Building Software Systems at Google and Lessons Learned

Table of Contents

Jeff Dean @ 2010

1 Google Web Search: 1999 vs. 2010

  • # docs: tens of millions to tens of billions ~1000X
  • queries processed/day: ~1000X
  • per doc info in index: ~3X
  • update latency: months to tens of secs ~50000X 这个似乎加快了不少,搜索实时性是一个很重要的问题
  • avg. query latency: <1s to <0.2s ~5X
  • More machines * faster machines: ~1000X
  • Continuous evolution:
    • 7 significant revisions in last 11 years
    • often rolled out without users realizing we’ve made major changes

1.1 Caching in Web Search

  • Cache servers:
    • cache both index results and doc snippets
    • hit rates typically 30-60%
      • depends on frequency of index updates, mix of query traffic, level of personalization, etc
  • Main benefits:
    • performance! a few machines do work of 100s or 1000s
    • much lower query latency on hits
      • queries that hit in cache tend to be both popular and expensive (common words, lots of documents to score, etc.)
  • Beware: big latency spike/capacity drop when index updated or cache flushed 使用cache系统就必须注意cache挂掉时候带来的latency spike


1.2 Indexing (circa 1998-1999)

  • Simple batch indexing system
    • No real checkpointing, so machine failures painful 在做索引的时候没有checkpoint,所以机器故障会非常麻烦
    • No checksumming of raw data, so hardware bit errors caused problems 对于大规模数据来说,checksum还是非常必须的。
    • Exacerbated by early machines having no ECC, no parity
    • Sort 1 TB of data without parity: ends up "mostly sorted"
    • Sort it again: "mostly sorted" another way
  • “Programming with adversarial memory”
    • Developed file abstraction that stores checksums of small records and can skip and resynchronize after corrupted records


1.3 Early 2001: In-Memory Index

  • 解决index server压力的办法是通过添加更多的index replicas,但是突然有一天发现 Eventually have enough replicas so that total memory across all index machines can hold ONE entire copy of index in memory replicas机器已经足够多,在内存完全可以存放下index. 然后变成如下结构
  • Many positives:
    • big increase in throughput
    • big decrease in query latency
  • Some issues:
    • Variance: query touches 1000s of machines, not dozens
      • 因为需要查询更多的机器,因此查询的变化也会非常大,不太容易控制差异
      • e.g. randomized cron jobs caused us trouble for a while
    • Availability: 1 or few replicas of each doc’s index data
      • Availability of index data when machine failed (esp for important docs): replicate important docs
      • Queries of death that kill all the backends at once: very bad
      • 存在一个问题就是,一个query可能会因为system bug造成crash. 如何解决这个问题,就是下面的canary request


1.4 Canary Requests

  • 主要就是针对特定query会造成system crash.但是我们不希望所有的nodes都crash.
  • Problem: requests sometimes cause server process to crash
    • testing can help reduce probability, but can’t eliminate
  • If sending same or similar request to 1000s of machines:
    • they all might crash!
    • recovery time for 1000s of processes pretty slow
  • Solution: send canary request first to one machine 可以首先将request发送到一个节点上,如果正常的话那么发送到其他节点,否则重试几次失败就放弃。
    • if RPC finishes successfully, go ahead and send to all the rest
    • if RPC fails unexpectedly, try another machine (might have just been coincidence)
    • if fails K times, reject request
  • Crash only a few servers, not 1000s

1.5 Query Serving System, 2004 edition

我觉得这个应该是google query serving system的最终模型了。leaf所有的数据都是全内存的,但是在GFS上面有on-disk数据做持久化,或者说leaf是一个简单的query system + bigtable吧。之所有使用这种multi-level tree结构在这篇文章后面的pattern部分会说到。


1.6 Encodings

  • Byte-Aligned Variable-length Encodings
    • 7 bits per byte with continuation bit
      • Con: Decoding requires lots of branches/shifts/masks
    • Encode byte length using 2 bits
      • Better: fewer branches, shifts, and masks
      • Con: Limited to 30-bit values, still some shifting to decode
  • Group Varint Encoding
    • encode groups of 4 32-bit values in 5-17 bytes
    • Pull out 4 2-bit binary lengths into single byte prefix
    • Much faster than alternatives:
      • 7-bit-per-byte varint: decode ~180M numbers/second
      • 30-bit Varint w/ 2-bit length: decode ~240M numbers/second
      • Group varint: decode ~400M numbers/second

