init

Dockerfile

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+# because of dotnet, we always build on amd64, and target platforms in cli
+# dotnet doesn't support QEMU for building or running. 
+# (errors common in arm/v7 32bit) https://github.com/dotnet/dotnet-docker/issues/1537
+# https://hub.docker.com/_/microsoft-dotnet
+# hadolint ignore=DL3029
+# to build for a different platform than your host, use --platform=<platform>
+# for example, if you were on Intel (amd64) and wanted to build for ARM, you would use:
+# docker buildx build --platform "linux/arm64/v8" .
+FROM --platform=${BUILDPLATFORM} mcr.microsoft.com/dotnet/sdk:7.0 as build
+ARG TARGETPLATFORM
+ARG TARGETARCH
+ARG BUILDPLATFORM
+RUN echo "I am running on $BUILDPLATFORM, building for $TARGETPLATFORM"
+
+WORKDIR /source
+COPY *.csproj .
+RUN dotnet restore -a $TARGETARCH
+
+COPY . .
+RUN dotnet publish -c release -o /app -a $TARGETARCH --self-contained false --no-restore
+
+# app image
+FROM mcr.microsoft.com/dotnet/runtime:7.0
+WORKDIR /app
+COPY --from=build /app .
+ENTRYPOINT ["dotnet", "Worker.dll"]

Program.cs

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+using System;
+using System.Data.Common;
+using System.Linq;
+using System.Net;
+using System.Net.Sockets;
+using System.Threading;
+using Newtonsoft.Json;
+using Npgsql;
+using StackExchange.Redis;
+
+namespace Worker
+{
+    public class Program
+    {
+        public static int Main(string[] args)
+        {
+            try
+            {
+                var pgsql = OpenDbConnection("Server=db;Username=postgres;Password=postgres;");
+                var redisConn = OpenRedisConnection("redis");
+                var redis = redisConn.GetDatabase();
+
+                // Keep alive is not implemented in Npgsql yet. This workaround was recommended:
+                // https://github.com/npgsql/npgsql/issues/1214#issuecomment-235828359
+                var keepAliveCommand = pgsql.CreateCommand();
+                keepAliveCommand.CommandText = "SELECT 1";
+
+                var definition = new { vote = "", voter_id = "" };
+                while (true)
+                {
+                    // Slow down to prevent CPU spike, only query each 100ms
+                    Thread.Sleep(100);
+
+                    // Reconnect redis if down
+                    if (redisConn == null || !redisConn.IsConnected) {
+                        Console.WriteLine("Reconnecting Redis");
+                        redisConn = OpenRedisConnection("redis");
+                        redis = redisConn.GetDatabase();
+                    }
+                    string json = redis.ListLeftPopAsync("votes").Result;
+                    if (json != null)
+                    {
+                        var vote = JsonConvert.DeserializeAnonymousType(json, definition);
+                        Console.WriteLine($"Processing vote for '{vote.vote}' by '{vote.voter_id}'");
+                        // Reconnect DB if down
+                        if (!pgsql.State.Equals(System.Data.ConnectionState.Open))
+                        {
+                            Console.WriteLine("Reconnecting DB");
+                            pgsql = OpenDbConnection("Server=db;Username=postgres;Password=postgres;");
+                        }
+                        else
+                        { // Normal +1 vote requested
+                            UpdateVote(pgsql, vote.voter_id, vote.vote);
+                        }
+                    }
+                    else
+                    {
+                        keepAliveCommand.ExecuteNonQuery();
+                    }
+                }
+            }
+            catch (Exception ex)
+            {
+                Console.Error.WriteLine(ex.ToString());
+                return 1;
+            }
+        }
+
+        private static NpgsqlConnection OpenDbConnection(string connectionString)
+        {
+            NpgsqlConnection connection;
+
+            while (true)
+            {
+                try
+                {
+                    connection = new NpgsqlConnection(connectionString);
+                    connection.Open();
+                    break;
+                }
+                catch (SocketException)
+                {
+                    Console.Error.WriteLine("Waiting for db");
+                    Thread.Sleep(1000);
+                }
+                catch (DbException)
+                {
+                    Console.Error.WriteLine("Waiting for db");
+                    Thread.Sleep(1000);
+                }
+            }
+
+            Console.Error.WriteLine("Connected to db");
+
+            var command = connection.CreateCommand();
+            command.CommandText = @"CREATE TABLE IF NOT EXISTS votes (
+                                        id VARCHAR(255) NOT NULL UNIQUE,
+                                        vote VARCHAR(255) NOT NULL
+                                    )";
+            command.ExecuteNonQuery();
+
+            return connection;
+        }
+
+        private static ConnectionMultiplexer OpenRedisConnection(string hostname)
+        {
+            // Use IP address to workaround https://github.com/StackExchange/StackExchange.Redis/issues/410
+            var ipAddress = GetIp(hostname);
+            Console.WriteLine($"Found redis at {ipAddress}");
+
+            while (true)
+            {
+                try
+                {
+                    Console.Error.WriteLine("Connecting to redis");
+                    return ConnectionMultiplexer.Connect(ipAddress);
+                }
+                catch (RedisConnectionException)
+                {
+                    Console.Error.WriteLine("Waiting for redis");
+                    Thread.Sleep(1000);
+                }
+            }
+        }
+
+        private static string GetIp(string hostname)
+            => Dns.GetHostEntryAsync(hostname)
+                .Result
+                .AddressList
+                .First(a => a.AddressFamily == AddressFamily.InterNetwork)
+                .ToString();
+
+        private static void UpdateVote(NpgsqlConnection connection, string voterId, string vote)
+        {
+            var command = connection.CreateCommand();
+            try
+            {
+                command.CommandText = "INSERT INTO votes (id, vote) VALUES (@id, @vote)";
+                command.Parameters.AddWithValue("@id", voterId);
+                command.Parameters.AddWithValue("@vote", vote);
+                command.ExecuteNonQuery();
+            }
+            catch (DbException)
+            {
+                command.CommandText = "UPDATE votes SET vote = @vote WHERE id = @id";
+                command.ExecuteNonQuery();
+            }
+            finally
+            {
+                command.Dispose();
+            }
+        }
+    }
+}

Worker.csproj

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+<Project Sdk="Microsoft.NET.Sdk">
+
+  <PropertyGroup>
+    <OutputType>Exe</OutputType>
+    <TargetFramework>net7.0</TargetFramework>
+  </PropertyGroup>
+
+  <ItemGroup>
+    <PackageReference Include="StackExchange.Redis" Version="2.2.4" />
+    <PackageReference Include="Npgsql" Version="4.1.9" />
+    <PackageReference Include="Newtonsoft.Json" Version="13.0.1" />
+  </ItemGroup>
+
+</Project>