Subscribe Services

Introduction

HP NonStop SQL/MX Queuing and Publish/Subscribe Services—523734-002

1-2

Why Use Publish/Subscribe?

of extra performance costs and application complexity. Figure 1-1 shows the

processing required.

If you simply started many instances of the application in an attempt to make this

application scalable, they would conflict with each other. As one instance held a lock

on the next invoice that needed processing, the others would have to wait. To avoid

conflict, you would have to devise a logical partitioning scheme so each instance

processed a subset of the invoices, adding complexity to the application and the

database schema.

Publish/Subscribe services provides solutions for each of these problems:

•

Rather than polling for changes to the invoice table, the application can use stream

access mode. Stream access mode changes the behavior of the table: an attempt

to fetch a row blocks, if necessary, until more rows are available. The application

code becomes simpler and performs better.

•

Instead of using a positioned UPDATE to mark the row as processed, the

application can use a special type of statement, an embedded UPDATE, to select

Figure 1-1. Invoice Processing Without Publish/Subscribe

Fetch

invoice

EOF?

Update

invoice

Close cursor

Sleep

Yes

Open cursor

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