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Phase 2: Dawn of the Shared Compute Clusters

Ultimately, HOD architecture had too little information to make intelligent decisions about its allocations, its resource granularity was too coarse, and its API forced users to provide misleading constraints to the resource management layer. This forced the next step of evolution—the majority of installations, including Yahoo!, moved to a model of a shared MapReduce cluster together with shared HDFS instances. The main components of this shared compute architecture were as follows:

  • A JobTracker: A central daemon responsible for running all the jobs in the cluster. This is the same daemon that used to run jobs for a single user in the HOD world, but with additional functionality.
  • TaskTrackers: The slave in the system, which executes one task at a time under directions from the JobTracker. This again is the same daemon as in HOD, but now runs the tasks of jobs from all users.

What follows is an exposition of shared MapReduce compute clusters. Shared MapReduce clusters working in tandem with shared HDFS instances is the dominant architecture of Apache Hadoop 1.x release lines. At the point of this writing, many organizations have moved beyond 1.x to the next-generation architecture, but at the same time multitudes of Hadoop deployments continue to the JobTracker/TaskTracker architecture and are looking forward to the migration to YARN-based Apache Hadoop 2.x release lines. Because of this, in what follows, note that we’ll refer to the age of shared MapReduce-only shared clusters as both the past and the present.

Evolution of Shared Clusters

Moving to shared clusters from HOD-based architecture was nontrivial, and replacement of HOD was easier said than done. HOD, for all its problems, was originally designed to specifically address (and thus masked) many of the multitenancy issues occurring in shared MapReduce clusters. Adding to that, HOD silently took advantage of some core features of the underlying traditional resource manager, which eventually became missing features when the clusters evolved to being native MapReduce shared clusters. In the remainder of this section, we’ll describe salient characteristics of shared MapReduce deployments and indicate how the architecture gradually evolved away from HOD.

HDFS Instances

In line with how a shared HDFS architecture was established during the days of HOD, shared instances of HDFS continue to advance. During Phase 2, HDFS improved its scalability, acquired more features such as file-append, the new File-Context API for applications, Kerberos-based security features, high availability, and other performance features such as local short-circuit to data-node files directly.

Central JobTracker Daemon

The first step in the evolution of the MapReduce subsystem was to start running the JobTracker daemon as a shared resource across jobs, across users. This started with putting an abstraction for a cluster scheduler right inside the JobTracker, the details of which we explore in the next subsection. In addition, and unlike in the phase in which HOD was the norm, both developer testing and user validation revealed numerous deadlocks and race conditions in the JobTracker that were earlier neatly shielded by HOD.

JobTracker Memory Management

Running jobs from multiple users also drew attention to the issue of memory management of the JobTracker heap. At large clusters in Yahoo!, we had seen many instances in which a user, just as he or she used to allocate large clusters in the HOD world, would submit a job with many thousands of mappers or reducers. The configured heap of the JobTracker at that time hadn’t yet reached the multiple tens of gigabytes observed with HDFS’s NameNode. Many times, the JobTracker would expand these very large jobs in its memory to start scheduling them, only to run into heap issues and memory thrash and pauses due to Java garbage collection. The only solution at that time once such a scenario occurred was to restart the JobTracker daemon, effectively causing a downtime for the whole cluster. Thus, the JobTracker heap itself became a shared resource that needed features to support multitenancy, but smart scheduling of this scarce resource was hard. The JobTracker heap would store in-memory representations of jobs and tasks—some of them static and easily accountable, but other parts dynamic (e.g., job counters, job configuration) and hence not bounded.

To avoid the risks associated with a complex solution, the simplest proposal of limiting the maximum number of tasks per job was first put in place. This simple solution eventually had to evolve to support more limits—on the number of jobs submitted per user, on the number of jobs that are initialized and expanded in the JobTracker’s memory at any time, on the number of tasks that any job might legally request, and on the number of concurrent tasks that any job can run.

