許多實時應用程序都有充當計數器的文檔。例如,您可以計算帖子的“贊數”或特定項目的“收藏夾”數。
在 Cloud Firestore 中,您無法無限制地更新單個文檔。如果您有一個基於單個文檔的計數器並且對其進行足夠頻繁的增量,您最終將看到對文檔更新的爭用。請參閱更新單個文檔。
解決方案:分佈式計數器
要支持更頻繁的計數器更新,請創建分佈式計數器。每個計數器都是一個包含“分片”子集合的文檔,計數器的值是分片值的總和。
寫入吞吐量隨分片數量線性增加,因此具有 10 個分片的分佈式計數器可以處理的寫入量是傳統計數器的 10 倍。
網絡
// counters/${ID}
{
"num_shards": NUM_SHARDS,
"shards": [subcollection]
}
// counters/${ID}/shards/${NUM}
{
"count": 123
}
迅速
注意:此產品不適用於 watchOS 和 App Clip 目標。
// counters/${ID} struct Counter { let numShards: Int init(numShards: Int) { self.numShards = numShards } } // counters/${ID}/shards/${NUM} struct Shard { let count: Int init(count: Int) { self.count = count } }
目標-C
注意:此產品不適用於 watchOS 和 App Clip 目標。
// counters/${ID} @interface FIRCounter : NSObject @property (nonatomic, readonly) NSInteger shardCount; @end @implementation FIRCounter - (instancetype)initWithShardCount:(NSInteger)shardCount { self = [super init]; if (self != nil) { _shardCount = shardCount; } return self; } @end // counters/${ID}/shards/${NUM} @interface FIRShard : NSObject @property (nonatomic, readonly) NSInteger count; @end @implementation FIRShard - (instancetype)initWithCount:(NSInteger)count { self = [super init]; if (self != nil) { _count = count; } return self; } @end
Kotlin+KTX
// counters/${ID} data class Counter(var numShards: Int) // counters/${ID}/shards/${NUM} data class Shard(var count: Int)
Java
// counters/${ID} public class Counter { int numShards; public Counter(int numShards) { this.numShards = numShards; } } // counters/${ID}/shards/${NUM} public class Shard { int count; public Shard(int count) { this.count = count; } }
Python
Python
節點.js
不適用,請參閱下面的計數器增量片段。
去
PHP
不適用,請參閱下面的計數器初始化片段。
C#
以下代碼初始化分佈式計數器:
網絡
function createCounter(ref, num_shards) { var batch = db.batch(); // Initialize the counter document batch.set(ref, { num_shards: num_shards }); // Initialize each shard with count=0 for (let i = 0; i < num_shards; i++) { const shardRef = ref.collection('shards').doc(i.toString()); batch.set(shardRef, { count: 0 }); } // Commit the write batch return batch.commit(); }
迅速
注意:此產品不適用於 watchOS 和 App Clip 目標。
func createCounter(ref: DocumentReference, numShards: Int) { ref.setData(["numShards": numShards]){ (err) in for i in 0...numShards { ref.collection("shards").document(String(i)).setData(["count": 0]) } } }
目標-C
注意:此產品不適用於 watchOS 和 App Clip 目標。
- (void)createCounterAtReference:(FIRDocumentReference *)reference shardCount:(NSInteger)shardCount { [reference setData:@{ @"numShards": @(shardCount) } completion:^(NSError * _Nullable error) { for (NSInteger i = 0; i < shardCount; i++) { NSString *shardName = [NSString stringWithFormat:@"%ld", (long)shardCount]; [[[reference collectionWithPath:@"shards"] documentWithPath:shardName] setData:@{ @"count": @(0) }]; } }]; }
Kotlin+KTX
fun createCounter(ref: DocumentReference, numShards: Int): Task<Void> { // Initialize the counter document, then initialize each shard. return ref.set(Counter(numShards)) .continueWithTask { task -> if (!task.isSuccessful) { throw task.exception!! } val tasks = arrayListOf<Task<Void>>() // Initialize each shard with count=0 for (i in 0 until numShards) { val makeShard = ref.collection("shards") .document(i.toString()) .set(Shard(0)) tasks.add(makeShard) } Tasks.whenAll(tasks) } }
Java
public Task<Void> createCounter(final DocumentReference ref, final int numShards) { // Initialize the counter document, then initialize each shard. return ref.set(new Counter(numShards)) .continueWithTask(new Continuation<Void, Task<Void>>() { @Override public Task<Void> then(@NonNull Task<Void> task) throws Exception { if (!task.isSuccessful()) { throw task.getException(); } List<Task<Void>> tasks = new ArrayList<>(); // Initialize each shard with count=0 for (int i = 0; i < numShards; i++) { Task<Void> makeShard = ref.collection("shards") .document(String.valueOf(i)) .set(new Shard(0)); tasks.add(makeShard); } return Tasks.whenAll(tasks); } }); }
Python
Python
節點.js
不適用,請參閱下面的計數器增量片段。
去
PHP
C#
紅寶石
要增加計數器,請選擇一個隨機分片並增加計數:
