Cache Rewrite #49

ychescale9 · 2019-12-28T14:07:53Z

Resolves #14

This is a rewrite of the cache module in Kotlin. The new implementation supports sized-based and time-based (time-to-live and time-to-idle) evictions.

Cache and Builder APIs

interface Cache<in Key, Value> {

    /**
     * Returns the value associated with [key] in this cache, or null if there is no
     * cached value for [key].
     */
    fun get(key: Key): Value?

    /**
     * Returns the value associated with [key] in this cache if exists,
     * otherwise gets the value by invoking [loader], associates the value with [key] in the cache,
     * and returns the cached value.
     *
     * Note that if an unexpired value for the [key] is present by the time the [loader] returns
     * the new value, the existing value won't be replaced by the new value. Instead the existing
     * value will be returned.
     */
    fun get(key: Key, loader: () -> Value): Value

    /**
     * Associates [value] with [key] in this cache. If the cache previously contained a
     * value associated with [key], the old value is replaced by [value].
     */
    fun put(key: Key, value: Value)

    /**
     * Discards any cached value for key [key].
     */
    fun invalidate(key: Key)

    /**
     * Discards all entries in the cache.
     */
    fun invalidateAll()

    /**
     * Main entry point for creating a [Cache].
     */
    interface Builder {

        /**
         * Specifies that each entry should be automatically removed from the cache once a fixed duration
         * has elapsed after the entry's creation or the most recent replacement of its value.
         *
         * When [duration] is zero, the cache's max size will be set to 0
         * meaning no values will be cached.
         */
        fun expireAfterWrite(duration: Long, unit: TimeUnit): Builder

        /**
         * Specifies that each entry should be automatically removed from the cache once a fixed duration
         * has elapsed after the entry's creation, the most recent replacement of its value, or its last
         * access.
         *
         * When [duration] is zero, the cache's max size will be set to 0
         * meaning no values will be cached.
         */
        fun expireAfterAccess(duration: Long, unit: TimeUnit): Builder

        /**
         * Specifies the maximum number of entries the cache may contain.
         * Cache eviction policy is based on LRU - i.e. least recently accessed entries get evicted first.
         *
         * When [size] is 0, entries will be discarded immediately and no values will be cached.
         *
         * If not set, cache size will be unlimited.
         */
        fun maximumCacheSize(size: Long): Builder

        /**
         * Guides the allowed concurrent update operations. This is used as a hint for internal sizing,
         * actual concurrency will vary.
         *
         * If not set, default concurrency level is 16.
         */
        fun concurrencyLevel(concurrencyLevel: Int): Builder

        /**
         * Specifies [Clock] for this cache.
         *
         * This is useful for controlling time in tests
         * where a fake [Clock] implementation can be provided.
         *
         * A [SystemClock] will be used if not specified.
         */
        fun clock(clock: Clock): Builder

        /**
         * Builds a new instance of [Cache] with the specified configurations.
         */
        fun <K : Any, V : Any> build(): Cache<K, V>
    }
}

Note that currently only APIs required by RealStore are supported. Please let know if more APIs are required.

Usage

// build a new cache
val cache = Cache.Builder.newBuilder()
            .expireAfterWrite(30, TimeUnit.MINUTES)
            .expireAfterAccess(24, TimeUnit.HOURS)
            .maximumCacheSize(100)
            .build<Long, String>()

// cache a value
cache.put(1, "dog")

// get cached value by key
cache.get(1)

// get cached value by key, using the provided loader to compute and cache the value if none exists
val value = cache.get(2) { "cat" }

// remove a cached value by key
cache.invalidate(1)

// remove all values from cache
cache.invalidateAll()

Tests

./gradlew cache:test

100% Test Coverage:

Migration

All usages have been switched to use the new cache implementation.

I've also converted the remaining Java files to Kotlin, except MultiParserTest.java which is commented out.

