1. Developing polyglot persistence applications
Chris Richardson
Author of POJOs in Action, Founder of the original CloudFoundry.com
@crichardson chris.richardson@springsource.com
http://plainoldobjects.com/

2. Presentation goal
The benefits and drawbacks of
polyglot persistence
and
How to design applications that
use this approach

3. About Chris

4. (About Chris)

5. About Chris()

6. About Chris

7. About Chris
http://www.theregister.co.uk/2009/08/19/springsource_cloud_foundry/

8. vmc push About-Chris
Developer Advocate for
CloudFoundry.com
Signup at http://cloudfoundry.com

9. Agenda
• Why polyglot persistence?
• Using Redis as a cache
• Optimizing queries using Redis materialized views
• Synchronizing MySQL and Redis
• Tracking changes to entities

10. Food to Go
• Take-out food delivery
service
• “Launched” in 2006

11. Food To Go Architecture
RESTAURANT
CONSUMER
OWNER
Order Restaurant
taking Management
MySQL
Database

12. Success Growth challenges
• Increasing traffic
• Increasing data volume
• Distribute across a few data centers
• Increasing domain model complexity

13. Limitations of relational
databases
• Scalability
• Distribution
• Schema updates
• O/R impedance mismatch
• Handling semi-structured data

14. Solution: Spend Money
http://upload.wikimedia.org/wikipedia/commons/e/e5/Rising_Sun_Yacht.JPG
OR
http://www.trekbikes.com/us/en/bikes/road/race_performance/madone_5_series/madone_5_2/#

15. Solution: Use NoSQL
Benefits Drawbacks
• Higher performance • Limited transactions
• Higher scalability • Limited querying
• Richer data-model • Relaxed consistency
• Schema-less • Unconstrained data

16. Example NoSQL Databases
Database Key features
Cassandra Extensible column store, very scalable, distributed
Neo4j Graph database
Document-oriented, fast, scalable
MongoDB
Redis Key-value store, very fast
http://nosql-database.org/ lists 122+ NoSQL
databases

17. Redis
• Advanced key-value store K1 V1
• C-based server
K2 V2
• Very fast, e.g. 100K reqs/sec
... ...
• Optional persistence
• Transactions with optimistic locking
• Master-slave replication
• Sharding using client-side consistent hashing

18. Sorted sets
Value
Key
a b
myset
5.0 10.
Members are Score
sorted by score

19. Adding members to a sorted set
Redis Server
Key Score Value
a
zadd myset 5.0 a myset
5.0

20. Adding members to a sorted set
Redis Server
a b
zadd myset 10.0 b myset
5.0 10.

21. Adding members to a sorted set
Redis Server
c a b
zadd myset 1.0 c myset
1.0 5.0 10.

22. Retrieving members by index range
Start End
Key
Index Index Redis Server
zrange myset 0 1
c a b
myset
1.0 5.0 10.
c a

23. Retrieving members by score
Min Max
Key
value value Redis Server
zrangebyscore myset 1 6
c a b
myset
1.0 5.0 10.
c a

24. Redis use cases
• Replacement for Memcached • Handling tasks that overload an RDBMS
• Session state • Hit counts - INCR
• Cache of data retrieved from • Most recent N items - LPUSH and
system of record (SOR) LTRIM
• Replica of SOR for queries • Randomly selecting an item –
needing high-performance SRANDMEMBER
• Queuing – Lists with LPOP, RPUSH, ….
• High score tables – Sorted sets and
ZINCRBY
• …

25. Redis is great but there are
tradeoffs
• Low-level query language: PK-based access only
• Limited transaction model:
• Read first and then execute updates as batch
• Difficult to compose code
• Data must fit in memory
• Single-threaded server: run multiple with client-side sharding
• Missing features such as access control, ...

