Contadores distribuídos

Muitos apps em tempo real têm documentos que atuam como contadores. Por exemplo, é possível contar "curtidas" em uma postagem ou "favoritos" de um item específico.

No Cloud Firestore, você só pode atualizar um único documento cerca de uma vez por segundo, o que pode ser muito pouco para alguns aplicativos de alto tráfego.

Solução: contadores distribuídos

Para aceitar atualizações de contador mais frequentes, crie um contador distribuído. Cada contador é um documento com uma subcoleção de "fragmentos", e o valor do contador é a soma dos valores dos fragmentos.

A capacidade de gravação aumenta linearmente com o número de fragmentos, de modo que um contador distribuído com 10 fragmentos pode lidar com 10 vezes mais gravações que um contador tradicional.

Web

// counters/${ID}
{
  "num_shards": NUM_SHARDS,
  "shards": [subcollection]
}

// counters/${ID}/shards/${NUM}
{
  "count": 123
}

Swift

Observação: este produto não está disponível para watchOS e destinos de clipes de apps. 
// 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
    }
}
SolutionCountersViewController.swift

Objective-C

Observação: este produto não está disponível para watchOS e destinos de clipes de apps. 
// 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
FIRSolutionCountersViewController.m

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;
    }
}
SolutionCounters.java

Kotlin+KTX 

// counters/${ID}
data class Counter(var numShards: Int)

// counters/${ID}/shards/${NUM}
data class Shard(var count: Int)
SolutionCounters.kt

Python 

import random

from google.cloud import firestore

class Shard(object):
    """
    A shard is a distributed counter. Each shard can support being incremented
    once per second. Multiple shards are needed within a Counter to allow
    more frequent incrementing.
    """

    def __init__(self):
        self._count = 0

    def to_dict(self):
        return {"count": self._count}

class Counter(object):
    """
    A counter stores a collection of shards which are
    summed to return a total count. This allows for more
    frequent incrementing than a single document.
    """

    def __init__(self, num_shards):
        self._num_shards = num_shards
distributed_counters.py

Python 

import random

from google.cloud import firestore

class Shard(object):
    """
    A shard is a distributed counter. Each shard can support being incremented
    once per second. Multiple shards are needed within a Counter to allow
    more frequent incrementing.
    """

    def __init__(self):
        self._count = 0

    def to_dict(self):
        return {"count": self._count}

class Counter(object):
    """
    A counter stores a collection of shards which are
    summed to return a total count. This allows for more
    frequent incrementing than a single document.
    """

    def __init__(self, num_shards):
        self._num_shards = num_shards

distributed_counters.py

Node.js

Não relevante. Consulte abaixo o snippet de incremento do contador.

Go

import (
	"context"
	"fmt"
	"math/rand"
	"strconv"

	"cloud.google.com/go/firestore"
	"google.golang.org/api/iterator"
)

// Counter is a collection of documents (shards)
// to realize counter with high frequency.
type Counter struct {
	numShards int
}

// Shard is a single counter, which is used in a group
// of other shards within Counter.
type Shard struct {
	Count int
}

solution_counters.go

PHP

Não relevante. Consulte o snippet de inicialização do contador abaixo.

C# 

/// <summary>
/// Shard is a document that contains the count.
/// </summary>
[FirestoreData]
public class Shard
{
    [FirestoreProperty(name: "count")]
    public int Count { get; set; }
}
Program.cs

O código a seguir inicializa um contador distribuído:

Web

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();
}
test.solution-counters.js

Swift

Observação: este produto não está disponível para watchOS e destinos de clipes de apps. 
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])
        }
    }
}
SolutionCountersViewController.swift

Objective-C

Observação: este produto não está disponível para watchOS e destinos de clipes de apps. 
- (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) }];
    }
  }];
}
FIRSolutionCountersViewController.m

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);
                }
            });
}
SolutionCounters.java

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)
            }
}
SolutionCounters.kt

Python 

def init_counter(self, doc_ref):
    """
    Create a given number of shards as
    subcollection of specified document.
    """
    col_ref = doc_ref.collection("shards")

    # Initialize each shard with count=0
    for num in range(self._num_shards):
        shard = Shard()
        col_ref.document(str(num)).set(shard.to_dict())
distributed_counters.py

Python 

async def init_counter(self, doc_ref):
    """
    Create a given number of shards as
    subcollection of specified document.
    """
    col_ref = doc_ref.collection("shards")

    # Initialize each shard with count=0
    for num in range(self._num_shards):
        shard = Shard()
        await col_ref.document(str(num)).set(shard.to_dict())

distributed_counters.py

Node.js

Não relevante. Consulte abaixo o snippet de incremento do contador.

