Friday, 17 March 2017

Composite Database example with details

This post gives more details of the example contained in the posting Composite Databases in this blog.

The following transcripts use the beta version 5.7 of Pyrrho dated 16 March 2017, and using localhost instead of servA, with servers A and B using folders \A and \B respectively. I have set a debugging –D flag on server A so that that we can see the use of RVVs and ETags.
We begin by setting up the databases on servers A and B:
Database A:
create table D (e int primary key, f char, g char)

insert into D values (1,'Joe','Soap'), (2,'Betty','Boop')

Database B:

create table H (e int primary key, k char, m int)
insert into H values (1,'Cleaner',12500), (2,'Manager',31400)
[create view W of (e int, f char, g char) as get

create view V as select * from W natural join H

The square brackets here are added because of the embedded newline added in the formatting of the page.
After setting up the databases on A and B with B’s views defined, we see the transaction log contents for A and B. Some of the numbers shown will be used in RVV and ETags in what follows.

We see at position 381 that the URL http://localhost:8180/A/A/D has been provided in metadata for the view W. W was defined in position 366 in terms of the anonymous structure with columns E, F, G declared at position 290.

The use of position numbers instead of identifiers in the definition of view V at position 439 is a standard feature of Pyrrho to allow renaming of objects.
In the blog post we now have the following operations on database B:
select e,f,m,check from V where e=1

Note that in normal use there is no need to request the check pseudocolumn: it is here so can show what is happening within the two databases. The database API uses it to implement the Versioned feature in client side “database model” classes.
Here, the check value was requested explicitly in the SELECT statement, and shows that this row of the join uses a row from A with defining position 209 placed there in transaction 193 (see the log for A) and a row from B with defining position 208 arising from transaction 192 (see the log for B).
The debug information for server A shows the REST request from B to A, and the ETag it constructed.

The ETag consists of an RVV for the first row of the result (mentioned above), and a readCheck for the read operation carried out by A. This was a specific row in table D (position 69) with key (1) .
The next operation is
update v set f='Elizabeth' where e=2

We see there is now an updated ETag supplied by A showing the new transaction that has updated the record defined at 241.
Also check B’s view using the join:

The next operation is an Insert into the View/RestView/Join combination:
[insert into v(e,f,g,k,m)

This time the readCheck information indicates it will conflict with any read operation on table 69.
And again verifying the view from B:

Finally, we try a deletion from the View/RestView/Join combination:

No comments:

Post a Comment