Saturday, May 28, 2011

Realtime Hadoop usage at Facebook -- Part 2 - Workload Types

This is the second part of our SIGMOD-2011 paper that describes our use case for Apache Hadoop and Apache HBase in realtime workloads. You can find the first part here. We describe why Hadoop and HBase fits the requirements of each of these applications.


Before deciding on a particular software stack and whether or not to move away from our MySQL-based architecture, we looked at a few specific applications where existing solutions may be problematic. These use cases would have workloads that are challenging to scale because of very high write throughput, massive datasets, unpredictable growth, or other patterns that may be difficult or suboptimal in a sharded RDBMS environment.

1. Facebook Messaging

The latest generation of Facebook Messaging combines existing Facebook messages with e-mail, chat, and SMS. In addition to persisting all of these messages, a new threading model also requires messages to be stored for each participating user. As part of the application server requirements, each user will be sticky to a single data center.

1.1 High Write Throughput
With an existing rate of millions of messages and billions of instant messages every day, the volume of ingested data would be very large from day one and only continue to grow. The denormalized requirement would further increase the number of writes to the system as each message could be written several times.

1.2 Large Tables
As part of the product requirements, messages would not be deleted unless explicitly done so by the user, so each mailbox would grow indefinitely. As is typical with most messaging applications, messages are read only a handful of times when they are recent, and then are rarely looked at again. As such, a vast majority would not be read from the database but must be available at all times and with low latency, so archiving would be difficult. Storing all of a user’’s thousands of messages meant that we’’d have a database schema that was indexed by the user with an ever-growing list of threads and messages. With this type of random write workload, write performance will typically degrade in a system like MySQL as the number of rows in the table increases. The sheer number of new messages would also mean a heavy write workload, which could translate to a high number of random IO operations in this type of system.

1.3 Data Migration
One of the most challenging aspects of the new Messaging product was the new data model. This meant that all existing user’’s messages needed to be manipulated and joined for the new threading paradigm and then migrated to the new system. The ability to perform large scans, random access, and fast bulk imports would help to reduce the time spent migrating users to the new system.

2 Facebook Insights

Facebook Insights provides developers and website owners with access to real-time analytics related to Facebook activity across websites with social plugins, Facebook Pages, and Facebook Ads. Using anonymized data, Facebook surfaces activity such as impressions, click through rates and website visits. These analytics can help everyone from businesses to bloggers gain insights into how people are interacting with their content so they can optimize their services. Domain and URL analytics were previously generated in a periodic, offline fashion through our Hadoop and Hive analytics data warehouse. However, this does not yield a rich user experience as the data is only available several hours after it has occurred.

2.1 Realtime Analytics
The insights teams wanted to make statistics available to their users within seconds of user actions rather than the hours previously supported. This would require a large-scale, asynchronous queuing system for user actions as well as systems to process, aggregate, and persist these events. All of these systems need to be fault-tolerant and support more than a million events per second.

2.2 High Throughput Increments
To support the existing insights functionality, time and demographic-based aggregations would be necessary. However, these aggregations must be kept up-to-date and thus processed on the fly, one event at a time, through numeric counters. With millions of unique aggregates and billions of events, this meant a very large number of counters with an even larger number of operations against them.

3. Facebook Metrics System

At Facebook, all hardware and software feed statistics into a metrics collection system called ODS (Operations Data Store). For example, we may collect the amount of CPU usage on a given server or tier of servers, or we may track the number of write operations to an HBase cluster. For each node or group of nodes we track hundreds or thousands of different metrics, and engineers will ask to plot them over time at various granularities. While this application has hefty requirements for write throughput, some of the bigger pain points with the existing MySQL-based system are around the resharding of data and the ability to do table scans for analysis and time roll-ups. This use-case is gearing up to be in production very shortly.

3.1 Automatic Sharding
The massive number of indexed and time-series writes and the unpredictable growth patterns are difficult to reconcile on a sharded MySQL setup. For example, a given product may only collect ten metrics over a long period of time, but following a large rollout or product launch, the same product may produce thousands of metrics. With the existing system, a single MySQL server may suddenly be handling much more load than it can handle, forcing the team to manually re-shard data from this server onto multiple servers.

3.2 Fast Reads of Recent Data and Table Scans
A vast majority of reads to the metrics system is for very recent, raw data, however all historical data must also be available. Recently written data should be available quickly, but the entire dataset will also be periodically scanned in order to perform time- based rollups.

(Credit to the authors of the paper: Dhruba Borthakur Kannan Muthukkaruppan Karthik Ranganathan Samuel Rash Joydeep Sen Sarma Jonathan Gray Nicolas Spiegelberg Hairong Kuang Dmytro Molkov Aravind Menon Rodrigo Schmidt Amitanand Aiyer)

Tuesday, May 17, 2011

Realtime Hadoop usage at Facebook -- Part 1

Facebook recently deployed Facebook Messages, its first ever user-facing application built on the Apache Hadoop platform. It uses HDFS and HBase as core technologies for this solution. Since then, there are many more applications that have started to used HBase. We have gained some experience in deploying and operating HDFS and HBase at peta-byte scale for realtime-workloads and decided to write a paper detailing some of these insights. This paper will be published in SIGMOD 2011.