Management of Completed Jobs

The JobTracker would also remember completed jobs so that users could learn about their status once the jobs finished. Initially, completed jobs would have a memory footprint similar to that of any other running job. Completed jobs are, by definition, unbounded as time progresses. To address this issue, the JobTracker was modified to start remembering only partial but critical information about completed jobs, such as job status and counters, thereby minimizing the heap footprint per completed job. Even after this, with ever-increasing completed jobs, the JobTracker couldn’t cope after sufficient time elapsed. To address this issue, the straightforward solution of remembering only the last N jobs per user was deployed. This created still more challenges: Users with a very high job-churn rate would eventually run into situations where they could not get information about recently submitted jobs. Further, the solution was a per-user limit, so given enough users; the JobTracker would eventually exhaust its heap anyway.

The ultimate state-of-the-art solution for managing this issue was to change the JobTracker to not remember any completed jobs at all, but instead redirect requests about completed jobs to a special server called the JobHistoryServer. This server offloaded the responsibility of serving web requests about completed jobs away from the JobTracker. To handle RPC requests in flight about completed jobs, the JobTracker would also persist some of the completed job information on the local or a remote file system; this responsibility of RPCs would also eventually transition to the JobHistoryServer in Hadoop 2.x releases.

Central Scheduler

When HOD was abandoned, the central scheduler that worked in unison with a traditional resource manager also went away. Trying to integrate existing schedulers with the newly proposed JobTracker-based architecture was a nonstarter due to the engineering challenges involved. It was proposed to extend the JobTracker itself to support queues of jobs. Users would interact with the queues, which are configured appropriately. In the HOD setting, nodes would be statically assigned to a queue—but that led to utilization issues across queues. In the newer architecture, nodes are no longer assigned statically. Instead, slots available on a node are dynamically allocated to jobs in queues, thereby also increasing the granularity of the scheduling from nodes to slots.

To facilitate innovations in the scheduling algorithm, an abstraction was put in place. Soon, several implementations came about. Yahoo! implemented and deployed the Capacity scheduler, which focused on throughput, while an alternative called the Fair scheduler also emerged, focusing on fairness.

Scheduling was done on every node’s heartbeat: The scheduler would look at the free capacity on this node, look at the jobs that need resources, and schedule a task accordingly. Several dimensions were taken into account while making this scheduling decision—scheduler-specific policies such as capacity, fairness, and, more importantly, per-job locality preferences. Eventually, this “one task per heartbeat” approach was changed to start allocating multiple tasks per heartbeat to improve scheduling latencies and utilization.

The Capacity scheduler is based on allocating capacities to a flat list of queues and to users within those queues. Queues are defined following the internal organizational structure, and each queue is configured with a guaranteed capacity. Excess capacities from idle queues are distributed to queues that are in demand, even if they have already made use of their guaranteed capacity. Inside a queue, users can share resources but there is an overarching emphasis on job throughput, based on a FIFO algorithm. Limits are put in place to avoid single users taking over entire queues or the cluster.

Moving to centralized scheduling and granular resources resulted in massive utilization improvements. This brought more users, more growth to the so-called research clusters, and, in turn, more requirements. The ability to refresh queues at run time to affect capacity changes or to modify queue Access Control Lists (ACLs) was desired and subsequently implemented. With node-level isolation (described later), jobs were required to specify their memory requirements upfront, which warranted intelligent scheduling of high-memory jobs together with regular jobs; the scheduler accordingly acquired such functionality. This was done through reservation of slots on nodes for high-RAM jobs so that they do not become starved while regular jobs come in and take over capacity.

