Examples

Examples

Workflow Simulation
Financial Transactions

A simple workflow is simulated in the following example:

'simulate' transaction creates and executes an workflow instance for every work item, W
'source' predicate stores the work items
'workflow' transaction applies the workflow, which is a sequence of three tasks, to an work item W
'task(I,W)' transaction applies task I to work item W; since this is a simulation, the task does nothing but monitor its own execution
'load2' transaction creates two work items

Database file: 'workflow.db':
updatable source/1.
finished :- not(unfinished).
unfinished :- source(W).

Transaction file 'workflow.ctr':
simulate :- getItem(W) * (workflow(W) # simulate).
simulate :- finished.
getItem(W) :- del(source(W)).
workflow(W) :- task(W,1) * task(W,2) * task(W,3).
task(I,W) :- monitor(task(I,W)).
load2 :- ins(source(a)) * ins(source(b)).
\| ?- ctr_comp(workflow).	- program is compiled
yes
?- exec(load2).	- load2 inserts two work items into the database
yes
\|
\| ?- exec(simulate).	- execute simulate transaction
committing task(a,1)	- the first workflow instance processes item a
committing task(a,2)
committing task(b,1)	- the second workflow instance processes item b
committing task(a,3)
committing task(b,2)
committing task(b,3)
yes

The following example, which simulates financial transactions, shows how updates can be combined with queries to define complex transactions.

'balance(Acct,Amt)' relation contains the balance Amt for account Acct
'change_balance(Acct,Bal1,Bal2)' transaction updates the balance of the account Acct from Bal1 to Bal2
'withdraw(Amt,Acct)' transaction withdraws the amount Amt from account Acct
'deposit(Amt,Acct)' transaction deposits amount Amt to account Acct
'transfer(Amt,Acct1,Acct2)' transaction transfers the amount Amt from account Acct1 to account Acct2
'init' transaction sets up some accounts
'test(Amt)' transaction transfers amount Amt from account Acct1 to Acct2, Acct3 and Acct4

Transaction file 'financial.ctr':
\| ?- ctr_comp(workflow).	- program is compiled
transfer(Amt,Acct1,Acct2) :- o1(withdraw(Amt,Acct1) # deposit(Amt,Acct2)).
withdraw(Amt,Acct) :- o((Amt > 0) * bal(Acct,B1) * (Amt < B1)
* del(bal(Acct,B1)) * (B2 is B1-Amt)
* ins(bal(Acct,B2)) * monitor(withdraw(Amt,Acct))).
deposit(Amt,Acct) :- o ((Amt > 0) * del(bal(Acct,B1)) * (B2 is B1+Amt)
* ins(bal(Acct,B2)) * monitor(deposit(Amt,Acct))).
init :- ins(bal(acct1,100)) * ins(bal(acct2,100)) * ins(bal(acct3,100))
* ins(bal(acct4,100)) * ins(bal(acct5,100)) * ins(bal(acct6,100)).
test(Amt) :- o1(transfer(Amt,acct1,acct2) # transfer(Amt,acct1,acct3)
# transfer(Amt,acct1,acct4)).

Database file 'financial.db':
updatable bal/2.
\| ?- ctr_comp(financial).	- program is compiled
yes
\| ?- exec(init).	- init inserts $100 in 6 accounts
yes
\| ?- exec(test(10)).	- test transfers $10 from the first account to other 3 accounts
committing withdraw(10,acct1)	- $10 are withdrawn from acct1
committing deposit(10,acct2)	- $10 are deposited into acct2
committing withdraw(10,acct1)	- $10 are withdrawn from acct1
committing deposit(10,acct3)	- $10 are deposited into acct3
committing withdraw(10,acct1)	- $10 are withdrawn from acct1
committing deposit(10,acct4)	- $10 are deposited into acct4
yes	- test completed with acct1 balance $70
\| ?- exec(test(30)).	- test transfers $30 from the first account to other 3 accounts
committing withdraw(30,acct1)
committing deposit(30,acct2)
committing withdraw(30,acct1)
committing deposit(30,acct3)
undoing deposit(30,acct3)
undoing withdraw(30,acct1)	- the third withdraw failed, so test execution is rolled back
undoing deposit(30,acct2)
undoing withdraw(30,acct1)
no