feat: Add Preferred Replica model and prefer those replicas when disc…

…overing replicas for shards (#65)

* feat: Add Preferred Replica model and prefer those replicas when discovering replicas for shards

* shard->replica for consistency and correctness

housewatch/admin/__init__.py

@@ -0,0 +1,6 @@
+from django.contrib import admin
+
+from housewatch.models.preferred_replica import PreferredReplica
+
+
+admin.site.register(PreferredReplica)

housewatch/clickhouse/clusters.py

@@ -1,6 +1,8 @@
 from collections import defaultdict
 from housewatch.clickhouse.client import run_query
 
+from housewatch.models.preferred_replica import PreferredReplica
+
 
 def get_clusters():
     QUERY = """Select cluster, shard_num, shard_weight, replica_num, host_name, host_address, port, is_local, user, default_database, errors_count, slowdowns_count, estimated_recovery_time FROM system.clusters"""
@@ -28,8 +30,19 @@ def get_shards(cluster):
 
 
 def get_node_per_shard(cluster):
+    # We want to return a node per shard, but if we have preferred replicas we should use those
+
     shards = get_shards(cluster)
     nodes = []
+
+    preferred = PreferredReplica.objects.filter(cluster=cluster).values_list("replica", flat=True)
     for shard, n in shards.items():
-        nodes.append((shard, n[0]))
+        preferred_replica_found = False
+        for node in n:
+            if node["host_name"] in preferred:
+                nodes.append((shard, node))
+                preferred_replica_found = True
+                break
+        if not preferred_replica_found:
+            nodes.append((shard, n[0]))
     return nodes

housewatch/migrations/0012_preferredreplica.py

@@ -0,0 +1,23 @@
+# Generated by Django 4.1.1 on 2024-05-29 16:34
+
+from django.db import migrations, models
+import uuid
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('housewatch', '0011_scheduledbackup_is_sharded_and_more'),
+    ]
+
+    operations = [
+        migrations.CreateModel(
+            name='PreferredReplica',
+            fields=[
+                ('id', models.UUIDField(default=uuid.uuid4, editable=False, primary_key=True, serialize=False)),
+                ('created_at', models.DateTimeField(auto_now_add=True)),
+                ('cluster', models.CharField(max_length=255)),
+                ('replica', models.CharField(max_length=255)),
+            ],
+        ),
+    ]

housewatch/models/preferred_replica.py

@@ -0,0 +1,9 @@
+import uuid
+from django.db import models
+
+
+class PreferredReplica(models.Model):
+    id: models.UUIDField = models.UUIDField(primary_key=True, default=uuid.uuid4, editable=False)
+    created_at: models.DateTimeField = models.DateTimeField(auto_now_add=True)
+    cluster: models.CharField = models.CharField(max_length=255)
+    replica: models.CharField = models.CharField(max_length=255)