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
// counters/${ID}
data class Counter(var numShards: Int)
// counters/${ID}/shards/${NUM}
data class Shard(var count: Int)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++) {
let 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
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)
}
}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
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))
}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
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::class.java)
count += shard.count
}
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.
- Cost - The cost of reading a counter value increases linearly with the number of shards, because the entire shards subcollection must be loaded.