How to configure an incremental database crawl process

Having a "full crawl" process is useful in many cases. For instance :

  • A problem occured and a full re-crawling is needed
  • It is imperative to ensure that all data is properly indexed

A common practice is to run this process weekly.

However, it can be time consuming. A lighter alternative is an incremental crawl process, which only crawls recently modified data.

Common answer

A common practice is to write a dedicated crawl process that uses an SQL query patterned after the full one, but with a slight addition. A WHERE clause is used to restrict results.

This WHERE clause must be applied on a field in which the date of last modification (or the date of creation) is stored for each row. For instance, assume an articles tab with a datetime field called date_modification. We could use this SQL query:

SELECT title, author, content, date_modification FROM articles WHERE date_modification > subdate(CURRENT_DATE, 1)

This example uses the MySQL function subdate. This function can be used to subtract a number of days from a particular date. The MySQL constant CURRENT_DATE is, unsurprisingly, equal to the current date. Thus, subdate(CURRENT_DATE, 1) is SQLese for "one day ago".

Thus, this query will retrieve every row modified during the last day.

This example crawl process could be run daily.

Using a single crawl process for both full and incremental crawling

Cloning a "full" process to derive an "incremental" process can lead to difficulties - as each modification in a process should be immediately echoed in the other. Therefore, the practice explained above may not be suitable.

Thankfully, OpenSearchServer offers its Scheduler and its variables. We can use this for an unified crawl process where SQL Query uses one such variable, for instance:

SELECT title, author, content, date_modification FROM articles %whereClause%

Two Scheduler jobs using the Database crawler - run task then get created:

  • In the first job the variable %whereClause% is replaced by "nothing" (whereClause=)
  • In the second job the variable is replaced by WHERE date_modification > subdate(CURRENT_DATE, 1) (whereClause=WHERE date_modification > subdate(CURRENT_DATE, 1))

Read the How to use variables with the Database Crawler page for more.

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