26. And don’t forget:
An RDBMS is fine for many
applications

27. The future is polyglot
e.g. Netflix
• RDBMS
• SimpleDB
• Cassandra
• Hadoop/Hbase
IEEE Software Sept/October 2010 - Debasish Ghosh / Twitter @debasishg

28. Agenda
• Why polyglot persistence?
• Using Redis as a cache
• Optimizing queries using Redis materialized views
• Synchronizing MySQL and Redis
• Tracking changes to entities

29. Increase scalability by caching
RESTAURANT
CONSUMER
OWNER
Order Restaurant
taking Management
MySQL
Cache
Database

30. Caching Options
• Where:
• Hibernate 2nd level cache
• Explicit calls from application code
• Caching aspect
• Cache technologies: Ehcache, Memcached, Infinispan, ...
Redis is also an option

31. Using Redis as a cache
• Spring 3.1 cache abstraction
• Annotations specify which methods to cache
• CacheManager - pluggable back-end cache
• Spring Data for Redis
• Simplifies the development of Redis applications
• Provides RedisTemplate (analogous to JdbcTemplate)
• Provides RedisCacheManager

32. Using Spring 3.1 Caching
@Service
public class RestaurantManagementServiceImpl implements RestaurantManagementService {
private final RestaurantRepository restaurantRepository;
@Autowired
public RestaurantManagementServiceImpl(RestaurantRepository restaurantRepository) {
this.restaurantRepository = restaurantRepository;
}
@Override
public void add(Restaurant restaurant) {
Cache result
restaurantRepository.add(restaurant);
}
@Override
@Cacheable(value = "Restaurant")
public Restaurant findById(int id) {
return restaurantRepository.findRestaurant(id);
Evict from
} cache
@Override
@CacheEvict(value = "Restaurant", key="#restaurant.id")
public void update(Restaurant restaurant) {
restaurantRepository.update(restaurant);
}

33. Configuring the Redis Cache
Manager
Enables caching
<cache:annotation-driven />
<bean id="cacheManager"
class="org.springframework.data.redis.cache.RedisCacheManager" >
<constructor-arg ref="restaurantTemplate"/>
</bean>
Specifies CacheManager The RedisTemplate used
implementation to access Redis

34. Domain object to key-value
mapping?
Restaurant
K1 V1
TimeRange
TimeRange MenuItem
MenuItem K2 V2
... ...
ServiceArea

35. RedisTemplate
• Analogous to JdbcTemplate
• Encapsulates boilerplate code, e.g. connection management
• Maps Java objects Redis byte[]’s

36. Serializers: object byte[]
• RedisTemplate has multiple serializers
• DefaultSerializer - defaults to JdkSerializationRedisSerializer
• KeySerializer
• ValueSerializer
• HashKeySerializer
• HashValueSerializer

37. Serializing a Restaurant as JSON
@Configuration
public class RestaurantManagementRedisConfiguration {
@Autowired
private RestaurantObjectMapperFactory restaurantObjectMapperFactory;
private JacksonJsonRedisSerializer<Restaurant> makeRestaurantJsonSerializer() {
JacksonJsonRedisSerializer<Restaurant> serializer =
new JacksonJsonRedisSerializer<Restaurant>(Restaurant.class);
...
return serializer;
}
@Bean
@Qualifier("Restaurant")
public RedisTemplate<String, Restaurant> restaurantTemplate(RedisConnectionFactory factory) {
RedisTemplate<String, Restaurant> template = new RedisTemplate<String, Restaurant>();
template.setConnectionFactory(factory);
JacksonJsonRedisSerializer<Restaurant> jsonSerializer = makeRestaurantJsonSerializer();
template.setValueSerializer(jsonSerializer);
return template;
}
Serialize restaurants using Jackson
} JSON

38. Caching with Redis
RESTAURANT
CONSUMER
OWNER
Order Restaurant
taking Management
Redis MySQL
First Second
Cache Database

39. Agenda
• Why polyglot persistence?
• Using Redis as a cache
• Optimizing queries using Redis materialized views
• Synchronizing MySQL and Redis
• Tracking changes to entities

