<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"> <html> <head> <title>Using Poly/ML - Chapter 8: Using Multiple Databases</title> </head> <body> <h1><font SIZE="6"><a name="MultipleDatabases8">Chapter 8: Using Multiple Databases</a></font></h1> <p><font color="#FF0000" size="6" face="Arial, Helvetica, sans-serif">This section applies only to Poly/ML version 4 and older.</font></p> <h2><b><font SIZE="4"><a name="ChildDatabases8.1">8.1 Child Databases</a></font></b></h2> <p><font SIZE="3">So far, we have assumed that only one database is being used. In fact, it is possible, and often useful, to create and use multiple databases. New databases are created using </font><font FACE="Courier" SIZE="3"><b>PolyML.make_database</b></font><font SIZE="3">. A full path name must be used for the database name on the command line so that the child database can use the same full path name to locate its parent database in future sessions. For example:</font></p> <p><font FACE="Courier" SIZE="3"><b>unix% poly /usr/staff/john/john_dbase<br> > PolyML.make_database "child_dbase";<br> val it = () : unit<br> > PolyML.quit();</b></font></p> <p><font SIZE="3">will create a new Unix file, </font><font FACE="Courier" SIZE="3"><b>child_dbase</b></font><font SIZE="3">. You can now run ML using the new database by entering:</font></p> <p><font FACE="Courier" SIZE="3"><b>unix% poly child_dbase</b></font></p> <p><font SIZE="3">at the Unix prompt. The new database, </font><font FACE="Courier" SIZE="3"><b>child_dbase</b></font><font SIZE="3">, is initially empty except that it 'inherits' all the definitions contained in its parent database (here </font><font FACE="Courier" SIZE="3"><b>john dbase</b></font><font SIZE="3">). New objects are created in the heap as usual and, if the database is committed, will be saved in </font><font FACE="Courier" SIZE="3"><b>child_dbase</b></font><font SIZE="3">. For example:</font></p> <p><font FACE="Courier" SIZE="3"><b>> val demo = "demo";<br> val demo = "demo" : string<br> > PolyML.commit();<br> val it = () : unit</b></font></p> <p><font SIZE="3">will add the string </font><b><font FACE="Courier" SIZE="3">"demo"</font></b><font SIZE="3"> to the child database. If an object from the parent database is changed, the heap will contain the updated object. </font><b><font FACE="Courier" SIZE="3">PolyML.commit()</font></b><font SIZE="3"> will nor update the parent database, however. Poly/ML will only update the database named on the Unix command line (here </font><font FACE="Courier" SIZE="3"><b>child_dbase</b></font><font SIZE="3">). This means that any change to an object in a parent database will not persist between ML sessions.</font></p> <p><font SIZE="3">The child database is not accessible from the parent database. For example:</font></p> <p><font FACE="Courier" SIZE="3"><b>unix% poly john_dbase<br> > demo;<br> Error- Value or constructor (demo) has not been declared<br> Found near demo<br> Exception static-errors raised<br> ></b></font></p> <p><font SIZE="3">The distributed database for the current version of the system is several megabytes large. Giving each person their own copy of this database would use a lot of disk space. A better solution is to give each person a different child of the distributed database. For example, assuming the distributed database is named </font><font FACE="Courier" SIZE="3"><b>ML_dbase</b></font><font SIZE="3">, then typing</font></p> <p><font FACE="Courier" SIZE="3"><b>unix% poly /usr/share/ML_dbase<br> > PolyML.make_database "/usr/staff/john/john_dbase";<br> val it = () :...<br> > PolyML.make_database "/usr/staff/mary/mary_dbase";<br> > val it = () :...