Will update cache/README.md in a followup.

filesystem/src/main/java/com/dropbox/android/external/fs3/filesystem/FileSystemImpl.kt

digitalbuddha · 2019-12-29T13:15:04Z

Wow this is incredible! Thank you so much for all the hard work. I left one comment about cache loader otherwise looks good from first glance. After the holidays we will take this through a more detailed review

ychescale9 · 2019-12-30T10:25:15Z

cache/src/test/kotlin/com/dropbox/android/external/cache4/CacheLoaderTest.kt

+
+    @Test
+    fun `get(key, loader) returns existing value when an entry with the associated key is absent initially but present after executing the loader`() =
+        runBlocking {


I wanted to use TestCoroutineScope.runBlockingTest here to advance the delays, but it doesn't seem to wait for the coroutines to finish when I'm launching with a different dispatcher: launch(Dispatchers.Default) which I need to be able to test concurrency. Is there a better way to test this?

@yigit has strategies for testing async coroutines maybe he can chime in

yigit · 2019-12-30T16:17:27Z

cache/src/main/kotlin/com/dropbox/android/external/cache4/Cache.kt

+     * the new value, the existing value won't be replaced by the new value. Instead the existing
+     * value will be returned.
+     */
+    fun get(key: Key, loader: () -> Value): Value


as far as i remember, we never use this loader in Store.

The FileSystemImpl.kt was using the CacheLoader implementation from guava so I added this as suggested by @digitalbuddha

can you document what happens if loader throws an exception

lets also document that loader needs to be quick (i.e it's not a suspend function).

@digitalbuddha mentioned the loader could be expensive (currently used by filesystem module).
I also don't think we should make the loader suspend as we would have to also make the get(...) API itself suspend in order to execute it since the cache module does not depend on the coroutines library.

Can we document that the loader will be executed on the caller's thread instead?

Since we want to be a replacement for Store3, I figured we should respect this api in case there are consumers of the cache only

cache/src/main/kotlin/com/dropbox/android/external/cache4/CacheBuilder.kt

yigit · 2019-12-30T16:23:01Z

cache/src/main/kotlin/com/dropbox/android/external/cache4/RealCache.kt

+    }
+
+    override fun get(key: Key, loader: () -> Value): Value {
+        synchronized(this) {


lets sync on an explicit object instead of this since this can be accessed outside the cache. I realize it is internal but it still means that can be accessed within the library

not super granular, but I guess we can improve it in the future to be key based if we want to.

Could we verify if old behavior blocked per key or not? Ideally we should be granular since an app might have a single store used among many features

Ok I'll add key-based locks then.

Added key-based synchronization with new tests.

yigit · 2019-12-30T17:14:47Z

cache/src/main/kotlin/com/dropbox/android/external/cache4/CacheEntry.kt

+ */
+internal data class CacheEntry<Key, Value>(
+    val key: Key,
+    @Volatile var value: Value,


as far as i can read, these are always accessed inside a lock which will put a memory barrier hence you should not need volatile for these.

Hmm I’m not sure I follow. These entries live in the ConcurrentHashMap and a couple of synchronized LinkedHashSet but we are updating these values directly (eg when replacing the value for an existing key) instead of recreating and putting a new entry into the map. And currently only get(key: Key, () -> Value): Value is protected by an explicit synchronized block to prevent calling the potentially expensive loader function multiple times concurrently. So I think those mutable fields in the CacheEntry should be the only things we need to protect from concurrent access and hence the volatile. Am I missing something here?

Although using access to ConcurrentHashMap as a memory barrier should be enough, we do not write Entry back into the map after updating it so I don't believe we relay on it. If we wanted to drop Volatile here we would probably need to make these changes.

Whatever we end up with here (either volatile or using the map as a barrier), we should add some documentation here on why we're reusing entries (to be efficient) and document the concurrency policy if it's not obvious.

Using map.put(...) as a barrier feels a bit subtle. Should we use an explicit lock to synchronize all updates to the CacheEntry objects?

I've studied this a bit more and now I believe explicit lock or volatile shouldn't be necessary :)

Will remove volatile and add documentation about reuse.

yigit · 2019-12-30T17:15:07Z

thanks for doing this btw, looks pretty good!

eyalgu

Wow nice PR. Thanks for doing all the work. I reviewed the interface and took a higher level look at the implementation and assumed that the tests covered most of the functionality. let me know if there's anything you'd like me to look at more closely.

cache/src/main/kotlin/com/dropbox/android/external/cache4/Cache.kt

eyalgu · 2019-12-30T18:44:24Z

cache/src/main/kotlin/com/dropbox/android/external/cache4/Cache.kt

@@ -0,0 +1,37 @@
+package com.dropbox.android.external.cache4
+
+interface Cache<in Key, Value> {


