com.yahoo.sketches.hll.UniqueCountMap Maven / Gradle / Ivy
/*
* Copyright 2016, Yahoo! Inc. Licensed under the terms of the
* Apache License 2.0. See LICENSE file at the project root for terms.
*/
package com.yahoo.sketches.hll;
import com.yahoo.sketches.SketchesArgumentException;
/**
* This is a real-time, key-value HLL mapping sketch that tracks approximate unique counts of
* identifiers (the values) associated with each key. An example might be tracking the number of
* unique user identifiers associated with each IP address. This map has been specifically designed
* for the use-case where the number of keys is quite large (many millions) and the distribution of
* identifiers per key is very skewed. A typical distribution where this works well is a
* power-law distribution of identifiers per key of the form y = Cx-α,
* where α < 0.5, and C is roughly ymax.
* For example, with 100M keys, over 75% of the keys would have only
* one identifier, 99% of the keys would have less than 20 identifiers, 99.9% would have less than
* 200 identifiers, and a very tiny fraction might have identifiers in the thousands.
*
* The space consumed by this map is quite sensitive to the actual distribution of identifiers
* per key, so you should characterize and or experiment with your typical input streams.
* Nonetheless, our experiments on live streams of over 100M keys required space less than 2GB.
*
*
Given such highly-skewed distributions, using this map is far more efficient space-wise than
* the alternative of dedicating an HLL sketch per key. Based on our use cases, after
* subtracting the space required for key storage, the average bytes per key required for unique
* count estimation ({@link #getAverageSketchMemoryPerKey()}) is about 10.
*
*
Internally, this map is implemented as a hierarchy of internal hash maps with progressively
* increasing storage allocated for unique count estimation. As a key acquires more identifiers it
* is "promoted" up to a higher internal map. The final map of keys is a map of compact HLL
* sketches.
*
*
The unique values in all the internal maps, except the final HLL map, are stored in a special
* form called a coupon. A coupon is a 16-bit value that fully describes a k=1024 HLL bin.
* It contains 10 bits of address and a 6-bit HLL value.
*
*
All internal maps use a prime number size and Knuth's Open Addressing Double Hash (OADH)
* search algorithm.
*
*
The internal base map holds all the keys and each key is associated with one 16-bit value.
* Initially, the value is a single coupon. Once the key is promoted, this 16-bit field contains a
* reference to the internal map where the key is still active.
*
*
The intermediate maps between the base map and the final HLL map are of two types.
* The first few of these are called traverse maps where the coupons are
* stored as unsorted arrays. After the traverse maps are the coupon hash maps, where the coupons
* are stored in small OASH hash tables.
*
*
All the intermediate maps support deletes and can dynamically grow and shrink as required by
* the input stream.
*
*
The sketch estimator algorithms are unbiased with a Relative Standard Error (RSE)
* of about 2.6% with 68% confidence, or equivalently, about 5.2% with a 95% confidence.
*
*
In a parallel package in the sketches-misc repository, there are 2 classes that can be used
* from the command line to feed this mapping sketch piped from standard-in for experimental
* evaluation. The first is ProcessIpStream, which processes simple IP/ID pairs and the second,
* ProcessDistributionStream, which processes pairs that describe a distribution.
* In this same package is the VariousMapRSETest class that was used to generate the error plots
* for the web site. Please refer to the javadocs for those classes for more information.
*
* @author Lee Rhodes
* @author Alexander Saydakov
* @author Kevin Lang
*/
public class UniqueCountMap {
private static final String LS = System.getProperty("line.separator");
private static final int NUM_LEVELS = 10; // total of single coupon + traverse + coupon maps + hll
private static final int NUM_TRAVERSE_MAPS = 3;
private static final int HLL_K = 1024;
private static final int INITIAL_NUM_ENTRIES = 1000003;
private static final int MIN_INITIAL_NUM_ENTRIES = 157;
private final int keySizeBytes_;
/** TraverseCouponMap or HashCouponMap instances */
private final Map[] maps_;
/**
* Constructs a UniqueCountMap with an initial capacity of one million entries.
* @param keySizeBytes must be at least 4 bytes to have sufficient entropy.
*/
public UniqueCountMap(final int keySizeBytes) {
this(INITIAL_NUM_ENTRIES, keySizeBytes);
}
/**
* Constructs a UniqueCountMap with a given initial number of entries.
