Persistent Data: PostgreSQL


PostgreSQL, is a powerful, open-source object-relational database system that has earned a strong reputation for reliability, robustness and performance. This tutorial will implement a persistent storage solution for Fermyon Cloud, using PostgreSQL. In this tutorial, we will be using ElephantSQL’s free PostgreSQL service.

Spin and Fermyon Cloud

First, you need to have Spin installed on your computer. Please use the official Fermyon Cloud Quickstart to both install Spin and also log in to Fermyon Cloud.

Using Spin Application Templates

The Spin CLI facilitates the creation of new Spin applications through the use of application templates. You can install Spin application templates using the official Spin CLI documentation. The template we are interested in, for this tutorial, is the experimental http-csharp template. We can go ahead and install it using the following command:

$ spin templates install --git --branch main --update

The output from the command above will be similar to the following:

Copying remote template source
Installing template http-csharp...
Installed 1 template(s)

| Name          Description                                  |
| http-csharp   HTTP request handler using C# (EXPERIMENTAL) |

Creating Our New Spin Application

The official Spin CLI documentation also has instructions on how to create a new Spin application, from an existing template. Using the docs as a reference, we can perform the following:

$ spin new http-csharp httpCSharpApplication
Project description: A new http-csharp spin application
HTTP base: /
HTTP path: /data


using Fermyon.Spin.Sdk;
using System.Net;
using System.Text;

namespace httpCSharpApplication;

public static class Handler {
  public static HttpResponse HandleHttpRequest(HttpRequest request) {
    if (request.Url == Warmup.DefaultWarmupUrl) {
      return new HttpResponse();

    var connectionString = "user=username password=password dbname=databasename host=";
    var result = PostgresOutbound.Query(connectionString, "SELECT * FROM myTable");

    var responseText = new StringBuilder();

    responseText.AppendLine($"Got {result.Rows.Count} row(s)");
    responseText.AppendLine($"COL: [{String.Join(" | ", result.Columns.Select(FmtCol))}]");

    string FmtEntry(DbValue v) {
      return v.Value() switch {
        null => "<DBNULL>",
          var val => val.ToString() ?? "<NULL>",

    foreach(var row in result.Rows) {
      responseText.AppendLine($"ROW: [{String.Join(" | ", row.Select(FmtEntry))}]");

    return new HttpResponse {
      StatusCode = HttpStatusCode.OK,
        BodyAsString = responseText.ToString(),
  private static string FmtCol(PgColumn c) {
    return $ "{c.Name} ({c.DataType})";


Wizer is required to successfully build this application. Please go ahead and install Wizer using the following command:

$ cargo install wizer --all-features

Spin Build

To build the application, use the following command:

$ spin build

Spin Deploy

To deploy the application, use the deploy command:

$ spin deploy

The above deploy command will produce similar output to the following:

Deployed httpCSharpApplication version 1.0.0+XXXXXXXX
Available Routes:

Visiting the above URL will show your data in the browser’s body.