1.7 2007: Universal Search


  • Performance: most of the corpora weren’t designed to deal with high QPS level of web search 性能匹配
  • Mixing: Which corpora are relevant to query? 相关性
  • UI: How to organize results from different corpora? UI布局


2 System Software Evolution

  • The Joys of Real Hardware (Typical first year for a new cluster):
    • ~1 network rewiring (rolling ~5% of machines down over 2-day span)
    • ~20 rack failures (40-80 machines instantly disappear, 1-6 hours to get back)
    • ~5 racks go wonky (40-80 machines see 50% packetloss)
    • ~8 network maintenances (4 might cause ~30-minute random connectivity losses)
    • ~12 router reloads (takes out DNS and external vips for a couple minutes)
    • ~3 router failures (have to immediately pull traffic for an hour)
    • ~dozens of minor 30-second blips for dns
    • ~1000 individual machine failures
    • ~thousands of hard drive failures
    • slow disks, bad memory, misconfigured machines, flaky machines, etc.
    • Long distance links: wild dogs, sharks, dead horses, drunken hunters, etc.
    • Reliability/availability must come from software!

3 System Building Experiences and Patterns

3.1 Many Internal Services

  • Break large complex systems down into many services!
  • Simpler from a software engineering standpoint
    • few dependencies, clearly specified
    • easy to test and deploy new versions of individual services
    • ability to run lots of experiments
    • easy to reimplement service without affecting clients
  • Development cycles largely decoupled
    • lots of benefits: small teams can work independently
    • easier to have many engineering offices around the world
  • e.g. search touches 200+ services
    • ads, web search, books, news, spelling correction, …

3.2 Designing Efficient Systems

  • Given a basic problem definition, how do you choose "best" solution?
    • Best might be simplest, highest performance, easiest to extend, etc.
  • Back of the Envelope Calculations
  • Know Your Basic Building Blocks
    • Core language libraries, basic data structures, protocol buffers, GFS, BigTable, indexing systems, MapReduce, …
    • Not just their interfaces, but understand their implementations (at least at a high level)
    • If you don’t know what’s going on, you can’t do decent back-of-the-envelope calculations!

3.3 Designing & Building Infrastructure

  • Identify common problems, and build software systems to address them in a general way 尝试从general角度解决问题,这样才能够做出infrastructure
  • Important to not try to be all things to all people 但是对不同需求需要不同对待,不一定需要将解决方案放在一个实现里面
    • Clients might be demanding 8 different things
    • Doing 6 of them is easy
    • …handling 7 of them requires real thought
    • …dealing with all 8 usually results in a worse system
    • more complex, compromises other clients in trying to satisfy everyone
  • Don't build infrastructure just for its own sake: 设计通用组件的话,还需要去排除那些潜在的不需要的需求,抑制复杂性
    • Identify common needs and address them
    • Don't imagine unlikely potential needs that aren't really there
  • Best approach: use your own infrastructure (especially at first!)
    • (much more rapid feedback about what works, what doesn't)

3.4 Design for Growth

  • Try to anticipate how requirements will evolve keep likely features in mind as you design base system
  • Don’t design to scale infinitely: 扩展性只需要考虑5x-50x左右的扩展即可
    • ~5X - 50X growth good to consider
    • >100X probably requires rethink and rewrite

3.5 Pattern: Single Master, 1000s of Workers

master主要完成全局性质的工作,其余工作交给worker完成。通常存在hot standby来做failover. 优点是可以很容易地进行全局控制,但是实现上必须小心,而缺点非常明显就是支撑worker不会很多,在1k级别上。如果涉及到更大规模集群的话,那么worker需要和master有更加频繁的交互,这对于master压力会非常大。