Key should be non-nullable, and in order to not have ambiguous get Value should be non-nullable too. Lets change to Cache<in Key: Any, Value: Any>

Thanks. I've also updated the generic types in Store interface to be bound by Any.

eyalgu · 2019-12-30T18:45:05Z

cache/src/main/kotlin/com/dropbox/android/external/cache4/Cache.kt

+     * the new value, the existing value won't be replaced by the new value. Instead the existing
+     * value will be returned.
+     */
+    fun get(key: Key, loader: () -> Value): Value


can you document what happens if loader throws an exception

cache/src/main/kotlin/com/dropbox/android/external/cache4/Cache.kt

cache/src/main/kotlin/com/dropbox/android/external/cache4/CacheBuilder.kt

eyalgu · 2019-12-30T19:10:54Z

cache/src/main/kotlin/com/dropbox/android/external/cache4/Cache.kt

+     * the new value, the existing value won't be replaced by the new value. Instead the existing
+     * value will be returned.
+     */
+    fun get(key: Key, loader: () -> Value): Value


lets also document that loader needs to be quick (i.e it's not a suspend function).

eyalgu · 2019-12-30T19:28:32Z

cache/src/main/kotlin/com/dropbox/android/external/cache4/CacheEntry.kt

+ */
+internal data class CacheEntry<Key, Value>(
+    val key: Key,
+    @Volatile var value: Value,


Although using access to ConcurrentHashMap as a memory barrier should be enough, we do not write Entry back into the map after updating it so I don't believe we relay on it. If we wanted to drop Volatile here we would probably need to make these changes.

Whatever we end up with here (either volatile or using the map as a barrier), we should add some documentation here on why we're reusing entries (to be efficient) and document the concurrency policy if it's not obvious.

eyalgu · 2019-12-30T19:31:01Z

cache/src/main/kotlin/com/dropbox/android/external/cache4/RealCache.kt

+    }
+
+    override fun get(key: Key, loader: () -> Value): Value {
+        synchronized(this) {


not super granular, but I guess we can improve it in the future to be key based if we want to.

eyalgu · 2019-12-30T19:34:12Z

cache/src/main/kotlin/com/dropbox/android/external/cache4/RealCache.kt

+        if (existingEntry != null) {
+            // cache entry found
+            existingEntry.recordWrite(nowNanos)
+            existingEntry.value = value


I think you can add cacheEntries[key] = existingEntry to create a memory barrier using the map access.

any reason not to make the loader suspend rather than blocking? Could be follow up

There's no CoroutineScope to execute the suspend function. If we made the get(key, loader) API itself suspend we will need to use Mutex which is also part of kotlinx-coroutines-core for synchronization. Or do we want to introduce coroutines library dependency to cache?

cache/src/test/kotlin/com/dropbox/android/external/cache4/CacheLoaderTest.kt

ychescale9 · 2019-12-30T23:00:13Z

@eyalgu @yigit Thanks for the review! Will address those comments soon.

… entry reuse and thread-safty assumption.

- Update docs. - Update Key, Value, Input, Output generic types to be bound by Any. - Replace spy with TestLoader for asserting invocation.

…ue): Value.

ychescale9

I believe all comments have been addressed except whether loader should be suspend.
Again thanks for the review!

ychescale9 · 2019-12-31T04:25:13Z

cache/src/main/kotlin/com/dropbox/android/external/cache4/Cache.kt

@@ -0,0 +1,37 @@
+package com.dropbox.android.external.cache4
+
+interface Cache<in Key, Value> {


Thanks. I've also updated the generic types in Store interface to be bound by Any.

cache/src/main/kotlin/com/dropbox/android/external/cache4/Cache.kt

cache/src/main/kotlin/com/dropbox/android/external/cache4/CacheBuilder.kt

cache/src/test/kotlin/com/dropbox/android/external/cache4/CacheLoaderTest.kt

ychescale9 · 2019-12-31T06:26:08Z

cache/src/main/kotlin/com/dropbox/android/external/cache4/RealCache.kt

+    }
+
+    override fun get(key: Key, loader: () -> Value): Value {
+        synchronized(this) {


