《互联网产品的服务端架构·规模化与架构演进》10.2 消息同步与顺序

// file: internal/immig/sync.go
// tier: L1
// verify: go test ./internal/immig/

package immig

import "sort"

// Message 是一条已落库的聊天消息。Seq 是会话内单调递增的序号，
// 由服务端在写入时分配；ID 是消息的业务标识，Body 仅用于演示负载。
type Message struct {
	Seq  int64
	ID   string
	Body string
}

// Merge 合并本地已缓存与服务端拉取到的两批消息，按 Seq 去重并升序排序。
// 同一个 Seq 出现两次时保留 local 的版本：本地版本是客户端已经展示过的，
// 切换节点时以它为准可以避免界面回退。返回新切片，不修改入参。
func Merge(local, remote []Message) []Message {
	bySeq := make(map[int64]Message, len(local)+len(remote))
	for _, m := range local {
		bySeq[m.Seq] = m
	}
	for _, m := range remote {
		if _, ok := bySeq[m.Seq]; ok {
			continue
		}
		bySeq[m.Seq] = m
	}

	out := make([]Message, 0, len(bySeq))
	for _, m := range bySeq {
		out = append(out, m)
	}
	sort.Slice(out, func(i, j int) bool { return out[i].Seq < out[j].Seq })
	return out
}

// Missing 返回 remote 中存在、local 中缺失的 Seq，升序去重。
// 它只回答“要补哪些洞”，不关心消息内容；补齐仍然走 Merge。
func Missing(local, remote []Message) []int64 {
	have := make(map[int64]struct{}, len(local))
	for _, m := range local {
		have[m.Seq] = struct{}{}
	}

	seen := make(map[int64]struct{}, len(remote))
	out := make([]int64, 0, len(remote))
	for _, m := range remote {
		if _, ok := have[m.Seq]; ok {
			continue
		}
		if _, ok := seen[m.Seq]; ok {
			continue
		}
		seen[m.Seq] = struct{}{}
		out = append(out, m.Seq)
	}
	sort.Slice(out, func(i, j int) bool { return out[i] < out[j] })
	return out
}

// file: internal/immig/sync_test.go
// tier: L1
// verify: go test ./internal/immig/

package immig

import (
	"reflect"
	"testing"
)

func TestMergeDedupesAndSortsBySeq(t *testing.T) {
	local := []Message{
		{Seq: 3, ID: "m3", Body: "local three"},
		{Seq: 1, ID: "m1", Body: "local one"},
	}
	remote := []Message{
		{Seq: 2, ID: "m2", Body: "remote two"},
		{Seq: 3, ID: "m3", Body: "remote three"},
		{Seq: 4, ID: "m4", Body: "remote four"},
	}

	got := Merge(local, remote)
	want := []Message{
		{Seq: 1, ID: "m1", Body: "local one"},
		{Seq: 2, ID: "m2", Body: "remote two"},
		{Seq: 3, ID: "m3", Body: "local three"},
		{Seq: 4, ID: "m4", Body: "remote four"},
	}
	if !reflect.DeepEqual(got, want) {
		t.Fatalf("Merge = %#v, want %#v", got, want)
	}
}

func TestMergeKeepsLocalVersionOnConflict(t *testing.T) {
	local := []Message{{Seq: 7, ID: "m7", Body: "shown locally"}}
	remote := []Message{{Seq: 7, ID: "m7", Body: "server copy"}}
	got := Merge(local, remote)
	if len(got) != 1 {
		t.Fatalf("Merge returned %d messages, want 1", len(got))
	}
	if got[0].Body != "shown locally" {
		t.Fatalf("Merge kept %q, want local version", got[0].Body)
	}
}

func TestMergeDoesNotMutateInputs(t *testing.T) {
	local := []Message{{Seq: 2, ID: "m2"}, {Seq: 1, ID: "m1"}}
	remote := []Message{{Seq: 3, ID: "m3"}}
	localCopy := append([]Message(nil), local...)
	remoteCopy := append([]Message(nil), remote...)

	Merge(local, remote)

	if !reflect.DeepEqual(local, localCopy) {
		t.Fatalf("local mutated: %#v", local)
	}
	if !reflect.DeepEqual(remote, remoteCopy) {
		t.Fatalf("remote mutated: %#v", remote)
	}
}

func TestMissingReturnsGapsSorted(t *testing.T) {
	local := []Message{{Seq: 1}, {Seq: 3}}
	remote := []Message{{Seq: 1}, {Seq: 2}, {Seq: 2}, {Seq: 3}, {Seq: 4}}

	got := Missing(local, remote)
	want := []int64{2, 4}
	if !reflect.DeepEqual(got, want) {
		t.Fatalf("Missing = %v, want %v", got, want)
	}
}

func TestMissingHandlesEmptyLocalAndCoveredRemote(t *testing.T) {
	remote := []Message{{Seq: 9}, {Seq: 5}}
	if got := Missing(nil, remote); !reflect.DeepEqual(got, []int64{5, 9}) {
		t.Fatalf("Missing(nil, remote) = %v, want [5 9]", got)
	}

	local := []Message{{Seq: 5}, {Seq: 6}, {Seq: 9}}
	if got := Missing(local, remote); len(got) != 0 {
		t.Fatalf("Missing(covered) = %v, want empty", got)
	}
}

cd .local/checks/webapp-lab && go test ./internal/immig/ -run 'TestMerge|TestMissing' -count=1 -v

=== RUN   TestMergeDedupesAndSortsBySeq
--- PASS: TestMergeDedupesAndSortsBySeq (0.00s)
=== RUN   TestMergeKeepsLocalVersionOnConflict
--- PASS: TestMergeKeepsLocalVersionOnConflict (0.00s)
=== RUN   TestMergeDoesNotMutateInputs
--- PASS: TestMergeDoesNotMutateInputs (0.00s)
=== RUN   TestMissingReturnsGapsSorted
--- PASS: TestMissingReturnsGapsSorted (0.00s)
=== RUN   TestMissingHandlesEmptyLocalAndCoveredRemote
--- PASS: TestMissingHandlesEmptyLocalAndCoveredRemote (0.00s)
PASS
ok  	webapp/internal/immig	0.245s