# Synapse Retention Discovery
# Generated: 2026-04-09 (Phase 6.0, Question 1)

## Current homeserver.yaml Values

### APPLIED 2026-04-12 (Archivist Phase 2)

Two settings added to homeserver.yaml:

```diff
 report_stats: false

+redaction_retention_period: null
+
 trusted_key_servers:
   - server_name: "matrix.org"
```

```diff
 experimental_features:
   msc3202_transaction_extensions: true
   msc2409_to_device_messages_enabled: true
+  msc2815_enabled: true
```

- Backup: `/opt/matrix/synapse/homeserver.yaml.bak-20260412`
- Applied: 2026-04-12 03:06 UTC
- Synapse restarted, health verified, bridge reconnected, Element login confirmed
- Rollback: restore backup and `docker compose restart synapse`

### Previous state (before 2026-04-12)

| Setting | Previous Value | Current Value | Source |
|---------|---------------|---------------|--------|
| redaction_retention_period | NOT SET (default 7d) | null (disabled) | synapse/config/server.py |
| msc2815_enabled | NOT SET (default false) | true | synapse/config/experimental.py |
| forgotten_room_retention_period | NOT SET | NOT SET (unchanged) | synapse/config/server.py |
| media_retention.local_media_lifetime | NOT SET | NOT SET (unchanged) | synapse/config/repository.py |
| media_retention.remote_media_lifetime | NOT SET | NOT SET (unchanged) | synapse/config/repository.py |

Full experimental_features block (current):
```yaml
experimental_features:
  msc3202_transaction_extensions: true
  msc2409_to_device_messages_enabled: true
  msc2815_enabled: true
```

## What `redaction_retention_period: null` Changes

Source: Synapse v1.147.1 synapse/config/server.py, synapse/storage/databases/main/events.py

When set to `null`:
- Synapse STOPS censoring (overwriting) redacted event content in the DB
- The original unredacted content stays in event_json table forever
- The 5-minute censoring job still runs but skips all events
- Redactions still WORK from the client perspective — clients see events as redacted
- The only difference is the server retains the pre-redaction content internally

When at default `7d`:
- After 7 days post-redaction, Synapse replaces stored event content with the redacted form
- Original content is permanently lost from the DB
- Any tool querying the DB after 7 days gets only the stripped event

## MSC2815 Support — CONFIRMED in Synapse 1.147.1

Config key: `experimental_features.msc2815_enabled` (default: false)
Source: synapse/config/experimental.py line 372
Feature flag advertised as: `fi.mau.msc2815: true` in /_matrix/client/versions

What MSC2815 does:
- Adds `include_unredacted_content=true` query param to GET /rooms/{roomId}/event/{eventId}
- Requires requester to have power level >= room's redact PL (default 50)
- Returns original unredacted content if still in DB
- Returns FI.MAU.MSC2815_UNREDACTED_CONTENT_DELETED if content was already censored

Critical interaction: MSC2815 + redaction_retention_period
- With default 7d: MSC2815 only works for 7 days after redaction
- With null: MSC2815 works forever (content never censored)

## Current DB Sizes

| Database | Size |
|----------|------|
| synapse | 259 MB |
| mas | 17 MB |
| mautrix_signal | 14 MB |

Top 5 tables in synapse:
| Table | Size |
|-------|------|
| state_groups_state | 91 MB |
| event_json | 20 MB |
| events | 15 MB |
| event_auth | 14 MB |
| device_lists_changes_in_room | 12 MB |

## Growth Rate Analysis

Backup sizes over 14 days (Mar 26 - Apr 9, gzipped):
- Mar 26: 5.28 MB → Apr 9: 5.41 MB
- Delta: ~130 KB compressed over 14 days = ~9 KB/day compressed
- Uncompressed estimate: ~50-100 KB/day growth

Event volume:
- Total events: 2,617 across 18 rooms
- Last 7 days: 2,356 events (bridge just deployed, bulk of these are portal creation)
- Steady-state will be much lower — Signal groups average 10-50 messages/day
- Only 3 redaction events in entire history

## Storage Impact Estimate (disabling redaction purge)

Current scale:
- 2,617 events = 259 MB total DB (but most of that is state, not event content)
- event_json table: 20 MB for 2,617 events = ~7.6 KB avg per event
- Redactions are 3 out of 2,617 (0.1%)

Even at 10x Signal bridge traffic (100 messages/day across all groups):
- 100 events/day × 7.6 KB = 760 KB/day event_json growth
- Redactions typically <5% of messages = <5 events/day × 7.6 KB = 38 KB/day saved by NOT censoring
- Over 1 year: ~14 MB retained from not censoring

Verdict: Storage impact of `redaction_retention_period: null` is NEGLIGIBLE.
The entire Synapse DB after months of use is 259 MB. Even with aggressive Signal
bridge traffic, retaining redacted content adds <15 MB/year.

## MAS Interaction with Redacted Content

MAS does NOT interact with redacted content. MAS handles:
- Authentication (login/logout/refresh)
- Token management
- Upstream OIDC delegation

MAS has no audit policy, no event content access, and no retention policy of its own.
Redaction handling is entirely within Synapse's event store. No conflict.

## Rollback Procedure

If `redaction_retention_period: null` and `msc2815_enabled: true` are added and need reverting:

1. Remove both settings from homeserver.yaml
2. Restart Synapse: `docker compose restart synapse`
3. Synapse returns to default 7d censoring
4. Already-retained content will be censored within 7 days + 5 minutes
5. No DB migration needed — Synapse's censoring job handles cleanup automatically

The rollback is clean and non-destructive.