Status
Current state: Under discussion
Discussion thread: https://lists.apache.org/thread/1vokqdnnt01yycl7y1p74g556cc8yvtq
JIRA: TBD
Released: Not released yet
Please keep the discussion on the mailing list rather than commenting on the wiki (wiki discussions get unwieldy fast).
This FLIP is a joint work of Yuan Zhu (zstraw@163.com) and Qingsheng Ren .
Motivation
As a widely-used feature in Flink SQL jobs, the performance of lookup table source is essential not only for users but also source developers for tuning their implementations. Most lookup table sources use cache to achieve better performance, but there are some features missing in the current design of cache:
- Missing cache related metrics, which is the key to debug and optimize SQL tasks
- Duplicated implementations. Currently every lookup source needs to implement or use its own cache.
- Inconsistent. Table options related to caching are defined differently in sources
In order to address the issues above, we propose here to define a unified abstraction for lookup source cache and its related metrics.
Proposed Changes
We'd like to split the proposal into two kinds of caching mode: partial caching and full caching.
Partial caching
Partial caching loads data into the cache along with the access to the external system. If the key to lookup does not exist in the cache, a lookup action to the external system will be triggered and the lookup result will be stored in the cache for further loading. Users and lookup table developers are able to config the eviction policy and maximum size of the cache.
In order to support partial caching, we propose to introduce several new interfaces to simplify the work for developers to implement lookup table functions and enable cache:
LookupFunction
/AsyncLookupFunction
, an extended version of TableFunction to clarify the semantic of lookup.LookupCache
/LookupCacheFactory
, defining the cache and its factory used in lookup table.DefaultLookupCacheFactory
, a default implementation of a cache that suitable for most use cases.LookupCacheMetricGroup
, defining metrics should be reported by the lookup cache.LookupFunctionProvider
/AsyncLookupFunctionProvider
, as the API interacting with table source to get LookupFunction and LookupCacheFactory.
The cache serves as a component in LookupJoinRunner, and would be pluggable by specifying LookupCacheFactory in LookupFunctionProvider. The developer of a lookup table needs to define a LookupFunctionProvider / AsyncLookupProvider in their implementation of LookupTableSource to specify the LookupFunction and the factory of the cache, then the planner will take over the cache factory, pass it to the LookupJoinRunner, and the cache will be instantiated during the runtime execution.
Full Caching
If the size of lookup table is relatively small to fit into the memory, and the lookup table doesn't change frequently, it'll be more efficient to load all entries of the lookup table into the cache to reduce network I/O, and refresh the table periodically. We'd like to name this use case as "full cache". Logically the reload operation is a kind of scan, so we'd like to reuse the ScanRuntimeProvider so that developers could reuse the scanning logic implemented in Source / SourceFunction / InputFormat. Considering the complexity of Source API, we'd like to support SourceFunction and InputFormat API first. Supporting Source API might require new topology and will be discussed later in another FLIP.
We propose to introduce a new interface FullCachingLookupProvider
in order to reuse the ability of scanning.
Public Interfaces
Lookup Functions
As the usage of TableFunction interface is not quite straight forward to lookup table developers, we'd like to introduce a new interface for sync and async lookup tables. Caching will be only supported on LookupFunction / AsyncLookupFunction.
/** * A wrapper class of {@link TableFunction} for synchronously lookup rows matching the lookup keys * from external system. * * <p>The output type of this table function is fixed as {@link RowData}. */ @PublicEvolving public abstract class LookupFunction extends TableFunction<RowData> { /** * Synchronously lookup rows matching the lookup keys. * * @param keyRow - A {@link RowData} that wraps keys to lookup. * @return A collections of all matching rows in the lookup table. */ public abstract Collection<RowData> lookup(RowData keyRow) throws IOException; /** Invoke {@link #lookup} and handle exceptions. */ public final void eval(Object... keys) { try { lookup(GenericRowData.of(keys)).forEach(this::collect); } catch (IOException e) { throw new RuntimeException("Failed to lookup values with given key", e); } } }
/** * A wrapper class of {@link AsyncTableFunction} for asynchronously lookup rows matching the lookup * keys from external system. * * <p>The output type of this table function is fixed as {@link RowData}. */ @PublicEvolving public abstract class AsyncLookupFunction extends AsyncTableFunction<RowData> { /** * Asynchronously lookup rows matching the lookup keys. * * @param keyRow - A {@link RowData} that wraps keys to lookup. * @return A collections of all matching rows in the lookup table. */ public abstract CompletableFuture<Collection<RowData>> asyncLookup(RowData keyRow); /** Invokes {@link #asyncLookup} and chains futures. */ public final void eval(CompletableFuture<Collection<RowData>> future, Object... keys) { asyncLookup(GenericRowData.of(keys)) .whenCompleteAsync( (result, exception) -> { if (exception != null) { future.completeExceptionally(exception); return; } future.complete(result); }); } }
LookupCache
Considering there might be custom caching strategies and optimizations, we'd like to expose the cache interface as public API for developers to make the cache pluggable.