Recovery and Upgrades

The JobTracker was clearly a single point of failure for the whole cluster. Whenever a software bug surfaced or a planned upgrade needed to be done, the JobTracker would bring down the whole cluster. Anytime it needed to be restarted, even though the submitted job definitions were persistent in HDFS from the clients themselves, the state of running jobs would be completely lost. A feature was needed to let jobs survive JobTracker restarts. If a job was running at the time when the JobTracker restarted, along with the ability to not lose running work, the user would expect to get all information about previously completed tasks of this job transparently. To address this requirement, the JobTracker had to record and create persistent information about every completed task for every job onto highly available storage.

This feature was eventually implemented, but proved to be fraught with so many race conditions and corner cases that it eventually couldn’t be pushed to production because of its instability. The complexity of the feature partly arose from the fact that JobTracker had to track and store too much information—first about the cluster state, and second about the scheduling state of each and every job. Referring to [Requirement 2] Serviceability, the shared MapReduce clusters in a way had regressed compared to HOD with respect to serviceability.

Isolation on Individual Nodes

Many times, tasks of user Map/Reduce applications would get extremely memory intensive. This could occur due to many reasons—for example, due to inadvertent bugs in the users’ map or reduce code, because of incorrectly configured jobs that would unnecessarily process huge amounts of data, or because of mappers/reducers spawning children processes whose memory/CPU utilization couldn’t be controlled by the task JVM. The last issue was very possible with the Hadoop streaming framework, which enabled users to write their MapReduce code in an arbitrary language that was then run under separate children processes of task JVMs. When this happened, the user tasks would start to interfere with the proper execution of other processes on the node, including tasks of other jobs, even Hadoop daemons like the DataNode and the TaskTracker. In some instances, runaway user jobs would bring down multiple DataNodes on the cluster and cause HDFS downtime. Such uncontrolled tasks would cause nodes to become unusable for all purposes, leading to a need for a way to prevent such tasks from bringing down the node.

Such a situation wouldn’t happen with HOD, as every user would essentially bring up his or her own Hadoop MapReduce cluster and each node belonged to only one user at any single point of time. Further, HOD would work with the underlying resource manager to set resource limits prior to the TaskTracker getting launched. This made the entire TaskTracker process chain—the daemon itself together, with the task JVMs and any processes further spawned by the tasks themselves—to be bounded. Whatever system needed to be designed to throttle runaway tasks had to mimic this exact functionality.

We considered multiple solutions—for example, the host operating system facilitating user limits that are both static and dynamic, putting caps on individual tasks, and setting a cumulative limit on the overall usage across all tasks. We eventually settled on the ability to control individual tasks by killing any process trees that surpass pre-determined memory limits. The TaskTracker uses a default admin configuration or a per-job user-specified configuration, continuously monitors tasks’ memory usage in regular cycles, and shoots down any process tree that has overrun the memory limits.

Distributed Cache was another feature that was neatly isolated by HOD. With HOD, any user’s TaskTrackers would download remote files and maintain a local cache only for that user. With shared clusters, TaskTrackers were forced to maintain this cache across users. To help manage this distribution, the concepts of a public cache, private cache, and application cache were introduced. A public cache would include public files from all users, whereas a private cache would restrict itself to be per user. An application-level cache included resources that had to be deleted once a job finished. Further, with the TaskTracker concurrently managing several caches at once, several locking problems with regard to the Distributed Cache emerged, which required a minor redesign/reimplementation of this part of the TaskTracker.

Security

Along with enhancing resource isolation on individual nodes, HOD shielded security issues with multiple users by avoiding sharing of individual nodes altogether. Even for a single user, HOD would start the TaskTracker, which would then spawn the map and reduce tasks, resulting in all of them running as the user who had submitted the HOD job. With shared clusters, however, the tasks needed to be run as the job owner for security and accounting purposes, rather than as the user running the TaskTracker daemon itself.