Go


// initCounter creates a given number of shards as
// subcollection of specified document.
func (c *Counter) initCounter(ctx context.Context, docRef *firestore.DocumentRef) error {
	colRef := docRef.Collection("shards")

	// Initialize each shard with count=0
	for num := 0; num < c.numShards; num++ {
		shard := Shard{0}

		if _, err := colRef.Doc(strconv.Itoa(num)).Set(ctx, shard); err != nil {
			return fmt.Errorf("Set: %v", err)
		}
	}
	return nil
}

solution_counters.go

PHP 

$numShards = 10;
$ref = $db->collection('samples/php/distributedCounters');
for ($i = 0; $i < $numShards; $i++) {
    $doc = $ref->document($i);
    $doc->set(['Cnt' => 0]);
}
solution_sharded_counter_create.php

C# 

/// <summary>
/// Create a given number of shards as a
/// subcollection of specified document.
/// </summary>
/// <param name="docRef">The document reference <see cref="DocumentReference"/></param>
private static async Task CreateCounterAsync(DocumentReference docRef, int numOfShards)
{
    CollectionReference colRef = docRef.Collection("shards");
    var tasks = new List<Task>();
    // Initialize each shard with Count=0
    for (var i = 0; i < numOfShards; i++)
    {
        tasks.Add(colRef.Document(i.ToString()).SetAsync(new Shard() { Count = 0 }));
    }
    await Task.WhenAll(tasks);
}
Program.cs

Ruby 

# project_id = "Your Google Cloud Project ID"
# num_shards = "Number of shards for distributed counter"
# collection_path = "shards"

require "google/cloud/firestore"

firestore = Google::Cloud::Firestore.new project_id: project_id

shards_ref = firestore.col collection_path

# Initialize each shard with count=0
num_shards.times do |i|
  shards_ref.doc(i).set({ count: 0 })
end

puts "Distributed counter shards collection created."
distributed_counters.rb

Para incrementar o contador, escolha um fragmento aleatório e aumente a contagem:

Web

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));
}
test.solution-counters.js

Swift

Observação: este produto não está disponível para watchOS e destinos de clipes de apps. 
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))
    ])
}
SolutionCountersViewController.swift

Objective-C

Observação: este produto não está disponível para watchOS e destinos de clipes de apps. 
- (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]
  }];
}
FIRSolutionCountersViewController.m

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));
}
SolutionCounters.java

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))
}
SolutionCounters.kt

Python 

def increment_counter(self, doc_ref):
    """Increment a randomly picked shard."""
    doc_id = random.randint(0, self._num_shards - 1)

    shard_ref = doc_ref.collection("shards").document(str(doc_id))
    return shard_ref.update({"count": firestore.Increment(1)})
distributed_counters.py

Python 

async def increment_counter(self, doc_ref):
    """Increment a randomly picked shard."""
    doc_id = random.randint(0, self._num_shards - 1)

    shard_ref = doc_ref.collection("shards").document(str(doc_id))
    return await shard_ref.update({"count": firestore.Increment(1)})

distributed_counters.py

Node.js 

function incrementCounter(docRef, numShards) {
  const shardId = Math.floor(Math.random() * numShards);
  const shardRef = docRef.collection('shards').doc(shardId.toString());
  return shardRef.set({count: FieldValue.increment(1)}, {merge: true});
}
solution-counters.js

Go


// incrementCounter increments a randomly picked shard.
func (c *Counter) incrementCounter(ctx context.Context, docRef *firestore.DocumentRef) (*firestore.WriteResult, error) {
	docID := strconv.Itoa(rand.Intn(c.numShards))

	shardRef := docRef.Collection("shards").Doc(docID)
	return shardRef.Update(ctx, []firestore.Update{
		{Path: "Count", Value: firestore.Increment(1)},
	})
}

solution_counters.go

PHP 

$ref = $db->collection('samples/php/distributedCounters');
$numShards = 0;
$docCollection = $ref->documents();
foreach ($docCollection as $doc) {
    $numShards++;
}
$shardIdx = random_int(0, $numShards - 1);
$doc = $ref->document($shardIdx);
$doc->update([
    ['path' => 'Cnt', 'value' => FieldValue::increment(1)]
]);
solution_sharded_counter_increment.php

C# 

/// <summary>
/// Increment a randomly picked shard by 1.
/// </summary>
/// <param name="docRef">The document reference <see cref="DocumentReference"/></param>
/// <returns>The <see cref="Task"/></returns>
private static async Task IncrementCounterAsync(DocumentReference docRef, int numOfShards)
{
    int documentId;
    lock (s_randLock)
    {
        documentId = s_rand.Next(numOfShards);
    }
    var shardRef = docRef.Collection("shards").Document(documentId.ToString());
    await shardRef.UpdateAsync("count", FieldValue.Increment(1));
}
Program.cs