/** * A semi-persistent mapping from keys to values for storing entries of lookup table. * * <p>The type of the caching key is a {@link RowData} with lookup key fields packed inside. The * type of value is a {@link Collection} of {@link RowData}, which are rows matching lookup key * fields. * * <p>Cache entries are manually added using {@link #put}, and are stored in the cache until either * evicted or manually invalidated. * * <p>Implementations of this interface are expected to be thread-safe, and can be safely accessed * by multiple concurrent threads. */ @PublicEvolving public interface LookupCache { /** * Returns the value associated with key in this cache, or null if there is no cached value for * key. */ @Nullable Collection<RowData> getIfPresent(RowData key); /** * Associates the specified value rows with the specified key row in the cache. If the cache * previously contained value associated with the key, the old value is replaced by the * specified value. * * @return the previous value rows associated with key, or null if there was no mapping for key. * @param key - key row with which the specified value is to be associated * @param value – value rows to be associated with the specified key */ Collection<RowData> put(RowData key, Collection<RowData> value); /** Discards any cached value for the specified key. */ void invalidate(RowData key); /** Returns the number of key-value mappings in the cache. */ long size(); }
LookupCacheFactory
As the cache should be instantiated during runtime execution to avoid serialization / deserialization, a factory is required for creating the cache.
/** Factory for creating an instance of {@link LookupCache}. */ @PublicEvolving public interface LookupCacheFactory extends Serializable { /** * Create a {@link LookupCache}. * * @param metricGroup - The lookup cache metric group in which the cache register predefined and * custom metrics. */ LookupCache createCache(LookupCacheMetricGroup metricGroup); }
DefaultLookupCacheFactory
In order to simplify the usage of developer, we provide a default factory for building a default cache.
/** Factory for creating instance of {@link DefaultLookupCache}. */ @PublicEvolving public class DefaultLookupCacheFactory implements LookupCacheFactory { private final Duration expireAfterAccessDuration; private final Duration expireAfterWriteDuration; private final Integer initialCapacity; private final Long maximumSize; public static DefaultLookupCacheFactory.Builder newBuilder() { return new Builder(); } private DefaultLookupCacheFactory( Duration expireAfterAccessDuration, Duration expireAfterWriteDuration, Long maximumSize) { this.expireAfterAccessDuration = expireAfterAccessDuration; this.expireAfterWriteDuration = expireAfterWriteDuration; this.initialCapacity = initialCapacity; this.maximumSize = maximumSize; } @Override public LookupCache createCache(LookupCacheMetricGroup metricGroup) { // Create instance of DefaultLookupCache } /** Builder of {@link DefaultLookupCacheFactory}. */ public static class Builder { private Duration expireAfterAccessDuration; private Duration expireAfterWriteDuration; private Integer initialCapacity; private Long maximumSize; public DefaultLookupCacheFactory.Builder expireAfterAccess(Duration duration) { expireAfterAccessDuration = duration; return this; } public DefaultLookupCacheFactory.Builder expireAfterWrite(Duration duration) { expireAfterWriteDuration = duration; return this; } public DefaultLookupCacheFactory.Builder maximumSize(long maximumSize) { this.maximumSize = maximumSize; return this; } public DefaultLookupCacheFactory build() { return new DefaultLookupCacheFactory( expireAfterAccessDuration, expireAfterWriteDuration, maximumSize); } } }
LookupCacheMetricGroup
An interface defining all cache related metric:
/** * Pre-defined metrics for {@code LookupCache}. * * <p>Please note that these methods should only be invoked once. Registering a metric with same * name for multiple times would lead to an undefined behavior. */ @PublicEvolving public interface LookupCacheMetricGroup extends MetricGroup { /** The number of cache hits. */ void hitCounter(Counter hitCounter); /** The number of cache misses. */ void missCounter(Counter missCounter); /** The number of times to load data into cache from external system. */ void loadCounter(Counter loadCounter); /** The number of load failures. */ void numLoadFailuresCounter(Counter numLoadFailuresCounter); /** The time spent for the latest load operation. */ void latestLoadTimeGauge(Gauge<Long> latestLoadTimeGauge); /** The number of records in cache. */ void numCachedRecordsGauge(Gauge<Long> numCachedRecordsGauge); /** The number of bytes used by cache. */ void numCachedBytesGauge(Gauge<Long> numCachedBytesGauge); }
LookupFunctionProvider
This is the API between table framework and user's table source. Implementation should define how to create a lookup function and whether to use cache.
