[riasvdv]

Description

Laravel's read/write connections mostly work in Craft already, but three things still went to the write connection (the primary). Front-end requests kept querying the primary, and a site couldn't serve pages from its replica while the primary was down.

 Cms::isInstalled()
-  DB::connection()->getPdo()            # opens the primary; 503 on every page when it's down
+  DB::connection()->getReadPdo()

 Fields / Sections / EntryTypes / ElementQuery / EloquentElement column checks
-  Schema::hasColumn() / getColumnListing()   # Laravel always introspects the write connection
+  DB::getReadSchemaBuilder()->…              # new Connection macro

 ConnectionConfig::dsn()
-  ':host='.$connection->getConfig('host')    # "Array to string conversion" when host is a list
+  ':host='.Arr::first(Arr::wrap(...))

getReadSchemaBuilder() introspects the read connection through Laravel's {name}::read connection. If the connection would read from the write connection anyway (in the console, inside a transaction, or after a write when sticky is on), it uses the normal schema builder instead. That way, a request that has just migrated never reads stale schema from a lagging replica.

Evidence

I tested a workbench in an orb with streaming replication on PostgreSQL 15 and on MySQL 8.4 (GTID, super_read_only replica). I counted the statements each server logged for every request.

Request (anonymous front end)	Before	After
Entry page, primary up	5–6 queries on the primary	0 on the primary, all on the replica
Entry page, primary down	503 (isInstalled()), then 500 (schema checks)	200
Search, eager loading, pagination, {% cache %}, users, relations, existing transforms; primary down	503 / 500	200, 0 queries on the primary
Control panel entry save, primary up	—	Writes go to the primary and replicate; reads stick to the primary after the first write

New regression tests (each fails on the old code):

CmsTest: returns true when only the read connection is reachable
  before: false   after: true
ConnectionConfigTest: builds requirement checker dsn values from read/write connection config
  before: ErrorException "Array to string conversion"   after: pgsql:host=10.0.0.1;port=5433;…
DatabaseServiceProviderTest: introspects the read connection unless the connection reads from the write connection
  before: undefined macro   after: replica columns, then primary columns after useWriteConnectionWhenReading()

Known limits that still need the primary:

• Generating a transform for the first time (it inserts an imagetransformindex row).
• Anything that writes.
• Database-backed sessions, cache, and queues.
• Legacy Yii queries (Craft::$app->db).

[github-actions]

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