40. Finding available restaurants
Available restaurants =
Serve the zip code of the delivery address
AND
Are open at the delivery time
public interface AvailableRestaurantRepository {
List<AvailableRestaurant>
findAvailableRestaurants(Address deliveryAddress, Date deliveryTime);
...
}

41. Food to Go – Domain model (partial)
class Restaurant { class TimeRange {
long id; long id;
String name; int dayOfWeek;
Set<String> serviceArea; int openTime;
Set<TimeRange> openingHours; int closeTime;
List<MenuItem> menuItems;
}
}
class MenuItem {
String name;
double price;
}

42. Database schema
ID Name …
RESTAURANT table
1 Ajanta
2 Montclair Eggshop
Restaurant_id zipcode
RESTAURANT_ZIPCODE table
1 94707
1 94619
2 94611
2 94619
RESTAURANT_TIME_RANGE table
Restaurant_id dayOfWeek openTime closeTime
1 Monday 1130 1430
1 Monday 1730 2130
2 Tuesday 1130 …

43. Finding available restaurants on Monday, 6.15pm
for 94619 zipcode
Straightforward three-way join
select r.*
from restaurant r
inner join restaurant_time_range tr
on r.id =tr.restaurant_id
inner join restaurant_zipcode sa
on r.id = sa.restaurant_id
where ’94619’ = sa.zip_code
and tr.day_of_week=’monday’
and tr.openingtime <= 1815
and 1815 <= tr.closingtime

44. How to scale queries?

45. Option #1: Query caching
• [ZipCode, DeliveryTime] ⇨ list of available restaurants
BUT
• Long tail queries
• Update restaurant ⇨ Flush entire cache
Ineffective

46. Option #2: Master/Slave replication
Writes Consistent reads
Queries
MySQL
(Inconsistent reads)
Master
MySQL MySQL MySQL
Slave 1 Slave 2 Slave N

47. Master/Slave replication
• Mostly straightforward
BUT
• Assumes that SQL query is efficient
• Complexity of administration of slaves
• Doesn’t scale writes

48. Option #3: Redis materialized
views
RESTAURANT
CONSUMER
OWNER
Order Restaurant
taking Management System
of
Copy update() Record
findAvailable()
MySQL
Redis Cache
Database

49. BUT how to implement findAvailableRestaurants()
with Redis?!
?
select r.*
from restaurant r K1 V1
inner join restaurant_time_range tr
on r.id =tr.restaurant_id
inner join restaurant_zipcode sa
on r.id = sa.restaurant_id
K2 V2
where ’94619’ = sa.zip_code
and tr.day_of_week=’monday’
and tr.openingtime <= 1815 ... ...
and 1815 <= tr.closingtime
Solution: Build an index using sorted
sets and ZRANGEBYSCORE

50. Where we need to be
ZRANGEBYSCORE myset 1 6
=
sorted_set
select value,score key value score
from sorted_set
where key = ‘myset’
and score >= 1
and score <= 6

51. We need to denormalize
Think materialized view

52. Simplification #1:
Denormalization
Restaurant_id Day_of_week Open_time Close_time Zip_code
1 Monday 1130 1430 94707
1 Monday 1130 1430 94619
1 Monday 1730 2130 94707
1 Monday 1730 2130 94619
2 Monday 0700 1430 94619
…
SELECT restaurant_id
FROM time_range_zip_code
WHERE day_of_week = ‘Monday’ Simpler query:
§ No joins
AND zip_code = 94619 § Two = and two <
AND 1815 < close_time
AND open_time < 1815

53. Simplification #2: Application
filtering
SELECT restaurant_id, open_time
FROM time_range_zip_code
WHERE day_of_week = ‘Monday’ Even simpler query
• No joins
AND zip_code = 94619
• Two = and one <
AND 1815 < close_time
AND open_time < 1815