Added key-based synchronization with new tests.

cache/src/main/kotlin/com/dropbox/android/external/cache4/CacheEntry.kt

eyalgu · 2019-12-31T18:41:03Z

cache/src/main/kotlin/com/dropbox/android/external/cache4/RealCache.kt

+        return keyBasedLocks.getLock(key).withLock {
+            action()
+        }.also {
+            keyBasedLocks.remove(key)


you need some sort of reference counting to make this work. you would have:
thread 1 takes lock.
thread 2 gets lock from map and waits
thread 1 finishes and removes key
thread 3 comes in and does not find a lock in map. proceeds with a new lock
thread 2 resumes on old lock

OTTOMH I can't think of a better way than a to add a counter to each lock entry in the map and have a global lock for adding/removing entries (that is aside from using Guava's ConcurrentHashMultiset which defeats the porpuse of rewriting this cache)

val LockMapSynchronizer = Any() val keyBasedLocks = HashMap<Key, Pair<Lock, Int>> private fun <Key> MutableMap<Key, Pair<Lock, Int>>.getLock(key: Key): Lock { synchronzied(lockMapSynchronizder) { val entry = get(key) if (entry != null) { entry.second++ return entry.first } map.put(key, Pair(ReentrantLock(), AtomicInt(1))) return get(key).first } } private fun <Key> MutableMap<Key, Pair<Lock, Int>>.removeLock(key: Key) { synchronzied(lockMapSynchronizder) { val entry = get(key) ?: return entry.second-- if (entry.second == 0) { remove(key) } } }

alternatively, we can take an alternative approach and not use the key as the synchronizer. we can instead use lock striping: create an array of 8/16/254/whatever entries of Lock. hash the key and grab the lock that matches the key's hash (mod size of array). if you lazy init this array you'll need to sync that too

what does guava do here?

As far as I can tell Guava doesn't synchronize loader execution by key, but relies on key hash and Segments to guard concurrent get in general but on a less granular level.

you need some sort of reference counting to make this work. you would have:
thread 1 takes lock.
thread 2 gets lock from map and waits
thread 1 finishes and removes key
thread 3 comes in and does not find a lock in map. proceeds with a new lock
thread 2 resumes on old lock

Thanks for this failing case. I'll try to implement the reference counting lock map as suggested and add more tests.

eyalgu · 2019-12-31T18:41:54Z

cache/src/main/kotlin/com/dropbox/android/external/cache4/RealCache.kt

+     * otherwise associates a new [Lock] with the specified [key] in the map and returns the new lock.
+     */
+    private fun <Key> MutableMap<Key, Lock>.getLock(key: Key): Lock {
+        val lock: Lock? = get(key) ?: put(key, ReentrantLock())


at the very least this would need to be putIfAbsent

Suggested change

val lock: Lock? = get(key) ?: put(key, ReentrantLock())

val lock: Lock? = get(key) ?: putIfAbsent(key, ReentrantLock())

On Android putIfAbsent is only available on API 24+, so I implemented the same logic here by checking get(key) first.

…Cache.kt as a private class.

cache/src/test/kotlin/com/dropbox/android/external/cache4/testutil/ConcurrencyTestRule.kt

…ve docs.

cache/src/test/kotlin/com/dropbox/android/external/cache4/testutil/ConcurrencyTestRule.kt

eyalgu

We're really getting close to done with this! Thank you for keeping at it. After taking a deeper look and taking memory issues into account I found 2 more issues that needs to be fixed. I think these are the final 2 issues

cache/src/main/kotlin/com/dropbox/android/external/cache4/RealCache.kt

eyalgu

Sorry I found one more bug. Caching is hard

cache/src/main/kotlin/com/dropbox/android/external/cache4/RealCache.kt

ychescale9 · 2020-01-07T01:03:45Z

Sorry I found one more bug. Caching is hard

I thought I knew what I signed up for when I decided to do this...

eyalgu

Thanks for all the hard work!

