Scale-out storage: PostgreSQL and MySQL read replicas (ST-5 to ST-15)

A data store with readReplicas becomes a replicated store. Writes,
operator-written SQL and everything outside a read scope go to the
primary. JMAP reads before a request's first write, IMAP LIST, STATUS,
SEARCH, SORT and FETCH, POP3 RETR and TOP, DAV GET, PROPFIND and REPORT,
and blob downloads run in a read scope. Only account data (properties,
indexes, change logs, counters, ACLs, blobs, the search index) is read
from a replica; the registry, in-memory values, the task queue and the
rest stay on the primary.

In a scope, the first read picks a replica round-robin among those up
and under the lag limit, and only if it has every change this node has
written or heard of for the scope's accounts: marks come from write
results, the cluster's state-change broadcasts, a sinceState the client
presents, and, with more than one node, Redis. A write inside the scope
sends the rest of it to the primary. A miss on a replica is looked up on
the primary, and a replica error retries the read there and marks the
replica down.

Each node samples lag every second (WAL positions on PostgreSQL; GTID
sets or Seconds_Behind_Source on MySQL), stops reading from a replica
over 5 s and starts again under 2.5 s, and probes a down replica every
10 s. At startup a replica is left out if it's the primary, isn't
read-only, applies out of commit order, or doesn't show a marker written
to the primary within six tries.

replica_tests (postgres, STORE=PostgreSqlReplicated) runs a primary and a
streaming hot standby in containers: tests 9, 10, 12, 13, 14 and 15 pass.

crates/store/src/backend/scaleout/mod.rs

@@ -12,6 +12,27 @@
 pub mod blob;
 pub mod layout;
 pub mod memory;
+pub mod replica;
+pub mod replica_health;
+
+/// Calls a PostgreSQL or MySQL backend directly. Replicated stores use it
+/// instead of going back through `Store`, whose futures would then contain
+/// themselves.
+#[macro_export]
+#[doc(hidden)]
+macro_rules! sql_backend {
+    ($store:expr, $backend:ident => $call:expr) => {
+        match $store {
+            #[cfg(feature = "postgres")]
+            $crate::Store::PostgreSQL($backend) => $call,
+            #[cfg(feature = "mysql")]
+            $crate::Store::MySQL($backend) => $call,
+            _ => Err(trc::StoreEvent::NotConfigured
+                .into_err()
+                .details("A replicated store's member isn't PostgreSQL or MySQL")),
+        }
+    };
+}
 
 pub use blob::ShardedBlobStore;
 pub use memory::ShardedInMemoryStore;