Samza Table API

Introduction

Samza Table API is an abstraction for data sources that support random access by key, which simplifies stream-table-join. It is the natural evolution of the existing storage API, and it offers support for both local and remote data sources and composition through hybrid tables.

For various reasons, a real-time stream often only contain minimal or a small amount of information, and may need to be augmented with richer information fetched from adjunct data sources through joining. This is quite common in advertising, relevance ranking, fraud detection, and other domains. However, there exists a wide variety of data stores with different characteristics and levels of sophistication. Samza Table API simplifies the application developer experience by hiding the details of individual technologies, while making it easier to migrate from one technology to another.

For remote data sources, the Samza remote table provides optimized access such as caching, rate-limiting, retry and batching (future) support.

In addition, more advanced functionalities can be provided through table composition. For example, bootstrapping a stream is often used to build an authoritative local cache, and today stream processing has to wait until bootstrap is completed. A hybrid table can provide access to remote data source while the local cache is being built, so that stream processing could begin earlier.

Application developers can now take advantage of the aforementioned benefits, which are all encapsulated under the Samza Table API.

Sample Applications

Sample applications demonstrating how to use Samza Table API can be found here.

Architecture and Concepts

The diagram below illustrates the overall architecture of Samza Table API.

Let’s look at a few concepts before diving into the API.

Table - This interface represents a dataset that can be accessed by key. We support two types of tables: read-only and read-write. A table can be accessed either synchronously or asynchronously and a request may contain one or more keys. There are three broad categories of tables: local, remote and hybrid.

ReadableTable - Interface that represents a read-only table. It implements Table.

ReadWriteTable - Interface that represents a read-write table. It implements Table.

TableDescriptor - User-facing object that contains metadata that completely describes a table. It may include identifier, serialization, provider, configuration, etc. Example implementations of this interface are

• RemoteTableDescriptor facilitates access to remotely stored data,
• InMemoryTableDescriptor describes a table stored in-memory, and
• RocksDbTableDescriptor describes a table stored in RocksDB.

Table Sync and Async API

Samza Table supports both synchronous and asynchronous API. Below is an example for the get operation.

 /**
   * Gets the value associated with the specified {@code key}.
   *
   * @param key the key with which the associated value is to be fetched.
   * @return if found, the value associated with the specified {@code key}; 
   * otherwise, {@code null}.
   * @throws NullPointerException if the specified {@code key} is {@code null}.
   */
  V get(K key);

 /**
   * Asynchronously gets the value associated with the specified {@code key}.
   *
   * @param key the key with which the associated value is to be fetched.
   * @return completableFuture for the requested value
   * @throws NullPointerException if the specified {@code key} is {@code null}.
   */
  CompletableFuture<V> getAsync(K key);

Using Table with Samza High Level API

The code snippet below illustrates the usage of table in Samza high level API.

 1  class SamzaStreamApplication implements StreamApplication {
 2    @Override
 3    public void describe(StreamApplicationDescriptor appDesc) {
 4      TableDescriptor<Integer, Profile> desc = new InMemoryTableDescriptor(
 5          "t1", KVSerde.of(new IntegerSerde(), new ProfileJsonSerde()));
 6 
 7      Table<KV<Integer, Profile>> table = appDesc.getTable(desc);
 8 
 9      appDesc.getInputStream("PageView", new NoOpSerde<PageView>())
10          .map(new MyMapFunc())
11          .join(table, new MyJoinFunc())
12          .sendTo(anotherTable);
13    }
14  }
15
16  static class MyMapFunc implements MapFunction<PageView, KV<Integer, PageView>> {
17    private ReadableTable<Integer, Profile> profileTable;
18
19    @Override
20    public void init(Config config, TaskContext context) {
21      profileTable = (ReadableTable<Integer, Profile>) context.getTable("t1");
22    }
23 
24    @Override
25    public KV<Integer, PageView> apply(PageView message) {
26      return new KV.of(message.getId(), message);
27    }
28  }
29
30  static class MyJoinFunc implements StreamTableJoinFunction
31      <Integer, KV<Integer, PageView>, KV<Integer, Profile>, EnrichedPageView> {
32
33    @Override
34    public EnrichedPageView apply(KV<Integer, PageView> m, KV<Integer, Profile> r) {
35      counterPerJoinFn.get(this.currentSeqNo).incrementAndGet();
36        return r == null ? null : new EnrichedPageView(
37            m.getValue().getPageKey(), m.getKey(), r.getValue().getCompany());
38    }
39  }

In the code snippet above, we read from an input stream, perform transformation, join with a table and finally write the output to another table.

