The document discusses the development of Creson, a system designed for efficient support of shared objects over a NoSQL database, providing strong consistency guarantees for concurrent accesses. It highlights the shortcomings of traditional object-relational mappings (ORMs) and introduces Creson's unique architecture, which utilizes a listenable key-value store to manage remote operations on objects stored in the cloud. Additionally, it presents a use case of Creson with Stacksync, an open-source cloud storage alternative to Dropbox, detailing its metadata management enhancements over traditional relational databases.

1. Des objets dans le cloud, et qui y restent :
L’expérience du développement de
CRESON, support pour des objets distants
fortement cohérents dans Infinispan
Prof Etienne Rivière,
Ecole Polytechnique de Louvain, UCLouvain, Belgique
(je ne fais pas d’IA non plus, ni de Machine Learning)
OSIS: Open Source pour le Cloud
14 juin 2019, Paris
etienne.riviere@uclouvain.be

2. Open Source pour le Cloud - OSIS - Paris - Juin 2019
Menu for today
• Object-oriented programming and data in the cloud
• Why ORMs/OGM are ill-suited for sharing concurrent objects
• How we can do better by keeping objects in the cloud: CRESON
• Joint work with: Pierre Sutra (now Télécom SudParis), Cristian Cotes,
Marc Sánchez Artigas, Pedro Garcia Lopez (Uni. Rovira iVirgili, Spain),
Emmanuel Bernard,William Burns and Galder Zamarreño (Red Hat)
• Published at IEEE ICDCS 2017
• Result of the LEADS FP7 EU project, with Uni. Neuchâtel and Red Hat
• With another open source FP7 EU project, CloudSpaces
• Some reflexions on the academia-open source interaction
2

3. Open Source pour le Cloud - OSIS - Paris - Juin 2019
Your typical Cloud application
✓ Scalability and elasticity (pay-as-you-go model)
✓ Availability even in the presence of failures
✓ Simplicity and separation of concerns
3
Presentation
Users
Presentation
Presentation
Application
logic
Application
logic
NoSQL
Data store
➡ NoSQL database: elastic and scalable storage for application state
• But typically offering a simple key/value store interface
➡ Application data shared and accessed concurrently by many service instances
🤔 Can we make the storage and access to application data in the database transparent?
Figure 1-5. Database architecture for the taxi-hailing application.
PASSENGER
MANAGEMENT
PASSENGER
MANAGEMENT
DATABASE
DRIVER
MANAGEMENT
DATABASE
TRIP
MANAGEMENT
DATABASE
REST
API
DRIVER
MANAGEMENT
REST
API
TRIP
MANAGEMENT
REST
API
DATABASE
ADAPTER
DATABASE
ADAPTER
DATABASE
ADAPTER
On the surface, the Microservices Architecture pattern is similar to SOA. With both
approaches, the architecture consists of a set of services. However, one way to think
about the Microservices Architecture pattern is that it’s SOA without the commercialization
and perceived baggage of web service specifications (WS-*) and an Enterprise Service
Bus (ESB). Microservice-based applications favor simpler, lightweight protocols such as
REST, rather than WS-* They also very much avoid using ESBs and instead implement
ESB-like functionality in the microservices themselves. The Microservices Architecture
pattern also rejects other parts of SOA, such as the concept of a canonical schema for
data access
The Benefits of Microservices
The Microservices Architecture pattern has a number of important benefits. First, it
tackles the problem of complexity. It decomposes what would otherwise be a monstrous
monolithic application into a set of services. While the total amount of functionality is
unchanged, the application has been broken up into manageable chunks or services
Each service has a well-defined boundary in the form of a remote procedure call
(RPC)-driven or message-driven API. The Microservices Architecture pattern enforces

