A Big Week for the Critter Stack: Per-Tenant Event Partitioning, F# Codegen, and 33 Releases

What happens when a .NET stack ships 33 NuGet releases in 7 days, lands two major new feature tracks across five repos, and absorbs 17+ community bug reports — all without breaking GA?

That was the past week in the JasperFx organization (Marten, Wolverine, Polecat, JasperFx, Weasel, CritterWatch). Here's a tour for .NET developers building event-sourced, multi-tenant, or high-throughput systems.

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📦 The Release Wave (May 30 → June 6)

Marten: 11 releases (V9.3.4 → V9.6.0, plus an 8.37.2 backport) Wolverine: 8 releases (V6.3.0 → V6.5.0) JasperFx: 6 releases (V2.2.4 → V2.8.1) Polecat: 2 releases (V4.2.0, V4.2.1) Weasel: 1 release (V9.0.3)

Most of those are coordinated patch waves. The big version bumps (Marten 9.4.0, Wolverine 6.3.0/6.5.0, JasperFx 2.5.0) each anchor a major feature track.

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🎯 The Big Tracks

🆕 Per-Tenant Event Partitioning

The headline feature of the week. In Marten 9.4.0+, opt in with one line:

opts.Events.UseTenantPartitionedEvents = true;

What that unlocks:

• Native PostgreSQL LIST partitioning of mt_events and mt_streams by tenant_id
• A per-tenant event sequence (mt_events_sequence_{suffix})
• mt_event_progression keyed by (name, tenant_id) via a folded {Name}:{ShardKey}:{tenantId} shard grammar (no new column needed)
• Vectorized per-tenant high-water detection — one round-trip per database emits a high-water vector for every active tenant
• Per-tenant rebuild isolation + cross-tenant rebuild fan-out
• Composite single-pass rebuild executor (read-once / fan-out)

Constraints (validated at DocumentStore construction):

• Requires conjoined tenancy
• EventAppendMode.Quick or QuickWithServerTimestamps only (Rich is out of scope)
• Can't currently combine with UseArchivedStreamPartitioning (planned follow-up)

The flag defaults to false; existing stores keep the global append path byte-for-byte.

Wolverine 6.5.0 followed through with a full handler-side matrix: [ReadAggregate] / [WriteAggregate], required-write isolation, optimistic versioning, cascading tenant inheritance, MartenOps tenant overloads, AlwaysEnforceConsistency, natural keys, multi-node distribution, ancillary stores, and the IEventStream<T> compound-handler append path.

Polecat (the SQL Server-backed sibling) started its parallel track immediately.

This is the kind of feature that removes a real scalability ceiling — the single shared event store stops being the bottleneck across tenants. If you're running an event-sourced multi-tenant SaaS on PostgreSQL, this is the upgrade to plan for.

📖 Per-Tenant Event Partitioning docs: https://martendb.io/events/multitenancy#per-tenant-event-partitioning

🆕 F# Pre-Generated Code

Wolverine 6.3.0 added a one-line CLI command that turns your handlers into runnable F# source files you can check into your repo:

dotnet run -- codegen write --language fsharp

The end-to-end slices that ship as compile-gated runnable samples:

• EF Core handlers
• Marten document handlers
• Marten event-sourced aggregate handlers
• FluentValidation + CosmosDB
• HTTP routing (JSON, route binding, query binding — both string and typed)
• In-memory sagas
• Behavioural Static-mode run-step

This positions Wolverine as a first-class F# option — not just "C# that happens to compile" but actual idiomatic F# emission from the code generator. There's a tutorial at https://wolverinefx.io/tutorials/fsharp.html and an open community-input discussion on idiomatic F# handler discovery (module-level functions, DUs, records) that would benefit from real F# adopter input.

🆕 JasperFx.Aspire — CLI Verbs as Dashboard Commands

If you're using .NET Aspire for your dev loop, JasperFx CLI verbs (describe, codegen, openapi, wolverine-diagnostics describe-handlers, etc.) are now exposed as Aspire dashboard commands with startup gates. Click a button instead of hopping to a terminal.

Pairs with the existing build-time openapi command and the source-generated discovery work that shipped the prior week.

🆕 Inline Request/Reply Over Wolverine.Http and Wolverine.Grpc

InvokeAsync<T> now flows efficiently over Wolverine's own HTTP and gRPC sender transports. Combined with the Result<T> work that shipped the week before, this rounds out the RPC-style story for Wolverine — you can do remote request/reply in the idiomatic Wolverine way without dropping down to bare HttpClient/Grpc.