We tried to avoid running the TaskTracker daemon as a privileged user (such as root) to solve this requirement. The TaskTracker would perform several operations on behalf of users, and running it as a privileged user would leak a lot of surface area that was vulnerable to attacks. Ultimately, we solved this problem by creating a setuid executable called taskcontroller that would be owned by root but runnable only by the TaskTracker. The TaskTracker would launch this executable with appropriate commands when needed. It would first run as root, do some very basic operations, and then immediately drop privileges by using setuid POSIX call to run the remaining operations as the user. Because this was very platform-specific code, we implemented a TaskController Java abstraction and shipped an implementation for Linux called LinuxTaskController with all the platform-specific code written in C.

The directories and files used by the task also needed to have appropriate permissions. Many of these directories and files were created by the TaskTracker, but were used by the task. A few were written by the user code but then used or accessed by the daemon. For security reasons, the permissions needed to be very strict and readable/writable by only the user or the TaskTracker. This step was done by making the taskcontroller first change the permissions from the TaskTracker to the user, and then letting the task run. Any files that needed to be read by the TaskTracker after the task finished had to have been created with appropriate permissions by the tasks.

Authentication and Access Control

As Hadoop managed more tenants, diverse use-cases, and raw data, its requirements for isolation became more stringent. Unfortunately, the system lacked strong, scalable authentication and an authorization model—a critical feature for multitenant clusters. This capability was added and backported to multiple versions of Hadoop.

A user can submit jobs to one or more MapReduce clusters, but he or she should be authenticated by Kerberos or a delegation mechanism before job submission. A user can disconnect after job submission and then reconnect to get the job status by using the same authentication mechanism. Once such an authenticated user sends requests to the JobTracker, it records all such accesses in an audit log that can be postprocessed for analyzing over time—thereby creating a kind of audit trail.

Tasks run as the user need credentials to securely talk to HDFS, too. For this to happen, the user needs to specify the list of HDFS clusters for a job at job submission either implicitly by input/output paths or explicitly. The job client then uses this list to reach HDFS and obtain credentials on users’ behalf. Beyond HDFS, communication with the TaskTracker for both task heartbeats and shuffle by the reduce tasks is also secured through a JobToken-based authentication mechanism.

A mechanism was needed to control who can submit jobs to a specified queue. Jobs can be submitted to only those queues the user is authorized to use. For this purpose, administrators set up Queue ACLs before the cluster is initialized. Administrators can dynamically change a queue’s ACL to allow a specific user or group to access it at run time. Specific users and groups, called the cluster administrators and queue administrators, are able to manage the ACL on the queue as well to access or modify any job in the queue.

On top of queue-level ACLs, users are allowed to access or modify only their own MapReduce jobs or jobs to which others have given them access via Job ACLs. A Job ACL governs two types of job operations: viewing a job and modifying a job. The web UI also shows information only about jobs run by the current user or about those jobs that are explicitly given access to via Job ACLs.

As one can see, MapReduce clusters acquired a lot of security features over time to manage more tenants on the same shared hardware. This [Requirement 6] Secure and Auditable Operation must be preserved in YARN.

  • [Requirement 6] Secure and Auditable Operation

    The next-generation compute platform should continue to enable secure and auditable usage of cluster resources.

Miscellaneous Cluster Management Features

So far, we have described in great detail the evolution of the central JobTracker daemon and the individual nodes. In addition to those, HOD made use of a few other useful features in the underlying resource manager such as addition and decommissioning of nodes that needed to be reimplemented in the JobTracker to facilitate cluster management. Torque also exposed a functionality to run an arbitrary program that could dynamically recognize any issues with specific nodes. To replace this functionality, TaskTrackers would run a similar health-check script every so often and figure out if a node had turned bad. This information would eventually reach the JobTracker, which would in turn remove this node from scheduling. In addition to taking nodes offline after observing their (poor) health status, heuristics were implemented to track task failures on each node over time and to blacklist any nodes that failed to complete a greater-than-mean number of tasks across jobs.