</b></font></p> <p><font SIZE="3">would create a child database called </font><b><font FACE="Courier" SIZE="3">/usr/staff/john/john_dbase</font></b><font SIZE="3"> and another child database called </font><font FACE="Courier" SIZE="3"><b>/usr/staff/mary/mary_dbase</b></font><font SIZE="3">. After all users have been given child databases, the parent database (</font><font FACE="Courier" SIZE="3"><b>ML_dbase</b></font><font SIZE="3">) should be made read-only. (The parent database must be made readable by all users of the system, as this database contains the actual code for the ML compiler.) In this way, the database containing the system will be shared, economising on disk space, but each person will have their own database for their own particular work.</font></p> <h2><b><font SIZE="4"><a name="CopyingLinkedDatabases8.2">8.2 Copying Linked Databases</a></font></b></h2> <p><font SIZE="3">Suppose John has a database called <b>john_dbase</b> which is a child database of the main Poly/ML database, </font><font FACE="Courier" SIZE="3"><b>ML_dbase</b></font><font SIZE="3">. In addition, he has created a child database of his own called <b>child_dbase.</b></font></p> <p><font SIZE="3">Mary has no databases, and wishes to copy John's. She can copy the database files with the following commands.</font></p> <p><font FACE="Courier" SIZE="3"><b>unix% cp /usr/staff/john/john_dbase /usr/staff/mary/mary_dbase<br> unix% cp /usr/staff/john/child_dbase /usr/staff/mary/child_dbase</b></font></p> <p><font SIZE="3">Unfortunately, this does not adjust the full pathnames inside the databases that point upwards to parent databases - Mary's </font><b><font FACE="Courier" SIZE="3">child_dbase</font></b><font SIZE="3"> still points to</font><font FACE="Courier" SIZE="3"> <b>john_dbase</b></font><font SIZE="3">. Mary can correct this using </font><font FACE="Courier" SIZE="3"><b>changeParent</b></font><font SIZE="3">.</font></p> <p><font SIZE="3">The program </font><font FACE="Courier" SIZE="3"><b>changeParent </b></font><font SIZE="3">takes two arguments. The first is the child database that contains a parent pointer, and the second is the new parent name.</font></p> <p><font FACE="Courier" SIZE="3"><b>unix% changeParent child_dbase /usr/staff/mary/mary_dbase</b></font></p> <p><font SIZE="3">This has made the new </font><b><font FACE="Courier" SIZE="3">child_dbase</font></b><font SIZE="3"> point to </font><font FACE="Courier" SIZE="3"><b>mary_dbase</b></font><font SIZE="3">. Note that full pathnames must be used for parent names.</font></p> <h2><font SIZE="4"><b><a name="DatabaseHeapLimits8.3">8.3 Database and Heap Limits</a></b></font></h2> <p><font SIZE="3">Do not run </font><b><font FACE="Courier" SIZE="3">discgarb</font></b><font SIZE="3"> on a parent database because this rearranges all the objects in that database without updating the references held in its child databases. You can add extra bindings into a parent database safely, as long as none of its children are in use.</font></p> <p><font SIZE="3">Poly/ML can create up to 3 levels of database hierarchy, but there is no limit on the number of child databases at each level.</font></p> <p><font SIZE="3">In ML, most objects that are created in the heap or in a database are immutable, that is they cannot be changed once they have been created. The only mutable objects, that is objects that can have their contents changed, are </font><font FACE="Courier" SIZE="3"><b>refs</b></font><font SIZE="3">. Mutable objects rarely account for more than 25% of the objects in a database.</font></p> <p><font SIZE="3">A database may grow to a total of 36Mb, where 32Mb is reserved for immutable objects, and 4Mb is reserved for mutable objects.</font></p> <p><font SIZE="3">The <a href="HeapParms9.html">maximum size of the local heap</a> is set by the </font><b><font FACE="Courier" SIZE="3">-h</font></b><font SIZE="3"> flag but may not exceed an architecturally-imposed limit.</font></p> <p><font size="2"><b>Copyright (c) 2000 CUTS and contributers.</b></font></p> </body> </html>
