Add cache for parallel HTTP requests (#4879)

beacon_node/beacon_chain/src/chain_config.rs

@@ -15,6 +15,9 @@ pub const DEFAULT_PREPARE_PAYLOAD_LOOKAHEAD_FACTOR: u32 = 3;
 /// Fraction of a slot lookahead for fork choice in the state advance timer (500ms on mainnet).
 pub const FORK_CHOICE_LOOKAHEAD_FACTOR: u32 = 24;
 
+/// Cache only a small number of states in the parallel cache by default.
+pub const DEFAULT_PARALLEL_STATE_CACHE_SIZE: usize = 2;
+
 #[derive(Debug, PartialEq, Eq, Clone, Deserialize, Serialize)]
 pub struct ChainConfig {
     /// Maximum number of slots to skip when importing an attestation.
@@ -83,6 +86,8 @@ pub struct ChainConfig {
     pub progressive_balances_mode: ProgressiveBalancesMode,
     /// Number of epochs between each migration of data from the hot database to the freezer.
     pub epochs_per_migration: u64,
+    /// Size of the promise cache for de-duplicating parallel state requests.
+    pub parallel_state_cache_size: usize,
 }
 
 impl Default for ChainConfig {
@@ -114,6 +119,7 @@ impl Default for ChainConfig {
             always_prepare_payload: false,
             progressive_balances_mode: ProgressiveBalancesMode::Fast,
             epochs_per_migration: crate::migrate::DEFAULT_EPOCHS_PER_MIGRATION,
+            parallel_state_cache_size: DEFAULT_PARALLEL_STATE_CACHE_SIZE,
         }
     }
 }