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Timestamp-based downloads

The timestamp method is the most useful general technique for efficient synchronization. This technique involves tracking the last time that each user synchronized and only downloading rows that have changed since then.

MobiLink maintains a timestamp value indicating when each MobiLink user last downloaded data. This value is called the last download time.

See Using last download times in scripts.

To implement timestamp-based synchronization for a table

1. At the consolidated database, add a column that holds the most recent time the row was modified. The column is typically declared as follows:

   DBMS	last modified column
   SQL Anywhere	timestamp DEFAULT timestamp
   Adaptive Server Enterprise	datetime
   Microsoft SQL Server	datetime
   Oracle	date
   IBM DB2 UDB	timestamp

2. In scripts for the download_cursor and download_delete_cursor events, compare the first parameter to the value in the timestamp column.

Example

The following table declaration and scripts implement timestamp-based synchronization on the Customer table in the Contact sample:

• Table definition:

  CREATE TABLE "DBA"."Customer"(
     "cust_id"  integer NOT NULL DEFAULT GLOBAL AUTOINCREMENT,
     "name"  char(40) NOT NULL,
     "rep_id"  integer NOT NULL,
     "last_modified" timestamp NULL DEFAULT timestamp,
     "active"  bit NOT NULL,
     PRIMARY KEY ("cust_id") )

• download_delete_cursor script:

  SELECT cust_id
  FROM Customer JOIN SalesRep
  ON Customer.rep_id = SalesRep.rep_id
  WHERE Customer.last_modified >= {ml s.last_table_download}
      AND ( SalesRep.ml_username != {ml s.username}
            OR Customer.active = 0 )

• download_cursor script:

  SELECT cust_id, Customer.name, Customer.rep_id
  FROM Customer KEY JOIN SalesRep
  WHERE Customer.last_modified >= {ml s.last_table_download}
      AND SalesRep.ml_username = {ml s.username}
      AND Customer.active = 1

See Synchronization logic source code and Synchronizing contacts in the Contact sample.

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Using last download times in scripts
Dealing with daylight savings time