WIP Prepare postgres for migration by replication

doc/postgres-migration.md

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+# Migrating PostgreSQL cluster to a new major version
+
+## Summary
+
+1. Dump from a replica
+2. Restore to fresh VM running new major version
+3. Add logical replication for delta sync from current/old primary
+4. Switch primary to new server
+5. Remove logical replication on new server
+
+## Runbook
+
+* Primary host: `PRIMARY_HOST`
+* Replica host: `REPLICA_HOST`
+* New PG14 host: `NEW_HOST`
+* PostgreSQL superuser: `postgres`
+* Running locally on each machine via `sudo -u postgres`
+
+Adjust hostnames/IPs/etc. where needed.
+
+---
+
+### 🟢 0. PRIMARY — Pre-checks
+
+```bash
+sudo -u postgres psql -c "SHOW wal_level;"
+sudo -u postgres psql -c "SHOW max_replication_slots;"
+```
+
+If needed, edit config:
+
+```bash
+sudo -u postgres vi $PGDATA/postgresql.conf
+```
+
+Ensure:
+
+```conf
+wal_level = logical
+max_replication_slots = 10
+```
+
+Restart if changed:
+
+```bash
+sudo systemctl restart postgresql
+```
+
+---
+
+### 🔵🟡 3. Create keypair for syncing dump later
+
+🔵 On NEW_HOST:
+
+```bash
+sudo mkdir -p /home/postgres/.ssh && \
+sudo chown -R postgres:postgres /home/postgres && \
+sudo chmod 700 /home/postgres/.ssh && \
+sudo -u postgres bash -c 'ssh-keygen -t ecdsa -b 256 -f /home/postgres/.ssh/id_ecdsa -N "" -C "postgres@$(hostname)"' && \
+sudo cat /home/postgres/.ssh/id_ecdsa.pub
+```
+
+Copy the public key from the above output
+
+🟡 On replica:
+
+```bash
+sudo mkdir -p /home/postgres/.ssh && \
+sudo chown -R postgres:postgres /home/postgres && \
+sudo chmod 700 /home/postgres/.ssh && \
+echo [public_key] | sudo tee /home/postgres/.ssh/authorized_keys > /dev/null && \
+sudo chmod 700 /home/postgres/.ssh
+```
+
+---
+
+### 🟢 1. PRIMARY — Create publication and replication slots
+
+```bash
+sudo -u postgres pg_create_replication_publications
+```
+
+or
+
+```bash
+sudo -u postgres pg_create_replication_publication [db_name]
+```
+
+Listing publications and slots:
+
+```bash
+sudo -u postgres pg_list_replication_publications
+sudo -u postgres pg_list_replication_slots
+```
+
+---
+
+### 🟡 3. REPLICA — Pause replication
+
+```bash
+sudo -u postgres psql -c "SELECT pg_wal_replay_pause();"
+```
+
+Verify:
+
+```bash
+sudo -u postgres psql -c "SELECT pg_is_wal_replay_paused();"
+```
+
+---
+
+### 🟡 4. REPLICA — Run dump
+
+```bash
+sudo -u postgres pg_dump_all_databases
+```
+
+or
+
+```bash
+sudo -u postgres bash -c "pg_dumpall --globals-only > /tmp/globals.sql"
+sudo -u postgres pg_dump_database [db_name]
+```
+
+---
+
+### 🟡 5. REPLICA — Resume replication
+
+```bash
+sudo -u postgres psql -c "SELECT pg_wal_replay_resume();"
+```
+
+---
+
+### 🔵 6. COPY dumps to NEW HOST
+
+From NEW_HOST:
+
+```bash
+export REPLICA_HOST=[private_ip] && \
+cd /tmp && \
+sudo -u postgres scp "postgres@$REPLICA_HOST:/tmp/globals.sql" . && \
+sudo -u postgres scp "postgres@$REPLICA_HOST:/tmp/dump_*.tar.zst" .
+```
+
+---
+
+### 🔵 7. NEW HOST (PostgreSQL 14) — Restore
+
+#### 7.1 Restore globals
+
+```bash
+sudo -u postgres psql -f /tmp/globals.sql
+```
+
+---
+
+#### 7.2 Create databases
+
+```bash
+sudo -u postgres psql -Atqc "SELECT datname FROM pg_database WHERE datallowconn AND datname NOT IN ('template1')" | \
+xargs -I{} sudo -u postgres createdb {}
+```
+
+or
+
+```bash
+sudo -u postgres createdb [db_name]
+```
+
+---
+
+#### 7.3 Restore each database
+
+```bash
+sudo -u postgres pg_restore_all_databases
+```
+
+or
+
+```bash
+sudo -u postgres pg_restore_database [db_name]
+```
+
+---
+
+### 🔵 8. NEW HOST — Create subscriptions
+
+```bash
+sudo -u postgres pg_create_replication_subscriptions
+```
+
+or
+
+```bash
+sudo -u postgres pg_create_replication_subscription [db_name]
+```
+
+---
+
+### 🔵 9. NEW HOST — Monitor replication
+
+```bash
+sudo -u postgres pg_list_replication_subscriptions
+```
+
+---
+
+### 🔴 11. CUTOVER
+
+#### 11.1 Stop writes on old primary
+
+(app maintenance mode, stop app/daemons)
+
+---
+
+#### 11.2 Wait for replication to catch up
+
+TODO: not the best way to check, since WAL LSNs keep increasing
+
+```bash
+sudo -u postgres psql -d [db_name] -c "SELECT * FROM pg_stat_subscription;"
+```
+
+---
+
+#### 11.3 Fix sequences
+
+Run per DB:
+
+```bash
+sudo -u postgres pg_fix_sequences_in_all_databases
+```
+
+or
+
+```bash
+sudo -u postgres pg_fix_sequences [db_name]
+```
+
+---
+
+#### 11.4 Point app to NEW_HOST
+
+* Update pg.kosmos.local in /etc/hosts on app server(s) (maybe override
+  attribute for role and converge)
+* Start app/daemons, deactivate maintenance mode
+
+---
+
+### 🧹 12. CLEANUP (NEW_HOST)
+
+```bash
+sudo -u postgres pg_drop_replication_subscriptions
+```
+
+---
+
+### 🧹 13. CLEANUP (PRIMARY)
+
+TODO: Looks like slots are dropped automatically, when subscriptions are dropped
+
+```bash
+sudo -u postgres pg_drop_replication_publications
+```
+
+---
+
+### ✅ DONE
+
+---