*
* @param initialNumEntries The initial number of entries provides a tradeoff between
* wasted space, if too high, and wasted time resizing the table, if too low.
* @param keySizeBytes must be at least 4 bytes to have sufficient entropy
*/
public UniqueCountMap(final int initialNumEntries, final int keySizeBytes) {
checkConstructorKeySize(keySizeBytes);
int initEntries = Math.max(initialNumEntries, MIN_INITIAL_NUM_ENTRIES);
keySizeBytes_ = keySizeBytes;
maps_ = new Map[NUM_LEVELS]; // includes base level and top level
maps_[0] = SingleCouponMap.getInstance(initEntries, keySizeBytes);
}
/**
* Updates the map with a given key and identifier and returns the estimate of the number of
* unique identifiers encountered so far for the given key.
* @param key the given key
* @param identifier the given identifier for unique counting associated with the key
* @return the estimate of the number of unique identifiers encountered so far for the given key.
*/
public double update(final byte[] key, final byte[] identifier) {
if (key == null) { return Double.NaN; }
checkMethodKeySize(key);
if (identifier == null) { return getEstimate(key); }
final short coupon = (short) Map.coupon16(identifier);
final int baseMapIndex = maps_[0].findOrInsertKey(key);
final double baseMapEstimate = maps_[0].update(baseMapIndex, coupon);
if (baseMapEstimate > 0) { return baseMapEstimate; }
int level = -(int) baseMapEstimate; // base map is level 0
if (level == 0) {
return promote(key, coupon, maps_[0], baseMapIndex, level, baseMapIndex, 0);
}
final Map map = maps_[level];
final int index = map.findOrInsertKey(key);
double estimate = map.update(index, coupon);
if (estimate > 0) { return estimate; }
return promote(key, coupon, map, index, level, baseMapIndex, -estimate);
}
/**
* Retrieves the current estimate of unique count for a given key.
* @param key given key
* @return estimate of unique count so far
*/
public double getEstimate(final byte[] key) {
if (key == null) { return Double.NaN; }
checkMethodKeySize(key);
double est = maps_[0].getEstimate(key);
if (est >= 0.0) { return est; }
//key has been promoted
final int level = -(int)est;
final Map map = maps_[level];
return map.getEstimate(key);
}
/**
* Returns the upper bound cardinality with respect to {@link #getEstimate(byte[])} associated
* with the given key.
* @param key the given key
* @return the upper bound cardinality with respect to {@link #getEstimate(byte[])} associated
* with the given key.
*/
public double getUpperBound(final byte[] key) {
if (key == null) { return Double.NaN; }
checkMethodKeySize(key);
double est = maps_[0].getEstimate(key);
if (est >= 0.0) { return est; }
//key has been promoted
final int level = -(int)est;
final Map map = maps_[level];
return map.getUpperBound(key);
}
/**
* Returns the lower bound cardinality with respect to {@link #getEstimate(byte[])} associated
* with the given key.
* @param key the given key
* @return the lower bound cardinality with respect to {@link #getEstimate(byte[])} associated
* with the given key.
*/
public double getLowerBound(final byte[] key) {
if (key == null) { return Double.NaN; }
checkMethodKeySize(key);
double est = maps_[0].getEstimate(key);
if (est >= 0.0) { return est; }
//key has been promoted
final int level = -(int)est;
final Map map = maps_[level];
return map.getLowerBound(key);
}
/**
* Returns the number of active, unique keys across all internal maps
* @return the number of active, unique keys across all internal maps
*/
public int getActiveEntries() {
return maps_[0].getCurrentCountEntries();
}
/**
* Returns total bytes used by all internal maps
* @return total bytes used by all internal maps
*/
public long getMemoryUsageBytes() {
long total = 0;
for (int i = 0; i < maps_.length; i++) {
if (maps_[i] != null) {
total += maps_[i].getMemoryUsageBytes();
}
}
return total;
}
/**
* Returns total bytes used for key storage
* @return total bytes used for key storage
*/
public long getKeyMemoryUsageBytes() {
long total = 0;
for (int i = 0; i < maps_.length; i++) {
if (maps_[i] != null) {
total += maps_[i].getActiveEntries() * keySizeBytes_;
}
}
return total;
}
/**
* Returns the average memory storage per key that is dedicated to sketching the unique counts.
* @return the average memory storage per key that is dedicated to sketching the unique counts.
