Overview

Use a sequence item to create a series of unique values. You can use the values in an attribute.

A sequence requires a Start Value and a Block Size. PhixFlow:

• reserves values in its database, from the Start Value up to the Block Size
  
• marks the last reserved value in the database
• stores the values in memory, as it is quicker to retrieve the values from memory.

When all the numbers in the block have been allocated, PhixFlow reserves the next block in the database from the marked value. 

Assigning a Sequence

To assign a sequence value from memory to an attribute in a record, use the following functions in an Attribute properties → Basic Settings → Expression:

• nextValue: allocate the next available value in the sequence.
  When all the numbers in the sequence have been used, the next time the nextValue function is called PhixFlow reserves the next block of numbers before assigning the next value. 

• curValue: find the last assigned number.

Primary Key Values

By default, PhixFlow treats an attribute called UID as the primary key and automatically allocates a unique integer to any new records that are created.

If the primary key attribute has a name other than UID, you must configure PhixFlow to allocate a unique integers to any records that are added.

For actionflow → insert action nodes, specify the following Table properties → Advanced:

• Primary Key Generator: Sequence
• Primary Key Generator Sequence: select a configured sequence from the drop-down list.

For legacy table-actions, use the expression field in the Record-Action Attribute properties to allocate new primary key values based on a sequence.

Gaps in the Number Sequence

Sequence numbers are unique, but there may be gaps between sequence numbers. This depends on:

• the block size specified when the sequence is set up
• unexpected server shut-down.

Server shut-down clears any remaining numbers from memory. When the server restarts, PhixFlow reserves a new block from the last reserved value. This can leave a gap between:

• the last number allocated before PhixFlow was shut down
• and the first number allocated after PhixFlow is restarted.

For example,

1. You create a sequence with a block size of 100. It has the values 0-99.
2. You have allocated the numbers 1 to 50 from memory.
3. PhixFlow shuts down.
4. On restart, PhixFlow reserves a new block from 100 to 199.
5. PhixFlow starts issuing numbers from memory, starting with 100. 
6. The numbers from 51 to 99 will never be issued.

Block Size and Performance 

Anchor
block block

Use the block size to specify the range of values in the sequence. The block size affects PhixFlow performance. 

• A very small block size increases the number of times that PhixFlow has to go to the database to reserve another block.  This slows down performance.
• A very large block size increases the risk that PhixFlow is shut down with unallocated numbers in memory.  

Sequence Properties

To create a sequence, in the repository, right-click Sequences and click 

Basic Settings

Example: Using a Sequence

To set

an attribute to a unique order ID:

1. Create a sequence and set its Name to order_id.

2. Create a

1. table with an Integer attribute.

2. Set its attribute expression to: 
     nextValue('order_id')

3. Every time a

1. new record is generated in this

1. table, nextValue will return a new unique value for the attribute.