---
title: Closing stream constraint
editUrl: https://github.com/Cratis/Chronicle/edit/main/Documentation/constraints/model-bound/closing-stream.mdx
description: Adorn an event type with [ClosesStream] so that appending it closes a stream scope, and name the events that reopen it.
---

import { Tabs, TabItem } from '@astrojs/starlight/components';


Adorn the event type class with `[ClosesStream]` to close a stream scope whenever an event of that type is appended. The closure is written in the same step as the event, and later appends inside the scope are rejected with a `StreamClosed` violation. For the concept, the guarantees and when to prefer it over `CompleteStream`, see [Closing events](/chronicle/closing-streams/closing-events/).

Chronicle discovers `[ClosesStream]`-adorned types automatically and registers the constraints with the Kernel when the client starts.

## Defining a closing constraint

Choose the dimensions that make up the closed scope:

<Tabs syncKey="chronicle-client">
<TabItem label="C#">

```csharp
using Cratis.Chronicle.EventSequences;
using Cratis.Chronicle.Events;
using Cratis.Chronicle.Events.Constraints;

[EventType]
[ClosesStream(Dimensions = ClosedStreamDimensions.EventSourceId | ClosedStreamDimensions.EventStreamType)]
public record ConstraintsModelBoundClosingReportDelivered(string Recipient);
```

[View C# snippet source on GitHub](https://github.com/Cratis/Chronicle/blob/main/Documentation/client-snippets/constraints/model-bound/closing-stream/basic.md)

</TabItem>
</Tabs>

`Dimensions` is a `ClosedStreamDimensions` flags value: `EventSourceId`, `EventSourceType`, `EventStreamType` and `EventStreamId`. Without it the scope is event source id, stream type and stream id of the append.

## Take the stream id from a property

Set `EventStreamIdFrom` to the name of an event property to use its value as the stream id. This implies the `EventStreamId` dimension:

<Tabs syncKey="chronicle-client">
<TabItem label="C#">

```csharp
using Cratis.Chronicle.Events;
using Cratis.Chronicle.Events.Constraints;

[EventType]
[ClosesStream(EventStreamIdFrom = nameof(Period))]
public record ConstraintsModelBoundClosingBooksClosed(string Period, decimal ClosingBalance);
```

[View C# snippet source on GitHub](https://github.com/Cratis/Chronicle/blob/main/Documentation/client-snippets/constraints/model-bound/closing-stream/event-stream-id-from.md)

</TabItem>
</Tabs>

The property must have a non-empty value on every append. The kernel refuses an append where it is missing, null or empty. Avoid a nullable property; the analyzer [CHR0012](/chronicle/code-analysis/chr0012/) warns on nullable event properties.

## Name and reopening events

`Name` sets the owner of the closure and defaults to the event type name. `ReopenedBy` lists the event types that remove this constraint's closure for the exact scope they resolve to:

<Tabs syncKey="chronicle-client">
<TabItem label="C#">

```csharp
using Cratis.Chronicle.Events;
using Cratis.Chronicle.Events.Constraints;

[EventType]
[ClosesStream(Name = "close-month", EventStreamIdFrom = nameof(Month), ReopenedBy = [typeof(ConstraintsModelBoundClosingMonthReopened)])]
public record ConstraintsModelBoundClosingMonthClosed(string Month);

[EventType]
public record ConstraintsModelBoundClosingMonthReopened(string Month, string Reason);
```

[View C# snippet source on GitHub](https://github.com/Cratis/Chronicle/blob/main/Documentation/client-snippets/constraints/model-bound/closing-stream/reopened-by.md)

</TabItem>
</Tabs>

A reopening event type must carry the `EventStreamIdFrom` property too, or the client throws `PropertyDoesNotExistOnEventType` when it discovers the constraint.

## Attribute reference

| Property | Meaning |
| --- | --- |
| `Dimensions` | Dimensions of the closed scope. Defaults to event source id, stream type and stream id |
| `EventStreamIdFrom` | Payload property that provides the stream id |
| `ReopenedBy` | Event types that reopen this constraint's scope |
| `Name` | Owner name. Defaults to the event type name. Cannot be empty |
| `EventSequences` | Event sequences the declaration applies to. Empty means all |

Declarations that share a `Name` merge into one constraint and must use the same dimensions and stream id property, otherwise the client throws `ConflictingClosesStreamDeclarations`.

Registering or changing the declaration does not close anything that is already in the log.