A HashMap is a Java data structure maintained in RAM. Talend Open Studio lets you load manipulate data in HashMaps using the tHashInput and tHashOutput components. Because the data exists in RAM, lookups are faster, especially if secondary storage is based on spinning hard disks.
Without a HashMap
This Talend Open Studio job loads data from two spreadsheets, EmployeeHires and EmployeeTerminations, into a target table, EmployeeActions. The spreadsheet sources contain a data (hireDate and terminationDate) that is used as a key into a table called BusinessDates. Although the date could simply be carried over into the target table (without the lookup), many data warehouses maintain date information in a separate table. This is because calendar-related business information is merged with the timestamp.
This data includes flags for Holiday, Payday, and Weekday that supplement that timestamp (Month, Day, Year, Quarter) fields. The spreadsheet has been loaded into the MS SQL Server table "BusinessDates".
|Basic Date Fields PLUS Business-specific Information|
|BusinessDates is Repeated for Each Subjob|
|Employee Action Source (Hires)|
The preceding job works. 5 Hire and 3 Termination records are written to the database. However, the job has a drawback. Even if caching is enabled, the lookup is read at least once for each subjob, leading to poorer performance. An alternative is to use the Talend Hash components: tHashInput and tHashOutput.
|Job Rewritten with Talend Hash Component|
The tHashOutput component is configured as follows.
The tHashOutput schema can now be applied to joins (tMap) by adding tHashInput components as lookup flows. This is the configuration of tHashInput_1 which is identical to tHashInput_2. More inputs can be added for other data loading subjobs.
|tHashInput Configuration Links to tHashOutput|
The join is performed as an Inner Join on three columns. Because the dates are represented differently, I use a TalendDate routine. Note the important +1 which dealing with the zero-based Java month and day values.
|Multi-Part Join on a Hash Map Input|
|Data Loading Results|