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数据平台系列:In-Stream Big Data Processing

英文原文:In-Stream-big-data-processing,有人翻译了中文版,也有直接中文注释英文版

The shortcomings and drawbacks of batch-oriented data processing were widely recognized by the Big Data community quite a long time ago. It became clear that real-time query processing and in-stream processing is the immediate need in many practical applications. In recent years, this idea got a lot of traction and a whole bunch of solutions like Twitter’s Storm, Yahoo’s S4, Cloudera’s Impala, Apache Spark, and Apache Tez appeared and joined the army of Big Data and NoSQL systems. This article is an effort to explore techniques used by developers of in-stream data processing systems, trace the connections of these techniques to massive batch processing and OLTP/OLAP databases, and discuss how one unified query engine can support in-stream, batch, and OLAP processing at the same time.

At Grid Dynamics, we recently faced a necessity to build an in-stream data processing system that aimed to crunch about 8 billion events daily providing fault-tolerance and strict transactioanlity i.e. none of these events can be lost or duplicated. This system has been designed to supplement and succeed the existing Hadoop-based system that had too high latency of data processing and too high maintenance costs. The requirements and the system itself were so generic and typical that we describe it below as a canonical model, just like an abstract problem statement.

A high-level overview of the environment we worked with is shown in the figure below:

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One can see that this environment is a typical Big Data installation: there is a set of applications that produce the raw data in multiple datacenters, the data is shipped by means of Data Collection subsystem to HDFS located in the central facility, then the raw data is aggregated and analyzed using the standard Hadoop stack (MapReduce, Pig, Hive) and the aggregated results are stored in HDFS and NoSQL, imported to the OLAP database and accessed by custom user applications. Our goal was to equip all facilities with a new in-stream engine (shown in the bottom of the figure) that processes most intensive data flows and ships the pre-aggregated data to the central facility, thus decreasing the amount of raw data and heavy batch jobs in Hadoop. The design of the in-stream processing engine itself was driven by the following requirements:

To find out how such a system can be implemented, we discuss the following topics in the rest of the article:

First, we explore relations between in-stream data processing systems, massive batch processing systems, and relational query engines to understand how in-stream processing can leverage a huge number of techniques that were devised for other classes of systems.

Second, we describe a number of patterns and techniques that are frequently used in building of in-stream processing frameworks and systems. In addition, we survey the current and emerging technologies and provide a few implementation tips.

The article is based on a research project developed at Grid Dynamics Labs. Much of the credit goes to Alexey Kharlamov and Rafael Bagmanov who led the project and other contributors: Dmitry Suslov, Konstantine Golikov, Evelina Stepanova, Anatoly Vinogradov, Roman Belous, and Varvara Strizhkova.

Basics of Distributed Query Processing

It is clear that distributed in-stream data processing has something to do with query processing in distributed relational databases. Many standard query processing techniques can be employed by in-stream processing engine, so it is extremely useful to understand classical algorithms of distributed query processing and see how it all relates to in-stream processing and other popular paradigms like MapReduce.

Distributed query processing is a very large area of knowledge that was under development for decades, so we start with a brief overview of the main techniques just to provide a context for further discussion.

Partitioning and Shuffling

Distributed and parallel query processing heavily relies on data partitioning to break down a large data set into multiple pieces that can be processed by independent processors. Query processing could consist of multiple steps and each step could require its own partitioning strategy, so data shuffling is an operation frequently performed by distributed databases.

Although optimal partitioning for selection and projection operations can be tricky (e.g. for range queries), we can assume that for in-stream data filtering it is practically enough to distribute data among the processors using a hash-based partitioning.

Processing of distributed joins is not so easy and requires a more thorough examination. In distributed environments, parallelism of join processing is achieved through data partitioning, i.e. the data is distributed among processors and each processor employs a serial join algorithm (e.g. nested-loop join or sort-merge join or hash-based join) to process its part of the data. The final results are consolidated from the results obtained from different processors.

There are two main data partitioning techniques that can be employed by distributed join processing:

Disjoint data partitioning technique shuffles the data into several partitions in such a way that join keys in different partitions do not overlap. Each processor performs the join operation on each of these partitions and the final result is obtained as a simple concatenation of the results obtained from different processors. Consider an example where relation R is joined with relation S on a numerical key k and a simple modulo-based hash function is used to produce the partitions (it is assumes that the data initially distributed among the processors based on some other policy):

The divide and broadcast join algorithm is illustrated in the figure below. This method divides the first data set into multiple disjoint partitions (R1, R2, and R3 in the figure) and replicates the second data set to all processors. In a distributed database, division typically is not a part of the query processing itself because data sets are initially distributed among multiple nodes.