4. Open Source pour le Cloud - OSIS - Paris - Juin 2019
Object-oriented apps with NoSQL
💡 Encapsulate application data in objects stored in some NoSQL DB
• Object-DB mapper (Hibernate OGM)
• Language integration (Java Persistence API)
☹ Object access: get serialized object, new instance in local memory
• Methods called locally
• Data structure traversal = multiple back-and-forth with DB
☹ State-based replication of entire serialized object
☹ Weak or no consistency guarantees for shared objects
• e.g. Objectify (part of Google App Engine) not thread-safe
4
mappingO
serialized objectin-memory object
net
state-based replication

5. Open Source pour le Cloud - OSIS - Paris - Juin 2019
Our proposal: CRESON
💡 Efficient support for shared objects over a NoSQL database
➡ Callable distributed objects
• Objects instantiated from DB representation but remain at server side
➡ Replication at the level of operations (method calls)
• Support for arbitrary-large objects
➡ Strong consistency guarantees for concurrent accesses
• Including for composed operations accessing multiple objects
5
mappingO
serialized objectin-memory object
net
O
state-based replication
proxy
net
operation
O
O O
operation-based replication
Traditional Object-NoSQL mapping
CRESON: callable and replicated shared objects

6. Open Source pour le Cloud - OSIS - Paris - Juin 2019
CRESON: components
• LKVS: a novel NoSQL storage abstraction
• Listenable Key-Value Store
• Object management logic built atop the LKVS
• Handle method calls for shared objects
• Maintain multiple replicas of in-memory objects
• Implement state-machine (operation-based) replication
• Client-side integration with the Java language
• Using annotations similar to JPA
6

7. Open Source pour le Cloud - OSIS - Paris - Juin 2019
Listenable Key/Value Store
• Extend classical Key/Value Store API …
• void put(key k, value v)
• value_type get(key k)
• … with two additional calls
• void regListener(key k, client_id c, handler h)
• void unregListener(key k, client_id c)
• put() call for key k call all handlers associated to k
• Handler receives new value for the key
• For handlers who return something, notify listener client
7

8. Open Source pour le Cloud - OSIS - Paris - Juin 2019
LKVS illustrated
8
Client application instances
C1 C3C2
Key-Value Store
v1put(k1,v1)
v1

9. Open Source pour le Cloud - OSIS - Paris - Juin 2019
LKVS illustrated
8
regListener(k1,c2,h2)
Client application instances
C1 C3C2
Key-Value Store
h2
v1

10. Open Source pour le Cloud - OSIS - Paris - Juin 2019
LKVS illustrated
8
Client application instances
C1 C3C2
Key-Value Store
h2
v1
handlers for k1

11. Open Source pour le Cloud - OSIS - Paris - Juin 2019
LKVS illustrated
8
Client application instances
C1 C3C2
Key-Value Store
h2
v1
handlers for k1
regListener(k1,c3,h3)
h3

12. Open Source pour le Cloud - OSIS - Paris - Juin 2019
LKVS illustrated
8
Client application instances
C1 C3C2
Key-Value Store
h2
v1
handlers for k1
h3

13. Open Source pour le Cloud - OSIS - Paris - Juin 2019
LKVS illustrated
8
Client application instances
C1 C3C2
Key-Value Store
h2
v1
handlers for k1
h3
v1’put(k1,v1’)

14. Open Source pour le Cloud - OSIS - Paris - Juin 2019
LKVS illustrated
8
Client application instances
C1 C3C2
Key-Value Store
h2
v1
handlers for k1
h3
v1’
v1’
false true
notification of
listener C3
success

15. Open Source pour le Cloud - OSIS - Paris - Juin 2019
Object management in CRESON (1)
• Client-side proxy + a session handler
• Object handler owns actual object
• Method calls and object creation/
closing sent as operations through
regular put() calls for key k
• Intercepted by handlers registered for k
• Client calling method receives its result
in its listener notification
• Lifetime of object for key k
• First use: on server, map from serialized
object in DB, or instantiate new object
• Object closed by last client for key k: on
server, object serialized and stored in DB
9
Client
application
instance
O1
C1
Listenable
Key-Value Store
kO1
Object
handler
Session
handler
Proxy
method calls
put(kO1,op)
listener for
kO1
O1

