Schemaless Databases

Overview of schemaless database technologies, from MUMPS to Mnesia.

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「Schemaless Databases」の内容

  1. Computational Research Division Lawrence Berkeley National Laboratory Dan Gunter
  2. Introduction About this talk It is not "hands-on" (sorry) Most of it is history and overview It's about databases, not explicitly "clouds" Relation to cloud computing Cloud computing and scalable databases go hand-in-hand There are a lot of open-source NOSQL projects right now Understanding what they do, and what features of the commercial implementations they're imitating, gives insight into scalability issues for distributed computing in general
  3. Terminology: NOSQL and "Schemaless" First: not terribly important or deep in meaning But "NOSQL" has gained currency Original, and best, meaning: Not Only SQL Wikipedia credits it to Carlo Strozzi in 1998, re-introduced in 2009 by Eric Evans of Rackspace May use non-SQL, typically simpler, access methods Don't need to follow all the rules for RDBMS'es Lends itself to "No (use of) SQL", but this is misleading Also referred to as "schemaless" databases Implies dynamic schema evolution
  4. NOSQL past and present Pre-RDBMS RDBMS era NOSQL
  5. Pre-relational structured storage systems Hierarchical storage and sparse multi-dimensional arrays MUMPS (Massachusetts General Hospital Utility Multi-Programming System), later ANSI M sparse multi-dimensional array global variables, prefixed with "^", are automatically persisted: ^Car("Door","Color") = "Blue" " Pick" OS/database everything is hash table IBM Information Management System (IMS), [DB1] Computer Systems News , 11/28/83
  6. The relational model Introduced with E. F. Codd's 1970 paper " A Relational Model of Data for Large Shared Data Banks" Relational algebra provided declarative means of reasoning about data sets SQL is loosely based on relational algebra A 1 ... A n Value 1 ... Value n R Relation (Table) Relation variable (Table name) Attribute (Column) {unordered} Heading Tuple (Row) {unordered}
  7. Recent NOSQL database products Columnar or Extensible record Google BigTable HBase Cassandra HyperTable SimpleDB Document Store CouchDB MongoDB Lotus Domino Graph DB Neo4j FlockDB InfiniteGraph Key/Value Store Mnesia Memcached Redis Tokyo Cabinet Dynamo Project Voldemort Dynomite Riak
  8. Why NOSQL? Renewed interest originated with global internet companies (Google, Amazon, Yahoo!, FaceBook, etc.) that hit limitations of standard RDBMS solutions for one or more of: Extremely high transaction rates Dynamic analysis of huge volumes of data Rapidly evolving and/or semi-structured data At the same time, these companies - unlike the financial and health services industries using M and friends - did not particularly need "ACID" transactional guarantees Didn't want to run z/OS on mainframes And had to deal with the ugly reality of distributed computing: networks break your $&#!
  9. CAP Theorem Introduced by Eric Brewer in a PODC keynote on July 2000, thus also known as "Brewer's Theorem" CAP = C onsistency, A vailability, P artition-tolerance Theorem states that in any "shared data" system, i.e. any distributed system, you can have at most 2 out of 3 of CAP (at the same time) This was later proved formally (w/asynchronous model) Three possibilities: All robust distributed systems live here Forfeit partition-tolerance Forfeit availability Forfeit consistency Single-site databases, cluster databases, LDAP Distributed databases w/pessimistic locking, majority protocols Coda, web caching, DNS, Dynamo
  10. CAP, ACID, and BASE RDBMS systems and research focus on ACID: A tomicity, C onsistency, I solation, and D urability concurrent operations act as if they are serialized Brewer's point is that this is one end of a spectrum , one that sacrifices Partition-tolerance and Availability for Consistency So, at the other end of the spectrum we have BASE : B asically A vailable S oft-state with E ventual consistency Stale data may be returned Optimistic locking (e.g., versioned writes) Simpler, faster, easier evolution ACID BASE
  11. Pioneers Google BigTable Amazon Dynamo These implementations are not publicly available, but the distributed-system techniques that they integrated to build huge databases have been imitated, to a greater or lesser extent, by every implementation that followed.