54. Simplification #3: Eliminate multiple =’s with
concatenation
Restaurant_id Zip_dow Open_time Close_time
1 94707:Monday 1130 1430
1 94619:Monday 1130 1430
1 94707:Monday 1730 2130
1 94619:Monday 1730 2130
2 94619:Monday 0700 1430
…
SELECT restaurant_id, open_time
FROM time_range_zip_code
WHERE zip_code_day_of_week = ‘94619:Monday’
AND 1815 < close_time
key
range

55. Simplification #4: Eliminate multiple RETURN
VALUES with concatenation
zip_dow open_time_restaurant_id close_time
94707:Monday 1130_1 1430
94619:Monday 1130_1 1430
94707:Monday 1730_1 2130
94619:Monday 1730_1 2130
94619:Monday 0700_2 1430
...
SELECT open_time_restaurant_id,
FROM time_range_zip_code
WHERE zip_code_day_of_week = ‘94619:Monday’
AND 1815 < close_time
✔

56. Using a Redis sorted set as an index
zip_dow open_time_restaurant_id close_time
94707:Monday 1130_1 1430
94619:Monday 1130_1 1430
94707:Monday 1730_1 2130
94619:Monday 1730_1 2130
94619:Monday 0700_2 1430
...
Key Sorted Set [ Entry:Score, …]
94619:Monday [0700_2:1430, 1130_1:1430, 1730_1:2130]
94707:Monday [1130_1:1430, 1730_1:2130]

57. Querying with ZRANGEBYSCORE
Key Sorted Set [ Entry:Score, …]
94619:Monday [0700_2:1430, 1130_1:1430, 1730_1:2130]
94707:Monday [1130_1:1430, 1730_1:2130]
Delivery zip and day Delivery time
ZRANGEBYSCORE 94619:Monday 1815 2359
è
{1730_1}
1730 is before 1815 è Ajanta is open

58. Adding a Restaurant
@Component
public class AvailableRestaurantRepositoryImpl implements AvailableRestaurantRepository {
@Override
public void add(Restaurant restaurant) {
addRestaurantDetails(restaurant);
Store as
addAvailabilityIndexEntries(restaurant); JSON
}
Text
private void addRestaurantDetails(Restaurant restaurant) {
restaurantTemplate.opsForValue().set(keyFormatter.key(restaurant.getId()), restaurant);
}
private void addAvailabilityIndexEntries(Restaurant restaurant) {
for (TimeRange tr : restaurant.getOpeningHours()) {
String indexValue = formatTrId(restaurant, tr); key member
int dayOfWeek = tr.getDayOfWeek();
int closingTime = tr.getClosingTime();
for (String zipCode : restaurant.getServiceArea()) {
redisTemplate.opsForZSet().add(closingTimesKey(zipCode, dayOfWeek), indexValue,
closingTime);
}
}
} score

59. Finding available Restaurants
@Component
public class AvailableRestaurantRepositoryImpl implements AvailableRestaurantRepository {
@Override
public List<AvailableRestaurant>
findAvailableRestaurants(Address deliveryAddress, Date deliveryTime) { Find those that
String zipCode = deliveryAddress.getZip(); close after
int dayOfWeek = DateTimeUtil.dayOfWeek(deliveryTime);
int timeOfDay = DateTimeUtil.timeOfDay(deliveryTime);
String closingTimesKey = closingTimesKey(zipCode, dayOfWeek);
Set<String> trsClosingAfter =
redisTemplate.opsForZSet().rangeByScore(closingTimesKey, timeOfDay, 2359);
Set<String> restaurantIds = new HashSet<String>();
for (String tr : trsClosingAfter) { Filter out those that
String[] values = tr.split("_"); open after
if (Integer.parseInt(values[0]) <= timeOfDay)
restaurantIds.add(values[1]);
}
Collection<String> keys = keyFormatter.keys(restaurantIds);
return availableRestaurantTemplate.opsForValue().multiGet(keys); Retrieve open
} restaurants

