Started draft 4.0.0 release notes. Added table of API changes.

_posts/release/2025-04-30-accumulo-4.0.0.md

@@ -0,0 +1,66 @@
+---
+title: Apache Accumulo 4.0.0
+sortableversion: '04.00.00'
+draft: true
+---
+## About
+
+Apache Accumulo 4.0.0 is a non-LTM major version release that contains
+a large number of internal changes that allow a lot of flexibility in
+the way that Accumulo is deployed with a small number of API changes.
+A high level overview of some of the changes are described in [this][blog]
+blog post. Not contained in the blog post are the changes to tablet
+[merging][merge], specifically the ability to automatically merge
+tablets.
+
+## Notable Changes
+
+* Accumulo 4.0.0 requires a Java 17 runtime.
+
+* The CompactionCoordinator process no longer exists, its functionality was moved into the Manager.
+
+* Non-system tablets will be optionally hosted by default. Operations requiring a tablet to be
+  hosted (live ingest and immediate consistency scans) will instruct the Manager to temporarily
+  assign the Tablet. Users can change the default hosting semantics for Tablets when creating
+  the table, using the TableOperations api, or the appropriate command in the Shell. Tablets
+  that are temporarily assigned are unloaded according to the logic in the OnDemandTabletUnloader
+  plugin being used, specified by the property 'tserver.ondemand.tablet.unloader'. The default
+  implementation will unload tablets that have not been used in 10 minutes, which can be changed
+  by the property 'tserver.ondemand.tablet.unloader.interval'.
+
+* Considering TabletServers optionally host Tablets, all Tablet management functions (e.g. merge,
+  split, compaction initiation, etc.) have been moved into the Manager. TabletServers no longer
+  perform major compactions making Compactor processes mandatory (ScanServers remain an optional
+  feature).
+
+* Split and Merge operations, which are now just metadata operations, can be much faster for tablets
+  that are not hosted. For tablets that are hosted in a TabletServer, they are temporarily unloaded
+  before the operation executes.
+
+## API Changes
+
+{% capture api_changes %}{% include_relative 4_0_api_changes.md %}{% endcapture %}
+{{ api_changes | markdownify }}
+
+## Upgrading
+
+View the [Upgrading Accumulo documentation][upgrade] for guidance as
+the upgrade procedure has changed. Note that during the upgrade
+all user tablets will be set to have on-demand tablet availability.
+Users will need to change the tablet availability post-upgrade for
+tablets that they want always or never hosted on a tablet server.
+The upgrade process also sets the tablet mergeability to never for
+all existing tablets to preserve any user created split points.
+Tablet mergeability can be set when adding new splits and will be
+set to always when the split is created by the system based on user
+settings.
+
+## Useful Links
+
+* [All tickets related to this release][milestone]
+
+
+[blog]: /blog/2024/10/07/accumulo4-preview.html
+[merge]: /docs/4.x/administration/merging
+[upgrade]: /docs/4.x/administration/upgrading
+[milestone]: https://github.com/apache/accumulo/milestone/20