  12. Google BigTable Internal Google back-end, scaling to thousands of nodes, for web indexing, Google Earth, Google Finance Scales to petabytes of data, with highly varied data size & latency requirements Data model is (3D) sparse, multi-dimensional, sorted map (row_key, column_key, timestamp) -> string Technologies: Google File System, to store data across 1000's of nodes 3-level indexing with Tablets SSTable for efficient lookup and high throughput Distributed locking with Chubby
  13. BigTable's Data Model Google's Bigtable is essentially a massive, distributed 3-D spreadsheet. It doesn't do SQL, there is limited support for atomic transactions, nor does it support the full relational database model. In short, in these and other areas, the Google team made design trade-offs to enable the scalability and fault-tolerance Google apps require. - Robin Harris, StorageMojo (blog), 2006-09-08 t 6 t 5 t 3 name contents: ... ... " com.cnn.www" " CNN" ... "" ... " <html>..." " <html>..." " <html>..."
  14. Tablets and SSTables Tablets represent contiguous groups of rows Automatically split when grow too big One "tablet server" holds many tablets 3-level indexing scheme similar to B+-tree Root tablet -> Metadata tablets -> Data (leaf) tablets With 128MB metadata tablets, can addr. 2 34 leaves Client communicates directly with tablet server, so data does not go through root (i.e. locate, then transfer) Client also caches information Values written to memory, to disk in a commit log; periodically dumped into read-only SSTables . Better throughput at the expense of some latency
  15. Use of Bloom Filters to optimize lookups Review: What is a Bloom filter? Can test whether an element is a member of a set probabilistic: can only say "no" with certainty Here, tests if an SSTable has a row/column pair NO: Stop YES: Need to load & retrieve data anyways Useful optimization in this space.. w is not in { x, y, z } because it hashes to one position with a 0 1 1 1 0 0 1 0 1 0 1 0 0 1 0 { x, w y, z }
  16. Chubby and Paxos Chubby is a distributed locking service. Requests go the current Master. If the Master fails, Paxos is used to elect a new one Each "DB" is a replica Each server runs on its own host Google tends to run 5 servers, with only one being the "master" at any one time Chubby server DB Chubby server DB Chubby server DB Chubby server DB Chubby server DB Master
  17. What about CAP? For bookkeeping tasks, Chubby's replication allows tolerance of node failures ( P ) and consistency ( C ) at the price of availability ( A ), during time to elect a new master and synchronize the replicas. Tablets have "relaxed consistency" of storage, GFS: A single master that maps files to servers Multiple replicas of the data Versioned writes Checksums to detect corruption (with periodic handshakes)
  18. Amazon's Dynamo Used by Amazon's "core services", for very high A and P at the price of C ("eventual consistency") Data is stored and retrieved solely by key (key/value store) Techniques used: Consistent hashing - for partitioning Vector clocks - to allow MVCC and read repairs rather than write contention Merkle trees -a data structure that can diff large amounts of data quickly using a tree of hash values Gossip - A decentralized information sharing approach that allows clusters to be self-maintaining Techniques not new, but their synthesis at this scale, in a real system, was
  19. Dynamo data partitioning and replication Virtual node Host "node" Host "node" Virtual node Virtual node Virtual node Virtual node Virtual node Virtual node . . Hash ring using consistent hashing Host "node" Virtual node Virtual node Virtual node Virtual node 4 4 3 Item Hashes to this spot coordinator node replicas
  20. Eventual consistency and sloppy quorum R = Number of healthy nodes from the preference list (roughly, list of "next" nodes on hash ring) needed for a read W = Number of healthy nodes from preference list needed for a write N = number of replicas of each data item You can tune your performance R << N, high read availability W << N, high write availability R + W > N, consistent, but sloppy quorum R + W < N, at best, eventual consistency Hinted handoff keeps track of the data "missed" by nodes that go down, and updates them when they come back online