網絡
function incrementCounter(ref, num_shards) { // Select a shard of the counter at random const shard_id = Math.floor(Math.random() * num_shards).toString(); const shard_ref = ref.collection('shards').doc(shard_id); // Update count return shard_ref.update("count", firebase.firestore.FieldValue.increment(1)); }
迅速
注意:此產品不適用於 watchOS 和 App Clip 目標。
func incrementCounter(ref: DocumentReference, numShards: Int) { // Select a shard of the counter at random let shardId = Int(arc4random_uniform(UInt32(numShards))) let shardRef = ref.collection("shards").document(String(shardId)) shardRef.updateData([ "count": FieldValue.increment(Int64(1)) ]) }
目標-C
注意:此產品不適用於 watchOS 和 App Clip 目標。
- (void)incrementCounterAtReference:(FIRDocumentReference *)reference shardCount:(NSInteger)shardCount { // Select a shard of the counter at random NSInteger shardID = (NSInteger)arc4random_uniform((uint32_t)shardCount); NSString *shardName = [NSString stringWithFormat:@"%ld", (long)shardID]; FIRDocumentReference *shardReference = [[reference collectionWithPath:@"shards"] documentWithPath:shardName]; [shardReference updateData:@{ @"count": [FIRFieldValue fieldValueForIntegerIncrement:1] }]; }
Kotlin+KTX
fun incrementCounter(ref: DocumentReference, numShards: Int): Task<Void> { val shardId = Math.floor(Math.random() * numShards).toInt() val shardRef = ref.collection("shards").document(shardId.toString()) return shardRef.update("count", FieldValue.increment(1)) }
Java
public Task<Void> incrementCounter(final DocumentReference ref, final int numShards) { int shardId = (int) Math.floor(Math.random() * numShards); DocumentReference shardRef = ref.collection("shards").document(String.valueOf(shardId)); return shardRef.update("count", FieldValue.increment(1)); }
Python
Python
節點.js
去
PHP
C#
紅寶石
要獲得總數,請查詢所有分片並對它們的count
字段求和:
網絡
function getCount(ref) { // Sum the count of each shard in the subcollection return ref.collection('shards').get().then((snapshot) => { let total_count = 0; snapshot.forEach((doc) => { total_count += doc.data().count; }); return total_count; }); }
迅速
注意:此產品不適用於 watchOS 和 App Clip 目標。
func getCount(ref: DocumentReference) { ref.collection("shards").getDocuments() { (querySnapshot, err) in var totalCount = 0 if err != nil { // Error getting shards // ... } else { for document in querySnapshot!.documents { let count = document.data()["count"] as! Int totalCount += count } } print("Total count is \(totalCount)") } }
目標-C
注意:此產品不適用於 watchOS 和 App Clip 目標。
- (void)getCountWithReference:(FIRDocumentReference *)reference { [[reference collectionWithPath:@"shards"] getDocumentsWithCompletion:^(FIRQuerySnapshot *snapshot, NSError *error) { NSInteger totalCount = 0; if (error != nil) { // Error getting shards // ... } else { for (FIRDocumentSnapshot *document in snapshot.documents) { NSInteger count = [document[@"count"] integerValue]; totalCount += count; } NSLog(@"Total count is %ld", (long)totalCount); } }]; }
Kotlin+KTX
fun getCount(ref: DocumentReference): Task<Int> { // Sum the count of each shard in the subcollection return ref.collection("shards").get() .continueWith { task -> var count = 0 for (snap in task.result!!) { val shard = snap.toObject<Shard>() count += shard.count } count } }
Java
public Task<Integer> getCount(final DocumentReference ref) { // Sum the count of each shard in the subcollection return ref.collection("shards").get() .continueWith(new Continuation<QuerySnapshot, Integer>() { @Override public Integer then(@NonNull Task<QuerySnapshot> task) throws Exception { int count = 0; for (DocumentSnapshot snap : task.getResult()) { Shard shard = snap.toObject(Shard.class); count += shard.count; } return count; } }); }
Python
Python
節點.js
去
PHP
C#
紅寶石
限制
上面顯示的解決方案是在 Cloud Firestore 中創建共享計數器的可擴展方式,但您應該注意以下限制:
- 分片計數- 分片數控制分佈式計數器的性能。分片太少,一些事務可能需要重試才能成功,這會減慢寫入速度。分片太多,讀取變得更慢且更昂貴。您可以通過將計數器總數保存在一個單獨的匯總文檔中來抵消讀取費用,該文檔以較慢的節奏更新,並讓客戶從該文檔中讀取以獲得總數。權衡是客戶端將不得不等待更新匯總文檔,而不是通過在任何更新後立即讀取所有分片來計算總數。
- 成本- 讀取計數器值的成本隨分片數量線性增加,因為必須加載整個分片子集合。