This strategy is applicable for joining of a large relation with a small relation or two small relations. In-stream data processing systems can employ this technique for stream enrichment i.e. joining a static data (admixture) to a data stream.

Processing of GroupBy queries also relies on shuffling and fundamentally similar to the MapReduce paradigm in its pure form. Consider an example where the data is grouped by a string key and sum of the numerical values is computed in each group:

In this example, computation consists of two steps: local aggregation and global aggregation. These steps basically correspond to Map and Reduce operations. Local aggregation is optional and raw records can be emitted, shuffled, and aggregated on a global aggregation phase.

The whole point of this section is that all the algorithms above can be naturally implemented using a message passing architectural style i.e. the query execution engine can be considered as a distributed network of nodes connected by the messaging queues. It is conceptually similar to the in-stream processing pipelines.

Pipelining

In the previous section, we noted that many distributed query processing algorithms resemble message passing networks. However, it is not enough to organize efficient in-stream processing: all operators in a query should be chained in such a way that the data flows smoothly through the entire pipeline i.e. neither operation should block processing by waiting for a large piece of input data without producing any output or by writing intermediate results on disk. Some operations like sorting are inherently incompatible with this concept (obviously, a sorting block cannot produce any output until the entire input is ingested), but in many cases pipelining algorithms are applicable. A typical example of pipelining is shown below:

In this example, the hash join algorithm is employed to join four relations: R1, S1, S2, and S3 using 3 processors. The idea is to build hash tables for S1, S2 and S3 in parallel and then stream R1 tuples one by one though the pipeline that joins them with S1, S2 and S3 by looking up matches in the hash tables. In-stream processing naturally employs this technique to join a data stream with the static data (admixtures).

In relational databases, join operation can take advantage of pipelining by using the symmetric hash join algorithm or some of its advanced variants [1,2]. Symmetric hash join is a generalization of hash join. Whereas a normal hash join requires at least one of its inputs to be completely available to produce first results (the input is needed to build a hash table), symmetric hash join is able to produce first results immediately. In contrast to the normal hash join, it maintains hash tables for both inputs and populates these tables as tuples arrive:

As a tuple comes in, the joiner first looks it up in the hash table of the other stream. If match is found, an output tuple is produced. Then the tuple is inserted in its own hash table.

However, it does not make a lot of sense to perform a complete join of infinite streams. In many cases join is performed on a finite time window or other type of buffer e.g. LFU cache that contains most frequent tuples in the stream. Symmetric hash join can be employed if the buffer is large comparing to the stream rate or buffer is flushed frequently according to some application logic or buffer eviction strategy is not predictable. In other cases, simple hash join is often sufficient since the buffer is constantly full and does not block the processing:

It is worth noting that in-stream processing often deals with sophisticated stream correlation algorithms where records are matched based on scoring metrics, not on field equality condition. A more complex system of buffers can be required for both streams in such cases.

In-Stream Processing Patterns

In the previous section, we discussed a number of standard query processing techniques that can be used in massively parallel stream processing. Thus, on a conceptual level, an efficient query engine in a distributed database can act as a stream processing system and vice versa, a stream processing system can act as a distributed database query engine. Shuffling and pipelining are the key techniques of distributed query processing and message passing networks can naturally implement them. However, things are not so simple. In a contrast to database query engines where reliability is not critical because a read-only query can always be restarted, streaming systems should pay a lot of attention to reliable events processing. In this section, we discuss a number of techniques that are used by streaming systems to provide message delivery guarantees and some other patterns that are not typical for standard query processing.

Stream Replay

Ability to rewind data stream back in time and replay the data is very important for in-stream processing systems because of the following reasons: This is the only way to guarantee correct data processing. Even if data processing pipeline is fault-tolerant, it is very problematic to guarantee that the deployed processing logic is defect-free. One can always face a necessity to fix and redeploy the system and replay the data on a new version of the pipeline.

Issue investigation could require ad hoc queries. If something goes wrong, one could need to rerun the system on the problematic data with better logging or with code alternations.

Although it is not always the case, the in-stream processing system can be designed in such a way that it re-reads individual messages from the source in case of processing errors and local failures, even if the system in general is fault-tolerant.

As a result, the input data typically goes from the data source to the in-stream pipeline via a persistent buffer that allows clients to move their reading pointers back and forth.

Kafka messaging queue is well known implementation of such a buffer that also supports scalable distributed deployments, fault-tolerance, and provides high performance. As a bottom line, Stream Replay technique imposes the following requirements of the system design:

Lineage Tracking

In a streaming system, events flow through a chain of processors until the result reaches the final destination (like an external database). Each input event produces a directed graph of descendant events (lineage) that ends by the final results. To guarantee reliable data processing, it is necessary to ensure that the entire graph was processed successfully and to restart processing in case of failures.

