Updated March 14, 2023
Introduction to Logstash if
Logstash if expression is used for the specification of conditional behavior in the Logstash processing. There are many scenarios that occur inside the application that we want some of the tasks to be carried out only and only if some of the required conditions are met or satisfied. For this, in Logstash we have special functionality of using the conditional statements of if, else, and else-if which is also referred to as Logstash Conditionals. We can keep on extending the ladder of else if as long as we want according to the requirement of our application.
In this article, we will learn about Logstash if and will try to explore the topic more in-depth by discussing subtopics including What is Logstash if, how to use Logstash if, Logstash if examples, and Conclusion about the same.
What is Logstash if?
It is used for specification of conditional behavior while execution of the code. If can be accompanied with else and zero or more elseif statements as well. This depends on the use case of implementation.
Similar to that of another programming language the conditionals in Logstash also follow the same pattern, rules, and implementation. We can also go for nesting multiple or single conditional statements inside one or more conditional statements. As discussed, priorly, conditional statements include the usage of three main statements which are if, else, and else if.
The syntax of the Logstash conditionals is as specified below.
if conditional_expression {
// statements to be performed if conditional_expression goes true
} else if conditional_expression_2 {
// Statements to be executed on evaluation of conditional_expression_2 to true and conditiona_expression to false.
} else {
// Statements to be executed if not even the conditional_expression of conditional_expression_2 evaluated to true.
}
In order to understand the above syntax, let us first understand what the conditional_expressions mentioned above are. The conditional expression can be any of the tests that involves comparison or the logical statement which evaluates to Boolean value or anything like that.
How to use Logstash if?
We can make the use of if statement in Logstash for executing certain code only on the basis of the result of conditional expression which involves checking, verifying, and comparison of values, expressions, fields, and tags. The syntax that needs to be followed is as shown in the above section. There are some more points that you need to consider while specifying the expression inside the if condition.
In order to create the conditional expression using if in Logstash we can go for using the operators of comparison which are specified below –
- To carry out the comparison for the test of equality we can use operators such as == (exactly equal to), != (not equal to), > (greater than), < (less than), <= (less than or equal to) or >= which stands for (greater than or equal to).
- For regular expressions, we can use =~ or !~ which helps in testing the pattern that will be present on the right side of the expression for the value of string specified on the left side of the expression.
- We can use the expressions involving the implementation of inclusion logic which contains operators like in or not in.
- Other than the above ones in order to create the conditional expressions, we can also make the use of boolean operators out of which the supported ones include nand, xor, and, or.
- We can make the use of unary operators which include! which stands for negating the specified value or expression whichever is specified.
We need to note one thing here the specified conditional expressions may be long and complex in nature sometimes as they might contain in them the use of other expressions as well like in case of negate! what we do is we just negate that calculates the opposite of that value is actually derived from the specified expression. Also, one more functionality and feature that makes the expressions sometimes more complex is the use of parenthesis to group together multiple subexpressions.
Logstash if Examples
We will be having a look at some of the examples in this section to understand the use of Logstash conditionals.
Example #1
Let us consider one example, in the below code we will make the use of mutate filter which will allow us to write conditional code in it. We will remove the existing field in our event named educba_field only of the article_name field contains the value of “Logstash” in it –
filter {
if [article] == "Logstash" {
mutate { remove_field => "educba_field" }
}
}
Which gives the following output on running as the field named article contained Logstash as its value –
Example #2
Let us consider one example where we will be specifying multiple expressions inside the same conditional statement –
output {
if [level_of_log] == " erroneous _report" and [deployment_to] == "production_environment" {
handleTransaction {
...
}
}
}
Which looks as shown below in the file –
What we actually do in the above code snippet is in case any occurrence of erroneous report is found and the deployment is made to the production environment then all the erroneous reports are forwarded to handle transactions.
We can make use of in and not in operators to check for the existence of a specific or particular element, list, key, or string value in the field. The meaning of in semantically changes depending on the target field type that you are checking in. For example, if you have a string field and making use of it then it is considered that you are checking whether the specified value is a substring of the target string value while in case if the target field type contains the collection of values, then it means that you are checking whether the specified element is present in the collection or not.
Conclusion
In this way, we can make use of Logstash if in the Logstash to implement the behavior such that the execution of certain steps can only be executed provided if the specified conditions are set to true.
Recommended Articles
This is a guide to Logstash if. Here we discuss the Introduction, What is Logstash if, How to use Logstash if? Examples with code implementation. You may also have a look at the following articles to learn more –