*/
public double getAverageSketchMemoryPerKey() {
return (double) (getMemoryUsageBytes() - getKeyMemoryUsageBytes()) / getActiveEntries();
}
/**
* Returns the number of active internal maps so far.
* Only the base map is initialized in the constructor, so this method would return 1.
* As more keys are promoted up to higher level maps, the return value would grow until the
* last level HLL map is allocated.
* @return the number of active levels so far
*/
int getActiveMaps() {
int levels = 0;
int iMapsLen = maps_.length;
for (int i = 0; i < iMapsLen; i++) {
if (maps_[i] != null) { levels++; }
}
return levels;
}
/**
* Returns the base map
* @return the base map
*/
Map getBaseMap() {
return maps_[0];
}
/**
* Returns the top-level HllMap. It may be null.
* @return the top-level HllMap.
*/
Map getHllMap() {
return maps_[maps_.length - 1];
}
/**
* Returns a string with a human-readable summary of the UniqueCountMap and all the internal maps
* @return human-readable summary
*/
@Override
public String toString() {
final long totKeys = getActiveEntries();
final long totMem = getMemoryUsageBytes();
final long keyMem = getKeyMemoryUsageBytes();
final double avgValMemPerKey = getAverageSketchMemoryPerKey();
final String ksb = Map.fmtLong(keySizeBytes_);
final String alvls = Map.fmtLong(getActiveMaps());
final String tKeys = Map.fmtLong(totKeys);
final String tMem = Map.fmtLong(totMem);
final String kMem = Map.fmtLong(keyMem);
final String avgValMem = Map.fmtDouble(avgValMemPerKey);
final StringBuilder sb = new StringBuilder();
final String thisSimpleName = this.getClass().getSimpleName();
sb.append("## ").append(thisSimpleName).append(" SUMMARY: ").append(LS);
sb.append(" Key Size Bytes : ").append(ksb).append(LS);
sb.append(" Active Map Levels : ").append(alvls).append(LS);
sb.append(" Total keys : ").append(tKeys).append(LS);
sb.append(" Total Memory Bytes : ").append(tMem).append(LS);
sb.append(" Total Key Memory Bytes : ").append(kMem).append(LS);
sb.append(" Avg Sketch Memory Bytes/Key: ").append(avgValMem).append(LS);
sb.append(LS);
for (int i = 0; i < maps_.length; i++) {
final Map cMap = maps_[i];
if (cMap != null) {
sb.append(cMap.toString());
sb.append(LS);
}
}
sb.append("## ").append("END UNIQUE COUNT MAP SUMMARY");
sb.append(LS);
return sb.toString();
}
private void setLevelInBaseMap(final int index, final int level) {
((SingleCouponMap) maps_[0]).setLevel(index, level);
}
private double promote(final byte[] key, final short coupon, final Map fromMap, final int fromIndex,
final int fromLevel, final int baseMapIndex, final double estimate) {
final Map newMap = getMapForLevel(fromLevel + 1);
final int newMapIndex = newMap.findOrInsertKey(key);
final CouponsIterator it = fromMap.getCouponsIterator(fromIndex);
while (it.next()) {
final double est = newMap.update(newMapIndex, it.getValue());
assert est > 0;
}
fromMap.deleteKey(fromIndex);
newMap.updateEstimate(newMapIndex, estimate);
final double newEstimate = newMap.update(newMapIndex, coupon);
setLevelInBaseMap(baseMapIndex, fromLevel + 1);
assert newEstimate > 0; // this must be positive since we have just promoted
return newEstimate;
}
private Map getMapForLevel(final int level) {
if (maps_[level] == null) {
final int newLevelCapacity = 1 << level;
if (level <= NUM_TRAVERSE_MAPS) {
maps_[level] = CouponTraverseMap.getInstance(keySizeBytes_, newLevelCapacity);
} else if (level < maps_.length - 1) {
maps_[level] = CouponHashMap.getInstance(keySizeBytes_, newLevelCapacity);
} else {
maps_[level] = HllMap.getInstance(keySizeBytes_, HLL_K);
}
}
return maps_[level];
}
private static final void checkConstructorKeySize(final int keySizeBytes) {
if (keySizeBytes < 4) {
throw new SketchesArgumentException("KeySizeBytes must be >= 4: " + keySizeBytes);
}
}
private final void checkMethodKeySize(final byte[] key) {
if (key.length != keySizeBytes_) {
throw new SketchesArgumentException("Key size must be " + keySizeBytes_ + " bytes.");
}
}
}