Efficient lineage tracking is not a trivial problem. Let us first consider how Twitter’s Storm tracks the messages to guarantee at-least-once delivery semantics (see the diagram below):

The described approach is elegant due to its decentrilized nature: nodes act independently sending acknowledgement messages, there is no cental entity that tracks all lineages explicitly. However, it could be difficult to manage transactional processing in this way for flows that maintain sliding windows or other buffers. For example, processing on a sliding window can involve hundreds of thousands events at each moment of time, so it becomes difficult to manage acknowledgements because many events stay uncommitted or computational state should be persisted frequently.

An alternative approach is used in Apache Spark [3]. The idea is to consider the final result as a function of the incoming data. To simplify lineage tracking, the framework processes events in batches, so the result is a sequence of batches where each batch is a function of the input batches. Resulting batches can be computed in parallel and if some computation fails, the framework simply reruns it. Consider an example:

In this example, the framework joins two streams on a sliding window and then the result passes through one more processing stage. The framework considers the incoming streams not as streams, but as set of batches. Each batch has an ID and the framework can fetch it by the ID at any moment of time. So, stream processing can be represented as a bunch of transactions where each transaction takes a group of input batches, transforms them using a processing function, and persists a result. In the figure above, one of such transactions is highlighted in red. If the transaction fails, the framework simply reruns it. It is important that transactions can be executed in parallel.

This simple but powerful paradigm enables centralized transaction management and inherently provides exactly-once message processing semantics. It is worth noting that this technique can be used both for batch processing and for stream processing because it treats the input data as a set of batches regardless to their streaming of static nature.

State Checkpointing

In the previous section we have considered the lineage tracking algorithm that uses signatures (checksums) to provide at-least-one message delivery semantics. This technique improves reliability of the system, but it leaves at least two major open questions:

Twitter’s Storm addresses these issues by using the following protocol:

This technique allows one to achieve exactly-once processing semantics assuming that data sources are fault-tolerant and can be replayed. However, persistent state updates can cause serious performance degradation even if large batches are used. By this reason, the intermediate computational state should be minimized or avoided whenever possible.

As a footnote, it is worth mentioning that state writing can be implemented in different ways. The most straightforward approach is to dump in-memory state to the persistent store as part of the transaction commit process. This does not work well for large states (sliding windows an so on). An alternative is to write a kind of transaction log i.e. a sequence of operations that transform the old state into the new one (for a sliding window it can be a set of added and evicted events). This approach complicates crash recovery because the state has to be reconstructed from the log, but can provide performance benefits in a variety of cases.

Additive State and Sketches

Additivity of intermediate and final computational results is an important property that drastically simplifies design, implementation, maintenance, and recovery of in-stream data processing systems. Additivity means that the computational result for a larger time range or a larger data partition can be calculated as a combination of results for smaller time ranges or smaller partitions. For example, a daily number of page views can be calculated as a sum of hourly numbers of page views. Additive state allows one to split processing of a stream into processing of batches that can be computed and re-computed independently and, as we discussed in the previous sections, this helps to simplify lineage tracking and reduce complexity of state maintenance.

It is not always trivial to achieve additivity:

Sketches is a very efficient way to transform non-additive values into additive. In the previous example, lists of ID can be replaced by compact additive statistical counters. These counters provide approximations instead of precise result, but it is acceptable for many practical applications. Sketches are very popular in certain areas like internet advertising and can be considered as an independent pattern of in-stream processing. A thorough overview of the sketching techniques can be found in [5].

Logical Time Tracking

It is very common for in-stream computations to depend on time: aggregations and joins are often performed on sliding time windows; processing logic often depends on a time interval between events and so on. Obviously, the in-stream processing system should have a notion of application’s view of time, instead of CPU wall-clock. However, proper time tracking is not trivial because data streams and particular events can be replayed in case of failures. It is often a good idea to have a notion of global logical time that can be implemented as follows:

Aggregation in a Persistent Store

We already have discussed that persistent store can be used for state checkpointing. However, it not the only way to employ an external store for in-stream processing. Let us consider an example that employs Cassandra to join multiple data streams over a time window. Instead of maintaining in-memory event buffers, one can simply save all incoming events from all data streams to Casandra using a join key as row key, as it shown in the figure below:

On the other side, the second process traverses the records periodically, assembles and emits joined events, and evicts the events that fell out of the time window. Cassandra even can facilitate this activity by sorting events according to their timestamps. It is important to understand that such techniques can defeat the whole purpose of in-stream data processing if implemented incorrectly – writing individual events to the data store can introduce a serious performance bottleneck even for fast stores like Cassandra or Redis. On the other hand, this approach provides perfect persistence of the computational state and different performance optimizations – say, batch writes – can help to achieve acceptable performance in many use cases.