16. Open Source pour le Cloud - OSIS - Paris - Juin 2019
State Machine Replication
• Objects replicated at the LKVS side
• (dormant) Copies of serialized objects
• In-memory instances of currently-used shared objects
• Fault-tolerance: survive up to f faults with f+1 servers
• In-memory copies must remain consistent under
concurrent accesses
➡ Operation-based (state machine) replication
• Copies receive the exact same stream of operations
⚠ Constraint: objects must be deterministic
✓ Applying operations in the same order to deterministic objects
ensures strong consistency (linearizability)
10

17. Open Source pour le Cloud - OSIS - Paris - Juin 2019
Putting everything together
11
Client
application
instances
O1
C1 C3C2
Listenable
Key-Value Store
trigger
kO1
call
Object
handlers
Session
handlers
Proxies
1
3
4
5
listener for kO2
2
method calls
put(kO1,op)
notify
kO2
ServersClients
linearize &
replicate op
listeners for
kO1
O1 O2
O1

18. Open Source pour le Cloud - OSIS - Paris - Juin 2019
CRESON guarantees
✓ Strong consistency: linearizability
✓ Composition
• A shared object can call other objects
• Maintains linearizability
✓ Persistence
✓ Disjoint-access parallelism
• Accesses to distinct objects use distinct LKVS components
✓ Elasticity
• Can add/remove storage nodes without restarting the service
12

19. Open Source pour le Cloud - OSIS - Paris - Juin 2019
Use case and Interface
• StackSync: open-source equivalent of Dropbox
• Synchronization of user file system with cloud-stored file system
• Sharing of folders and files between users spaces
• Trace collected from Ubuntu1 personal cloud service
• Data stored in immutable object store (OpenStack Swift)
• Metadata requires strong consistency
13

20. Open Source pour le Cloud - OSIS - Paris - Juin 2019
Original metadata
management in StackSync
• PostgreSQL relational database
• Stores the association between users, files, authorized devices, etc.
• Accessed from the application using SQL directly (manual object-relational mapping)
• Storage and querying handled by a Facade object
• Performance: use of stored procedures implementing app. logic at server side
• Scalability: sharded (partitioned) database using PL/Proxy
☹ No support for elastic scaling
☹ No consistency (ACID) guarantees across shards
14
Chunk ItemVersion
Item
WorkspaceUser Device
Facade
⭑
⭑⭑
⭑
⭑
⭑
⭑
⭑⭑⭑