Evolution of the MapReduce Framework

In addition to the changes in the underlying resource management, the MapReduce framework itself went through many changes. New MapReduce APIs were introduced in an attempt to fill some gaps in the old APIs, the algorithm for running speculative duplicate JVMs to work around slow tasks went through several iterations, and new features like reusing JVMs across tasks for performance were introduced. As the MapReduce framework was tied to the cluster management layer, this evolution would eventually prove to be difficult.

Issues with Shared MapReduce Clusters

Issues with the shared MapReduce clusters developed over time.

Scalability Bottlenecks

As mentioned earlier, while HDFS had scaled gradually over years, the JobTracker had been insulated from those forces by HOD. When that guard was removed, MapReduce clusters suddenly became significantly larger and job throughput increased dramatically, but issues with memory management and coarse-grained locking to support many of the features added to the JobTracker became sources of significant scalability bottlenecks. Scaling the JobTracker to clusters containing more than about 4000 nodes would prove to be extremely difficult.

Part of the problem arose from the fact that the JobTracker was keeping in memory data from user jobs that could potentially be unbounded. Despite the innumerable limits that were put in place, the JobTracker would eventually run into some other part of the data structure that wasn’t limited. For example, a user job might generate so many counters (which were then not limited) that TaskTrackers would spend all their time uploading those counters. The JobTracker’s RPCs would then slow down to a grinding halt, TaskTrackers would get lost, resulting in a vicious circle that ended only with a downtime and a long wild goose chase for the offending application.

This problem would eventually lead to one of the bigger design points of YARN—to not load any user data in the system daemons to the greatest extent possible.

The JobTracker could logically be extended to support larger clusters and heterogeneous frameworks, if only with significant engineering investments. Heartbeat latency could be as high as 200 ms in large clusters, leading to node heartbeat intervals of as much as 40 seconds due to coarse-grained locking of its internal data structures. This problem could be improved with carefully designed fine-grained locking. The internal data structures in the JobTracker were often inefficient but they could be redesigned to occupy less memory. Many of the functions of the JobTracker could also be offloaded to separate, multitenant daemons. For example, serving the status of historical jobs could be—and eventually was—offloaded to the separate service JobHistoryServer. In other words, evolution could ideally continue by iterating on the existing code.

Although logical in theory, this scheme proved infeasible in practice. Changes to the JobTracker had become extremely difficult to validate. The continuous push for ill-thought-out features had produced a working, scalable, but very fragile system. It was time to go back to the drawing board for a complete overhaul. Scalability targets also anticipated clusters of 6000 machines running 100,000 concurrent tasks from 10,000 concurrent jobs, and there was no way the JobTracker could support such a massive scale without a major rewrite.

Reliability and Availability

While the move to shared clusters improved utilization and locality compared to HOD, it also brought concerns for serviceability and availability into sharp focus. Instead of losing a single workflow, a JobTracker failure caused an outage that would lose all of the running jobs in a cluster and require users to manually resubmit and recover their workflows. Upgrading a cluster by deploying a new version of Hadoop in a shared cluster was a rather common event and demanded very careful planning. To fix a bug in the MapReduce implementation, operators would necessarily schedule a cluster downtime, shut down the cluster, deploy the new binaries, validate the upgrade, and then admit new jobs. Any downtime created a backlog in the processing pipelines; when the jobs were eventually resubmitted, they would put a significant strain on the JobTracker. Restarts sometimes involved manually killing users’ jobs until the cluster recovered.

Operating a large, multitenant Hadoop cluster is hard. While fault tolerance is a core design principle, the surface exposed to user applications is vast. Given the various availability issues exposed by the single point of failure, it was critical to continuously monitor workloads in the cluster for offending jobs. All of these concerns may be grouped under the need for [Requirement 7] Reliability and Availability.

  • [Requirement 7] Reliability and Availability

    The next-generation compute platform should have a very reliable user interaction and support high availability.