Aggregation on a Sliding Window

In-stream data processing frequently deals with queries like “What is the sum of the values in the stream over last 10 minutes?” i.e. with continuous queries on a sliding time window. A straightforward approach to processing of such queries is to compute the aggregation function like sum for each instance of the time window independently. It is clear that this approach is not optimal because of the high similarity between two sequential instances of the time window. If the window at the time T contains samples {s(0), s(1), s(2), …, s(T-1), s(T)}, then the window at the time T+1 contains samples {s(1), s(2), s(3), …, s(T), s(T+1)}. This observation suggests that incremental processing might be used.

Incremental computations over sliding windows is a group of techniques that are widely used in digital signal processing, in both software and hardware. A typical example is a computation of the sum function. If the sum over the current time window is known, then the sum over the next time window can be computed by adding a new sample and subtracting the eldest sample in the window:

Similar techniques exist not only for simple aggregations like sums or products, but also for more complex transformations. For example, the SDFT (Sliding Discreet Fourier Transform) algorithm [4] is a computationally efficient alternative to per-window calculation of the FFT (Fast Fourier Transform) algorithm.

Query Processing Pipeline: Storm, Cassandra, Kafka

Now let us return to the practical problem that was stated in the beginning of this article. We have designed and implemented our in-stream data processing system on top of Storm, Kafka, and Cassandra adopting the techniques described earlier in this article. Here we provide just a very brief overview of the solution – a detailed description of all implementation pitfalls and tricks is too large and probably requires a separate article.

The system naturally uses Kafka 0.8 as a partitioned fault-tolerant event buffer to enable stream replay and improve system extensibility by easy addition of new event producers and consumers. Kafka’s ability to rewind read pointers also enables random access to the incoming batches and, consequently, Spark-style lineage tracking. It is also possible to point the system input to HDFS to process the historical data.

Cassandra is employed for state checkpointing and in-store aggregation, as described earlier. In many use cases, it also stores the final results.

Twitter’s Storm is a backbone of the system. All active query processing is performed in Storm’s topologies that interact with Kafka and Cassandra. Some data flows are simple and straightforward: the data arrives to Kafka; Storm reads and processes it and persist the results to Cassandra or other destination. Other flows are more sophisticated: one Storm topology can pass the data to another topology via Kafka or Cassandra. Two examples of such flows are shown in the figure above (red and blue curved arrows).

Towards Unified Big Data Processing

It is great that the existing technologies like Hive, Storm, and Impala enable us to crunch Big Data using both batch processing for complex analytics and machine learning, and real-time query processing for online analytics, and in-stream processing for continuous querying. Moreover, techniques like Lambda Architecture [6, 7] were developed and adopted to combine these solutions efficiently. This brings us to the question of how all these technologies and approaches could converge to a solid solution in the future. In this section, we discuss the striking similarity between distributed relational query processing, batch processing, and in-stream query processing to figure out the technologies that could cover all these use cases and, consequently, have the highest potential in this area.

The key observation is that relational query processing, MapReduce, and in-stream processing could be implemented using exactly the same concepts and techniques like shuffling and pipelining. At the same time:

The two statement above imply that tunable persistence (in-memory message passing versus on-disk materialization) and reliability are the distinctive features of the imaginary query engine that provides a set of processing primitives and interfaces to the high-level frameworks:

Among the emerging technologies, the following two are especially notable in the context of this discussion:

References

  1. A. Wilschut and P. Apers, “Dataflow Query Execution in a Parallel Main-Memory Environment “
  2. T. Urhan and M. Franklin, “XJoin: A Reactively-Scheduled Pipelined Join Operator“
  3. M. Zaharia, T. Das, H. Li, S. Shenker, and I. Stoica, “Discretized Streams: An Efficient and Fault-Tolerant Model for Stream Processing on Large Clusters”
  4. E. Jacobsen and R. Lyons, “The Sliding DFT“
  5. A. Elmagarmid, Data Streams Models and Algorithms
  6. N. Marz, “Big Data Lambda Architecture”
  7. J. Kinley, “The Lambda architecture: principles for architecting realtime Big Data systems”
  8. http://hortonworks.com/hadoop/tez/
  9. http://hortonworks.com/stinger/
  10. http://spark-project.org/

杂谈

这次看到dirtysalt的文章:In-stream-big-data-processing(英文版+中文注释),顿时解决了困扰自己的一个问题,英文的好文章,如何做记录?直接翻译中文?直接转载中文?NO,最佳方式当然是在原文基础上,添加自己的注释,OK,作者dirtysalt已经在做了,今后我也这么办。

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