1. Line 4-5: A TableDescriptor for an in-memory table is created, and then the serde is set.
2. Line 7: A table object is created from the TableDescriptor; internally, the TableDescriptor is converted to a TableSpec, and registered with the TaskApplicationDescriptor. The table object has a reference to the TableSpec.
3. Line 9: creates an InputStream
4. Line 10: applies a map operator defined in line 16-28, all table instances can be accessed from the task context in Task.init(). In this example, it is stored in a local variable.
5. Line 11: joins the mapped stream with the table using the supplied join function defined in lines 30-39.
6. Line 12: writes the join result stream to another table

Using Table with Samza Low Level API

The code snippet below illustrates the usage of table in Samza Low Level Task API.

 1  class SamzaTaskApplication implements TaskApplication {
 2  
 3  @Override
 4  public void describe(TaskApplicationDescriptor appDesc) {
 5    DelegatingSystemDescriptor ksd = new DelegatingSystemDescriptor("mySystem");
 6 
 7    TableDescriptor<Integer, Profile> tableDesc = new InMemoryTableDescriptor(
 8        "t1", KVSerde.of(new IntegerSerde(), new ProfileJsonSerde()));
 9    GenericInputDescriptor<Profile> profileISD = ksd.getInputDescriptor("Profile", new NoOpSerde<>());
10 
11   appDesc.addTable(tableDesc); 
12   appDesc.addInputStream(profileISD);  
13 }
14
15
16  public class MyStreamTask implements StreamTask, InitableTask {
17    private ReadWriteTable<Integer, Profile> profileTable;
18 
19    @Override
20    public void init(Config config, TaskContext context) {
21      profileTable = (ReadWriteTable<Integer, Profile>) context.getTable("t1");
22    }
23 
24    @Override
25    public void process(IncomingMessageEnvelope envelope, MessageCollector collector, TaskCoordinator coordinator) {
26      String key = (String)message.getKey();
27      Profile profile = (Profile)message.getMessage();
28      profileTable.put(key, profile);
29    }
30  }

In the code snippet above, we read from an input stream, perform transformation, join with a table and finally write the output to another table.

1. Line 7-8: A TableDescriptor for an in-memory table is created with tableId “t1”, and then the serde is set.
2. Line 9: creates an InputStreamDescriptor.
3. Line 11-12: adds TableDescriptor and InputStreamDescriptor to the TaskApplicationDescriptor.
4. Line 16: InitiableStreamTask is implemented.
5. Line 21: A reference to table “t1” is obtained in the Task.init() method.
6. Line 26-28: gets the profile key and record from the incoming stream and writes to the table.

ReadableTable or ReadWriteTable can be used in the StreamTask.process() method on the table reference obtained in the InitableTask.init() method.

Table Metrics

The table below summarizes table metrics:

Table Types

Remote Table

RemoteTable provides a unified abstraction for Samza applications to access any remote data store through stream-table join in High Level Streams API or direct access in Low Level Task API. Remote Table is a store-agnostic abstraction that can be customized to access new types of stores by writing pluggable I/O “Read/Write” functions, implementations of TableReadFunction and TableWriteFunction interfaces. Remote Table also provides common functionality, eg. rate limiting (built-in) and caching (hybrid).

The async APIs in Remote Table are recommended over the sync versions for higher throughput. They can be used with Samza with Low Level Task API to achieve the maximum throughput.

Remote Tables are represented by class RemoteReadableTable and RemoteReadWriteTable. All configuration options of a Remote Table can be found in the RemoteTableDescriptor class.

