Question: IBM Corporation. Distributed Relational Database Architecture Reference. IBM Form No. SC26-4651. IBM's DRDA defines four levels of distributed database functionality. as follows:

1. Remote request

2. Remote unit of work

3. Distributed unit of work

4. Distributed request Since these terms show signs of becoming de facto standards in the industry, we briefly explain them here.

Note: "Request" and "unit of work" are IBM's terms for SQL statement and transaction, respectively.

1. Remote request means that an application at one site X can send an individual SQL statement to some remote site Y for execution. That request is executed and committed (or rolled back) entirely at site Y. The original application at site X can subsequently send another request to site Y (or possibly to another site Z), regardless of whether the first request was successful or unsuccessful.

2. Remote unit of work (abbreviated RUW) means that an application at one site X can send all of the database requests in a given "unit of work" (i.e., transaction) to some remote site Y for execution. The database processing for the transaction is thus executed in its entirety at the remote site Y; however, the local site X decides whether the transaction is to be com-mitted or rolled back. Note: RUW is effectively client/server processing with a single server.

3. Distributed unit of work (abbreviated DUW) means that an application at one site X can send some or all of the database requests in a given unit of work (transaction) to one or more remote sites Y, Z ... for execution. The database processing for the transaction is thus spread across multiple sites, in general; each individual request is still executed in its en. tirety at a single site, but different requests can be executed at different sites. However, site X is still the c000rdinating site, i.e., the site that decides whether the transaction is to be committed or rolled back.

4. Distributed request is the only one of the four levels that approaches what is commonly accepted as true distributed database support. Distributed request means everything that distributed unit of work means, plus it permits individual database requests (SQL statements) to span multiple sites-for example, a request originating from site X might ask for a join or union to be performed between a table at site Y and a table at site Z. Note that it is only at this level that the system can be said to be providing genuine location independence; in all three previous cases, users do have to have some knowledge regarding the physical location of data.