METRON-1618: Stellar boolean expressions should treat missing variables as false

[cestella]

Contributor Comments

Right now, because fields may not exist, users can have an awkward time. For instance, checking for is_alert, you end up having to preface checks with exists(is_alert).

For instance, in one of our use-cases we use

"is_alert := exists(is_alert) && is_alert",
"is_alert := is_alert || (geo_outlier != null && geo_outlier == true)"

instead of :

"is_alert := is_alert || geo_outlier == true"

I suggest that we adopt a convention from javascript whereby we assume a field not existing or being null should act as false in boolean expressions. This will simplify stellar's use and hopefully result in less awkwardness.

You can test this via the REPL. This is what I did, in addition to unit tests:

{12:48}[system]~/Documents/workspace/metron/fork/incubator-metron:stellar_falsey ✓ ➭ mvn exec:java -Dexec.mainClass="org.apache.metron.stellar.common.shell.cli.StellarShell" -pl metron-stellar/stellar-common
[INFO] Scanning for projects...
[INFO]
[INFO] ------------------------------------------------------------------------
[INFO] Building stellar-common 0.5.0
[INFO] ------------------------------------------------------------------------
[INFO]
[INFO] --- exec-maven-plugin:1.6.0:java (default-cli) @ stellar-common ---
log4j:WARN No appenders could be found for logger (org.apache.metron.stellar.common.shell.DefaultStellarShellExecutor).
log4j:WARN Please initialize the log4j system properly.
log4j:WARN See http://logging.apache.org/log4j/1.2/faq.html#noconfig for more info.
Stellar, Go!
Functions are loading lazily in the background and will be unavailable until loaded fully.
{}
[Stellar]>>> is_alert || null
false
[Stellar]>>> NOT(is_alert)
true
[Stellar]>>> is_alert
[Stellar]>>> if is_alert then 'alert' else 'not'
not
[Stellar]>>> if is_alert || true then 'alert' else 'not'
alert
[Stellar]>>> MAP_GET(is_alert, { false : 'blah', true : 'foo' })
[Stellar]>>>

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[ottobackwards]

A couple of comments.

• Please add tests for MATCH expressions.
• Please add some doc comment to the logic being applied in the apply().

[simonellistonball]

Should the example target state not be:
is_alert := is_alert || geo_outlier

We should probably have a test for that, but I assume the null is falsey would apply on both sides of the boolean and that things like short cutting is_thing && FUNC(thing) would be unaffected (i.e if is_thing is null, FUNC would never run).

Should we also include updates to the configs in the use cases folder (since these are kinda documentation) with this PR?

[merrimanr]

I tested this in full dev and it worked fine. I feel like this is a nice improvement and makes it easier to use. I would give it my +1 when the doc and testing requests made by others are done.

[cestella]

I think I'm going to move this discussion to #1064 as it's getting kinda hard to treat this and #1064 as separate without incurring merge conflicts. I'll be closing this once I've addressed comments here in commits to #1064