BigDebug: Debugging Primitives for Interactive Big Data Processing in Spark
An overview of a recent paper outlining BigDebug, which provides real-time interactive debugging support for Data-Intensive Scalable Computing (DISC) systems, or more particularly, Apache Spark.
BigDebug: Debugging primitives for interactive big data processing in Spark– Gulzar et al. ICSE 2016
BigDebug provides real-time interactive debugging support for Data-Intensive Scalable Computing (DISC) systems, or more particularly, Apache Spark. It provides breakpoints, watchpoints, latency monitoring, forward and backward tracing, crash monitoring, and a real-time fix-and-resume capability. The overheads are low for a tool of this nature – less than 24% for record-level tracing, 19% for crash monitoring, and 9% for an on-demand watchpoint. The tool and manual are both available online at http://web.cs.ucla.edu/~miryung/software.html.
Debugging challenges with Apache Spark
Currently, developers do not have an easy means to debug DISC applications. The use of cloud computing makes application development feel more like batch jobs and the nature of debugging is therefore post-mortem. Developers are notified of runtime failures or incorrect outputs after many hours of wasted computing cycles on the cloud. DISC systems such as Spark do provide execution logs of submitted jobs. However, these logs present only the physical view of Big Data processing, as they report the number of worker nodes, the job status at individual nodes, the overall job progress rate, the messages passed between nodes, etc. These logs do not provide the logical view of program execution e.g., system logs do not convey which intermediate outputs are produced from which inputs, nor do they indicate what inputs are causing incorrect results or delays, etc. Alternatively, a developer may test their program by downloading a small subset of Big Data from the cloud onto their local disk, and then run the DISC application in a local mode. However, using a local mode, she may not encounter the same failure, because the faulty data may not be included in the given data sample.
A running case study in the paper resolves around Alice and her program to analyse logs from election polling.
It works fine when run with a subset of the data on Alice’s own machine, but crashes when run against the billions of logs in the full dataset.
Though Spark reports the task ID of a crash, it is impossible for Alice to know which records were assigned to the crashed executor and which specific entry is causing the crash. Even if she identifies a subset of input records assigned to the task, it is not feasible for her to manually inspect millions of records assigned to the failed task. She tries to rerun the program several times but the crash is persistent, making it less probable to occur due to a hardware failure in the cluster.
Crash culprits and remediation
Using BigDebug, Alice is instead provided with the specific record that causes the crash: “312-222-904 Trump Illinois 2015-10-11”. She can see that the date is not in the format her program expects, causing a crash at line 4 in the program.
When a crash occurs at an executor, BIGDEBUG sends all the required information to the driver, so that the user can examine crash culprits and take actions as depicted in Figure 8 [below]. When a crash occurs, BIGDEBUG reports (1) a crash culprit—an intermediate record causing a crash (2) a stack trace, (3) a crashed RDD, and (4) the original input record inducing a crash by leveraging backward tracing…
The user can elect to either skip the crash culprit record, correct it, or supply a code fix so that it can be processed. While waiting for user resolution, BigDebug continues processing the remaining records so that throughput is not affected. Only once an executor reaches end of task does it wait for the user. If there are multiple crash culprits, BigDebug lets them all accumulate and only waits at the end of the very last executor.
The last executor on hold then processes the group of corrected records provided from the user, before the end of the stage. This method applies to the pre-shuffle stage only, because the record distribution must be consistent with existing record-to-worker mappings. This optimization of replacing crash-inducing records in batch improves performance.
If the user elects to provide a code fix, a repair function is provided by the user that is then run on the offending records to transform them in such a way that processing can continue. Alternatively, BigDebug supports a Realtime Code Fixfeature that allows the user to supply a new function that will be used to processall records. The function is compiled using Scala’s NSC library and shipped to each worker.
In the general case, after seeing a record that causes a particular executor to fail, Alice will want to understand the provenance of that record – how it relates to the original input sources. BigDebug is able to issue a data provenance query on the fly, implemented through an extension of Spark’s RDD abstraction calledLineageRDD.
Provenance data is captured at the record level granularity, by tagging records with identifiers and associating output record identifiers with the relevant input record identifier, for a given transformation. From any given RDD, a Spark programmer can obtain a LineageRDD reference and use it to perform data tracing—i.e., the ability to transition backward (or forward) in the Spark program dataflow, at the record level. BIGDEBUG instruments submitted Spark programs with tracing agents that wrap transformations at stage boundaries. These agents implement the LineageRDD abstraction and have two responsibilities: (1) tag input and output records with unique identifiers for a given transformation and (2) store the associations between input and output record identifiers as a data provenance table in Spark’s native storage system.
Based on this information, BigDebug supports a goBackAll() query which given a result record returns all source input records that were used to compute it. Likewise goNextAll() returns all result records that a starting record contributes to. (There are also single-step goBack() and goNext() operations).
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