  21. Replica synchronization with Merkle trees When things go really bad, the "hinted" replicas may be lost and nodes may need to synchronize their replicas To make synchronization efficient, all the keys for a given virtual node are stored in a hash tree or Merkle tree which stores data at the leaves and recursive hashes in the nodes Same hash => Same data at leaves For Dynamo, the "data" are the keys stored in a given virtual node Each node is a hash of its children If two top hashes match, then the trees are the same
  22. Infrastructure (at scale) is fractal This ability to be effective at multiple scales is crucial to the rise in NOSQL (schemaless) database popularity Why didn't Amazon or Google just run a big machine with something like GT.M, Vertica, or KDB (etc.)? The answer must be partially to do something new, but partially that it wasn't just shopping carts or search
  23. The Gold Rush Columnar or Extensible record Google BigTable HBase Cassandra HyperTable SimpleDB Document Store CouchDB MongoDB Lotus Domino Graph DB Neo4j FlockDB InfiniteGraph Key/Value Store Mnesia Memcached Redis Tokyo Cabinet Dynamo Project Voldemort Dynomite Riak Hibari
  24. Basic operations are simply get, put, and delete All systems can distribute keys over nodes Vector clocks are used as in Dynamo (or just locks) Replication: common Transactions: not common Multiple storage engines: common Key/Value Store Memcached Redis Tokyo Cabinet Dynamo Project Voldemort Dynomite Riak Hibari
  25. Dynamo-like features: Automatic partitioning with consistent hashing MVCC with vector clocks Eventual consistency (N, R, and W) Also: combines cache with storage to avoid sep. cache layer pluggable storage layer RAM, disk, other.. Project Voldemort Type Key/Value Store License Apache 2.0 Language Java Company Linked-In Web
  26. Dynamo-like features: Consistent hashing MVCC with vector clocks Eventual consistency (N, R, and W) Also: Hadoop-like M/R queries in either JS or Erlang REST access API result = self.client .add(bucket.get_name()) .map(&quot;Riak.mapValuesJson" .reduce(&quot;Riak.reduceSum" .run() Riak Example: Map/reduce with the Python API Type Key/Value Store License Open-Source Language Erlang Company Basho Web
  27. Dynamo-like features: consistent hashing Unique features: Chain replication Each node may function as head, middle, or end of a chain associated with a position on the hash ring; head gets requests & tail services them. See Durability (fsync) in exchange for slower writes Hibari Type Key/Value Store License Open-Source Language Erlang Company Gemini Mobile Web
  28. All share BigTable data model rows and columns " column families" that can have new columns added Consistency models vary: MVCC distributed locking Need to run on a different back-end than BigTable (GFS ain't for sale) Columnar or Extensible record Google BigTable HBase Cassandra HyperTable
  29. Marriage of BigTable and Dynamo Consistent hashing Structured values Columns / column families Slicing with predicates Tunable consistency: W = 0, Any, 1, Quorum, All R = 1, Quorum, All Write commit log, memtable, and uses SSTables Cassandra Used at: Facebook, Twitter, Digg, Reddit, Rackspace Type Extensible column store License Apache 2.0 Language Java Company Apache Software Foundation Web
  30. Store objects (not really documents) think: nested maps Varying degrees of consistency, but not ACID Allow queries on data contents (M/R or other) May provide atomic read-and-set operations SimpleDB Document Store CouchDB MongoDB Lotus Domino Mnesia
  31. Objects are grouped in "collections" REST API not very efficient for throughput Read scalability through asynchronous replication with eventual consistency No sharding Incrementally updated M/R "views" ACID? Uses MVCC and flush on commit. So, kinda.. CouchDB Type Document store License Apache 2.0 Language Erlang Company Apache Software Foundation Web
