diff --git a/hermes/Cargo.lock b/hermes/Cargo.lock
index bfa025aa..b902a9a1 100644
--- a/hermes/Cargo.lock
+++ b/hermes/Cargo.lock
@@ -1659,7 +1659,7 @@ checksum = "95505c38b4572b2d910cecb0281560f54b440a19336cbbcb27bf6ce6adc6f5a8"
 
 [[package]]
 name = "hermes"
-version = "0.1.3"
+version = "0.1.4"
 dependencies = [
  "anyhow",
  "async-trait",
@@ -1678,6 +1678,7 @@ dependencies = [
  "libc",
  "libp2p",
  "log",
+ "prometheus-client",
  "pyth-sdk",
  "pythnet-sdk",
  "rand 0.8.5",
@@ -3635,6 +3636,29 @@ dependencies = [
  "unicode-ident",
 ]
 
+[[package]]
+name = "prometheus-client"
+version = "0.21.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "78c2f43e8969d51935d2a7284878ae053ba30034cd563f673cde37ba5205685e"
+dependencies = [
+ "dtoa",
+ "itoa",
+ "parking_lot 0.12.1",
+ "prometheus-client-derive-encode",
+]
+
+[[package]]
+name = "prometheus-client-derive-encode"
+version = "0.4.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "72b6a5217beb0ad503ee7fa752d451c905113d70721b937126158f3106a48cc1"
+dependencies = [
+ "proc-macro2 1.0.56",
+ "quote 1.0.27",
+ "syn 1.0.109",
+]
+
 [[package]]
 name = "prost"
 version = "0.9.0"
diff --git a/hermes/Cargo.toml b/hermes/Cargo.toml
index 1bfc06eb..ec5f6796 100644
--- a/hermes/Cargo.toml
+++ b/hermes/Cargo.toml
@@ -1,6 +1,6 @@
 [package]
 name                   = "hermes"
-version                = "0.1.3"
+version                = "0.1.4"
 edition                = "2021"
 
 [dependencies]
@@ -35,6 +35,7 @@ libp2p                 = { version = "0.42.2", features = [
 ]}
 
 log                    = { version = "0.4.17" }
+prometheus-client      = { version = "0.21.1" }
 pyth-sdk               = { version = "0.7.0" }
 
 # Parse Wormhole attester price attestations.
diff --git a/hermes/src/store.rs b/hermes/src/store.rs
index f8a741b9..ae745db9 100644
--- a/hermes/src/store.rs
+++ b/hermes/src/store.rs
@@ -19,10 +19,7 @@ use {
             construct_message_states_proofs,
             store_wormhole_merkle_verified_message,
         },
-        storage::{
-            AccumulatorState,
-            CompletedAccumulatorState,
-        },
+        storage::CompletedAccumulatorState,
         types::{
             ProofSet,
             UnixTimestamp,
@@ -150,18 +147,14 @@ impl Store {
             Update::AccumulatorMessages(accumulator_messages) => {
                 let slot = accumulator_messages.slot;
                 log::info!("Storing accumulator messages for slot {:?}.", slot,);
-                let mut accumulator_state = self
-                    .storage
-                    .fetch_accumulator_state(slot)
-                    .await?
-                    .unwrap_or(AccumulatorState {
-                        slot,
-                        accumulator_messages: None,
-                        wormhole_merkle_state: None,
-                    });
-                accumulator_state.accumulator_messages = Some(accumulator_messages);
                 self.storage
-                    .store_accumulator_state(accumulator_state)
+                    .update_accumulator_state(
+                        slot,
+                        Box::new(|mut state| {
+                            state.accumulator_messages = Some(accumulator_messages);
+                            state
+                        }),
+                    )
                     .await?;
                 slot
             }
diff --git a/hermes/src/store/proof/wormhole_merkle.rs b/hermes/src/store/proof/wormhole_merkle.rs
index 7450a55b..99b5d2a0 100644
--- a/hermes/src/store/proof/wormhole_merkle.rs
+++ b/hermes/src/store/proof/wormhole_merkle.rs
@@ -1,7 +1,6 @@
 use {
     crate::store::{
         storage::{
-            AccumulatorState,
             CompletedAccumulatorState,
             MessageState,
         },
@@ -49,23 +48,18 @@ pub async fn store_wormhole_merkle_verified_message(
     root: WormholeMerkleRoot,
     vaa_bytes: Vec<u8>,
 ) -> Result<()> {
-    let mut accumulator_state = store
-        .storage
-        .fetch_accumulator_state(root.slot)
-        .await?
-        .unwrap_or(AccumulatorState {
-            slot:                  root.slot,
-            accumulator_messages:  None,
-            wormhole_merkle_state: None,
-        });
-
-    accumulator_state.wormhole_merkle_state = Some(WormholeMerkleState {
-        root,
-        vaa: vaa_bytes,
-    });
     store
         .storage
-        .store_accumulator_state(accumulator_state)
+        .update_accumulator_state(
+            root.slot,
+            Box::new(|mut state| {
+                state.wormhole_merkle_state = Some(WormholeMerkleState {
+                    root,
+                    vaa: vaa_bytes,
+                });
+                state
+            }),
+        )
         .await?;
     Ok(())
 }
diff --git a/hermes/src/store/storage.rs b/hermes/src/store/storage.rs
index 371d708c..1d1b10af 100644
--- a/hermes/src/store/storage.rs
+++ b/hermes/src/store/storage.rs
@@ -134,8 +134,20 @@ pub trait Storage: Send + Sync {
         filter: MessageStateFilter,
     ) -> Result<Vec<MessageState>>;
 
