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Austin Scales- Clickstream Analytics at Bazaarvoice
- 1. Clickstream Analytics at Bazaarvoice Evan Pollan, Engineering Lead @EvanPollan
- 2. Agenda • Infrastructure: lessons learned operating Hadoop in EC2 • Case study: uniques at scale using Hadoop and HBase Confidential and Proprietary. © 2012 Bazaarvoice, Inc.
- 3. Project Magpie • Bazaarvoice products – extremely large web surface area • Client-side instrumentation to measure interactions • Many event sources (apps) => one sink: Magpie • Consolidated HTTP event collection – Network-wide event correlation – Network ~ many apps and many “sites” (clients) • Clickstream == Topically segmented JSON event log files • Sense of scale – 10 - 20K events per second – 500M – 1B impressions per day – 25 – 50 GB compressed event log data per day Confidential and Proprietary. © 2012 Bazaarvoice, Inc.
- 4. Infrastructure Whys • Why Hadoop? – Experience scaling brute-force log processing via Hadoop • Everybody’s favorite: Akamai edge request logs • EMR, Apache Whirr – Needed online analytics – HBase fit the bill – Apache OSS ecosystem familiar to BV • Why Amazon Web Services? – Existing infrastructure hosting solution too inflexible and slow – Couldn’t scale R&D without an elastic infrastructure Confidential and Proprietary. © 2012 Bazaarvoice, Inc.
- 5. High-level architecture • Event collectors in auto-scale groups behind elastic load balancers • Event stream compressed and uploaded hourly to S3 • S3: store of record • Hadoop cluster: – HDFS: stores raw event logs, derived file-based data sets, and HBase HFiles/WALs – Oozie: job scheduling, data dependency management – MapReduce: analytics (mix of Pig, Java => 100% Java) – HBase: stores hourly/daily analytics results • Job Portal: job schedule viz, gap analysis & alerting • UI/API: Analytics available via JSON API and in Backbone.js UI Confidential and Proprietary. © 2012 Bazaarvoice, Inc.
- 6. EMR vs. roll our own • Neither • Cons: EMR – Price premium – Opaque Hadoop configuration – No way to mitigate SPOFs • Cons: Roll our own – Small group of engineers, no ops manpower at beginning • Solution: Cloudera – Cloudera Manager for config management and provisioning – CDH 3.X distribution Confidential and Proprietary. © 2012 Bazaarvoice, Inc.
- 7. Missteps • Problem: non-HA NameNode • Solution: EBS! • Problem: EC2 MTBF iffy • Solution: EBS! • Reality: When something goes wrong in AWS, it is invariably an outage or degradation in EBS. – Violates the whole concept of data locality. Hadoop + SAN = sadness • Problem: Where should HBase live? • Solution: Co-resident with MapReduce! Confidential and Proprietary. © 2012 Bazaarvoice, Inc.
- 8. Where we’ve ended up • Moved to the latest Cloudera CDH 4.X – HA NameNode! – Zookeeper for leader election – Quorum Journal Manager for edit logs • Learn to let go – Mitigate SPOF where possible, but plan for failure – End-to-end automation for DR/migration • Avoid EBS like the plague • HBase and MapReduce segmentation – Enables different hardware step size – Batch processing doesn’t affect HBase response time – Better understanding of HBase/HDFS locality (or lack thereof) Confidential and Proprietary. © 2012 Bazaarvoice, Inc.
- 9. Let’s talk sets Confidential and Proprietary. © 2012 Bazaarvoice, Inc.
- 10. Let’s talk sets • Common problem: uniques (e.g. unique visitors, users, etc.) • Naïve solution: SELECT DISTINCT(X) FROM Y • Not tenable given: – Massive, semi-structured data set – Thousands of grouping axes • OK: pre-calculate via MapReduce • But… – What would you pre-calculate? – Daily for each grouping? – How would you answer queries for other time ranges? Pre- calculate them, too? Confidential and Proprietary. © 2012 Bazaarvoice, Inc.
- 11. Set Unions • Definition: cardinality of a set is the number of elements in that set – A = {1, 2, 3}; |A| = 3 • Cardinality of the union of two sets cannot be determined from the cardinality of the two sets – |A U B| not necessarily equal to |A| + |B| – Only equal if A and B are disjoint – How do you know if they’re disjoint? – You need both sets • Imagine: – Set “a” are the visitors from yesterday – Set “b” are the visitors from today – To get uniques for both days, you have to look at both data sets Confidential and Proprietary. © 2012 Bazaarvoice, Inc.
- 12. An entirely different set: bit sets • Translate set members’ identifiers to an index in a bit set • Bit sets are combinable – yahtzee! • HBase is good at storing bits – MapReduce to build bit set for each grouping in your smallest desirable unit of time – Persist w/ row key as a function of date and grouping • Uniques for last month? – Scan: start and stop rows accounting for date range and grouping – Merge each day’s bit set with a single bit set representing the union – Count the number of “on” bits in the merged bit set => cardinality • But… – # bits for items whose identifiers number in the billions? – A billion bits is a lot of bits Confidential and Proprietary. © 2012 Bazaarvoice, Inc.
- 13. Bit sets – solving the size problem • 109 bits is an expensive way to store a combinable cardinality • Query I/O example: Uniques for last quarter – 120 MB/day * 90 days = 10.8 GB – Too much to pull out of HBase to answer an “online” query • Storage example: 10K different grouping axes – Clients, sites, favorite colors, whatever – 120 MB * 10K = 1.2 TB/day of storage • Possible mitigation: compression – Still need to generate a 120 MB data structure, then compress Confidential and Proprietary. © 2012 Bazaarvoice, Inc.
