Add store-agnostic projection dead-letter count read to IEventDatabase (#356)

src/EventTests/Daemon/DeadLetterCountDefaultsTests.cs

@@ -0,0 +1,62 @@
+using System;
+using System.Collections.Generic;
+using System.Threading;
+using System.Threading.Tasks;
+using JasperFx.Events;
+using JasperFx.Events.Daemon;
+using JasperFx.Events.Projections;
+using Shouldly;
+
+namespace EventTests.Daemon;
+
+public class DeadLetterCountDefaultsTests
+{
+    // A bare IEventDatabase that does NOT override the dead-letter count read members,
+    // so the calls below exercise the default interface implementations added for
+    // jasperfx#356 (the "return 0 / empty" stand-ins for stores that don't yet read
+    // dead letters). Everything else throws — those members are never invoked here.
+    private sealed class BareEventDatabase : IEventDatabase
+    {
+        public string Identifier => throw new NotImplementedException();
+        public Uri DatabaseUri => throw new NotImplementedException();
+        public ShardStateTracker Tracker => throw new NotImplementedException();
+        public string StorageIdentifier => throw new NotImplementedException();
+
+        public Task StoreDeadLetterEventAsync(object storage, DeadLetterEvent deadLetterEvent, CancellationToken token)
+            => throw new NotImplementedException();
+
+        public Task EnsureStorageExistsAsync(Type storageType, CancellationToken token)
+            => throw new NotImplementedException();
+
+        public Task WaitForNonStaleProjectionDataAsync(TimeSpan timeout)
+            => throw new NotImplementedException();
+
+        public Task<long> ProjectionProgressFor(ShardName name, CancellationToken token = default)
+            => throw new NotImplementedException();
+
+        public Task<long?> FindEventStoreFloorAtTimeAsync(DateTimeOffset timestamp, CancellationToken token)
+            => throw new NotImplementedException();
+
+        public Task<long> FetchHighestEventSequenceNumber(CancellationToken token)
+            => throw new NotImplementedException();
+
+        public Task<IReadOnlyList<ShardState>> AllProjectionProgress(CancellationToken token = default)
+            => throw new NotImplementedException();
+    }
+
+    private readonly IEventDatabase theDatabase = new BareEventDatabase();
+
+    [Fact]
+    public async Task count_dead_letters_default_returns_zero()
+    {
+        var count = await theDatabase.CountDeadLetterEventsAsync(new ShardName("Fake"));
+        count.ShouldBe(0);
+    }
+
+    [Fact]
+    public async Task fetch_dead_letter_counts_default_returns_empty()
+    {
+        var counts = await theDatabase.FetchDeadLetterCountsAsync();
+        counts.ShouldBeEmpty();
+    }
+}

src/JasperFx.Events/Daemon/DeadLetterShardCount.cs

@@ -0,0 +1,14 @@
+using JasperFx.Events.Projections;
+
+namespace JasperFx.Events.Daemon;
+
+/// <summary>
+///     A count of stored projection/subscription dead letter events for a single shard.
+///     <see cref="ProjectionName" /> aligns with <see cref="ShardName.Name" /> and
+///     <see cref="ShardKey" /> aligns with <see cref="ShardName.ShardKey" />, matching how
+///     <see cref="DeadLetterEvent" /> records them. See jasperfx#356.
+/// </summary>
+/// <param name="ProjectionName">The projection name (<see cref="ShardName.Name" />).</param>
+/// <param name="ShardKey">The shard key within the projection (<see cref="ShardName.ShardKey" />).</param>
+/// <param name="Count">The number of stored dead letter events for this shard.</param>
+public record DeadLetterShardCount(string ProjectionName, string ShardKey, long Count);

src/JasperFx.Events/IEventDatabase.cs

@@ -67,4 +67,28 @@ Task<long> ProjectionProgressFor(ShardName name,
     /// <returns></returns>
     Task<IReadOnlyList<ShardState>> AllProjectionProgress(
         CancellationToken token = default);
+
+    /// <summary>
+    ///     Count the stored dead letter events for a single projection/subscription shard. With
+    ///     <c>SkipApplyErrors</c> on (the JasperFx.Events 2.0 default), a failed <c>Apply()</c> is
+    ///     recorded as a <see cref="DeadLetterEvent" /> and the shard keeps advancing, so the
+    ///     accumulation of these is the primary "this projection is unhealthy" signal.
+    ///     The default implementation returns 0 as a stand-in; event stores that persist dead
+    ///     letters should override this. See jasperfx#356.
+    /// </summary>
+    /// <param name="shard">The projection/subscription shard to count dead letters for.</param>
+    /// <param name="token"></param>
+    Task<long> CountDeadLetterEventsAsync(ShardName shard, CancellationToken token = default)
+        => Task.FromResult(0L);
+
+    /// <summary>
+    ///     Fetch the stored dead letter event counts for this database, one row per shard
+    ///     (<see cref="DeadLetterShardCount.ProjectionName" /> + <see cref="DeadLetterShardCount.ShardKey" />).
+    ///     Mirrors the "give me every row" shape of <see cref="AllProjectionProgress" />.
+    ///     The default implementation returns an empty list as a stand-in; event stores that
+    ///     persist dead letters should override this. See jasperfx#356.
+    /// </summary>
+    /// <param name="token"></param>
+    Task<IReadOnlyList<DeadLetterShardCount>> FetchDeadLetterCountsAsync(CancellationToken token = default)
+        => Task.FromResult<IReadOnlyList<DeadLetterShardCount>>([]);
 }