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More complicated scenarios
If you want to do redundancy and load-balancing among three modules, the configuration is quite complex:
- load-balance {
- redundant {
- sql1
- load-balance {
- redundant {
- sql2
- sql3
- }
- redundant {
- sql3
- sql2
- }
- redundant {
- }
- } # sql1, etc.
- redundant {
- sql2
- load-balance {
- redundant {
- sql3
- sql1
- }
- redundant {
- sql1
- sql3
- }
- redundant {
- }
- } # sql2, etc.
- redundant {
- sql3
- load-balance {
- redundant {
- sql1
- sql2
- }
- redundant {
- sql2
- sql1
- }
- redundant {
- }
- } # sql3, etc.
- redundant {
- }
For four or more modules, it quickly becomes unmanageable.
The solution is to use the "redundant-load-balance" section, which combines the features of "load-balance", with "redundant" fail-over between members. The above complex configuration for three modules then becomes:
- redundant-load-balance {
- sql1
- sql2
- sql3
- }
Which means "load-balance evenly among all three servers. If the one picked for load-balancing is down, load-balance among the remaining two. If that one is down, pick the one remaining 'live' server".
The "redundant-load-balance" section can contain any number of modules.
Interaction with "if" and "else"
It's best to have "if" and "else" blocks contain "load-balance" or "redundant-load-balance" sections, rather than the other way around. The "else" and "elsif" sections cannot appear inside of a "load-balance" or "redundant-load-balance" section, because the "else" condition would be chose as one of the modules for load-balancing, which is not what you want.
It's OK to have a plain "if" block inside of a "load-balance" or "redundant-load-balance" section. In that case, the "if" condition checks the return code of the module or group that executed just before the "load-balance" section. It does *not* check the return code of the previous module in the section.
The following table illustrates which sections can be sub-sections of others. If an entry for a row/column is empty, then that combination is not allowed.
- x = allowed
- i = allowed if immediately after an 'if or 'elsif'
| Allowed: | group | redundant | l-b | r-l-b | if | else | elsif |
|---|---|---|---|---|---|---|---|
| Container: | | ||||||
| group | x | x | x | x | x | i | i |
| if | x | x | x | x | x | i | i |
| else | i | i | i | i | i | i | i |
| elsif | i | i | i | i | i | i | i |
| l-b | x | x | x | x | x | ||
| r-l-b | x | x | x | x | x | ||
| redundant | x | x | x | x | |||
e.g. "redundant" can contain "load-balance", but not "if", "else", or "eslif". "if" can contain any other section, but if it contains "else" or "elsif", they have to have be listed after a second "if" section, inside of the first "if".
