CAS is Elastic! Part 2
In Part 1, we introduced the concept of elasticity as it applies to CAS. Now, let's explore the setup and demonstrate how table rebalancing operates.
Setup for CAS Table Rebalancing
Enabling table rebalancing is only necessary when adding more workers or when a failed worker node is restarted.
For a detailed setup guide, Gilles provides step-by-step instructions in his post.
To activate table rebalancing, set the following environment variables to "true" in CAS:
- CAS_GLOBAL_TABLE_AUTO_BALANCE for permanent CAS tables
- SESSION_TABLE_AUTO_BALANCE for session CAS tables
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Indeed, you may choose to apply rebalancing only to global tables, as session tables may not always need it.
These variables unlock the rebalancing feature, but additional configuration is required to specify which tables should be rebalanced.
Now, SAS Administrators or Programmers must designate which tables are eligible for rebalancing when more workers are added. This is controlled by the tableRedistUpPolicy option, which has two settings:
- NOREDIST: Prevents redistribution of table data when the number of worker pods increases.
- REBALANCE: Enables rebalancing of table data when the number of worker pods increases.
You can set tableRedistUpPolicy at various levels, depending on whether you need a global or table-specific configuration. Here’s the order of precedence:
| Order of precedence | Option | Scope | Example |
| 1 | tableRedistUpPolicy CAS action casout option | Table | proc cas ; simple.freq / table={caslib="DM", name="prdsale"}, inputs={"country"}, casOut={caslib="DM", name="country_freq", tableRedistUpPolicy="REBALANCE"} ; quit ; |
| 2 | tableRedistUpPolicy CASLIB option | CASLIB | caslib DM datasource=(srctype="path") path="/gelcontent/" tableRedistUpPolicy=REBALANCE ; |
| 3 | sessionTableRedistUpPolicy CAS session option | Session table in a CAS session | cas mysession sessopts=(sessionTableRedistUpPolicy=rebalance) ; |
| 4 | cas.TABLEREDISTUPPOLICY CAS option | CAS server | cas.TABLEREDISTUPPOLICY = ‘REBALANCE’ |
How Does CAS Table Rebalancing Work?
Let’s explore how CAS table rebalancing affects each of our four scenarios. Watch the videos below to see rebalancing in action.
- Adding Workers for Peak Activity
When a SAS Administrator adds one or more new CAS workers to handle a surge in activity.
- Removing Workers to Return to Normal
When a SAS Administrator removes one or more CAS workers to scale back to normal resource levels.
- Handling Node Failure with Automatic Restart
When a CAS worker fails (node failure), and Kubernetes automatically restarts a new one to maintain the right number of workers.
- Handling Node Failure Without Restart
When a CAS worker fails, but Kubernetes is unable to restart a replacement within 60 seconds, triggering rebalancing on the remaining nodes.
That's it for this one. In the final part, we will cover additional considerations regarding CAS table rebalancing.
Thanks for reading!
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