60. Sorry Ted!
http://en.wikipedia.org/wiki/Edgar_F._Codd

61. Agenda
• Why polyglot persistence?
• Using Redis as a cache
• Optimizing queries using Redis materialized views
• Synchronizing MySQL and Redis
• Tracking changes to entities

62. MySQL & Redis
need to be consistent

63. Two-Phase commit is not an
option
• Redis does not support it
• Even if it did, 2PC is best avoided http://www.infoq.com/articles/ebay-scalability-best-practices

64. Atomic
Consistent Basically Available
Isolated Soft state
Durable Eventually consistent
BASE: An Acid Alternative http://queue.acm.org/detail.cfm?id=1394128

65. Updating Redis #FAIL
begin MySQL transaction
update MySQL Redis has update
update Redis MySQL does not
rollback MySQL transaction
begin MySQL transaction
update MySQL
MySQL has update
commit MySQL transaction
Redis does not
<<system crashes>>
update Redis

66. Updating Redis reliably
Step 1 of 2
begin MySQL transaction
update MySQL
ACID
queue CRUD event in MySQL
commit transaction
Event Id
Operation: Create, Update, Delete
New entity state, e.g. JSON

67. Updating Redis reliably
Step 2 of 2
for each CRUD event in MySQL queue
get next CRUD event from MySQL queue
If CRUD event is not duplicate then
Update Redis (incl. eventId)
end if
begin MySQL transaction
mark CRUD event as processed
commit transaction

68. Step 1 Step 2
Timer
EntityCrudEvent EntityCrudEvent apply(event) Redis
Repository Processor Updater
INSERT INTO ... SELECT ... FROM ...
ENTITY_CRUD_EVENT
ID JSON processed?
Redis

69. Optimistic
locking Updating Redis
WATCH restaurant:lastSeenEventId:≪restaurantId≫
lastSeenEventId = GET restaurant:lastSeenEventId:≪restaurantId≫
Duplicate
if (lastSeenEventId >= eventId) return; detection
MULTI
SET restaurant:lastSeenEventId:≪restaurantId≫ eventId
Transaction
... update the restaurant data...
EXEC

70. Agenda
• Why polyglot persistence?
• Using Redis as a cache
• Optimizing queries using Redis materialized views
• Synchronizing MySQL and Redis
• Tracking changes to entities

71. How do we generate CRUD
events?

72. Change tracking options
• Explicit code
• Hibernate event listener
• Service-layer aspect
• CQRS/Event-sourcing

73. HibernateEvent EntityCrudEvent
Listener Repository
ENTITY_CRUD_EVENT
ID JSON processed?

74. Hibernate event listener
public class ChangeTrackingListener
implements PostInsertEventListener, PostDeleteEventListener, PostUpdateEventListener {
@Autowired
private EntityCrudEventRepository entityCrudEventRepository;
private void maybeTrackChange(Object entity, EntityCrudEventType eventType) {
if (isTrackedEntity(entity)) {
entityCrudEventRepository.add(new EntityCrudEvent(eventType, entity));
}
}
@Override
public void onPostInsert(PostInsertEvent event) {
Object entity = event.getEntity();
maybeTrackChange(entity, EntityCrudEventType.CREATE);
}
@Override
public void onPostUpdate(PostUpdateEvent event) {
Object entity = event.getEntity();
maybeTrackChange(entity, EntityCrudEventType.UPDATE);
}
@Override
public void onPostDelete(PostDeleteEvent event) {
Object entity = event.getEntity();
maybeTrackChange(entity, EntityCrudEventType.DELETE);
}

75. Summary
• Each SQL/NoSQL database = set of tradeoffs
• Polyglot
persistence: leverage the strengths of SQL and
NoSQL databases
• Use Redis as a distributed cache
• Store denormalized data in Redis for fast querying
• Reliable database synchronization required

76. @crichardson chris.richardson@springsource.com
http://plainoldobjects.com
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