+    /// Store the accumulator state. Please note that this call will replace the
+    /// existing accumulator state for the given state's slot. If you wish to
+    /// update the accumulator state, use `update_accumulator_state` instead.
     async fn store_accumulator_state(&self, state: AccumulatorState) -> Result<()>;
     async fn fetch_accumulator_state(&self, slot: Slot) -> Result<Option<AccumulatorState>>;
+
+    /// Update the accumulator state inplace using the provided callback. The callback
+    /// takes the current state and returns the new state. If there is no accumulator
+    /// state for the given slot, the callback will be called with an empty accumulator state.
+    async fn update_accumulator_state(
+        &self,
+        slot: Slot,
+        callback: Box<dyn (FnOnce(AccumulatorState) -> AccumulatorState) + Send>,
+    ) -> Result<()>;
 }
 
 pub type StorageInstance = Box<dyn Storage>;
diff --git a/hermes/src/store/storage/local_storage.rs b/hermes/src/store/storage/local_storage.rs
index b6cbc09d..c081dde2 100644
--- a/hermes/src/store/storage/local_storage.rs
+++ b/hermes/src/store/storage/local_storage.rs
@@ -87,6 +87,25 @@ impl LocalStorage {
             None => None,
         }
     }
+
+    /// Store the accumulator state in the cache assuming that the lock is already acquired.
+    fn store_accumulator_state_impl(
+        &self,
+        state: AccumulatorState,
+        cache: &mut VecDeque<AccumulatorState>,
+    ) {
+        cache.push_back(state);
+
+        let mut i = cache.len().saturating_sub(1);
+        while i > 0 && cache[i - 1].slot > cache[i].slot {
+            cache.swap(i - 1, i);
+            i -= 1;
+        }
+
+        if cache.len() > self.cache_size as usize {
+            cache.pop_front();
+        }
+    }
 }
 
 #[async_trait]
@@ -158,18 +177,7 @@ impl Storage for LocalStorage {
 
     async fn store_accumulator_state(&self, state: super::AccumulatorState) -> Result<()> {
         let mut accumulator_cache = self.accumulator_cache.write().await;
-        accumulator_cache.push_back(state);
-
-        let mut i = accumulator_cache.len().saturating_sub(1);
-        while i > 0 && accumulator_cache[i - 1].slot > accumulator_cache[i].slot {
-            accumulator_cache.swap(i - 1, i);
-            i -= 1;
-        }
-
-        if accumulator_cache.len() > self.cache_size as usize {
-            accumulator_cache.pop_front();
-        }
-
+        self.store_accumulator_state_impl(state, &mut accumulator_cache);
         Ok(())
     }
 