Ruby 

# project_id = "Your Google Cloud Project ID"
# num_shards = "Number of shards for distributed counter"
# collection_path = "shards"

require "google/cloud/firestore"

firestore = Google::Cloud::Firestore.new project_id: project_id

# Select a shard of the counter at random
shard_id = rand 0...num_shards
shard_ref = firestore.doc "#{collection_path}/#{shard_id}"

# increment counter
shard_ref.update({ count: firestore.field_increment(1) })

puts "Counter incremented."
distributed_counters.rb

Para obter a contagem total, consulte todos os fragmentos e some os campos count:

Web

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;
    });
}
test.solution-counters.js

Swift

Observação: este produto não está disponível para watchOS e destinos de clipes de apps. 
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)")
    }
}
SolutionCountersViewController.swift

Objective-C

Observação: este produto não está disponível para watchOS e destinos de clipes de apps. 
- (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);
        }
  }];
}
FIRSolutionCountersViewController.m

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;
                }
            });
}
SolutionCounters.java

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
            }
}
SolutionCounters.kt

Python 

def get_count(self, doc_ref):
    """Return a total count across all shards."""
    total = 0
    shards = doc_ref.collection("shards").list_documents()
    for shard in shards:
        total += shard.get().to_dict().get("count", 0)
    return total
distributed_counters.py

Python 

async def get_count(self, doc_ref):
    """Return a total count across all shards."""
    total = 0
    shards = doc_ref.collection("shards").list_documents()
    async for shard in shards:
        total += (await shard.get()).to_dict().get("count", 0)
    return total

distributed_counters.py

Node.js 

async function getCount(docRef) {
  const querySnapshot = await docRef.collection('shards').get();
  const documents = querySnapshot.docs;

  let count = 0;
  for (const doc of documents) {
    count += doc.get('count');
  }
  return count;
}
solution-counters.js

Go


// getCount returns a total count across all shards.
func (c *Counter) getCount(ctx context.Context, docRef *firestore.DocumentRef) (int64, error) {
	var total int64
	shards := docRef.Collection("shards").Documents(ctx)
	for {
		doc, err := shards.Next()
		if err == iterator.Done {
			break
		}
		if err != nil {
			return 0, fmt.Errorf("Next: %v", err)
		}

		vTotal := doc.Data()["Count"]
		shardCount, ok := vTotal.(int64)
		if !ok {
			return 0, fmt.Errorf("firestore: invalid dataType %T, want int64", vTotal)
		}
		total += shardCount
	}
	return total, nil
}

solution_counters.go

PHP 

$result = 0;
$docCollection = $db->collection('samples/php/distributedCounters')->documents();
foreach ($docCollection as $doc) {
    $result += $doc->data()['Cnt'];
}
solution_sharded_counter_get.php

C# 

/// <summary>
/// Get total count across all shards.
/// </summary>
/// <param name="docRef">The document reference <see cref="DocumentReference"/></param>
/// <returns>The <see cref="int"/></returns>
private static async Task<int> GetCountAsync(DocumentReference docRef)
{
    var snapshotList = await docRef.Collection("shards").GetSnapshotAsync();
    return snapshotList.Sum(shard => shard.GetValue<int>("count"));
}
Program.cs

Ruby 

# project_id = "Your Google Cloud Project ID"
# collection_path = "shards"

require "google/cloud/firestore"

firestore = Google::Cloud::Firestore.new project_id: project_id

shards_ref = firestore.col_group collection_path

count = 0
shards_ref.get do |doc_ref|
  count += doc_ref[:count]
end

puts "Count value is #{count}."
distributed_counters.rb

Limitações

A solução mostrada acima é uma maneira escalonável de criar contadores compartilhados no Cloud Firestore, mas esteja ciente das seguintes limitações:

  • Contagem de fragmentos: o número de fragmentos controla o desempenho do contador distribuído. Com poucos fragmentos, algumas transações podem ter que tentar novamente antes de serem bem-sucedidas, o que deixará as gravações mais lentas. Com fragmentos demais, as leituras ficam mais lentas e caras. Você pode compensar a despesa com leituras mantendo o contador total em um documento de visualização completa, que é atualizado em uma cadência mais lenta (por exemplo, uma vez por segundo) e fazendo com que os clientes leiam esse documento para obter o total. A desvantagem é que os clientes terão que esperar o documento de visualização completa ser atualizado, em vez de calcular o total lendo todos os fragmentos imediatamente após qualquer atualização.
  • Custo: o custo da leitura de um valor de contador aumenta linearmente com o número de fragmentos, porque toda a subcoleção de fragmentos precisa ser carregada.