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Thursday, October 18, 2012

DataStage Configuration file : Explained - 1

The Datastage configuration file is a master control file (a textfile which sits on the server side) for jobs which describes the parallel system resources and architecture. The configuration file provides hardware configuration for supporting such architectures as SMP (Single machine with multiple CPU , shared memory and disk), Grid , Cluster or MPP (multiple CPU, mulitple nodes and dedicated memory per node). DataStage understands the architecture of the system through this file.

This is one of the biggest strengths of Datastage. For cases in which you have changed your processing configurations, or changed servers or platform, you will never have to worry about it affecting your jobs since  all the jobs depend on this configuration file for execution. Datastage jobs determine which node to run the process on, where to store the temporary data, where to store the dataset data, based on the entries provide in the configuration file. There is a default configuration file available whenever the server is installed.

The configuration files have extension ".apt". The main outcome from having the configuration file is to separate software and hardware configuration from job design. It allows changing hardware and software resources without changing a job design. Datastage jobs can point to different configuration files by using job parameters, which means that a job can utilize different hardware architectures without being recompiled.

The configuration file contains the different processing nodes and also specifies the disk space provided for each processing node which are logical processing nodes that are specified in the configuration file. So if you have more than one CPU this does not mean the nodes in your configuration file correspond to these CPUs. It is possible to have more than one logical node on a single physical node. However you should be wise in configuring the number of logical nodes on a single physical node. Increasing nodes, increases the degree of parallelism but it does not necessarily mean better performance because it results in more number of processes. If your underlying system should have the capability to handle these loads then you will be having a very inefficient configuration on your hands.

1.    APT_CONFIG_FILE is the file using which DataStage determines the configuration file (one can have many configuration files for a project) to be used. In fact, this is what is generally used in production. However, if this environment variable is not defined then how DataStage determines which file to use ??

1.    If the APT_CONFIG_FILE environment variable is not defined then DataStage look for default configuration file (config.apt) in following path:
1.    Current working directory.
2.    INSTALL_DIR/etc, where INSTALL_DIR ($APT_ORCHHOME) is the top level directory of DataStage installation. 

2.    Define Node in configuration file
A Node is a logical processing unit. Each node in a configuration file is distinguished by a virtual name and defines a number and speed of CPUs, memory availability, page and swap space, network connectivity details, etc. 

3.    What are the different options a logical node can have in the configuration file? 

1.    fastname – The fastname is the physical node name that stages use to open connections for high volume data transfers. The attribute of this option is often the network name. Typically, you can get this name by using Unix command ‘uname -n’.
2.    pools – Name of the pools to which the node is assigned to. Based on the characteristics of the processing nodes you can group nodes into set of pools.
1.    A pool can be associated with many nodes and a node can be part of many pools.
2.    A node belongs to the default pool unless you explicitly specify apools list for it, and omit the default pool name (“”) from the list.
3.    A parallel job or specific stage in the parallel job can be constrained to run on a pool (set of processing nodes).
1.    In case job as well as stage within the job are constrained to run on specific processing nodes then stage will run on the node which is common to stage as well as job.
3.    resource resource resource_type “location” [{pools “disk_pool_name”}]  | resource resource_type “value” . resource_type can be canonicalhostname (Which takes quoted ethernet name of a node in cluster that is unconnected to Conductor node by the hight speed network.) or disk (To read/write persistent data to this directory.) or scratchdisk (Quoted absolute path name of a directory on a file system where intermediate data will be temporarily stored. It is local to the processing node.) or RDBMS Specific resourses (e.g. DB2, INFORMIX, ORACLE, etc.) 

4.    How datastage decides on which processing node a stage should be run? 

1.    If a job or stage is not constrained to run on specific nodes then parallel engine executes a parallel stage on all nodes defined in the default node pool. (Default Behavior)
2.    If the node is constrained then the constrained processing nodes are chosen while executing the parallel stage.

Parts of this tutorial --

njoy the simplicity.......
Atul Singh

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