- 14. Cardinality Estimation • Many different approaches to estimate the cardinality of a set – General goal: calculate cardinality in small RAM footprint • Big breakthrough in 2007: the HyperLogLog estimator • What’s the big deal? – Tunable accuracy – Incredible information density – Combinable • Analog: lossy compression of bit sets • How good? – Estimate cardinality of 109 unique elements +/-2% in 1.5 KB Confidential and Proprietary. © 2012 Bazaarvoice, Inc.
- 15. Nuts & Bolts • http://github.com/clearspring/stream-lib – Java impls of top-K, frequency, and cardinality for streams • A ha moment: combining estimators from distributed counters is no different than combining them across different time periods! • MapReduce algorithm – map(Event) : (key, identifier) • key is what ever grouping you want uniques for – Shuffle sorts all key, identifier tuples by key – reduce(key, Iterable<identifier>) : estimator bytes • Reducer simply updates the estimator in-place – tiny RAM footprint Confidential and Proprietary. © 2012 Bazaarvoice, Inc.
- 16. Nuts & Bolts • Reducer output: HBase Put • HBase “schema”, e.g. daily uniques aggregated by brand: • Scan: Row Key Estimator – brandX – Jan 2-3 brandX-20130101 [0100110111000] [0110100111000] brandX-20130102 [0110100111000] [0100000101011] brandX-20130103 [0100000101011] brandY-20130101 [0101100011000] [0110100111011] brandY-20130102 [0100100111001] Cardinality = N Confidential and Proprietary. © 2012 Bazaarvoice, Inc.
- 17. Nuts & Bolts • HBase scan is the key to making this fast – First result: instantiate HyperLogLog estimator – Remaining results: update estimator in-place • O(n) to compute result, n ~ number of bits in estimator (1.5KB) • Freedom to build a data set of unique estimators that can be arbitrarily sliced quickly – Quarterly, daily, weekly, ad-hoc date ranges – HBase client pulls 1.5KB * number of days, returns a long – Perf anecdote: REST API call to get network-wide uniques for current month-to-date • 66 ms over the internet • 12 ms server-side latency Confidential and Proprietary. © 2012 Bazaarvoice, Inc.
Notas del editor
- August – 2012 Version
- A magpie is a bird that suffers an irresistible urge to collect and hoard things Sense of scaleAt our current level of instrumentation and app penentration
- HBase fit the bill…Given its storage model and affinity to timeseries dataGiven its clean, out of the box integration with MapReduce
- I can’t, and therefore don’t, do diagrams. You’re stuck with word-dense slidesHadoop clusterOf note: we sync S3 to HDFS for optimized job execution and to enable Oozie’s data dependency managementJob PortalOozie web UI is painful to use
- When most people get ready to deploy Hadoop to EC2, they choose between Elastic Map Reduce or a custom deploymentCDH distributionCurated, don’t have to worry about mixing and matching various apache component versions
- non-HANameNodeEven CDH3 was not immune from SPOFEC2 MTBF iffy…The Magpie team was definitely not the first to foray in to EC2 – BV had been using EC2 for quite some time at this point
- Quorum Journal Manager for edit logs- Doesn’t push the SPOF further upstream with a NFS NAS solution for shared storage of the edit logs - This system works really well. Leader election is lightning fast, and we haven’t encountered any failures of reads or writes during out “pull the plug” testingEnd-to-end automation for DR:- And by DR, I mean AZ outages; loss of 3+ data nodes; loss of 2+ “master nodes” - When our SLAs require it, we’ll run an HBase replica in another region, but still treat the MapReduce cluster as expendable HBase/HDFS locality: - Region Server and HFile blocks are not co-resident after a region has been reassigned
- We have a solid hadoop infrastructure running in AWS, let’s crunch some big dataNot tenable given…Well, not tenable w/out a very, very large OLAP data store. We’ve got a hadoop cluster, though…Pre-calculate them, too?Large, expensive jobs re-processing the same data sets, lack of flexibility to the end-user
- We have a solid hadoop infrastructure running in AWS, let’s crunch some big dataNot tenable given…Well, not tenable w/out a very, very large OLAP data store. We’ve got a hadoop cluster, though…Pre-calculate them, too?Large, expensive jobs re-processing the same data sets, lack of flexibility to the end-user
- Conclusion: need some way to calculate and persist a representation of cardinality for an incremental time period that would not be prohibitive to scan over arbitrary time ranges and combine into a single representation of the cardinality of all the subsets.
- Bit sets are combinable…Meaning you could take a bit set representation of one day’s cardinality, OR it with another day’s bit set and have a bit set that would tell you the cardinality of the union of the two daysMapReduce to build …For example, unique users at site XYZ on January 31, 2013Scan: start and stop…HBase is very good at scans over reasonable sets of data, even without the benefit of block cache, when rows are (a) reasonably narrow, and (b) the ordering of the keys leads to linear readsA billion bits is a lot of bits…It’s not big data, but it can quickly become big data
- Possible mitigation: compressionStill need to generate a 120 MB data structure in RAM, then compressRetrieval non-trivial given decompression costs and heap pressure
- -Calculate cardinality in small RAM footprintE.g. for stream processingBig breakthrough in 2007: HyperLogLogNew algorithm and representational data structureTeam of French mathematicians, led by FlajoletTimely: engineers at Google just published a refinement called HLL++ that is more accurate on the low and high end.Combinable…Not unique to HyperLogLogAnalog: lossy compression…BUT: doesn’t require large intermediate heap and associated CPU cycles for compression
- I don’t peruse the proceedings of math conferences – but I do keep up on hacker news and high scalability.comLast April, Matt Abrams of Clearspring wrote a blog post on using HyperLogLog estimators to merge cardinality estimators from a bunch of distributed stream-processing machines