@@ -180,6 +188,30 @@ impl Storage for LocalStorage {
             Err(_) => Ok(None),
         }
     }
+
+    async fn update_accumulator_state(
+        &self,
+        slot: Slot,
+        callback: Box<dyn (FnOnce(AccumulatorState) -> AccumulatorState) + Send>,
+    ) -> Result<()> {
+        let mut accumulator_cache = self.accumulator_cache.write().await;
+        match accumulator_cache.binary_search_by_key(&slot, |state| state.slot) {
+            Ok(idx) => {
+                let state = accumulator_cache.get_mut(idx).unwrap();
+                *state = callback(state.clone());
+            }
+            Err(_) => {
+                let state = callback(AccumulatorState {
+                    slot,
+                    accumulator_messages: None,
+                    wormhole_merkle_state: None,
+                });
+                self.store_accumulator_state_impl(state, &mut accumulator_cache);
+            }
+        }
+
+        Ok(())
+    }
 }
 
 #[cfg(test)]
@@ -187,12 +219,16 @@ mod test {
     use {
         super::*,
         crate::store::{
-            proof::wormhole_merkle::WormholeMerkleMessageProof,
+            proof::wormhole_merkle::{
+                WormholeMerkleMessageProof,
+                WormholeMerkleState,
+            },
             types::{
                 AccumulatorMessages,
                 ProofSet,
             },
         },
+        futures::future::join_all,
         pyth_sdk::UnixTimestamp,
         pythnet_sdk::{
             accumulators::merkle::MerklePath,
@@ -201,6 +237,7 @@ mod test {
                 Message,
                 PriceFeedMessage,
             },
+            wire::v1::WormholeMerkleRoot,
         },
     };
 
