, Id ClientB, Index 3, Event: "Exported"
Unhandled Event: Category Misc, Id x, Index 0, Event: "Dummy"

Features: FsCodec.Box.Codec

FsCodec.Box.Codec is a drop-in-equivalent for FsCodec.(Newtonsoft|SystemText)Json.Codec with equivalent .Create overloads that encode as ITimelineEvent<obj> (as opposed to ITimelineEvent<ReadOnlyMemory<byte>> / ITimelineEvent<JsonElement>).

This is useful when storing events in a MemoryStore as it allows one to take the perf cost and ancillary yak shaving induced by round-tripping arbitrary event payloads to the concrete serialization format out of the picture when writing property based unit and integration tests.

NOTE this does not imply one should avoid testing this aspect; the opposite in fact -- one should apply the Test Pyramid principles:

• have a focused series of tests that validate that the various data representations in the event bodies are round-trippable a. in the chosen encoding format (i.e. UTF8 JSON) b. with the selected concrete json encoder (i.e. Newtonsoft.Json for now 🙁)
• integration tests can in general use BoxEncoder and MemoryStore

You should absolutely have acceptance tests that apply the actual serialization encoding with the real store for a representative number of scenarios at the top of the pyramid

RELATED ARTICLES / BLOG POSTS etc

• Contracts for Event Sourced Systems with FsCodec by @deviousasti
• A Contract Pattern for Schemaless DataStores by Eirik Tsarpalis

CONTRIBUTING

The intention is to keep this set of converters minimal and interoperable, e.g., many candidates are deliberately being excluded from this set; its definitely a non-goal for this to become a compendium of every possible converter. So, especially in this repo, the bar for adding converters will be exceedingly high and hence any contribution should definitely be preceded by a discussion.

Examples, tests and docs are welcomed with open arms.

General guidelines:

• Less [converters] is more - has a converter really proved itself broadly applicable ?
• this is not the final complete set of converters; Json.NET and System.Text.Json are purposefully extensible and limited only by your imagination, for better or worse. However such specific conversions are best kept within the app.
• If the upstream library (Newtonsoft.Json, System.Text.Json) can or should be made to do something, it should. Also for System.Text.Json, if it's an F#-specific, the powerful and complete FSharp.SystemTextJson library may be much more aligned.

Please raise GitHub issues for any questions so others can benefit from the discussion.

Building

# verify the integrity of the repo wrt being able to build/pack/test
./dotnet build build.proj