Versions Compared

Version Old Version 15 New Version 16
Changes made by

Fiona Sargeant

Fiona Sargeant

Saved on

Key

  • This line was added.
  • This line was removed.
  • Formatting was changed.

Overview

To make it easy to create a sequence of unique numbers, PhixFlow has a Sequence item. In the repository, add a new sequence, and use its properties to set up a sequence of unique numbers that you want to allocate to stream-items (records) in a stream. 

You need to specify a start value and a block size for the sequence. The block size specifies how often you go back to the database to update the number up to which you have allocated. This is referred to as the 'tidemark'. PhixFlow then allocates those numbers from the database and stores them in memory. When PhixFlow reserves a block, it marks the last reserved value in the database. 

To assign a number from memory to stream records-items, use the nextValue function in an expression. You can also find the last assigned number using the curValue function. 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. 


Panel
borderColor#7da054
titleColorwhite
titleBGColor#7da054
borderStylesolid
titleSections on this page

Table of Contents
maxLevel3
indent12px
stylenone



Tip

It is much quicker for PhixFlow to assign a number from memory than it is to assign a number from the database. It also takes time to reserve the next block of numbers.

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.  
Tip

We recommend you set a large block size, to optimise performance.

If you require a continuous sequence of values for items:

  • in a small stream set, you may want to accept the overhead of assigning every value in the sequence from the database. In this case, set the Block Size to 1.
  • in larger stream sets, consider an alternative method of assigning the value.

Sequence Properties

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

Insert excerpt
_new
_new
nopaneltrue
.

Insert excerpt
_standard_settings
_standard_settings
nopaneltrue

Basic Settings

FieldDescription
NameEnter the name of the sequence.
Start Value

Enter the first value in the sequence.

Note

This will only have an effect when the sequence is created. No further changes to the start value will alter the next value to be allocated.


Block Size

Enter a number to set the size of the sequence of numbers. This will depend on the number of records stream-items in the stream-set to which you are allocating unique numbers. For example, if the stream-set has 10,000 recordsstream-items, the block size should be at least 10,000.

Note

Sequences provide unique numbers, but they the numbers may not always be contiguous.

Block size has an effect on performance; see Block Size and Performance, above.

Current ValuePhixFlow displays the highest value already allocated.

Example: Using a Sequence

To set a stream attribute to a unique order ID:

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

  2. Create a stream with an Integer attribute.

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

  4. Every time a new record stream-item is generated in this stream, nextValue will return a new unique value for the attribute.

Insert excerpt
_terms_changing
_terms_changing
nopaneltrue