meetup cassandra for_java_cql

PowerPoint Presentation

Building Java Applications
with
Apache Cassandra

Nate [email protected]@zznate

What is Apache Cassandra?

CAP Theorem

ConsistencyAvailability Partition Tolerance

Thou shalt have but 2

- Conjecture made by Eric Brewer in 2000- Published as formal proof in 2002- See: http://en.wikipedia.org/wiki/CAP_theorem for more

Apache Cassandra Concepts

- Explicit choice of partition tolerance and availability. Consistency is tunable.- No read before write- Merge on read- Idempotent- Schema Optional- All nodes share the same role- Still performs well with larger-than-memory data sets

Generally complements another system(s)

(Not intended to be one-size-fits-all)

*** You should always use the right tool for the right job anyway

How does this differ from an RDBMS?

How does this differ from an RDBMS?

Substantially.

vs. RDBMS - No Joins

Unless: - you do them on the client - you do them via Map/Reduce

vs. RDBMS - Schema Optional

(Though you can add meta information for validation and type checking)

*** Supports secondary indexes too: WHERE state = 'TX'

vs. RDBMS - Prematerialized and Transaction-less

- No ACID transactions - Limited support for ad-hoc queries

vs. RDBMS - Prematerialized and Transaction-less

- No ACID transactions - Limited support for ad-hoc queries

*** You are going to give up both of these anyway when you shard an RDBMS ***

vs. RDBMS - Facilitates Consolidation

It can be your caching layer * Off-heap cache (provided you install JNA)

It can be your analytics infrastructure * true map/reduce * pig driver * hive driver coming soon

vs. RDBMS - Shared-Nothing Architecture

Every node plays the same role: no masters, no slaves, no special nodes

*** No single point of failure

vs. RDBMS - Real Linear Scalability

Want 2x performance? Add 2x nodes.

*** 'No downtime' included!

vs. RDBMS - Performance

Reads on par with writes

Storage (Briefly)

Storage (Briefly)

Understanding the on-disk format is extremely helpful in designing your data model correctly

Storage - SSTable

- SSTables are immutable (Merge on read)- Newest timestamp wins

Storage Compaction

Merge SSTables keeping count down making Merge on Read more efficientDiscards Tombstones (more on this later!)

Data Model

Data Model

"...sparse, persistent, distributed, multi-dimensional sorted map."

(The Bigtable paper)

Data Model

Keyspace- Collection of Column Families

- Controls replication

Column Family

- Similar to a table

- Columns ordered by name

Data Model Column Family

Static Column Family- Model my object data

Dynamic Column Family

- Pre-calculated query results

Nothing stopping you from mixing them!

Data Model Static CF

GOOG

AAPL

NFLX

NOK

price: 589.55

price: 401.76

price: 78.73

name: Google

name: Apple

name: Netflix

price: 6.90

name: Nokia

exchange: NYSE

Stocks

Data Model Prematerialized Query

StockHist

10/25/2011: 6.71

GOOG

AAPL

NFLX

NOK

10/24/2011: 6.76

10/21/2011: 6.61

10/25/2011: 77.37

10/24/2011: 118.84

10/21/2011: 117.04

10/25/2011: 397.77

10/24/2011: 405.77

10/21/2011: 392.87

10/25/2011: 583.16

10/24/2011: 596.42

10/21/2011: 590.49

API Operations

Five general categories

RetrievingWriting/Updating/Removing (all the same op!)Increment counters

Meta InformationSchema ManipulationCQL Execution

Using a Client

Hector Client:http://hector-client.org- Most popular Java client - In use at very large installations- A number of tools and utilities built on top- Very active community- MIT Licensed

*** like any open source project fully dependent on another open source project it has its worts

Sample Project for Experimenting

https://github.com/zznate/cassandra-tutorialhttps://github.com/zznate/hector-examplesBuilt using Hector Really basic designed to be beginner level w/ very few moving partsModify/abuse/alter as needed

*** Descriptions of what is going on and how to run each example are in the Javadoc comments.

ColumnFamilyTemplate

Familiar, type-safe approach- based on template-method design pattern- generic: ColumnFamilyTemplate (K is the key type, N the column name type)

ColumnFamilyTemplate template = new ThriftColumnFamilyTemplate(keyspaceName, columnFamilyName, StringSerializer.get(), StringSerializer.get());

*** (no generics for clarity)


new ThriftColumnFamilyTemplate(keyspaceName, columnFamilyName, StringSerializer.get(), StringSerializer.get());

Key Format

Column Name Format- Cassandra calls this a comparator- Remember: defines column order in on-disk format


ColumnFamilyResult res = cft.queryColumns("zznate");

String value = res.getString("email");

Date startDate = res.getDate(startDate);

Key Format

Column Name Format


ColumnFamilyResult wrapper = template.queryColumns("GOOG", "AAPL", "NFLX");

String googName = wrapper.getString("name");

wrapper.next();

String aaplName = wrapper.getString("name");

wrapper.next(); String nflxName = wrapper.getString("name");

Querying multiple rows and iterating over results


ColumnFamilyUpdater updater = template.createUpdater("AAPL");

updater.setString("exchange","NASDAQ");updater.addKey("GOOG");updater.setString("exchange","NASDAQ");

template.update(updater);

Inserting data with ColumnFamilyUpdater


template.deleteColumn("AAPL", "notNeededStuff");template.deleteColumn("GOOG", "somethingElse");template.deleteColumn("GOOG", "aDifferentColumnName");...template.deleteRow(NOK);

template.executeBatch();

Deleting Data with ColumnFamilyTemplate

Deletion

Deletion

Again: Every mutation is an insert!