/** * Provider for creating {@link LookupFunction} and {@link LookupCacheFactory} if caching should be * enabled for the lookup table. */ @PublicEvolving public interface LookupFunctionProvider extends LookupTableSource.LookupRuntimeProvider { /** Creates a builder of {@link LookupFunctionProvider}. */ static Builder newBuilder() { return new Builder(); } /** Creates an {@link LookupFunction} instance. */ LookupFunction createLookupFunction(); /** * Gets the {@link LookupCacheFactory} for creating lookup cache. * * <p>This factory will be used for creating an instance of cache during runtime execution for * optimizing the access to external lookup table. * * @return an {@link Optional} of {@link LookupCacheFactory}, or an empty {@link Optional} if * caching shouldn't be applies to the lookup table. */ Optional<LookupCacheFactory> getCacheFactory(); /** * Whether the missing key (key fields without any matching value rows) should be stored in the * cache. * * <p>Please note that this option is required if {@link #getCacheFactory()} returns a non-empty * instance. If the cache factory is empty, the return value of this function will be ignored. * * @return true if a null or empty value should be stored in the cache. */ Optional<Boolean> cacheMissingKey(); /** Builder class for {@link LookupFunctionProvider}. */ class Builder { private LookupFunction lookupFunction; private LookupCacheFactory cacheFactory; private Boolean enableCacheMissingKey; /** Sets lookup function. */ public Builder withLookupFunction(LookupFunction lookupFunction) { this.lookupFunction = lookupFunction; return this; } /** Enables caching and sets the cache factory. */ public Builder withCacheFactory(LookupCacheFactory cacheFactory) { this.cacheFactory = cacheFactory; return this; } /** * Enables storing missing key in the cache. * * <p>See {@link LookupFunctionProvider#cacheMissingKey()} for more details. */ public Builder enableCacheMissingKey() { this.enableCacheMissingKey = true; return this; } public LookupFunctionProvider build() { // Build LookupFunctionProvider } } }
AsyncLookupFunctionProvider
@PublicEvolving public interface AsyncLookupFunctionProvider extends LookupTableSource.LookupRuntimeProvider { /** Creates a builder of {@link LookupFunctionProvider}. */ static Builder newBuilder() { return new Builder(); } /** Creates an {@link AsyncLookupFunction} instance. */ AsyncLookupFunction createAsyncLookupFunction(); /** * Gets the {@link LookupCacheFactory} for creating lookup cache. * * <p>This factory will be used for creating an instance of cache during runtime execution for * optimizing the access to external lookup table. * * @return an {@link Optional} of {@link LookupCacheFactory}, or an empty {@link Optional} if * caching shouldn't be applies to the lookup table. */ Optional<LookupCacheFactory> getCacheFactory(); /** * Whether the missing key (key fields without any matching value rows) should be stored in the * cache. * * <p>Please note that this option is required if {@link #getCacheFactory()} returns a non-empty * instance. If the cache factory is empty, the return value of this function will be ignored. * * @return true if a null or empty value should be stored in the cache. */ Optional<Boolean> cacheMissingKey(); /** Builder class for {@link AsyncLookupFunctionProvider}. */ class Builder { private AsyncLookupFunction asyncLookupFunction; private LookupCacheFactory cacheFactory; private Boolean enableCacheMissingKey; /** Sets lookup function. */ public Builder withAsyncLookupFunction(AsyncLookupFunction asyncLookupFunction) { this.asyncLookupFunction = asyncLookupFunction; return this; } /** Enables caching and sets the cache factory. */ public Builder withCacheFactory(LookupCacheFactory cacheFactory) { this.cacheFactory = cacheFactory; return this; } /** * Enables storing missing key in the cache. * * <p>See {@link AsyncLookupFunctionProvider#cacheMissingKey()} for more details. */ public Builder enableCacheMissingKey() { this.enableCacheMissingKey = true; return this; } public AsyncLookupFunctionProvider build() { // Build AsyncLookupFunctionProvider } } }
FullCachingLookupProvider
This interface is for supporting full cache strategy. It reuses ScanRuntimeProvider and defines interval of reload.
