---
title: "[runtime]"
description: "The [runtime] table — replication, changefeed capacity, and cascade depth, baked into database.rs at generate time."
purpose: "reference"
---
The `[runtime]` table carries generate-time behavior knobs, schema-blind and baked
into the emitted `database.rs`. Change one and run `forgedb generate` for it to take
effect. Defaults reproduce the prior output byte-for-byte; the one exception is
`replication`, which defaults to **off**.
```toml
[runtime]
replication = false # Tier A — attach the durable replication broker
changefeed_capacity = 1024 # Tier B — broadcast/broker buffer size
max_cascade_depth = 64 # Tier B — @on_delete(cascade) recursion bound
```
## Keys
| Key | Type | Default | Tier | Meaning |
|---|---|---|---|---|
| `replication` | bool | `false` | A | Attach the durable replication broker in `open_at`. Required for `/replicate` and the browser read-replica. |
| `changefeed_capacity` | usize | `1024` | B | In-process changefeed broadcast channel capacity and the durable broker's in-memory buffer. |
| `max_cascade_depth` | u32 | `64` | B | Maximum recursive `@on_delete(cascade)` depth — a structural safety bound against a pathological FK cycle. |
## `replication` (Tier A, default off)
When `true`, `open_at` attaches a `DurableBroker` that records every committed change to a
CRC-framed append-only log (`<root>/_replication.log`). It powers the generated
`/replicate` WebSocket endpoint and the browser read-replica follower.
When `false` (the default), no broker is emitted. The write path's `if let Some(broker)`
guard becomes static dead code the compiler removes.
<Callout type="warning" title="Replication costs a second barrier per write">
With `replication = true`, each `insert`/`update`/`delete` fsyncs the broker log *in
addition to* the WAL — a **second** `F_FULLFSYNC` barrier. This roughly doubles single-write
latency (measured ~3.95 ms → ~7.35 ms per insert). The default is off precisely so apps
that never consume `/replicate` don't pay it. See [durability](/docs/features/durability/)
and [benchmarks](/docs/benchmarks/).
</Callout>
Turn it on when you consume the [replication stream](/docs/reference/rest-api/) or run a
browser read-replica.
## `changefeed_capacity` (Tier B, default 1024)
The capacity of the in-process changefeed broadcast channel (used by the `/subscribe` and
`/live-query` WebSocket endpoints) and the durable broker's buffer. It is a fan-out buffer,
**not** on the durable write path, so raising it has no measured write-latency cost. Raise
it freely for a high subscriber count.
## `max_cascade_depth` (Tier B, default 64)
Bakes `const MAX_CASCADE_DEPTH`. When you delete a parent whose relation FK declares
`@on_delete(cascade)`, the generated delete recurses into referencing children. This bound
stops runaway recursion on a pathological FK cycle. Most schemas never approach 64.
## Related
- [`[storage]`](/docs/config/storage/) — the durability/compaction knobs.
- [Generated REST API](/docs/reference/rest-api/) — the `/replicate`, `/subscribe`, and `/live-query` routes `replication` and `changefeed_capacity` feed.