- Merge on read

- Sstables are immutable

- Highest timestamp wins

Deletion As Seen by CLI

[default@Tutorial] list StateCity;Using default limit of 100

-------------------

RowKey: CA Burlingame

=> (column=650, value=33372e3537783132322e3334, timestamp=1310340410528000)

-------------------

RowKey: TX Austin






Deletion As Seen by CLI

[default@Tutorial] list StateCity;Using default limit of 100

-------------------

RowKey: CA Burlingame

-------------------

RowKey: TX Austin






Deletion FYI

mutator.addDeletion("202230", "Npanxx", city, stringSerializer);

Does not exist? You just inserted a tombstone!

Sending a deletion for a non-existing row:

[default@Tutorial] list Npanxx; Using default limit of 100

. . .

-------------------

RowKey: 202230

-------------------

. . .

Integrating with existing patterns

Integrating with existing patterns

Hector Object Mapper:

https://github.com/rantav/hector/wiki/Hector-Object-Mapper-%28HOM%29

Hector JPA:

https://github.com/riptano/hector-jpa

CQL via JDBC

CQL via JDBC

- Integrate with existing tools (Spring Framework's JdbcTemplate in this case)

*** Still some caveats and missing features

CQL via JDBC

https://github.com/riptano/jdbc-conn-pool

- see portfolio_example sub project

CQL via JDBC: Components

- HCQLDataSource (from jdbc-pool)

- Spring Framework's JdbcTemplate

- DAO class with associated domain objects

- Junit

- Spring Framework's SpringJUnit4ClassRunner (context setup and injection)

- EmbededServerHelper from hector-test (manage Cassandra lifecycle, directories and configuration)

CQL via JDBC: Configuration

Pool Configuration

- Cluster name, keyspace and at least 1 host required

- Additional settings for:

* fail over semantics

* automatic host discovery

* timeout counters and thresholds

CQL via JDBC: Configuration (JNDI)

CQL via JDBC: Configuration (Spring)


private static final String PORTFOLIOS_INSERT =

"BEGIN BATCH "

+ "INSERT INTO Portfolios (KEY, BLU, CJS, DAL) VALUES (168,'19', '7', '38') "

+ "INSERT INTO Portfolios (KEY, BSX, CHK, DNB, MCI, SR) VALUES (236,'32', '27', '7','8','3') "

+ "APPLY BATCH";

...

jdbcTemplate.execute(PORTFOLIOS_INSERT);

Inserting Test Data


public Stock mapRow(ResultSet rs, int row) throws SQLException {

CassandraResultSet crs = (CassandraResultSet)rs;

Stock stock = new Stock();

stock.setTicker(new String(crs.getKey()));

stock.setPrice(crs.getDouble("price"));

return stock;

}

See PortfolioDao#loadStocks

Reading Data via RowMapper

Development Resources

CQL Documentation (and CQL Shell)
http://www.datastax.com/docs/1.0/dml/using_cqlHector Documentation
http://hector-client.orgCassandra Maven Plugin (exec-cql goal)
http://mojo.codehaus.org/cassandra-maven-plugin/CCM localhost cassandra cluster
https://github.com/pcmanus/ccmOpsCenter
http://www.datastax.com/products/opscenter

Cassandra AMIs
https://github.com/riptano/CassandraClusterAMI

Putting it Together

Take control of consistency

If you do need a high degree of consistency, use thresholds to trigger different behavior

- Bank account:

on values over $10,000, wait to here from all replicas

- Distributed Shopping Cart:

Show a confirmation page to verify order resolution

*** What is your appetite for risk?

Uniquely identify operations in the application

Facilitates idempotent behavior and out-of-order execution

Denormalization

The point of normalization is to avoid update anomalies

***But In an append-only system, we don't do updates

Summary

- Take advantage of strengths

- Look for idempotence and asynchronicity in your business processes

- If it's not in the API, you are probably doing it wrong

- Seek death is still possible if you model incorrectly

Questions

Nate [email protected]@zznate

Additional Resources

DataStax Documentation: http://www.datastax.com/docs/0.8/index

Apache Cassandra project wiki: http://wiki.apache.org/cassandra/

The Dynamo Paper

http://www.allthingsdistributed.com/files/amazon-dynamo-sosp2007.pdf

P. Helland. Building on Quicksand

http://arxiv.org/pdf/0909.1788

P. Helland. Life Beyond Distributed Transactions

http://www.ics.uci.edu/~cs223/papers/cidr07p15.pdf

S. Anand. Netflix's Transition to High-Availability Storage Systems

http://media.amazonwebservices.com/Netflix_Transition_to_a_Key_v3.pdf

The Megastore Paper

http://research.google.com/pubs/archive/36971.pdf

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