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Programs of the LDM System

• The LDM Program rpc.ldmd
• The Utility pqact
• Data-Product Ingesters
• Data-Product Decoders
• The ldmadmin Script
• Ancillary Utilities of the LDM

The LDM Program rpc.ldmd

An LDM system's main program is the binary executable rpc.ldmd. This program

• Is typically started by the ldmadmin script.
• Configures itself according to the LDM configuration-file, ldmd.conf.
• Starts-up the LDM ingesters.
• Forks itself to create downstream LDMs.
• Starts-up the pqact processes.
• Validates FEEDME requests by downstream LDMs and forks itself to create corresponding upstream LDM processes.
• Validates HIYA connections by upstream LDMs and forks itself to create corresponding downstream LDM processes. This is an uncommon operation.

The Utility pqact

The pqact utility reads data-products from the product-queue and disposes of them according to user-configurable instructions. One or more instances of this program are, typically, started by the top-level rpc.ldmd process as the result of EXEC entries in the LDM configuration-file, ldmd.conf. Instructions for the disposition of data-products are contained in a pqact configuration-file.

Certain instructions in the pqact configuration-file cause the pqact utility to spawn child processs to aid in the disposition of data-products. These instructions are the pqact PIPE action and the pqact EXEC action.

Typically, pqact processes are terminated by reception of a SIGINT or SIGTERM from the top-level rpc.ldmd process.

When a pqact process terminates, it saves the insertion-time of the last, successfully-processed data-product in a file. The pathname of the file is that of the pqact configuration-file with ".state" appended. This allows a subsequent pqact process that uses the same configuration-file to start processing where the previous process stopped. It also means only one pqact process should use any configuration-file.

Data-Product Ingesters

An LDM data-product ingester is a program that creates data-products and inserts them into the LDM product-queue using the pq(3) API. Such a program has the following characteristics:

• It should send a SIGCONT signal to its process group whenever it inserts a data-product into the product-queue.
• In general, it should be started by the top-level rpc.ldmd process as the result of an EXEC entry in the LDM configuration-file, ldmd.conf, so that the the LDM process group will immediately act upon the new data-product. Failure to be started in this way will result in larger than necessary data-product latency.
• In general, it should never self-terminate; instead, it should terminate upon reception of a SIGINT or SIGTERM, which will be sent to it from the top-level rpc.ldmd process.

It is, ultimately, irrelevant how ingesters obtain the data from which they create data-products because such information is lost once the data-product has been created and inserted into the product-queue. Data sources that have been used include the following: serial ports, files, and pseudo-random number generators.

The LDM package comes with the following ingesters. While some are useful in their own right, their primary purpose is to provide examples of how to write ingesters.

pqing

Reads meteorological data in WMO or AFOS format from a file or serial port. This is a "classical" data-product ingester.

pqinsert

Inserts files as data-products. This is an example of an ingester that not only runs outside the LDM process group but also self-terminates.

Data-Product Decoders

An LDM data-product decoder is a program that reads data-product data from standard input and disposes of the data in some fashion -- often by writing the data into a file that is specific to an analysis package (e.g., McIDAS or GEMPAK). Such programs have the following characteristics:

• They are typically started by a pqact process as the result of PIPE actions in the process's pqact configuration-file.
• They self-terminate if no data has been received within a certain period of time (e.g., 10 minutes) or upon reception of a SIGINT or SIGTERM from the pqact parent process.
• Due to a limitation on the number of output-files that the pqact program can have open at one time, decoders should
  • Continue reading until the end-of-file is encountered and then exit; otherwise, the decoder process might become a zombie (alias defunct) process.
  • Be written so that they can "start-up" in the middle of decoding something that spans multiple data-products (for example a single "sweep" of a NEXRAD radar actually comprises four data-products per parameter). The reason for this can be found in the documentation for the pqact configuration-file.

The ldmadmin Script

The ldmadmin utility is a Perl-5 script intended for high-level administration of an LDM system. The script acts as a facade for more primitive utilities -- coordinating their use for the sake of convenience. The things you can do with the ldmadmin script include the following:

Action	Command
Start the LDM system	ldmadmin start
Stop the LDM system	ldmadmin stop
Stop then restart the LDM system	ldmadmin restart
Determine if the LDM system is running	ldmadmin isrunning
Create the product-queue	ldmadmin mkqueue
Delete the product-queue	ldmadmin delqueue
Watch the most recent data-products arrive in the product-queue	ldmadmin watch
Check the syntax of all pqact configuration-files associated with active EXEC pqact entries in the LDM configuration-file .	ldmadmin pqactcheck
Rotate the LDM logfiles	ldmadmin newlog

Ancillary Utilities of the LDM

Utilities that are included in the LDM package but are less important than the separately-documented ones above include the following:

Utility	Description
feedme	Requests data-products from an upstream LDM and writes them to standard output. Deprecated.
ldmping	Temporarily connects to an upstream LDM to test its availability.
notifyme	Requests data-product metadata from an upstream LDM and, optionally, writes the metadata to an LDM logfile.
pqcat	Reads once through a product-queue and writes the data-products to a file or to standard output. Useful for ensuring that a product-queue isn't corrupt.
pqcheck	Checks the writer-counter in the product-queue. Useful for ensuring that a product-queue isn't corrupt.
pqcreate	Creates an empty LDM product-queue.
pqexpire	Deletes old data-products from a product-queue. Deprecated.
pqmon	Prints a summary of the state of a product-queue.
pqsend	Reads a product-queue and sends the data-products to a downstream LDM. Waits for more data-products.
pqsurf	Reads data-products that are composite WMO surface bulletins from the product-queue, splits them into individual bulletins, inserts them as data-products into a separate product-queue, and processes the individual bulletins similar to pqact. Deprecated.
pqutil	Interactive program for accessing the product-queue.
rtstats	Monitors the product-queue and periodically sends summary statistics to a downstream LDM (which, for the IDD, is at the Unidata Program Center).
scour	crontab-driven utility for deleting old data-files based on pathname patterns and retention-times.
ldmsend	Sends files as LDM data-products to a downstream LDM using HEREIS messages.

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