You can find the full paper here later, but here are some highlights:


The requirements for the storage system for our workloads can be summarized as follows:

1. Elasticity: We need to be able to add incremental capacity to our storage systems with minimal overhead and no downtime. In some cases we may want to add capacity rapidly and the system should automatically balance load and utilization across new hardware.

2. High write throughput: Most of the applications store (and optionally index) tremendous amounts of data and require high aggregate write throughput.

3. Efficient and low-latency strong consistency semantics within a data center: There are important applications like Messages that require strong consistency within a data center. This requirement often arises directly from user expectations. For example ‘‘unread’’ message counts displayed on the home page and the messages shown in the inbox page view should be consistent with respect to each other. While a globally distributed strongly consistent system is practically impossible, a system that could at least provide strong consistency within a data center would make it possible to provide a good user experience. We also knew that (unlike other Facebook applications), Messages was easy to federate so that a particular user could be served entirely out of a single data center making strong consistency within a single data center a critical requirement for the Messages project. Similarly, other projects, like realtime log aggregation, may be deployed entirely within one data center and are much easier to program if the system provides strong consistency guarantees.

4. Efficient random reads from disk: In spite of the widespread use of application level caches (whether embedded or via memcached), at Facebook scale, a lot of accesses miss the cache and hit the back-end storage system. MySQL is very efficient at performing random reads from disk and any new system would have to be comparable.

5. High Availability and Disaster Recovery: We need to provide a service with very high uptime to users that covers both planned and unplanned events (examples of the former being events like software upgrades and addition of hardware/capacity and the latter exemplified by failures of hardware components). We also need to be able to tolerate the loss of a data center with minimal data loss and be able to serve data out of another data center in a reasonable time frame.
6. Fault Isolation: Our long experience running large farms of MySQL databases has shown us that fault isolation is critical. Individual databases can and do go down, but only a small fraction of users are affected by any such event. Similarly, in our warehouse usage of Hadoop, individual disk failures affect only a small part of the data and the system quickly recovers from such faults.

7. Atomic read-modify-write primitives: Atomic increments and compare-and-swap APIs have been very useful in building lockless concurrent applications and are a must have from the underlying storage system.

8. Range Scans: Several applications require efficient retrieval of a set of rows in a particular range. For example all the last 100 messages for a given user or the hourly impression counts over the last 24 hours for a given advertiser.

It is also worth pointing out non-requirements:

1. Tolerance of network partitions within a single data center: Different system components are often inherently centralized. For example, MySQL servers may all be located within a few racks, and network partitions within a data center would cause major loss in serving capabilities therein. Hence every effort is made to eliminate the possibility of such events at the hardware level by having a highly redundant network design.

2. Zero Downtime in case of individual data center failure: In our experience such failures are very rare, though not impossible. In a less than ideal world where the choice of system design boils down to the choice of compromises that are acceptable, this is one compromise that we are willing to make given the low occurrence rate of such events. We might revise this non-requirement at a later time.

3. Active-active serving capability across different data centers: As mentioned before, we were comfortable making the assumption that user data could be federated across different data centers (based ideally on user locality). Latency (when user and data locality did not match up) could be masked by using an application cache close to the user.

Some less tangible factors were also at work. Systems with existing production experience for Facebook and in-house expertise were greatly preferred. When considering open-source projects, the strength of the community was an important factor. Given the level of engineering investment in building and maintaining systems like these –– it also made sense to choose a solution that was broadly applicable (rather than adopt point solutions based on differing architecture and codebases for each workload).

After considerable research and experimentation, we chose Hadoop and HBase as the foundational storage technology for these next generation applications. The decision was based on the state of HBase at the point of evaluation as well as our confidence in addressing the features that were lacking at that point via in- house engineering. HBase already provided a highly consistent, high write-throughput key-value store. The HDFS NameNode stood out as a central point of failure, but we were confident that our HDFS team could build a highly-available NameNode (AvatarNode) in a reasonable time-frame, and this would be useful for our warehouse operations as well. Good disk read-efficiency seemed to be within striking reach (pending adding Bloom filters to HBase’’s version of LSM Trees, making local DataNode reads efficient and caching NameNode metadata). Based on our experience operating the Hive/Hadoop warehouse, we knew HDFS was stellar in tolerating and isolating faults in the disk subsystem. The failure of entire large HBase/HDFS clusters was a scenario that ran against the goal of fault-isolation, but could be considerably mitigated by storing data in smaller HBase clusters. Wide area replication projects, both in-house and within the HBase community, seemed to provide a promising path to achieving disaster recovery.

HBase is massively scalable and delivers fast random writes as well as random and streaming reads. It also provides row-level atomicity guarantees, but no native cross-row transactional support. From a data model perspective, column-orientation gives extreme flexibility in storing data and wide rows allow the creation of billions of indexed values within a single table. HBase is ideal for workloads that are write-intensive, need to maintain a large amount of data, large indices, and maintain the flexibility to scale out quickly.

HBase is now being used by many other workloads internally at Facebook . I will describe these different workloads in a later post.

(Credit to the authors of the paper: Dhruba Borthakur Kannan Muthukkaruppan Karthik Ranganathan Samuel Rash Joydeep Sen Sarma Jonathan Gray Nicolas Spiegelberg Hairong Kuang Dmytro Molkov Aravind Menon Rodrigo Schmidt Amitanand Aiyer)