/** * Runtime provider for fully loading and periodically reloading all entries of the lookup table and * storing the table locally for lookup. * * <p>Implementations should provide a {@link ScanTableSource.ScanRuntimeProvider} in order to reuse * the ability of scanning for loading all entries from the lookup table. */ @PublicEvolving public interface FullCachingLookupProvider extends LookupTableSource.LookupRuntimeProvider { /** * Creates {@link FullCachingLookupProvider} with provided scan runtime provider and rescan * interval. */ static FullCachingLookupProvider of( ScanTableSource.ScanRuntimeProvider scanRuntimeProvider, Duration reloadInterval) { return new FullCachingLookupProvider() { @Override public ScanTableSource.ScanRuntimeProvider getScanRuntimeProvider() { return scanRuntimeProvider; } @Override public Duration getReloadInterval() { return reloadInterval; } }; } /** * Gets the {@link org.apache.flink.table.connector.source.ScanTableSource.ScanRuntimeProvider} * for executing the periodically reload. */ ScanTableSource.ScanRuntimeProvider getScanRuntimeProvider(); /** Gets the interval between two reload operations. */ Duration getReloadInterval(); }
TableFunctionProvider / AsyncTableFunctionProvider
We'd like to deprecate these two interfaces and let developers to switch to the new LookupFunctionProvider / AsyncLookupFunctionProvider / FullCachingLookupProvider instead.
Table Options for Default Lookup Cache
In order to unify the usage of DefaultLookupCache across all connectors, we'd like to introduce some common table options. Note that these options are not required to implement by all connectors.
Option | Type | Descriptions |
---|---|---|
lookup.cache | Enum of NONE, PARTIAL and FULL | The caching strategy for this lookup table. NONE: Do not use cache Partial: Use partial caching mode FULL: Use full caching mode |
lookup.async | Boolean | Whether to use asynchronous mode for the lookup table |
lookup.max-retries | Integer | The maximum allowed retries if a lookup operation fails |
lookup.partial-cache.expire-after-access | Duration | Duration to expire an entry in the cache after accessing |
lookup.partial-cache.expire-after-write | Duration | Duration to expire an entry in the cache after writing |
lookup.partial-cache.cache-missing-key | Boolean | Whether to store an empty value into the cache if the lookup key doesn't match any rows in the table |
lookup.partial-cache.max-rows | Long | The maximum number of rows to store in the cache |
lookup.full-cache.reload-interval | Duration | Interval of reloading all entries from the lookup table into cache |
Cache Metrics
It is important to mention that a cache implementation does not have to report all the defined metrics. But if a cache reports a metric of the same semantic defined below, the implementation should follow the convention.
Name | Type | Unit | Description |
---|---|---|---|
numCachedRecord | Gauge | Records | The number of records in cache. |
numCachedBytes | Gauge | Bytes | The number of bytes used by cache. |
hitCount | Counter | The number of cache hits | |
missCount | Counter | The number of cache misses, which might leads to loading operations | |
loadCount | Counter | The number of times to load data into cache from external system. For partial cache the load count should be equal to miss count, but for all cache this would be different. | |
numLoadFailure | Counter | The number of load failures | |
latestLoadTime | Gauge | ms | The time spent for the latest load operation |
Here we just define fundamental metrics and let the external metric system make the aggregation to get more descriptive values such as hitRate = hitCount / (hitCount + missCount).
Scope
The metric group for the cache would be a sub-group of the OperatorMetricGroup where the table function belongs to.
Future Works
In order to reduce network I/O with external systems and the usage of cache further, some optimizations implemented on scan source could be also applied on the lookup table, such as projection and filter pushdown. These features will be introduced separately in another FLIP.
Compatibility, Deprecation, and Migration Plan
Currently we have JDBC, Hive and HBase connector implemented lookup table source. All existing implementations will be migrated to the current design and the migration will be transparent to end users.
Test Plan
We will use unit and integration test for validating the functionality of cache implementations.
Rejected Alternatives
Add cache in TableFunction implementations
Compared with this design, adding cache in TableFunction implementations might lead to inconsistency between sync and async table function, and not suitable for applying optimizations.