  32. (Also) groups objects in "collections", within a "database" Data stored in binary JSON called BSON Replication just for failover Automatic sharding M/R queries, and simple filters User-defined indexes on fields of the objects Atomic update "modifiers" can increment value modify-if-current ..others MongoDB As of v1.6, can also do limited replication with replica sets Type Document store License GPL Language C++ Company 10gen Web
  33. Stores data in "tables" Data stored in memory Logged to selected disks Replication and sharding Queries are performed using Erlang list comprehensions (!) User-defined indexes on fields of the objects Transactions are supported (but optional) Optimizing query compiler and dynamic "rule" tables Embedded in Erlang OTP platform (similar to Pick ) Mnesia * Mozilla Public License modified to conform with laws of Sweden (more herring) Type Document store License EPL* Language Erlang Company Ericsson Web Papers
  34. Why do we care about Mnesia / OTP? Database for RabbitMQ (distributed messaging behind S3) Erlang seems to be gaining a popularity in the distributed-computing space females() -> F = fun() -> Q = query [ || E <- table(employee), = female] end, mnemosyne:eval(Q) end, mnesia:transaction(F). Erlang query for "all females" in company* *I know, but it's not my example. This is right out of the manual.
  35. Comparison of MongoDB and CouchDB Domain is monitoring a set of ongoing managed data transfers initial concern is handling the data in real-time So, did some very simple 1-node benchmarks of MongoDB and CouchDB load times (i.e on my laptop) for 200K records Of course this is just one (lame) test There is a need for a standard NOSQL benchmark suite; so far YCSB is the closest (from Yahoo!) Database Inserts/sec MongoDB 16,000 CouchDB 70 CouchDB, batch 1,800
  36. Example from distributed monitoring Consider semi-structured input like: ts=2010-02-20T23:14:06Z event=job.state level=Info wf_uuid=8bae72f2-31b9-45f4-bdd3-ce8032081a28 state=JOB_SUCCESS name=create_dir_montage_0_viz_glidein job_submit_seq=1 If the fields are likely to change, or new types of data will appear, how to model this kind of data? Blob Placeholders Entity-Attribute-Value All of these are data modeling "anti-patterns" for relational DBs
  37. What's wrong with EAV? It's terrible, I should know, I tried it You end up with queries that look like this to just extract a bunch of fields that started out in the same log line:
  38. What about queries?
  39. SQL vs. M/R and other models You need to think about this going in; you are throwing away much of the elegance of relational query optimization need to weigh against costs of static schemata Holistic approach: Spend lots of time on logical model, understand problem! What degree of normalization makes sense? Is your data well-represented as a hash table? Is it hierarchical? Graph-like? What degree of consistency do you really need? Or maybe multiple ones?
  40. Google's interactive analysis tool: Dremel see Uses a parallel "nested columnar storage" DB SQL-like query language SELECT A, COUNT(B) FROM T GROUP BY A Interactive query times (seconds) on "trillions of records" Of course it's not released open-source, but the glove has been thrown Now if we could only combine with visualization.. and link it all up to the cloud.. and make it free.. with ponies..
  41. Conclusions Anyone who says RDBMS is dead (and means it) is an idiot SQL is mostly a red herring Can be layered on top of NOSQL, e.g. BigQuery and Hive What really is interesting about NOSQL is scalability (given relaxed consistency) and lack of static schemas incremental scalability from local disk to large degrees of parallelism in the face of distributed failure easier schema evolution, esp. important at the "development" phase, which is often longer than anyone wants to admit Whether we should move towards the One True Database or a Unix-like ecosystem of tools is mostly a matter of philosophical bent; certainly both directions hold promise
  42. Selected references Cattell's overview of "scalable datastores" BigTable: Stonebraker et al. on columnar vs. map/reduce NOSQL "summer reading": "path throgh them": Varley's Master's Thesis on non-relational db's (modeling)


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