gzurowski commented on code in PR #1683:
URL: https://github.com/apache/camel-website/pull/1683#discussion_r3508000175
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content/blog/2026/07/camel421-whatsnew/index.md:
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+---
+title: "Apache Camel 4.21 What's New"
+date: 2026-07-03
+draft: false
+authors: [ davsclaus, squakez ]
+categories: [ "Releases" ]
+keywords: ["apache camel", "whats new", "camel 4", "release", "camel 4.21",
"integration framework"]
+preview: "Details of what we have done in the Camel 4.21 release."
+---
+
+Apache Camel 4.21 has just been [released](/blog/2026/07/RELEASE-4.21.0/).
+
+This release introduces a large set of new features and noticeable
improvements that we will cover in this blog post.
+
+## Camel Core
+
+### Error Registry
+
+The Error Registry has been enhanced to capture rich exchange data snapshots
at error time,
+following the same pattern as BacklogTracer. This includes the message body,
headers, exchange properties,
+and variables — giving you a complete picture of what was happening when the
error occurred.
+
+Error registry configuration has moved to a dedicated `camel.errorRegistry.*`
prefix:
+
+```properties
+camel.errorRegistry.enabled=true
+camel.errorRegistry.maximumEntries=100
+camel.errorRegistry.timeToLiveSeconds=300
+```
+
+The error registry integrates with the route diagram rendering, allowing you
to visualize the
+exact path a failed exchange took through your routes.
+
+### Route Topology
+
+A new route topology service computes inter-route relationships — which routes
send messages to
+which other routes (via `direct`, `seda`, `kafka`, etc.). The topology is
available through a dev console
+and powers the new topology diagram rendering in both the CLI and the
embeddable web component.
+
+See the dedicated [Route Topology](/blog/2026/06/camel-route-topology/) blog
post for more details.
+
+### Group-Scoped Variables
+
+Variables can now be scoped to a route group. When multiple routes share a
`routeGroup`,
+they can share variables within that group scope using `variable.group:myVar`:
+
+```java
+from("direct:a").routeGroup("myGroup")
+ .setVariable("group:counter", simple("${variable.group:counter}++"))
+ .to("direct:b");
+
+from("direct:b").routeGroup("myGroup")
+ .log("Counter is ${variable.group:counter}");
+```
+
+### Exchange Memory Optimization
+
+We made significant work to reduce memory pressure on the Exchange object:
+
+- **Copy-on-write headers**: Message headers are now shared across exchange
copies (multicast, splitter, etc.)
+ and only copied when actually modified, reducing allocation overhead.
+- **Lazy initialization**: Several internal data structures on the Exchange
are now initialized lazily,
+ reducing the baseline memory footprint.
+- **O(1) CaseInsensitiveMap**: The default `CaseInsensitiveMap` used for
message headers now uses a custom
+ hash table with zero-allocation lookups and header key deduplication, making
the external
+ `camel-headersmap` dependency unnecessary (now deprecated).
+
+### Virtual Threads
+
+When virtual threads are enabled, the `threads()` EIP now honors `maxQueueSize`
+for backpressure. Previously, `maxQueueSize` was silently ignored and tasks
were accepted unboundedly.
+
+A new `Block` rejected policy has been added to `ThreadPoolRejectedPolicy`,
where the caller blocks
+until capacity becomes available.
+
+### JSpecify Null Annotations
+
+The `camel-api` module now uses `@NullMarked` and `@Nullable` annotations from
JSpecify to document
+nullability contracts across the Camel API surface. This enables compile-time
null checking with tools like NullAway.
+
+### Simple Language
+
+Several improvements to the Simple language:
+
+- `floor`/`ceil` now return `long` instead of `int` to avoid overflow for
large values.
+- `range(N)` in csimple now starts at 1 (consistent with the `simple`
language).
+- The internal implementation has been refactored into domain-aligned builder
classes for better maintainability.
+
+## Camel CLI
+
+Camel JBang has been rebranded as **Camel CLI** in the documentation.
+
+### Camel TUI
+
+The Camel TUI (Terminal User Interface) is a new feature in heavy development,
and we will publish
Review Comment:
```suggestion
The Camel TUI (Terminal User Interface) is a new feature currently in heavy
development, and we will publish
```
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