How Jet Build Microservices with F#

Happy first day of F# Advent!

Lately, I’ve had a lot of questions about our microservices and how we’re using F# to build them. So, I wanted to take this opportunity to go a bit more in-depth on what they look like. Let’s start in the most basic place:


What do microservices even mean to us? We belong to the SRP camp:

A microservice is an application of the Single Responsibility Principle (SRP) at the service level.

This means that a microservice should have one — and only one — reason to exist. Not necessarily that there should be only one function within the service, just that the microservice should have only one job.

Generally, we also note that we should strive to create microservices that have an input and produce an output. This will eliminate many side effects when considering upfront the inputs, outputs, and transformations that the service will have and help us to do. While this isn’t always possible (logging and sending emails happen in the real world, but when they do, they should be isolated to their own service), abiding by this rule goes far in eliminating unnecessary side effects.


The benefits of microservices fall into three main categories:

  1. Easy scalability. It’s only a matter of scaling a single service as needed. Did we receive a large shipment to the warehouse today, putting the related services — and people πŸ˜‰ — under heavy load? We can scale out just those services without regard for the rest of the system.
  2. Independent releasability. While it is possible to release a single service at once, we organize ourselves into teams and our microservices into groups within our teams. Because we’re using F#, each group of microservices is usually within the same Visual Studio solution. When we talk about independent releasability, we usually mean that we’ll promote an entire solution of related services — often a group of 5-10 — at the same time.
  3. A more even distribution of complexity. By this, I mean that it generally becomes much more simple to create and maintain your services, but that it tends to be more difficult to manage all your services. Recently, during Jet’s Tech Talk Weekly series, we watched Tammer Saleh giving his “Microservices Anti-Patterns” talk. One of the anti-patterns he mentioned was doubling down on microservices as a pattern and creating an application from the ground up with microservices. He suggests instead starting with a monolithic application and breaking off services as it becomes obvious that they’re needed. This isn’t how we proceeded at Jet. I won’t speak to it as a piece of advice, but I will add that because of the additional layers of complexity needed to manage your services, it’s crucial to at least have your management story thought through, if not fully determined, right up front before you start to break off those services.


We use a couple different methods for our microservices to communicate: Kafka, EventStore, and Azure ServiceBus with Azure Queues. For my examples today, I’ll show off EventStore. In some cases, we were mis-using EventStore more as a message bus, but we have decided to start moving away from EventStore in favor of Kafka.


So, without further ado, I present some example code:

In this example, I’ve created a microservice that price checks a product on a competing site, in this case, the “Nile” shopping site. πŸ˜‰

We won’t get very far if we don’t define our types upfront. Let’s start with a Product record type that contains information about the product we want to price check.

type Product = {
    Sku : string
    ProductId : int
    ProductDescription : string
    CostPer : decimal

We also need some additional members on the Product so that it’s easily serializable: ToJson andFromJson, which convert a Product record type to and from JSON, using one of our internal libraries. We also want to create a type for the failure case. For that case, we’ll need just the product ID that we were trying to price check, and the failure message.

type Product with
    static member ToJson(x:Product) =
        jobj [|
            "sku" .= x.Sku
            "productId" .= x.ProductId
            "productDescription" .= x.ProductDescription
            "costper" .= x.CostPer

    static member FromJson (_:Product) =
        parseObj <| fun json -> jsonParse {
            let! sku = jget json "sku"
            let! productid = jget json "productId"
            let! productdescription = jget json "productDescription"
            let! costper = jget json "costper"
            return {
                Product.Sku = sku
                ProductId = productid
                ProductDescription = productdescription
                CostPer = costper
    static member EventCodec : EventCodec<Product> = jsonValueCodec "product"

type PriceCheckFailed = {
    ProductId : int
    Message : string

Once we have our types set up, we can start to construct the service itself. We tend to use a similar format for many of our services so that developers can get up to speed more quickly, as well as move across teams faster. Services have five main sections.

First, we set up our inputs and our outputs. Here, we are taking in the Product type that we just defined and we need to return either a tuple of Product type and a decimal cost, or a PriceCheckFailed type that will contain information about the reasons we can’t currently check this price on the Nile site.

type Input = 
    | Product of Product

type Output = 
    | ProductPriceNile of Product * decimal
    | ProductPriceCheckFailed of PriceCheckFailed

Second, we define how to handle the input — specifically, how to convert from the input to the output. In this case, I’ve cheated and constructed a tuple that fits the successful output type. However, this is the section of the microservice that would contain the longest bit of code, going out to the competing site and gathering the information, say through their API. Note, that we’re also using Option types, so that we have an additional check for failure. This will become important in our next step.

let handle (input:Input) =
    async {
        return Some(
                 ProductId = 17; 
                 ProductDescription = "My amazing product"; 

Third, it’s necessary to define how to interpret the output once we have received it. Here, we pattern match on what we received from the handle function. If we receive a successful response, we write to Kafka or EventStore. If we receive a failed response, we might log that failure and try again. The final case considers an unanticipated failure where we get no response at all. So we might log that failure and then raise an alert to the teams involved. By using an option type here, we have a third check — for an unknown and unanticipated failure — that would not have otherwise been used.

let interpret id output =
    match output with
    | Some (Output.ProductPriceNile (e, price))  -> async {()} // write to event store, kafka, etc. 
    | Some (Output.ProductPriceCheckFailed e) -> async {()} // log specific failure and try again
    | None -> async {()} // log failure and raise alert

Fourth, we gather these functions into a consume method. This calls another consume method in one of our shared libraries, passing in a decoded input, as well as our handle and interpret methods.

let consume = EventStoreQueue.consume (decodeT Input.Product) handle interpret

Finally, we can subscribe to a specific event stream, calling our consume function for each event we receive.

    (EventStore.connHost "MyEventStoreConnection") 
    (TimeSpan.FromSeconds 1.0) 
|> AsyncSeq.concatSeq
|> AsyncSeq.iterAsyncParThrottled 200 consume


I’ve set up an example to show off the style we use at Jet for our F# microservices code. We aim to keep our microservices set up in the same manner across projects so that folks are easily able to transfer teams, as well as to get up to speed when they first start at Jet. Hopefully the tools and techniques outlined here can help you and your company get started with microservices in F#.


  1. Is it possible to share the source code for this example?
    Trying to better understand the F# microservice approach.

    1. Hi Matt,

      Not yet. πŸ™‚ We’ll need to open-source a few libraries that I used first (and we plan to, but it’s a lower priority). Once those are out, we can definitely push out this code. Thanks for your interest!

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