@@ -642,4 +679,184 @@ mod test {
             accumulator_state_at_slot_30
         );
     }
+
+    #[tokio::test]
+    pub async fn test_update_accumulator_state_works() {
+        // Initialize a storage with a cache size of 2 per key and the accumulator state.
+        let storage = LocalStorage::new_instance(2);
+
+        // Create an empty accumulator state at slot 10.
+        create_and_store_empty_accumulator_state_at_slot(&storage, 10).await;
+
+        // Update the accumulator state with slot 10.
+        let accumulator_messages = AccumulatorMessages {
+            magic:        [0; 4],
+            slot:         10,
+            ring_size:    3,
+            raw_messages: vec![],
+        };
+
+        let accumulator_messages_clone = accumulator_messages.clone();
+
+        storage
+            .update_accumulator_state(
+                10,
+                Box::new(|mut accumulator_state| {
+                    accumulator_state.accumulator_messages = Some(accumulator_messages_clone);
+                    accumulator_state
+                }),
+            )
+            .await
+            .unwrap();
+
+        // Make sure the retrieved accumulator state is what we stored.
+        assert_eq!(
+            storage.fetch_accumulator_state(10).await.unwrap(),
+            Some(AccumulatorState {
+                slot:                  10,
+                accumulator_messages:  Some(accumulator_messages),
+                wormhole_merkle_state: None,
+            })
+        );
+    }
+
+    #[tokio::test]
+    pub async fn test_update_accumulator_state_works_on_nonexistent_state() {
+        // Initialize an empty storage with a cache size of 2 per key and the accumulator state.
+        let storage = LocalStorage::new_instance(2);
+
+        // Update the accumulator state with slot 10.
+        let accumulator_messages = AccumulatorMessages {
+            magic:        [0; 4],
+            slot:         10,
+            ring_size:    3,
+            raw_messages: vec![],
+        };
+        let accumulator_messages_clone = accumulator_messages.clone();
+        storage
+            .update_accumulator_state(
+                10,
+                Box::new(|mut accumulator_state| {
+                    accumulator_state.accumulator_messages = Some(accumulator_messages_clone);
+                    accumulator_state
+                }),
+            )
+            .await
+            .unwrap();
+
+        // Make sure the retrieved accumulator state is what we stored.
+        assert_eq!(
+            storage.fetch_accumulator_state(10).await.unwrap(),
+            Some(AccumulatorState {
+                slot:                  10,
+                accumulator_messages:  Some(accumulator_messages),
+                wormhole_merkle_state: None,
+            })
+        );
+    }
+
+    #[tokio::test]
+    pub async fn test_update_accumulator_state_works_on_concurrent_updates() {
+        // Initialize an empty storage with a cache size of 20 per key and the accumulator state.
+        let storage = LocalStorage::new_instance(20);
+
+
+        // Run this check 10 times to make sure the concurrent updates work.
+        let mut futures = vec![];
+
+        for slot in 1..10 {
+            futures.push(storage.update_accumulator_state(
+                slot,
+                Box::new(|mut accumulator_state| {
+                    accumulator_state.accumulator_messages = Some(AccumulatorMessages {
+                        magic:        [0; 4],
+                        slot:         123,
+                        ring_size:    3,
+                        raw_messages: vec![],
+                    });
+                    accumulator_state
+                }),
+            ));
+            futures.push(storage.update_accumulator_state(
+                slot,
+                Box::new(|mut accumulator_state| {
+                    accumulator_state.wormhole_merkle_state = Some(WormholeMerkleState {
+                        root: WormholeMerkleRoot {
+                            root:      [0; 20],
+                            slot:      123,
+                            ring_size: 3,
+                        },
+                        vaa:  vec![],
+                    });
+                    accumulator_state
+                }),
+            ));
+        }
+
+        join_all(futures).await;
+
+        for slot in 1..10 {
+            assert_eq!(
+                storage.fetch_accumulator_state(slot).await.unwrap(),
+                Some(AccumulatorState {
+                    slot,
+                    accumulator_messages: Some(AccumulatorMessages {
+                        magic:        [0; 4],
+                        slot:         123,
+                        ring_size:    3,
+                        raw_messages: vec![],
+                    }),
+                    wormhole_merkle_state: Some(WormholeMerkleState {
+                        root: WormholeMerkleRoot {
+                            root:      [0; 20],
+                            slot:      123,
+                            ring_size: 3,
+                        },
+                        vaa:  vec![],
+                    }),
+                })
+            )
+        }
+    }
+
+    #[tokio::test]
+    pub async fn test_update_accumulator_state_evicts_cache() {
+        // Initialize a storage with a cache size of 2 per key and the accumulator state.
+        let storage = LocalStorage::new_instance(2);
+
+        storage
+            .update_accumulator_state(10, Box::new(|accumulator_state| accumulator_state))
+            .await
+            .unwrap();
+        storage
+            .update_accumulator_state(20, Box::new(|accumulator_state| accumulator_state))
+            .await
+            .unwrap();
+        storage
+            .update_accumulator_state(30, Box::new(|accumulator_state| accumulator_state))
+            .await
+            .unwrap();
+
+        // The accumulator state at slot 10 should be evicted from the cache.
+        assert_eq!(storage.fetch_accumulator_state(10).await.unwrap(), None);
+
+        // Retrieve the rest of accumulator states and make sure it is what we stored.
+        assert_eq!(
+            storage.fetch_accumulator_state(20).await.unwrap().unwrap(),
+            AccumulatorState {
+                slot:                  20,
+                accumulator_messages:  None,
+                wormhole_merkle_state: None,
+            }
+        );
+
+        assert_eq!(
+            storage.fetch_accumulator_state(30).await.unwrap().unwrap(),
+            AccumulatorState {
+                slot:                  30,
+                accumulator_messages:  None,
+                wormhole_merkle_state: None,
+            }
+        );
+    }
 }