Rate Limiting

Remote Table has built-in client-side rate limiting support in both of its sync and async executions. This is useful when a remote data store does not have server-side rate limiting or is not sophisticated enough to handle heavy inbound requests.

Configuration

Rate limiting can be enabled with RemoteTableDescriptor in two ways:

1. Default: withReadRateLimit() and withWriteRateLimit(). The default implementation uses Guava for rate limiting and provides basic throttling on read/write rates.
2. User-defined instance: withRateLimiter(). Tailored for more advanced usages, eg. custom policies and/or rate limiter libraries

Quota

For the default rate limiter, a per-container quota needs to be specified, and is divided evenly among all task instances. Application developers are expected to calculate such quota from a global quota against the number of containers.

Retry

I/O failures are not uncommon given the inherently unreliable network and complex behaviors of distributed data stores. To be fault-tolerant, Remote Table provides built-in support for retrying failed I/O operations originated from the data store clients.

Retry capability can be added to a RemoteTableDescriptor by providing a TableRetryPolicy, which consists of three aspects:

• Backoff/Sleep policy - fixed, random, exponential with jitters
• Termination policy - by total attempts and/or delay
• Retriable exception classification - predicate on Throwable

By default, retry is disabled as such failed I/O operations will propagate up and the caller is expected to handle the exception. When enabled, retry is on a per-request basis such that each individual request is retried independently.

Lastly, Remote Table retry provides a set of standard metrics for monitoring. They can be found in RetryMetrics.

Local Table

A table is considered local when its data physically co-exists on the same host machine as its running job, e.g. in memory or on disk. Local tables are particularly useful when data needs to be accessed frequently with low latency, such as a cache. Samza Table API supports in-memory and RocksDB-based local tables, which are based on the current implementation of in-memory and RocksDB stores. Both tables provide feature parity to existing in-memory and RocksDB-based stores. For more detailed information please refer to RocksDbTableDescriptor and InMemoryTableDescriptor.

Hybrid Table

Hybrid Table consists of one or more tables, and it orchestrates operations between them to achieve more advanced functionality. Caching support for remote table is currently built on top of hybrid Table because cache can be naturally abstracted as a table, eg. local table is also a durable cache.

Caching

Despite the convenience of remote table, it still incurs the same latency as accessing the remote store directly. Whenever eventual consistency is acceptable, Samza applications can leverage the caching support in Table API to reduce such latency in addition to using the async methods.

CachingTable is the generic table type for combining a cache table (Guava, RocksDb, Couchbase) with a remote table. Both the cache and data tables are pluggable, and CachingTable handles the interactions between them for caching semantics.

Write Policies

Caching Table supports below write policies and you can configure them with CachingTableDescriptor.

• Write-through: records are written to both the data store and cache
• Write-around: records are written only to data store bypassing the cache
  • Useful when read-path has no locality with read-path

Synchronization

No synchronization is done between data store and cache in CachingTable because it is very cumbersome in the async code paths. Given eventual consistency is a presumed trade-off for enabling caching, it should be acceptable for the table and cache to not always be in-sync. Last but not least, unsynchronized operations in CachingTable deliver much higher throughput.

Configuration

Similar to RateLimiter configuration in remote table, caching can be configured in two ways:

1. Default: withCacheSize(), withReadTtl(), withWriteTtl()
2. Custom CacheTable instance: withCache()

The default CacheTable is an in-memory cache implemented on top of Guava Cache.

Implementing Your Own Tables

More Concepts

TableSpec - Internal representation of a table, containing all information about a table.

TableProvider - Provides the underlying table implementation that conforms to Table API.

TableManager - Responsible for lifecycle of all table instances within a task; it is created during instantiation of Samza container.

Lifecycle of a Table

The life of a table goes through a few phases

1. Declaration - at first one declares the table by creating a TableDescriptor. In both Samza High Level Streams API and Low Level Task API, the TableDescriptor is registered with stream graph, internally converted to TableSpec and in return a reference to a Table object is obtained that can participate in the building of the DAG.
2. Instantiation - during planning stage, configuration is generated from TableSpec, and the actual tables are instantiated during initialization of Samza container.
3. Usage - there are a few ways to access a table instance
   • In Samza high level API, a reference to Table can be obtained from a TableDescriptor, which can be used to participate in the DAG operations such as join() and sendTo()
   • In Samza high level API, all table instances can be retrieved from TaskContext using table-id during initialization of a InitableFunction.
   • In Samza Low Level Task API, all table instances can be retrieved from TaskContext using table-id during initialization of a InitableTask.
4. Cleanup - close() is invoked on all tables when a job is stopped.

