StandardStMan.h

Classes

StandardStMan -- The Standard Storage Manager (full description)

class StandardStMan : public SSMBase

Interface

Public Members
explicit StandardStMan (Int bucketSize = 0, uInt cacheSize = 1)
explicit StandardStMan (const String& dataManagerName, Int bucketSize = 0, uInt cacheSize = 1)
~StandardStMan()
Private Members
StandardStMan (const StandardStMan& that)
StandardStMan& operator= (const StandardStMan& that)

Description

Review Status

Programs:
Tests:

Prerequisite

Etymology

StandardStMan is the data manager which stores the data in a standard way. I.e. it does not use special techniques like other storage managers do.

Synopsis

StandardStMan is meant as the storage manager to be used standardly. Other storage managers like IncrementalStMan and the TiledStMan derivatives should only be used when appropriate.
Like the other storage managers StandardStMan uses bucket-based access to its data. where a bucket contains the number of columns and rows that fit best. Variable length strings are stored in separate buckets because they do not fit in the fixed bucket layout used for the other columns. Only fixed length strings and strings <= 8 characters are stored directly. Note that, in fact, fixed length string means maximum length strings. It can be set using the setMaxLength function in class ColumnDesc or class BaseColumnDesc.

The file size is at least the size of a bucket, even if only the table contains only a few rows, thus uses only a fraction of a bucket. The default bucketsize is 32 rows. This means that if it is known in advance that the table will contain many more rows, it might make sense to construct the StandardStMan with a larger bucketsize.

StandardStMan is a robust storage manager. Care has been taken that its index cannot be corrupted in case of exceptions like device full or crash.

StandardStMan supports the following functionality:

  1. Removal of rows. This leaves some empty space in a bucket. An empty bucket will be reused.
  2. Addition of rows. This is always done in the last bucket and a new bucket is added when needed.
  3. Removal of a column. This also leaves empty space, which will be reused when a newly added column fits in it.
  4. Addition of a column. If available, empty column space is used. Otherwise it creates as many new buckets as needed.
All direct data (scalars and direct arrays) is stored in the main file. Indirect arrays (except strings) are stored in a second file. Indirect string arrays are also stored in the main file, because in that way frequently rewriting indirect strings arrays wastes far less space.

As said above all string arrays and variable length scalar strings are stored in separate string buckets.

Motivation

StManAipsIO is the standard storage manager used so far. Its major drawback is that it is memory based which makes it not usable for large tables. Furthermore it is not a very robust storage manager. When a system crashes, tables might get corrupted.
These drawbacks have been adressed in this new StandardStman. It uses a bucket-based access scheme and makes sure that its indices are stored in a way that they can hardly get corrupted.

Example

The following example shows how to create a table and how to attach the storage manager to some columns. 
   SetupNewTable newtab("name.data", tableDesc, Table::New);
   StandardStMan stman;                     // define storage manager
   newtab.bindColumn ("column1", stman);    // bind column to st.man.
   newtab.bindColumn ("column2", stman);    // bind column to st.man.
   Table tab(newtab);                       // actually create table

The following example shows how to create a StandardStMan storage manager for a table with 16 rows. By giving the (expected) nr of rows to the storage manager, it can optimize its bucket size. 

   SetupNewTable newtab("name.data", tableDesc, Table::New);
   StandardStMan stman(-16);
   newtab.bindAll ("column1", stman);       // bind all columns to st.man.
   Table tab(newtab);                       // actually create table

Member Description

explicit StandardStMan (Int bucketSize = 0, uInt cacheSize = 1)
explicit StandardStMan (const String& dataManagerName, Int bucketSize = 0, uInt cacheSize = 1)

Create a Standard storage manager with the given name. If no name is used, it is set to "SSM" The name can be used to construct a ROStandardStManAccessor object (e.g. to set the cache size).
The cache size has to be given in buckets.
The bucket size can be given in 2 ways:
- A positive number gives the bucket size in bytes. The number of rows per bucket will be calculated from it.
- A negative number gives the number of rows per bucket. The bucket size in bytes will be calculated from it. Note that in this way the maximum bucketsize is 32768 (minimum is 128).
- The default 0 means that 32 rows will be stored in a bucket.
Note that the default is only suitable for small tables. In general it makes sense to give the expected number of table rows. In that way the buckets will be small enough for small tables and not too small for large tables.

~StandardStMan()

StandardStMan (const StandardStMan& that)

Copy constructor cannot be used.

StandardStMan& operator= (const StandardStMan& that)

Assignment cannot be used.