21. Open Source pour le Cloud - OSIS - Paris - Juin 2019
StackSync metadata
management with CRESON
• Logic for metadata management re-
implemented in plain Java, as methods in
StackSync’s classes
• Determining which objects to store
independently in CRESON depends on update
visibility requirements
• Embedding Item, etc. to Workspace= atomic
operations within one Workspace object
• Portage was less than a week of effort
• Code is simpler and more coherent than with SQL
15
to modify the application metadata. We depict the class schema
of the SyncService in Figure 3. At some SyncService instance,
a thread interacts with CRESON using a Facade object.
Classes including the Facade and below differ from one
persistence technology to another. Their portage is where we
spent most of our effort.
A Workspace object models a synced folder. It is
composed of files and directories (Item in Figure 3). For
each Item, the SyncService stores versioning information
as ItemVersion objects. Similarly to other personal cloud
storage services, StackSync operates at the sub-file level by
splitting files into chunks; this greatly reduces the cost of data
synchronization. A Chunk is immutable and identified with a
fingerprint. It may appear in one or more files.
Relational Approach. To scale up the original relational
implementation, we followed conventional wisdom and sharded
metadata across multiple servers. The key enabler of this pro-
cess is PL/Proxy, a stored procedure language for PostgreSQL.
PL/Proxy allows dispatching requests to several PostgreSQL
servers. It was originally developed by Skype to scale up their
services to millions of users.
Following this approach, we horizontally partition metadata
by hashing user identifiers with PostgreSQL built-in function.
As a result, all the metadata of a user is slotted into the
same shard. Any request for committing changes made by the
same user is redirected to the appropriate shard. In detail, we
accomplish this with the following PL/Proxy procedure (we
omit some parameters for readability):
1 @Entity(key = "id")
2 public class Workspace {
3
4 public UUID id;
5 private Item root;
6 private List<User> users;
7
8 /* ... */
9
10 public boolean isAllowed(User user) {
11 return users.contains(user.getId());
12 }
13 }
1 @Entity(key = "id")
2 public class Facade {
3
4 @Entity(key = "deviceIndex")
5 public static Map<UUID,Device> devices;
6
7 @Entity(key = "workspaceIndex")
8 public static Map<UUID,Workspace> workspaces;
9
10 @Entity(key = "userIndex")
11 public static Map<UUID,User> users;
12
13 public UUID id;
14
15 /* ... */
16
17 public boolean add(Device device) {
18 return deviceMap.putIfAbsent(
19 device.getId(),device) == null;
20 }
21 }
Fig. 4. Workspace and Facade classes
Chunk ItemVersion
Item
WorkspaceUser Device
Facade
⭑
⭑⭑
⭑
⭑
⭑
⭑
⭑⭑⭑
independent objects stored in CRESON
embedded objects

22. Open Source pour le Cloud - OSIS - Paris - Juin 2019
CRESON interface
• Integration in Java (using AspectJ)
• Similar to the Java Persistence API (Hibernate,
etc.) … but using remote calls
• @Entity(key = “id”) annotation
• Object o of this class stored in CRESON
under key (classname+”:”+o.id)
• Store static field in CRESON under key
(classname+”:”+id)
• Only applies to static fields!
• No further action required from developer
• Shared maps (e.g. deviceIndex) are
transparently stored as collections in LKVS
16
to modify the application metadata. We depict the class schema
of the SyncService in Figure 3. At some SyncService instance,
a thread interacts with CRESON using a Facade object.
Classes including the Facade and below differ from one
persistence technology to another. Their portage is where we
spent most of our effort.
A Workspace object models a synced folder. It is
composed of files and directories (Item in Figure 3). For
each Item, the SyncService stores versioning information
as ItemVersion objects. Similarly to other personal cloud
storage services, StackSync operates at the sub-file level by
splitting files into chunks; this greatly reduces the cost of data
synchronization. A Chunk is immutable and identified with a
fingerprint. It may appear in one or more files.
Relational Approach. To scale up the original relational
implementation, we followed conventional wisdom and sharded
metadata across multiple servers. The key enabler of this pro-
cess is PL/Proxy, a stored procedure language for PostgreSQL.
PL/Proxy allows dispatching requests to several PostgreSQL
servers. It was originally developed by Skype to scale up their
services to millions of users.
Following this approach, we horizontally partition metadata
by hashing user identifiers with PostgreSQL built-in function.
As a result, all the metadata of a user is slotted into the
same shard. Any request for committing changes made by the
same user is redirected to the appropriate shard. In detail, we
accomplish this with the following PL/Proxy procedure (we
omit some parameters for readability):
1 @Entity(key = "id")
2 public class Workspace {
3
4 public UUID id;
5 private Item root;
6 private List<User> users;
7
8 /* ... */
9
10 public boolean isAllowed(User user) {
11 return users.contains(user.getId());
12 }
13 }
1 @Entity(key = "id")
2 public class Facade {
3
4 @Entity(key = "deviceIndex")
5 public static Map<UUID,Device> devices;
6
7 @Entity(key = "workspaceIndex")
8 public static Map<UUID,Workspace> workspaces;
9
10 @Entity(key = "userIndex")
11 public static Map<UUID,User> users;
12
13 public UUID id;
14
15 /* ... */
16
17 public boolean add(Device device) {
18 return deviceMap.putIfAbsent(
19 device.getId(),device) == null;
20 }
21 }
Fig. 4. Workspace and Facade classes

