docs wip

Author: kjnilsson

docs/internals/COMPACTION.md

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+# Ra log compaction
+
+This is a living document capturing current work on log compaction.
+
+## Overview
+
+
+Compaction in Ra is intrinsically linked to the snapshotting
+feature. Standard Raft snapshotting removes all entries in the Ra log
+that precedes the snapshot where the snapshot is a full representation of
+the state machine state. 
+
+
+### Ra Server log worker responsibilities
+
+* Write checkpoints and snapshots
+* Perform compaction runs
+* report segments to be deleted back to the ra server (NB: the worker does
+not perform the segment deletion itself, it needs to report changes back to the
+ra server first). The ra server log worker maintains its own list of segments
+to avoid double processing
+
+
+
+### Compacted segments: naming
+
+Segment files in a  Ra log have numeric names incremented as they are written.
+This is essential as the order is required to ensure log integrity.
+
+
+Retain immutability, entries will never be deleted from a segment.
+
+do we need levels?
+
+how about 0000001.segment.1 where each segment has an ordered number of items
+but segments themselves can have non ordered ranges. Newer segments can completely
+
+eg.
+
+```
+[{0, 5}, {7, 22}] -> compacted to [{0, 5}, {7, 22}, {3, 20}] meaning
+{0, 5} can be deleted.
+
+
+```mermaid
+sequenceDiagram
+    participant segment-writer
+    participant ra-server
+    participant ra-server-log
+
+    segment-writer--)ra-server: new segments
+    ra-server-)+ra-server-log: new segments
+    ra-server-log->>ra-server-log: phase 1 compaction
+    ra-server-log-)-ra-server: segment changes (new, to be deleted)
+    ra-server-)+ra-server-log: new snapshot
+    ra-server-log->>ra-server-log: write snapshot
+    ra-server-log->>ra-server-log: phase 1 compaction
+    ra-server-log-)-ra-server: snapshot written, segment changes
+```
+