Lecture Notes – Evolution of Google Search Engine

Jeff Dean gave a keynote Building Large Scale Information Retrieval Systems at WSDM 2009. It's actually a presentation on how Google search engine evolves during the past 10 years. Here are my notes for this lecture.

Part I - Overview of Search Engine Evolution: 1999 VS 2009

Factors to Consider when Designing a Information Retrieval System:
1. Corpus Size(# docs to be indexed)
2. QPS(Query Per Second)
3. Freshness/Update Rate
4. Query Latency
5. Complexity/Cost of Scoring/Retrieval Algorithm

Parameter Change 1999 -> 2009:
1. Corpus Size: 70M -> *B ~100X
2. QPS: ~1000X
3. Refresh: Months -> Minutes ~10000X
4. Latency: <1s> <02.s style="font-weight: bold; font-style: italic;">~5X
5. Machine Scale: ~1000X

Consider 10x Growth when designing, Rewrite for 100x Growth!

Part II - Evolution of Google Search Engine

~1997 - Circa, Research Prototype

- Simple Architecture and Focus on System Distributing/Partitioning
- Term vs Doc based Partition: Doc based Win
- Disk Based Index, DocID+Posting List with Position Attributes, Byte Aligned Encoding

~1999 - Circa, Production

- Introduced Cache
-- hit rate is low 30~60% due to index refresh and long tail query
-- very beneficial, reduce large disk i/o
-- hot term first priority to cache, hot and costy request

- Replica Index Data
-- better performance
-- better availability

Some Summary in late 1990's:

Crawler is simple batch system
- start with very few urls
- queue it when found new urls
- stop when have enough docs

Index Serving using cheap machine
- no failure handling
- added record/chunk checksum

Index Update
- once/month
- wait traffic to low -> take replica offline -> do update -> start serving

~2000 - Dealing with Growth


- doc size:50m -> 1000m
- ~20% query traffic increase/month
- Yahoo! deal


add machines constantly
- more index shards for larger index size
- more index replica for bigger query capacity

And improve software constantly
- better disk scheduling
- better index encoding

~2001 - Adding In-Memory Index

- enough machine memory: holding all index in mem
- machine function: replica -> micro shard holding
- balancer: cordinator
- availability: replicate important docs

~2004 - Adding Infrastructure

- Generalize tree structured query flow
- Generalize balancer concept
- New index encoding: group varint encoding
- GFS appear in production

~2007 - Universal Search

- Universal search: combine results from multiple vertical corpus
- Realtime search: fast url finding -> crawling -> indexing -> serving cycle
Experiment supporting: have idea -> try it on real data offline and
tune -> live experiment on small piece -> roll out and launch

Part III - Future Trends

- Cross Language Information Retrieval
- ACL in large IR system with huge amount of user and dynamic requirement
Automatic Construction of Efficient IR system (one bin for realtime and
regular web index with different parameter configuration)
- Info extraction from semi-structured data


1. Challenges in Building Large Scale Information Retrieval Systems[PDF, Video]
2. Notes by Another Blogger - http://www.searchenginecaffe.com/2009/02/jeffrey-dean-wsdm-keynote-building.html

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