🐛 Critical Reliability Fixes Shipped

Highlights worth flagging if you're on the GA majors:

• DCB tag-concurrency race — concurrent DCB tag-boundary appends sharing a tag could both commit. Fixed by serializing through a side table.
• OnException middleware now cascades return-value messages — a long-standing UX gap.
• Concurrent health-check race that corrupted leadership locks under sustained load.
• Codegen scope priming for service-located MessageContext / IDocumentSession — closes the family of duplicate-variable codegen bugs that hit several adopters at GA.
• BatchedQuery.FetchForExclusiveWriting race that wedged Wolverine HTTP [Aggregate(LoadStyle = Exclusive)] endpoints under concurrency.
• SecondaryStoreProxyFactory regression in 9.5.1 — first concurrent build threw "Duplicate type name within an assembly". Fixed in 9.5.2 hours after report.
• Projection rebuild stress-test fixes: write-path *Projected variants and read-path LoadProjectedAsync now bypass session-shared trackers, and ProjectionDocumentSession routes user-code LoadAsync through a projection-safe path. Closes thread-safety side effects of the 9.0 changes.

🆕 Marten.ScaleTesting CLI

A new performance/scale harness landed: an event seeder, a TelehealthComposite 4+2+2 scenario, plus rebuild / validate / stress subcommands. This is becoming the official rebuild-correctness stress vehicle. Signals a near-term focus on rebuild correctness and projection performance at production scale.

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🌟 The Community Story

The thing I find most encouraging about this stack right now is how much real-world feedback is flowing through the issue tracker.

This week alone:

• 12 community issue reporters filed real, reproducible bugs
• 2 first-time PR contributors landed merges (@steve-ziegler reported AND fixed a race in BatchedQuery.FetchForExclusiveWriting<T> in their first PR; @midub fixed a NATS JetStream scheduled-send bug)
• 3 other community PRs merged from established contributors

A few standouts:

@erdtsieck continues to be the de facto QA partner for advanced Marten paths — filed four high-quality per-tenant-partitioning bug reports surfacing real adopter edge cases (sharded provisioning, StartStream semantics, optimistic-append compatibility, BulkInsertEventsAsync NULL types). Every one was real, reproducible, and closed within the week.

@RorySan found and fixed an IndexOutOfRangeException regression on tenant-mapped projection documents in their first contribution, then immediately found a second related regression (9.5.1 → 9.5.2 patch cycle).

@dmytro-pryvedeniuk caught a concurrent health-check race that could corrupt leadership locks under sustained load — exactly the kind of failure that only shows up in production-shaped workloads.

The patch cadence on community-reported bugs is typically sub-24-hour for high-severity issues. That tight loop is the real story.

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📈 Patterns Worth Watching

1. The per-tenant partitioning rollout was a model multi-repo execution. Umbrella issue → top-of-chain JasperFx surface → Marten implementation → Wolverine handler matrix → Polecat parallel track → community-reported edge cases → cascading patch wave. All inside a week.

2. Patch cadence is sub-12-hour for severe bugs. A 9.5.1 regression was fixed in 9.5.2 within hours of being reported. The GA messageContext codegen regression cluster from late May is now entirely closed.

3. Aspire is becoming the canonical dev-time UI for the Critter Stack. JasperFx.Aspire brings CLI verbs into the dashboard, the docs site picked up an Aspire integration page, and the friction to debug/inspect a Critter Stack app at dev time has dropped significantly.

4. Marten.ScaleTesting CLI signals a near-term focus on rebuild correctness and projection performance at production scale. Expect benchmark-driven changes in the next month.

5. Three open community Discussions are worth a triage pass — particularly one asking about the Marten release cadence and another asking what replaced ProjectEventAsync in Marten 9 (almost certainly a migration-docs gap).

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If You're Adopting

Safe upgrade matrix as of this morning:

• Marten 9.6.0 + Marten.AspNetCore 9.6.0
• Wolverine 6.5.0
• Polecat 4.2.1
• JasperFx 2.8.1 family (RuntimeCompiler stays on its 5.x line)

Two semantic changes from Marten 8 → 9 that are worth knowing before you upgrade:

1. The default IDocumentSession resolved from DI is now a lightweight session (no identity map, no dirty tracking). If your code depends on identity-map semantics, opt in explicitly.
2. Marten 9 uses source-generated projection dispatch. Your self-aggregating types compile out of the box now, but partial is still required on *Projection subclasses.

The 5 → 6 migration on Wolverine covers ServiceLocationPolicy defaults, the Lamar removal (the built-in ServiceProvider is now the container), and the AOT story.

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Net

33 releases, two major new feature tracks shipped end-to-end, 17+ community bug reports triaged or fixed, and Aspire integration begun — in one week.

If you're building event-sourced multi-tenant systems on .NET, the per-tenant partitioning feature alone removes a real scalability ceiling that traditionally pushed people toward either app-level sharding or completely separate databases per tenant. If you're an F# shop that has been quietly watching from the sidelines, the codegen write --language fsharp work means the Critter Stack is now a first-class option.

Find the work on GitHub (https://github.com/JasperFx), the docs at https://martendb.io and https://wolverinefx.io, or the JasperFx Discord. Issues and PRs from real adopters are landing within hours — if you're using this stack and hit something rough, file it.

#dotnet #csharp #fsharp #eventsourcing #postgresql #multitenancy #microservices #Marten #Wolverine #CritterStack