From 8eb1c733d047d696a0adb1aed59311cedb90255e Mon Sep 17 00:00:00 2001
From: chrislu <chris.lu@gmail.com>
Date: Thu, 20 Nov 2025 18:59:00 -0800
Subject: [PATCH] improve: add data center preference and URL shuffling for
 consistent performance
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Added missing data center preference and load distribution (URL shuffling)
to ensure consistent performance and behavior across all code paths.

Problem 1: PreferPublicUrl path missing DC preference and shuffling
Location: weed/wdclient/filer_client.go lines 184-192

The custom PreferPublicUrl implementation was simply iterating through
locations and building URLs without considering:
  1. Data center proximity (latency optimization)
  2. Load distribution across volume servers

Before:
  for _, loc := range locations {
    url := loc.PublicUrl
    if url == "" { url = loc.Url }
    fullUrls = append(fullUrls, "http://"+url+"/"+fileId)
  }
  return fullUrls, nil

After:
  var sameDcUrls, otherDcUrls []string
  dataCenter := fc.GetDataCenter()
  for _, loc := range locations {
    url := loc.PublicUrl
    if url == "" { url = loc.Url }
    httpUrl := "http://" + url + "/" + fileId
    if dataCenter != "" && dataCenter == loc.DataCenter {
      sameDcUrls = append(sameDcUrls, httpUrl)
    } else {
      otherDcUrls = append(otherDcUrls, httpUrl)
    }
  }
  rand.Shuffle(len(sameDcUrls), ...)
  rand.Shuffle(len(otherDcUrls), ...)
  fullUrls = append(sameDcUrls, otherDcUrls...)

Problem 2: Cache miss path missing URL shuffling
Location: weed/wdclient/vidmap_client.go lines 95-108

The cache miss path (fallback lookup) was missing URL shuffling, while
the cache hit path (vm.LookupFileId) already shuffles URLs. This
inconsistency meant:
  - Cache hit: URLs shuffled → load distributed
  - Cache miss: URLs not shuffled → first server always hit

Before:
  var sameDcUrls, otherDcUrls []string
  // ... build URLs ...
  fullUrls = append(sameDcUrls, otherDcUrls...)
  return fullUrls, nil

After:
  var sameDcUrls, otherDcUrls []string
  // ... build URLs ...
  rand.Shuffle(len(sameDcUrls), ...)
  rand.Shuffle(len(otherDcUrls), ...)
  fullUrls = append(sameDcUrls, otherDcUrls...)
  return fullUrls, nil

Benefits:
  ✓ Reduced latency by preferring same-DC volume servers
  ✓ Even load distribution across all volume servers
  ✓ Consistent behavior between cache hit/miss paths
  ✓ Consistent behavior between PreferUrl and PreferPublicUrl
  ✓ Matches behavior of existing vidMap.LookupFileId implementation

Impact on performance:
  - Lower read latency (same-DC preference)
  - Better volume server utilization (load spreading)
  - No single volume server becomes a hotspot

Note: Added math/rand import to vidmap_client.go for shuffle support.
---
 weed/wdclient/filer_client.go  | 16 ++++++++++++++--
 weed/wdclient/vidmap_client.go |  5 +++++
 2 files changed, 19 insertions(+), 2 deletions(-)

diff --git a/weed/wdclient/filer_client.go b/weed/wdclient/filer_client.go
index 0bcfd4d4b..f0dd5f2e6 100644
--- a/weed/wdclient/filer_client.go
+++ b/weed/wdclient/filer_client.go
@@ -181,14 +181,26 @@ func (fc *FilerClient) GetLookupFileIdFunction() LookupFileIdFunctionType {
 		// Volume found successfully - ignore any errors about other volumes
 		// (not relevant for single-volume lookup, but defensive for future batching)
 
-		// Build URLs with publicUrl preference
+		// Build URLs with publicUrl preference, and also prefer same DC
+		var sameDcUrls, otherDcUrls []string
+		dataCenter := fc.GetDataCenter()
 		for _, loc := range locations {
 			url := loc.PublicUrl
 			if url == "" {
 				url = loc.Url
 			}
-			fullUrls = append(fullUrls, "http://"+url+"/"+fileId)
+			httpUrl := "http://" + url + "/" + fileId
+			if dataCenter != "" && dataCenter == loc.DataCenter {
+				sameDcUrls = append(sameDcUrls, httpUrl)
+			} else {
+				otherDcUrls = append(otherDcUrls, httpUrl)
+			}
 		}
+		// Shuffle to distribute load across volume servers
+		rand.Shuffle(len(sameDcUrls), func(i, j int) { sameDcUrls[i], sameDcUrls[j] = sameDcUrls[j], sameDcUrls[i] })
+		rand.Shuffle(len(otherDcUrls), func(i, j int) { otherDcUrls[i], otherDcUrls[j] = otherDcUrls[j], otherDcUrls[i] })
+		// Prefer same data center
+		fullUrls = append(sameDcUrls, otherDcUrls...)
 		return fullUrls, nil
 	}
 }
diff --git a/weed/wdclient/vidmap_client.go b/weed/wdclient/vidmap_client.go
index caa8be99b..402eaf8c4 100644
--- a/weed/wdclient/vidmap_client.go
+++ b/weed/wdclient/vidmap_client.go
@@ -4,6 +4,7 @@ import (
 	"context"
 	"errors"
 	"fmt"
+	"math/rand"
 	"sort"
 	"strconv"
 	"strings"
@@ -103,6 +104,10 @@ func (vc *vidMapClient) LookupFileIdWithFallback(ctx context.Context, fileId str
 		}
 	}
 
+	// Shuffle to distribute load across volume servers
+	rand.Shuffle(len(sameDcUrls), func(i, j int) { sameDcUrls[i], sameDcUrls[j] = sameDcUrls[j], sameDcUrls[i] })
+	rand.Shuffle(len(otherDcUrls), func(i, j int) { otherDcUrls[i], otherDcUrls[j] = otherDcUrls[j], otherDcUrls[i] })
+
 	// Prefer same data center
 	fullUrls = append(sameDcUrls, otherDcUrls...)
 	return fullUrls, nil