Developing a Local Table

Developing a local table involves implementing a new table descriptor, provider and provider factory.

1. TableDescriptor - this is the user facing object that contains primarily configuration parameters. In addition, a few internal methods need to be implemented
   • generateTableSpecConfig() should convert parameters in the table descriptor to a Map<String, String>, so that information about a table can be transferred to a TableSpec.
   • getTableSpec() creates a TableSpec object, which is the internal representation of a table.
2. TableProvider - provides the implementation for a table. It ensures a table is properly constructed and also manages its lifecycle. Methods to implement are
   • generateConfig() generates all Samza configuration relevant to the table. Note in the case of RocksDB, store configuration is also generated here.

Note:

• Since the current local table implementation relies on Samza store implementation, adding a local table requires providing the store implementation first.

Developing a Remote Table

The generic design of remote table abstracts away mach common functionality. Therefore, adding a new remote table type is much more straightforward than writing a Samza table from scratch. You only need to implement the TableReadFunction and TableWriteFunction (if supported) interfaces, and the new table type is readily usable with the Remote Table framework.

Since the interfaces have sufficient javadocs and are self-explanatory, we present a high-level guideline to call out some less obvious aspects and help ensure consistency among future I/O (Read/Write) function implementations in the following sections.

Serializability

Samza Remote Table I/O function interfaces extend java.io.Serializable, which imposes a serializability expectation on their implementations, i.e.

1. All fields within I/O function classes must be serializable.
2. Non-serializable fields must be marked transient. Otherwise, serialization of I/O function will fail. Typical examples of such fields are store client objects.
3. Consequently, non-serializable fields must be initialized within the implementation of the init() method of the InitableFunction interface extended by both TableReadFunction and TableWriteFunction interfaces. Otherwise, non-serializable fields will be null after I/O function is deserialized.

class ReadFunction implements TableReadFunction {
  @Override
  public void init(Config config, TaskContext context) {
    /* Initialize all transient fields here. */
  }
}
 
class WriteFunction implements TableWriteFunction {
  @Override
  public void init(Config config, TaskContext context) {
    /* Initialize all transient fields here. */
  }
}

Logging

Our recommendation is to:

1. Avoid doing any informational logging in I/O functions that are typically invoked extensively in Samza applications, e.g.
   • TableReadFunction.get[All]()
   • TableWriteFunction.put[All]()
   • TableWriteFunction.delete[All]()
2. Log all initialization-related successes/failures occurring in overrides of InitableFunction.init() to improve diagnosability.

Handling Client Exceptions and Retrying Failed Requests

Implementations of I/O functions for remote stores are likely to utilize a client object for communicating with their corresponding store endpoints. In this setup, it is possible for an I/O function to run into situations where the client it uses throws, e.g. in response to networking or logical errors.

We recommend:

1. Catching all such errors, wrapping and throwing them in a SamzaException.
2. Attempting no retry in the face of client errors or failed requests. The intent of the current design of Samza Remote Table API is to handle retries at a higher and more abstract Remote Table level, which implies retrying is not a responsibility of I/O functions.

Caching

Samza Remote Table API can be configured to utilize user-supplied caches. You may refer to the Caching section under Hybrid Table for more details.

Rate Limiting

Samza Remote Table API offers generic rate limiting capabilities that can be used with all I/O function implementations. You may refer to the Rate Limiting section under Remote Table for more details.

Separate vs Combined Read/Write Implementations

It is up to the developer whether to implement both TableReadFunction and TableWriteFunction in one class or two separate classes. Defining them in separate classes can be cleaner if their implementations are elaborate and extended, whereas keeping them in a single class may be more practical if they share a considerable amount of code or are relatively short.