eyalgu · 2020-01-07T01:19:34Z

@yigit I unblocked only needs your ok now

digitalbuddha · 2020-01-07T18:14:57Z

merging tonight unless any qualms

digitalbuddha · 2020-01-07T22:37:09Z

Could you resolve conflicts and then I'll merge. Thank you

* master: fix dokka, update comments, integrate persister (MobileNativeFoundation#56) Switch POM packaging format to jar for cache, filesystem and store. (MobileNativeFoundation#54) Add an optional param to `Multicaster` to keep the fetches alive even if all downstreams are closed. (MobileNativeFoundation#40) Replace Store.fetch() on fresh() methods in ReadMe (MobileNativeFoundation#52) Properly implement multicast closing (MobileNativeFoundation#48) # Conflicts: # store/src/main/java/com/dropbox/android/external/store4/Store.kt # store/src/main/java/com/dropbox/android/external/store4/StoreBuilder.kt

ychescale9 · 2020-01-08T00:00:36Z

Resolved conflicts.

As a result of making the Cache's Key and Value bounded by Any to make sure neither can be null, these changes propagated to the Store and StoreBuilder APIs. I just found out that these have not been updated in DiskReader, DiskWriter and Persister (maybe more). Maybe we can clean up these in a followup (after alpha01 release)?

I've also removed the old cache/README.md and I'll write a new one later.
Also removed the last remaining java file MultiParserTest.java

digitalbuddha · 2020-01-08T11:56:44Z

Great job!! We will be doing our first alpha release today along with a blog post 😁

ychescale9 added 6 commits December 28, 2019 23:01

Rewrite cache module in Kotlin.

42eacac

Migrate RealStore to use new cache.

982e803

Migrate FileSystemImpl to use new cache.

e04e36c

Migrate StoreTest to new cache.

caa63e4

Remove Guava Cache.

38aff71

Migrate remaining Java file to Kotlin.

9658138

ychescale9 commented Dec 28, 2019

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filesystem/src/main/java/com/dropbox/android/external/fs3/filesystem/FileSystemImpl.kt Outdated Show resolved Hide resolved

ychescale9 added 2 commits December 29, 2019 01:15

Fix ktlint error.

42e7e3f

Fix package.

4a86fb4

digitalbuddha mentioned this pull request Dec 29, 2019

Create kotlin multiplatform targets #50

Closed

ychescale9 added 2 commits December 30, 2019 21:13

Add support for CacheLoader.

6eda8ff

Remove unnecessary mockito-inline.

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ychescale9 commented Dec 30, 2019

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ychescale9 added 3 commits December 31, 2019 14:42

Remove volatile keyword from CacheEntry fields and add docs re. cache…

7027288

… entry reuse and thread-safty assumption.

- Refactor CacheBuilder.

c5b1c3c

- Update docs. - Update Key, Value, Input, Output generic types to be bound by Any. - Replace spy with TestLoader for asserting invocation.

Support key-based synchronization for get(key: Key, loader: () -> Val…

59bedfe

…ue): Value.

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ychescale9 requested review from eyalgu and yigit December 31, 2019 06:38

ychescale9 added 2 commits December 31, 2019 17:50

Document loader exception propagation.

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Small clean ups.

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ychescale9 added 2 commits January 1, 2020 14:04

Make CacheEntry's Key and Value bound to Any, move CacheEntry to Real…

3028df6

…Cache.kt as a private class.

Add reference counting key-based synchronization for cache loader.

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ychescale9 dismissed digitalbuddha’s stale review via 5376d98 January 5, 2020 14:07

Allow retries (3 times) for concurrency tests.

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Move performMaintenance function to bottom of the class.

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Fix more typos.

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ychescale9 added 2 commits January 7, 2020 11:03

Only remove entries in map but not in queue.

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Replace memory barrier with @volatile fields in CacheEntry.

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ychescale9 added 2 commits January 8, 2020 10:47

Remove stale cache/README.md, remove commented out java file.

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digitalbuddha merged commit 062deda into MobileNativeFoundation:master Jan 8, 2020

		@@ -0,0 +1,37 @@
		package com.dropbox.android.external.cache4

		interface Cache<in Key, Value> {

	val lock: Lock? = get(key) ?: put(key, ReentrantLock())
	val lock: Lock? = get(key) ?: putIfAbsent(key, ReentrantLock())

Cache Rewrite #49

Cache Rewrite #49

Conversation

ychescale9 commented Dec 28, 2019 • edited

Cache and Builder APIs

Usage

Tests

Migration

digitalbuddha commented Dec 29, 2019

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