# Date-Based Indexing

You can use date-based indexing to distribute the indexing load between IDOL servers. Date-based indexing uses the date of the documents being indexed or in which you replace fields.

When you enable date-based indexing, DIH indexes each document in a `DREADD` and `DREADDDATA` action by its `#DREDATE` field, or another `DateType` field configured in the `[FieldProcessing]` section. It indexes each replace in a `DREREPLACE` action based on its `#DREDATE` line, if it exists. Otherwise it sends the action to all child servers. It sends all other actions to all child servers.

When you use date-based indexing, you cannot alter the number of child servers.

To enable date-based indexing, set `DistributeByDate` to `True` in the `[Server]` section of the DIH configuration file.

Configure the date ranges for child servers by using a `[DateRangeN]` subsection in the `[DistributionIDOLServers]` section of the DIH configuration file.

For both DIH stand-alone and unified configuration, you must configure `DateFormatCSVs` in the `[Server]` section for date-based indexing to work. For example:

    [Server]
    Port=9070
    DIHPort=9071
    MirrorMode=False
    DistributeByDate=True
    DateFormatCSVs=DD/MM/YYYY,YYYY/MM/DD,YYYY-MM-DD

    [DistributionIDOLServers]
    Number=2

    [IDOLServer0]
    Host=localhost
    Port=9100

    [IDOLServer1]
    Host=localhost
    Port=9500

    [DateRange0]
    FromDate=1980/01/01
    UpToDate=1990/01/01
    Engines=0

    [DateRange1]
    FromRelative=-3
    UpToRelative=5
    Engines=1

In this example, server 0 indexes documents dated from 1 January 1980 to 31 December 1989. If it is Tuesday (relative 0), server 1 gets documents dated from the previous Saturday (relative -3) and from the following Saturday (relative 4). The upper limit is exclusive.

NOTE: You can use `DistributeByDate` only when `MirrorMode` is set to `False`. DIH will not start if `DistributeByDate` and `MirrorMode` are both set to `True`.

In `DistributeByDate` mode, you can also use the `UnknownFieldValueAction` configuration parameter to determine how to treat documents where the date field is missing, contains a value that DIH cannot parse as a date, or contains a date value that does not match the configured date ranges. For more information, refer to the DIH Reference.

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