Abuse of the MapReduce Programming Model

While MapReduce supports a wide range of use-cases, it is not the ideal model for all large-scale computations. For example, many machine learning programs require multiple iterations over a data set to converge to a result. If one composes this flow as a sequence of MapReduce jobs, the scheduling overhead will significantly delay the result. Similarly, many graph algorithms are better expressed using a bulk-synchronous parallel model (BSP) with message passing to communicate between vertices, rather than the heavy, all-to-all communication barrier in a fault-tolerant, large-scale MapReduce job. This mismatch became an impediment to users’ productivity, but the MapReduce-centricity in Hadoop allowed no other alternative programming model.

The evolution of the software wired the intricacies of MapReduce so deeply into the platform that it took a multiple months’ effort to introduce job-level setup and cleanup tasks, let alone an alternative programming model. Users who were in dire need of such alternative models would write MapReduce programs that would spawn their custom implementations—for example, for a farm of web servers. To the central scheduler, they appeared as a collection of map-only jobs with radically different resource curves, causing poor utilization, potentially resource deadlocks, and instability. If YARN were to be the next-generation platform, it must declare a truce with its users and provide explicit [Requirement 8] Support for Programming Model Diversity.

  • [Requirement 8] Support for Programming Model Diversity

    The next-generation compute platform should enable diverse programming models and evolve beyond just being MapReduce-centric.

Resource Model

Beyond their mismatch with emerging framework requirements, the typed slots on the TaskTrackers harmed utilization. While the separation between map and reduce capacity on individual nodes (and hence the cluster) prevented cross-task-type deadlocks, it also caused bottleneck resources.

The overlap between the map and reduce stages is configured by the user for each submitted job. Starting reduce tasks later increases cluster throughput, while starting them earlier in a job’s execution reduces its latency. The number of map and reduce slots are fixed by the cluster administrators, so unused map capacity can’t be used to spawn reduce tasks, and vice versa. Because the two task types can potentially (and more often than not do) complete at different rates, no configuration will ever be perfectly ideal. When either slot type becomes completely utilized, the JobTracker is forced to apply back-pressure to job initialization despite the presence of available slots of the other type. Nonstatic definition of resources on individual nodes complicates scheduling, but it also empowers the scheduler to pack the cluster more tightly.

Further, the definition of slots was purely based on jobs’ memory requirements, as memory was the scarcest resource for much of this time. Hardware keeps evolving, however, and there are now many sites where CPU has become the most scarce resource, with memory being available in abundance, and the concept of slots doesn’t easily accommodate this conundrum of scheduling multiple resources. This highlights the need for a [Requirement 9] Flexible Resource Model.

  • [Requirement 9] Flexible Resource Model

    The next-generation compute platform should enable dynamic resource configurations on individual nodes and a flexible resource model.

Management of User Logs

The handling of user logs generated by applications had been one of the biggest selling points of HOD, but it turned into a pain point for shared MapReduce installations. User logs were typically left on individual nodes by the TaskTracker daemon after they were truncated, but only for a specific amount of time. If individual nodes died or were taken offline, their logs wouldn’t be available at all. Runaway tasks could also fill up disks with useless logs, and there was no way to shield other tasks or the system daemons from such bad tasks.

Agility

By conflating the platform responsible for arbitrating resource usage with the framework expressing that program, one is forced to evolve both structures simultaneously. While cluster administrators try to improve the allocation efficiency of the platform, it is the users’ responsibility to help incorporate framework changes into the new structure. Thus, upgrading a cluster should not require users to halt, validate, and restore their pipelines. But the exact opposite thing happened with shared MapReduce clusters: While updates typically required no more than recompilation, users’ assumptions about internal framework details or developers’ assumptions about users’ programs occasionally created incompatibilities, wasting more software development cycles.

As stated earlier, HOD was much better at supporting this agility of user applications. [Requirement 2] Serviceability covered this need for the next-generation compute platform to enable evolution of cluster software completely decoupled from users’ applications.

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