SLB
	Simple Load Balancer	Sergey Poznyakoff

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3.9 Test Mode

The slb configuration can be quite complex, therefore it is important to verify that it behaves as expected before actually implementing it in production. Three command line options are provided for that purpose.

The ‘--dry-run’ (or ‘-n’) option instructs the program to start in foreground mode and print to the standard output what would have otherwise been printed to the output file. Additional debugging information is displayed on the standard error.

The ‘--eval’ option initiates expression evaluation mode. In this mode slb evaluates the expression whose name is supplied as argument to the option. Actual values of the variables and constants used in the expression are supplied in the command line in form of variable assignments. The result is printed to the standard output. For example, if the configuration file contains

expression load "(la1/100 + usr/1024)/2";

then the command

slb --eval=load la1=30 usr=800

will print ‘.540625’.

If the expression contains calls to ‘d()’ (derivative), you will need several evaluations to compute its value. The minimal number of evaluations equals the order of the highest derivative computed by the expression, plus 1. Thus, computing the following expression:

expression load "sqrt(k * d(out)**2 + m * la1**2)";

requires at least two evaluations. To supply several values to a single variable, separate them with commas, e.g.: ‘out=16000,20000’. This will run first evaluation with ‘out=16000’ and the second one with ‘out=20000’. For example:

$ slb --eval=load k=1.5 m=3 out=16000,20000 la1=0.4
16.3446

Notice, that you don't need to supply the same number of values for each variable. If a variable is assigned a single value, this value will be used in all evaluations.

A more elaborate test facility is enabled by the ‘--test’ option. This option instructs slb to read SNMP variables and their values from a file and act as if they were returned by SNMP. The output is directed to the standard output, unless the ‘--output-file’ option is also given.

The input file name is taken from the first non-option argument. If it is ‘-’ (a dash) or if no arguments are given, the standard input will be read:

slb --test input.slb

The input file consists of sections separated by exactly one empty line. A section contains assignments to SNMP variables in the style of snmpset(2). Such assignments form several server groups, each group being preceded by a single word on a line, followed by a semicolon. This word indicates the ID of the server to which the data in the group belong. For example:

srv01:
UCD-SNMP-MIB::laLoadFloat.1 F 0.080000
IF-MIB::ifOutUcastPkts.3 c 346120120
srv02:
UCD-SNMP-MIB::laLoadFloat.1 F 0.020000
IF-MIB::ifOutUcastPkts.3 c 2357911693

This section supplies data for two servers, named ‘srv01’ and ‘srv02’.

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