Conditional Expressions

In LogScale the streaming style is not well-suited for procedural-style conditions. However, there are a few ways to do conditional evaluation:

Case Expressions

Using case expressions, you can define alternative flows in your queries. It is similar to case or cond you might know from many other functional programming languages.

The syntax looks like this:

logscale
case {
  pipeline1
| ... ;
  pipeline2
| ... ;
  pipeline3
| ... ;
  *
| ...
}

Each clause in the pipeline is terminated by a semicolon. For each clause within the case statement, the pipeline is run individually as if the expression were executed within LogScale as a typical query. If the expressions return events, the clause is considered to have matched.

Typically, each clause takes the form of one or more test expressions followed by one or more results. For example, a single test and result:

logscale
src="client-side"
| type:="client"

The first expression tests for a field value, the second creates a new field.

OR for a multiple expression test:

logscale
src="client-side"
| ip != "127.0.0.1" 
| type:="local"

In this example we are testing both the field src and ip.

The pipeline is considered to have matched if the pipeline returns events. If the test returns true and a value is set, then the clause has matched. If no clauses match, then events are dropped.

You can add a wildcard or default clause that will be used if no other clauses match by using an asterisk in the pipeline:

logscale
case { ... ;
*
| DEFAULT }

The default matches all events in the "default case", performing similar to the else part of an if-statement. If you do not add a wildcard clause, any events that do not match any of the explicit clauses will be dropped.

Conditional Case Example

Let's say we have logs from multiple sources that all have a field named time, and we want to group those hosts by a type identifying what host group to produce percentiles of the time fields, but one for each kind of source.

First, we try to match some text that distinguishes the different types of line. Then we can create a new field type and assign a value that we can use to group by.

logscale
time=*
| case { 
  src="client-side"
| type := "client";
  src="frontend-server"
| ip != 192.168.1.1
| type := "frontend";
*
| type := "server"
}
| groupBy(type, function=percentile(time))

Within the case statement, there are three separate clauses:

  • logscale
    src="client-side"
    | type := "client";

    When the src field contains client-side, set the type field to client.

  • logscale
    src="frontend-server"
    | ip != 192.168.1.1
    | type := "frontend";

    When the src field contains frontend-server and provided that the ip is not 192.168.1.1, set the type field to frontend.

  • logscale
    *
    | type := "server"

    If no other clause matches, set type to server.

Catch-All Clause

The * clause captures any output that has not matched a previous clause, and can be used either to ignore that information (resulting in no action or output) or to apply a default operation or value. For example, in the sample below the client field is being used to determine whether the IP address is localhost and setting the local field accordingly:

logscale
case { client = "::1"
| local := "true";
       client = "127.0.0.1"
| local := "true";
       *
| local := "false"}

The * clause in this instance sets local to false for any non-matching value.

However, the following is also valid:

logscale
case { client = "::1"
| local := "true";
       client = "127.0.0.1"
| local := "true";
       * }

The above ensures that the non-matching clauses are not processed, but does not create the field unless we have identified a local value.

The following example demonstrates a base match and default response example when looking at the Event_SimpleName field:

logscale
Event_SimpleName match{
/.*Network.*/ => network_tag := "Network";
* => new_tag := "Other"}

Match Expressions

If you you looking for the match() function, see match()

Using match expressions, you can describe alternative flows in your queries where the conditions all check the same field. It is similar to the switch operation you might recognize from many other programming languages. The matches on the field support the filters listed in Field Filters and in Regular Expression Filters.

The syntax looks like this:

logscale
field match {
  value => expression
| expression... ;
  /regex/ => expression
| ...;
  * => expression
| ...
}

You write a sequence of filter and pipeline clauses to run when the filter matches, separated by a semicolon (;). LogScale will apply each clause from top to bottom until one returns a value (matches the input).

You can use some functions as selectors (in addition to string patterns). More specifically, those functions which test a single field (and do not transform the event).

You can add a wildcard clause match { ... ; * => * } which matches all events as the "default case", essentially the else part of an if- statement. If you do not add a wildcard clause, then any events that do not match any of the explicit clauses will be dropped. You cannot use the empty clause — you must explicitly write * to match all.

As opposed to how a regular field test would work — where x=* checks for existence and x=true checks that x matches the string true — in the match (for case expressions) the guards * and true are equivalent and matches everything.

Because * no longer has the same meaning as in field tests, x match { ** => *} and x match { * => *} are not equivalent since the first checks for existence and the latter accepts everything.

This also means that "*" and * are not equivalent either, and "*" and "**" are equivalent.

Since, as in case expressions, the guards * and true match anything, the existence of a field can be checked using a quote glob "*" or two consecutive globs **.

The syntax looks like this:

logscale
field match {
  value => expression
| expression... ;
  /regex/ => expression
| ...;
  in(values=[1, 2, 3]) => expression
| ...; // match if in(field=field, values=[1, 2, 3]) does
  "*" => expression
| ...; // match if the field exists
  * => expression
| ... // match all events
}

match expressions accept any "pure" Filtering Query Functions (filter functions that do not mutate) that has a field parameter, for example:

logscale
x match { in(values=[5, 56, 567]) => *;}

Conditional Match Example

Let's say we have logs from multiple sources that all have a field that holds the time spent on some operation, but in different fields and units. We want to get percentiles of the time fields all in the same unit and in one field.

logscale
logtype match {
    "accesslog" => time:=response_time ;     // Access log is in seconds.
    /server_\d+/ => time:=server_time*1000 ; // These servers log in millis
  }
| groupBy(logtype, function=percentile(time))

Setting a Field's Default Value

You can use the function default() to set the value of a missing or empty field.