From ad4d3b5d622d90e634d5dcf5700f73143fa8c44f Mon Sep 17 00:00:00 2001
From: Chrysostomos Nanakos <chris@include.gr>
Date: Thu, 23 Oct 2025 21:44:49 +0300
Subject: [PATCH] feat(node): implement sliding window for block prefetching

Now fetchBatched maintains a sliding window of batchSize blocks in-flight.
When 75% complete, adds next chunk to maintain constant window size.
This ensures blocks are already pending or have been fetched when
StoreStream needs them.

Part of https://github.com/codex-storage/nim-codex/issues/974

Signed-off-by: Chrysostomos Nanakos <chris@include.gr>
---
 codex/blockexchange/engine/engine.nim        |  6 +-
 codex/blockexchange/engine/pendingblocks.nim |  2 +-
 codex/node.nim                               | 96 +++++++++++---------
 3 files changed, 59 insertions(+), 45 deletions(-)

diff --git a/codex/blockexchange/engine/engine.nim b/codex/blockexchange/engine/engine.nim
index 79eb9c44..bd3afc98 100644
--- a/codex/blockexchange/engine/engine.nim
+++ b/codex/blockexchange/engine/engine.nim
@@ -82,7 +82,6 @@ declareCounter(
 )
 
 const
-  DefaultMaxPeersPerRequest* = 10
   # The default max message length of nim-libp2p is 100 megabytes, meaning we can
   # in principle fit up to 1600 64k blocks per message, so 20 is well under
   # that number.
@@ -90,9 +89,10 @@ const
   DefaultTaskQueueSize = 100
   DefaultConcurrentTasks = 10
   # Don't do more than one discovery request per `DiscoveryRateLimit` seconds.
-  DiscoveryRateLimit = 1.seconds
+  DiscoveryRateLimit = 3.seconds
   DefaultPeerActivityTimeout = 1.minutes
-  PresenceBatchSize = 1024
+  # Match MaxWantListBatchSize to efficiently respond to incoming WantLists
+  PresenceBatchSize = MaxWantListBatchSize
 
 type
   TaskHandler* = proc(task: BlockExcPeerCtx): Future[void] {.gcsafe.}
diff --git a/codex/blockexchange/engine/pendingblocks.nim b/codex/blockexchange/engine/pendingblocks.nim
index 44bb26c1..2ff1062c 100644
--- a/codex/blockexchange/engine/pendingblocks.nim
+++ b/codex/blockexchange/engine/pendingblocks.nim
@@ -34,7 +34,7 @@ declareGauge(
 
 const
   DefaultBlockRetries* = 3000
-  DefaultRetryInterval* = 1.seconds
+  DefaultRetryInterval* = 2.seconds
 
 type
   RetriesExhaustedError* = object of CatchableError
diff --git a/codex/node.nim b/codex/node.nim
index 18fc4609..67145d22 100644
--- a/codex/node.nim
+++ b/codex/node.nim
@@ -53,8 +53,9 @@ logScope:
   topics = "codex node"
 
 const
-  DefaultFetchBatch = 2048
+  DefaultFetchBatch = 1024
   MaxOnBatchBlocks = 128
+  BatchRefillThreshold = 0.75 # Refill when 75% of window completes
 
 type
   Contracts* =
@@ -188,48 +189,61 @@ proc fetchBatched*(
   #     (i: int) => self.networkStore.getBlock(BlockAddress.init(cid, i))
   #   )
 
+  # Sliding window: maintain batchSize blocks in-flight
+  let
+    refillThreshold = int(float(batchSize) * BatchRefillThreshold)
+    refillSize = max(refillThreshold, 1)
+    maxCallbackBlocks = min(batchSize, MaxOnBatchBlocks)
+
+  var
+    blockData: seq[bt.Block]
+    failedBlocks = 0
+    successfulBlocks = 0
+    completedInWindow = 0
+
   var addresses = newSeqOfCap[BlockAddress](batchSize)
-  while not iter.finished:
-    addresses.setLen(0)
-    for i in 0 ..< batchSize:
-      if not iter.finished:
-        let address = BlockAddress.init(cid, iter.next())
-        if fetchLocal or not (await address in self.networkStore):
-          addresses.add(address)
-
-    let blockResults = await self.networkStore.getBlocks(addresses)
-
-    var
-      successfulBlocks = 0
-      failedBlocks = 0
-      blockData: seq[bt.Block]
-
-    for res in blockResults:
-      without blk =? await res:
-        inc(failedBlocks)
-        continue
-
-      inc(successfulBlocks)
-
-      # Only retains block data in memory if there's
-      # a callback.
-      if not onBatch.isNil:
-        blockData.add(blk)
-
-        if blockData.len >= MaxOnBatchBlocks:
-          if batchErr =? (await onBatch(blockData)).errorOption:
-            return failure(batchErr)
-          blockData = @[]
-
-    if failedBlocks > 0:
-      return failure("Some blocks failed (Result) to fetch (" & $failedBlocks & ")")
-
-    if not onBatch.isNil and blockData.len > 0:
-      if batchErr =? (await onBatch(blockData)).errorOption:
-        return failure(batchErr)
-
+  for i in 0 ..< batchSize:
     if not iter.finished:
-      await idleAsync()
+      let address = BlockAddress.init(cid, iter.next())
+      if fetchLocal or not (await address in self.networkStore):
+        addresses.add(address)
+
+  var blockResults = await self.networkStore.getBlocks(addresses)
+
+  while not blockResults.finished:
+    without blk =? await blockResults.next(), err:
+      inc(failedBlocks)
+      continue
+
+    inc(successfulBlocks)
+    inc(completedInWindow)
+
+    if not onBatch.isNil:
+      blockData.add(blk)
+      if blockData.len >= maxCallbackBlocks:
+        if batchErr =? (await onBatch(blockData)).errorOption:
+          return failure(batchErr)
+        blockData = @[]
+
+    if completedInWindow >= refillThreshold and not iter.finished:
+      var refillAddresses = newSeqOfCap[BlockAddress](refillSize)
+      for i in 0 ..< refillSize:
+        if not iter.finished:
+          let address = BlockAddress.init(cid, iter.next())
+          if fetchLocal or not (await address in self.networkStore):
+            refillAddresses.add(address)
+
+      if refillAddresses.len > 0:
+        blockResults =
+          chain(blockResults, await self.networkStore.getBlocks(refillAddresses))
+      completedInWindow = 0
+
+  if failedBlocks > 0:
+    return failure("Some blocks failed (Result) to fetch (" & $failedBlocks & ")")
+
+  if not onBatch.isNil and blockData.len > 0:
+    if batchErr =? (await onBatch(blockData)).errorOption:
+      return failure(batchErr)
 
   success()