许多实时应用程序都有充当计数器的文档。例如,您可以计算帖子的“赞数”或特定项目的“收藏夹”数。
在 Cloud Firestore 中,您大约每秒只能更新一次文档,这对于某些高流量应用程序来说可能太低了。
解决方案:分布式计数器
要支持更频繁的计数器更新,请创建分布式计数器。每个计数器都是一个包含“分片”子集合的文档,计数器的值是分片值的总和。
写入吞吐量随分片数量线性增加,因此具有 10 个分片的分布式计数器可以处理的写入量是传统计数器的 10 倍。
网络
// counters/${ID}
{
"num_shards": NUM_SHARDS,
"shards": [subcollection]
}
// counters/${ID}/shards/${NUM}
{
"count": 123
}
迅速
注意:此产品不适用于 watchOS 和 App Clip 目标。
// counters/${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 } }
目标-C
注意:此产品不适用于 watchOS 和 App Clip 目标。
// 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
节点.js
不适用,请参阅下面的计数器增量片段。
去
PHP
不适用,请参阅下面的计数器初始化片段。
C#
以下代码初始化分布式计数器:
网络
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:此产品不适用于 watchOS 和 App Clip 目标。
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]) } } }
目标-C
注意:此产品不适用于 watchOS 和 App Clip 目标。
- (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
节点.js
不适用,请参阅下面的计数器增量片段。
去
PHP
C#
红宝石
要增加计数器,请选择一个随机分片并增加计数:
网络
function incrementCounter(db, 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:此产品不适用于 watchOS 和 App Clip 目标。
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)) ]) }
目标-C
注意:此产品不适用于 watchOS 和 App Clip 目标。
- (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
节点.js
去
PHP
C#
红宝石
要获得总数,请查询所有分片并对它们的count
字段求和:
网络
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:此产品不适用于 watchOS 和 App Clip 目标。
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)") } }
目标-C
注意:此产品不适用于 watchOS 和 App Clip 目标。
- (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
节点.js
去
PHP
C#
红宝石
限制
上面显示的解决方案是在 Cloud Firestore 中创建共享计数器的可扩展方式,但您应该注意以下限制:
- 分片计数- 分片数控制分布式计数器的性能。分片太少,一些事务可能需要重试才能成功,这会减慢写入速度。分片太多,读取变得更慢且更昂贵。您可以通过将计数器总数保存在一个单独的汇总文档中来抵消读取费用,该文档以较慢的节奏(例如每秒一次)更新,并让客户从该文档中读取以获得总数。权衡是客户端将不得不等待更新汇总文档,而不是通过在任何更新后立即读取所有分片来计算总数。
- 成本- 读取计数器值的成本随分片数量线性增加,因为必须加载整个分片子集合。