1. Oracle GoldenGate Microservices Documentation
  2. Extract
  3. Manage Trail Files

Manage Trail Files

After data has been extracted, it must be processed into one or more trails, where it is stored for processing by another Oracle GoldenGate process. A trail is a sequence of files that are created and aged as needed. Processes that read a trail are:

  • Distribution Service: Extracts data from a local trail for further processing, if needed, and transfers it to the target system.

  • Receiver Service: Receives the trail and transfers to Replicat, which reads the trail and applies change data to the target database.

You can create more than one trail to separate the data of different tables or applications, or to satisfy the requirements of a specific replication topology, such as a cascading topology. You link tables specified with a TABLE statement to a trail specified with an EXTTRAIL or RMTTRAIL parameter statement in the Extract parameter file.

  • Assign Storage for Oracle GoldenGate Trails

  • Estimate Space for the Trail

  • Add a Trail

See About Oracle GoldenGate Trails.

Topics:

Assign Storage for Oracle GoldenGate Trails

In a typical configuration, there is at least one trail on the source system and one on the target system. Allocate enough disk space to allow for the following:

  • The primary Extract process captures transactional data from the source database and writes it to the local trail. There must be enough disk space to contain the data accumulation, or the primary Extract will abend.

  • For a trail at the target location, provide enough disk space to handle data accumulation according to the purge rules set with the PURGEOLDEXTRACTS parameter. Even with PURGEOLDEXTRACTS in use, data will always accumulate on the target because it is transferred across the network faster than it can be applied to the target database. Also see Purge Datastore.

To prevent trail activity from interfering with business applications, assign a separate disk or file system to contain the trail files. Trail files can reside on drives that are local to the Oracle GoldenGate installation, or they can reside on NAS or SAN devices. In an Oracle cluster, they can reside on ASM or DBFS storage. See Preparing DBFS for an Active-Active Configuration.

Estimate Space for the Trails

The following are guidelines for estimating the amount of disk space that will be required to store Oracle GoldenGate trail data.
  1. Estimate the longest time that the network could be unavailable. Plan to store enough data to withstand the longest possible outage, because otherwise you will need to resynchronize the source and target data if the outage outlasts disk capacity.
  2. Estimate how much transaction log volume your business applications generate in one hour.
  3. Use the following formula to calculate the required disk space. 
    [log volume in one hour] x [number of hours downtime] x .4 = trail disk space
    This equation uses a multiplier of 40 percent because only about 40 percent of the data in a transaction log is needed by Oracle GoldenGate.

    Note:

    This formula is a conservative estimate, and you should run tests once you have configured Oracle GoldenGate to determine exactly how much space you need.

To prevent trail activity from interfering with business applications, assign a separate disk or file system to contain the trail files. Trail files can reside on drives that are local to the Oracle GoldenGate installation, or they can reside on NAS or SAN devices. In an Oracle cluster, they can reside on ASM or DBFS storage.

Add a Trail

When you create, or add, a trail, you do not physically create any files on disk. The files are created automatically by an Extract process. Rather, you specify the name of the trail and associate it with the Extract group that writes to it.

To add a trail, issue the following command on the source system.

ADD {RMTTRAIL | EXTTRAIL} pathname, EXTRACT group [, MEGABYTES n]

Where:

  • RMTTRAIL specifies a trail on a remote system.

  • EXTTRAIL specifies a trail on the local system.

    • EXTTRAIL cannot be used for an Extract in PASSIVE mode.

    • EXTTRAIL must be used to specify a local trail.

  • pathname is the relative or fully qualified name of the trail, including a two-character name that can be any two alphanumeric characters, for example c:\ggs\dirdat\rt. Oracle GoldenGate appends a serial number to each trail file as it is created during processing. Typically, trails are stored in the dirdat sub-directory of the Oracle GoldenGate directory.

  • EXTRACT group specifies the name of the Extract group that writes to this trail. Only one Extract group can write to a trail.

  • MEGABYTES n is an optional argument with which you can set the size, in megabytes, of each trail file (default is 100).

Example: Create a Local Trail

This example creates a local trail named /ggs/dirdat/lt for Extract group exte. 

ADD EXTTRAIL /ggs/dirdat/lt, EXTRACT exte

Example: Create a Remote Trail

This example creates a trail named c:\ggs\dirdat\rt for Extract group finance, with each file sized at approximately 50 megabytes. 

ADD RMTTRAIL c:\ggs\dirdat\rt, EXTRACT finance, MEGABYTES 200