Many realtime apps have documents that act as counters. For example, you might count 'likes' on a post, or 'favorites' of a specific item.
In Cloud Firestore, you can only update a single document about once per second, which might be too low for some high-traffic applications.
Solution: Distributed counters
To support more frequent counter updates, create a distributed counter. Each counter is a document with a subcollection of "shards," and the value of the counter is the sum of the value of the shards.
Write throughput increases linearly with the number of shards, so a distributed counter with 10 shards can handle 10x as many writes as a traditional counter.
Web
// counters/${ID}
{
"num_shards": NUM_SHARDS,
"shards": [subcollection]
}
// counters/${ID}/shards/${NUM}
{
"count": 123
}
Swift
// counters/${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
}
}
Objective-C
// 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/${NUM}
@interface FIRShard : NSObject
@property (nonatomic, readonly) NSInteger count;
@end
@implementation FIRShard
- (instancetype)initWithCount:(NSInteger)count {
self = [super init];
if (self != nil) {
_count = count;
}
return self;
}
@end
Java
// counters/${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; } }
Kotlin+KTX
// counters/${ID} data class Counter(var numShards: Int) // counters/${ID}/shards/${NUM} data class Shard(var count: Int)
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
Node.js
Not applicable, see the counter increment snippet below.
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 }
PHP
Not applicable, see the counter initialization snippet below.
C#
/// <summary> /// Shard is a document that contains the count. /// </summary> [FirestoreData] public class Shard { [FirestoreProperty(name: "count")] public int Count { get; set; } }
Ruby
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
The following code initializes a distributed counter:
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();
}
Swift
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])
}
}
}
Objective-C
- (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) }];
}
}];
}
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); } }); }
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) } }
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())
Node.js
Not applicable, see the counter increment snippet below.
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 }
PHP
$numShards = 10; $colRef = $ref->collection('SHARDS'); for ($i = 0; $i < $numShards; $i++) { $doc = $colRef->document($i); $doc->set(['Cnt' => 0]); }
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); }
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."
To increment the counter, choose a random shard and increment the count:
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));
}
Swift
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))
])
}
Objective-C
- (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]
}];
}
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)); }
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)) }
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)})
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}); }
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)}, }) }
PHP
$colRef = $ref->collection('SHARDS'); $numShards = 0; $docCollection = $colRef->documents(); foreach ($docCollection as $doc) { $numShards++; } $shardIdx = random_int(0, $numShards-1); $doc = $colRef->document($shardIdx); $doc->update([ ['path' => 'Cnt', 'value' => FieldValue::increment(1)] ]);
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)); }
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."
To get the total count, query for all shards and sum their count
fields:
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;
});
}
Swift
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)")
}
}
Objective-C
- (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);
}
}];
}
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; } }); }
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 } }
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
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; }
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 }
PHP
$result = 0; $docCollection = $ref->collection('SHARDS')->documents(); foreach ($docCollection as $doc) { $result += $doc->data()['Cnt']; }
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")); }
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}."
Limitations
The solution shown above is a scalable way to create shared counters in Cloud Firestore, but you should be aware of the following limitations:
- Shard count - The number of shards controls the performance of the distributed counter. With too few shards, some transactions may have to retry before succeeding, which will slow writes. With too many shards, reads become slower and more expensive. You can offset the read-expense by keeping the counter total in a seprate roll-up document which is updated at a slower cadence (e.g. once per second), and having clients read from that document to get the total. The tradeoff is that clients will have to wait for the roll-up document to be updated, instead of computing the total by reading all of the shards immediately after any update.
- Cost - The cost of reading a counter value increases linearly with the number of shards, because the entire shards subcollection must be loaded.