DALi A database abstraction layer for multi-user web applications

1. DALi
A database abstraction layer for
multi-user web applications
Johan Brichau, Andy Kellens
Monday 22 August 11

2. Goals of DALi
‣ Independence of specific database platform
- Magma, GOODS
- Gemstone (GLASS)
- Image persistency
‣ Focus on “Develop in Pharo, deploy in Gemstone”
- ... but don’t limit your options
‣ Tx API for handling concurrency conflicts
- “Application-level” semantic concurrency conflicts
- Parallel web request execution conflicts
DALi - database abstraction layer 2
Monday 22 August 11

3. Use cases: shared data editing
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Monday 22 August 11

4. Transaction conflicts
‣ GOODS / Magma / ...
- Keep a separate database view per web session
- Concurrent web editing conflicts are database Tx conflicts
WASession
DB
WASession
WASession
VM
DALi - database abstraction layer 4
Monday 22 August 11

5. Transaction conflicts
‣ GOODS / Magma / ...
- Keep a separate database view per web session
- Concurrent web editing conflicts are database Tx conflicts
WASession
DB
WASession
WASession
VM
WASession
WASession
WASession
VM
DALi - database abstraction layer 4
Monday 22 August 11

6. Transaction conflicts
‣ GEMSTONE (GLASS)
- Keep a database view per VM (aka gem)
- Concurrent web editing conflicts are not database Tx conflicts
WASession
DB
WASession
WASession
VM
WASession
WASession
WASession
VM
DALi - database abstraction layer 5
Monday 22 August 11

7. DALi’s API: Transactions DS
G OO
“Native” db Magma
AbstractDAO
session
Gems
tone
pe Ima
rsi ge
ste
nc
YourAppDAO y
Text
Text
Tx code
dao
startTransaction: [ ... ]
forObject: ... Tx commit succeeded
onSuccess: [ ... ]
onFailure: [ ... ]
Tx aborted
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Monday 22 August 11

8. DALi’s API: change tracking
State change
DALObject
tracker
Is used internally to detect
YourAppDomainObject semantic application-level
tx conflicts
setValue: aNumber
Record the changed attribute
myValue := aNumber. in the state change tracker
self markAsChanged: #myValue
DALi - database abstraction layer 7
Monday 22 August 11

9. Why change tracking?
‣ Essential in GLASS
- Only means to detect conflicts between different Seaside sessions
- Integrates with GLASS’ 1 commit per web request
DALi’s Tx blocks prevent conflicts driven by the state tracker
Gemstone Tx block prevent actual db conflicts
Both do an abort and retry the request
DALi’s retried request will execute the onFailure blocks
‣ Finer-grained tx conflicts in GOODS
- Attribute-level instead of object-level conflicts
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Monday 22 August 11

10. Some more details
‣ Identification & change tracking
- Unique ids for each DALObject
- Versionstamps for each DALObject
‣ DALi-GOODS
- Nesting of transactions
- Manual write barrier
- Tracked collections
- Proper unique ids
‣ DALi-GEMSTONE
- Nesting of transactions
- Integrated with Seaside GLASS
- Use of oops for unique ids
DALi - database abstraction layer 9
Monday 22 August 11

11. Future
‣ Support for communicating state changes between
sessions
- We have change tracking already for conflict detection
- We could send out notifications on db updates on commit/abort
‣ Update backends
- Magma backend has not been maintained
- Need to try an RDB backend
DALi - database abstraction layer 10
Monday 22 August 11

12. Get it...
Want to give it a try?
http://www.squeaksource.com/DALi
http://ss3.gemstone.com/DALi
License: MIT
DALi - database abstraction layer 11
Monday 22 August 11