  • Master orchestrates global operation of system
    • load balancing, assignment of work, reassignment when machines fail, etc.
    • … but client interaction with master is fairly minimal
    • Often: hot standby of master waiting to take over
    • Always: bulk of data transfer directly between clients and workers
  • Examples:
    • GFS, BigTable, MapReduce, file transfer service, cluster scheduling system, …
  • Pro:
    • simpler to reason about state of system with centralized master
  • Caveats:
    • careful design required to keep master out of common case ops
    • scales to 1000s of workers, but not 100,000s of workers

3.6 Pattern: Tree Distribution of Requests

这个模型本质上是从single master模型发展过来的,是multi master实现。随着master管理worker数目增加,CPU以及network IO都会bounded. 以single master为例,如果每个master最多管理1k worker的话,那么1k master可以由另外一个master管理,这样就可以支持1k * 1k worker级别了。

  • Problem: Single machine sending 1000s of RPCs overloads NIC on machine when handling replies
    • wide fan in causes TCP drops/retransmits, significant latency
    • CPU becomes bottleneck on single machine
  • Solution: Use tree distribution of requests/responses
    • fan in at root is smaller
    • cost of processing leaf responses spread across many parents
  • Most effective when parent processing can trim/combine leaf data
    • can also co-locate parents on same rack as leaves

3.7 Pattern: Backup Requests to Minimize Latency

通过backup request来降低延迟,因为部分请求可能会成为straggler,这点在mapreduce里面的speculative非常经典。 #note: jeff dean在另外一篇文章tail at scale里面也提到即便如何也存在一些bad case

  • Problem: variance high when requests go to 1000s of machines
    • last few machines to respond stretch out latency tail substantially
  • Often, multiple replicas can handle same kind of request
  • When few tasks remaining, send backup requests to other replicas
  • Whichever duplicate request finishes first wins
    • useful when variance is unrelated to specifics of request
    • increases overall load by a tiny percentage
    • decreases latency tail significantly
  • Examples:
    • MapReduce backup tasks (granularity: many seconds)
    • various query serving systems (granularity: milliseconds)

3.8 Pattern: Multiple Smaller Units per Machine


  • Problems:
    • want to minimize recovery time when machine crashes
    • want to do fine-grained load balancing
  • Having each machine manage 1 unit of work is inflexible
    • slow recovery: new replica must recover data that is O(machine state) in size
    • load balancing much harder
  • Have each machine manage many smaller units of work/data
    • typical: ~10-100 units/machine
    • allows fine grained load balancing (shed or add one unit)
    • fast recovery from failure (N machines each pick up 1 unit)
  • Examples:
    • map and reduce tasks, GFS chunks, Bigtable tablets, query serving system index shards

3.9 Pattern: Elastic Systems


  • Problem: Planning for exact peak load is hard
    • overcapacity: wasted resources
    • undercapacity: meltdown
  • Design system to adapt:
    • automatically shrink capacity during idle period
    • automatically grow capacity as load grows
  • Make system resilient to overload:
    • do something reasonable even up to 2X planned capacity
      • e.g. shrink size of index searched, back off to less CPU intensive algorithms, drop spelling correction tips, etc.
    • more aggressive load balancing when imbalance more severe

3.10 Pattern: Combine Multiple Implementations

多种实现的结合,这点以realtime + batch说明非常直观。

  • Example: Google web search system wants all of these:
    • freshness (update documents in ~1 second)
    • massive capacity (10000s of requests per second)
    • high quality retrieval (lots of information about each document)
    • massive size (billions of documents)
  • Very difficult to accomplish in single implementation
  • Partition problem into several subproblems with different engineering tradeoffs. E.g.
    • realtime system: few docs, ok to pay lots of $$$/doc
    • base system: high # of docs, optimized for low $/doc
    • realtime+base: high # of docs, fresh, low $/doc

4 Final Thoughts

  • Today: exciting collection of trends: 未来趋势的一些思考
    • large-scale datacenters + 大规模数据中心建设
    • increasing scale and diversity of available data sets + 大量数据需要分析和挖掘
    • proliferation of more powerful client devices 各种设备接入
  • Many interesting opportunities: 值得去做的事情
    • planetary scale distributed systems 宇宙级别分布式系统
    • development of new CPU and data intensive services 新的CPU和数据密集服务
    • new tools and techniques for constructing such systems 以及构建这些服务的工具
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