Allow failover for replicas

CONFIG.md

@@ -309,6 +309,15 @@ If the client doesn't specify, PgCat routes traffic to this role by default.
 `replica` round-robin between replicas only without touching the primary,
 `primary` all queries go to the primary unless otherwise specified.
 
+### replica_to_primary_failover_enabled
+```
+path: pools.<pool_name>.replica_to_primary_failover_enabled
+default: "false"
+```
+
+If set to true, when the specified role is `replica` (either by setting `default_role` or manually)
+and all replicas are banned, queries will be sent to the primary (until a replica is back online).
+
 ### prepared_statements_cache_size
 ```
 path: general.prepared_statements_cache_size

README.md

@@ -175,7 +175,7 @@ The setting will persist until it's changed again or the client disconnects.
 By default, all queries are routed to the first available server; `default_role` setting controls this behavior.
 
 ### Failover
-All servers are checked with a `;` (very fast) query before being given to a client. Additionally, the server health is monitored with every client query that it processes. If the server is not reachable, it will be banned and cannot serve any more transactions for the duration of the ban. The queries are routed to the remaining servers. If all servers become banned, the ban list is cleared: this is a safety precaution against false positives. The primary can never be banned.
+All servers are checked with a `;` (very fast) query before being given to a client. Additionally, the server health is monitored with every client query that it processes. If the server is not reachable, it will be banned and cannot serve any more transactions for the duration of the ban. The queries are routed to the remaining servers. If `replica_to_primary_failover_enabled` is set to true and all replicas become banned, the query will be routed to the primary. If `replica_to_primary_failover_enabled` is false and all servers (replicas) become banned, the ban list is cleared: this is a safety precaution against false positives. The primary can never be banned.
 
 The ban time can be changed with `ban_time`. The default is 60 seconds.
 

src/config.rs

@@ -541,6 +541,9 @@ pub struct Pool {
     #[serde(default = "Pool::default_default_role")]
     pub default_role: String,
 
+    #[serde(default)] // False
+    pub replica_to_primary_failover_enabled: bool,
+
     #[serde(default)] // False
     pub query_parser_enabled: bool,
 
@@ -734,6 +737,7 @@ impl Default for Pool {
             pool_mode: Self::default_pool_mode(),
             load_balancing_mode: Self::default_load_balancing_mode(),
             default_role: String::from("any"),
+            replica_to_primary_failover_enabled: false,
             query_parser_enabled: false,
             query_parser_max_length: None,
             query_parser_read_write_splitting: false,

src/pool.rs

@@ -162,6 +162,9 @@ pub struct PoolSettings {
     // Default server role to connect to.
     pub default_role: Option<Role>,
 
+    // Whether or not we should use primary when replicas are unavailable
+    pub replica_to_primary_failover_enabled: bool,
+
     // Enable/disable query parser.
     pub query_parser_enabled: bool,
 
@@ -219,6 +222,7 @@ impl Default for PoolSettings {
             user: User::default(),
             db: String::default(),
             default_role: None,
+            replica_to_primary_failover_enabled: false,
             query_parser_enabled: false,
             query_parser_max_length: None,
             query_parser_read_write_splitting: false,
@@ -531,6 +535,8 @@ impl ConnectionPool {
                             "primary" => Some(Role::Primary),
                             _ => unreachable!(),
                         },
+                        replica_to_primary_failover_enabled: pool_config
+                            .replica_to_primary_failover_enabled,
                         query_parser_enabled: pool_config.query_parser_enabled,
                         query_parser_max_length: pool_config.query_parser_max_length,
                         query_parser_read_write_splitting: pool_config
@@ -731,6 +737,19 @@ impl ConnectionPool {
             });
         }
 
+        // If the role is replica and we allow sending traffic to primary when replicas are unavailble,
+        // we add primary address at the end of the list of candidates, this way it will be tried when
+        // replicas are all unavailable.
+        if role == Role::Replica && self.settings.replica_to_primary_failover_enabled {
+            let mut primaries = self
+                .addresses
+                .iter()
+                .flatten()
+                .filter(|address| address.role == Role::Primary)
+                .collect::<Vec<&Address>>();
+            candidates.insert(0, primaries.pop().unwrap());
+        }
+
         // Indicate we're waiting on a server connection from a pool.
         let now = Instant::now();
         client_stats.waiting();
@@ -935,24 +954,28 @@ impl ConnectionPool {
             return true;
         }
 
-        // Check if all replicas are banned, in that case unban all of them
-        let replicas_available = self.addresses[address.shard]
-            .iter()
-            .filter(|addr| addr.role == Role::Replica)
-            .count();
+        // If we have replica to primary failover we should not unban replicas
+        // as we still have the primary to server traffic.
+        if !self.settings.replica_to_primary_failover_enabled {
+            // Check if all replicas are banned, in that case unban all of them
+            let replicas_available = self.addresses[address.shard]
+                .iter()
+                .filter(|addr| addr.role == Role::Replica)
+                .count();
 
-        debug!("Available targets: {}", replicas_available);
+            debug!("Available targets: {}", replicas_available);
 
-        let read_guard = self.banlist.read();
-        let all_replicas_banned = read_guard[address.shard].len() == replicas_available;
-        drop(read_guard);
+            let read_guard = self.banlist.read();
+            let all_replicas_banned = read_guard[address.shard].len() == replicas_available;
+            drop(read_guard);
 
-        if all_replicas_banned {
-            let mut write_guard = self.banlist.write();
-            warn!("Unbanning all replicas.");
-            write_guard[address.shard].clear();
+            if all_replicas_banned {
+                let mut write_guard = self.banlist.write();
+                warn!("Unbanning all replicas.");
+                write_guard[address.shard].clear();
 
-            return true;
+                return true;
+            }
         }
 
         // Check if ban time is expired

src/query_router.rs

@@ -1443,6 +1443,7 @@ mod test {
             load_balancing_mode: crate::config::LoadBalancingMode::Random,
             shards: 2,
             user: crate::config::User::default(),
+            replica_to_primary_failover_enabled: false,
             default_role: Some(Role::Replica),
             query_parser_enabled: true,
             query_parser_max_length: None,
@@ -1522,6 +1523,7 @@ mod test {
             shards: 5,
             user: crate::config::User::default(),
             default_role: Some(Role::Replica),
+            replica_to_primary_failover_enabled: false,
             query_parser_enabled: true,
             query_parser_max_length: None,
             query_parser_read_write_splitting: true,