23. Open Source pour le Cloud - OSIS - Paris - Juin 2019
Implementation
• Built over the open source Infinispan NoSQL
• Basis for Red Hat JBosss Data Grid
• LKVS = 13,500 SLOC ; CRESON = 4,000 SLOC
• Along with other developments from the LEADS
EU project, CRESON was integrated to the
‘experimental’ features in Infinispan
• Red Hat developed and integrated the LKVS
• Also used for data curation and processing
• Under its previous name ‘Atomic Object Factory’
17

24. Open Source pour le Cloud - OSIS - Paris - Juin 2019
Evaluation
• Experiments
• Micro-benchmarks with single and multiple objects
✓ StackSync: throughput and latency using a real trace
• Using a real trace from the “Ubuntu One” drive
✓ StackSync: replication and elasticity
• Using trace collected from Ubuntu1 personal Cloud
• Cluster of 8-core/8GB Xeon 2.5 GHz, switched 1 Gbps
• 2 to 6 Infinispan servers (default = 3)
• Replication factor is 2 by default
18

25. Open Source pour le Cloud - OSIS - Paris - Juin 2019
StackSync performance: throughput
• 24 StackSync clients, each with 10 threads
• Dispatch commands from trace using RabbitMQ
• PostgreSQL using 2 additional PL/Proxy nodes (out of 6)
19
0
2000
4000
6000
8000
10000
2 4 6
Throughput(operations/s)
CRESON nodes
0
2000
4000
6000
8000
10000
2 4
Throughput(operations/s)
PostgreSQL shards
(higher is better)(higher is better)
+ 2 additional PL/Proxy nodes
median performance
using 6 servers: +50%

26. Open Source pour le Cloud - OSIS - Paris - Juin 2019
StackSync performance: latency
• About 300 doCommit() operations per second
• Most common method: adding new files to a personal space
20
0
20
40
60
80
100
5 10 15 20 25 30 35 40
CRESON PostgreSQL
CDF(%)
Latency (ms)
(leftmost is better)

27. Open Source pour le Cloud - OSIS - Paris - Juin 2019
Elasticity and replication
• Single StackSync client with synthetic workload
21
0
1000
2000
3000
4000
2 3 4 5 6
Throughput(operations/s)
CRESON nodes
no replication
replication factor of 2
(higher is better)
adding server with no
restart (elasticity)
Cost of replication

28. Open Source pour le Cloud - OSIS - Paris - Juin 2019
Academia contributing
to open source
• Open source was beneficial in LEADS
• Implement ideas in a state-of-the-art KVS
• Got feedback on code and integrated staging
• Much easier IP management (often complicated in EU projects)
• But is it enough?
• Non-maintained code gets deprecated
• EU (and generally, academic) project lifecycles does not match
requirements for high impact in open source
• Lack of funding and competencies for long-term support
• Funding agencies should act?
• Movement towards reproducible research artifacts
• Benefits from and to open source
22

29. Open Source pour le Cloud - OSIS - Paris - Juin 2019
Conclusion
• Client-side object-NoSQL mapping: costly and inefficient
for strongly-consistent shared objects
• CRESON: a solution to host callable shared objects
directly on a NoSQL DB
• Leverage the support from a novel LKVS abstraction
implemented in the Infinispan KVS
• Simple programming model and better performance and
elasticity than a state-of-the-art PostgreSQL solution
23

30. Des objets dans le cloud, et qui y restent :
L’expérience du développement de
CRESON, support pour des objets distants
fortement cohérents dans Infinispan
🤔??
questions?