The way that log and exception messages get formatted is an important part of a project. Specifically, it is
important that there is consistency in formatting such that someone can easily identify and interpret messages.
This consistency applies to both log *and* exception messages.
1. Messages should have something interpolated into them. Generally speaking, if the time is being taken to generate a message, it is usually valuable to interpolate something from the context into that message. There are exceptions to this, but all messages should start with the assumption that something should be interpolated and try to figure out what that is.
* Messages INFO level or above (this includes all Exceptions) cannot leak secrets or the content of data. When choosing what to interpolate, it is important to make sure that what is being added is not going to leak secrets or the contents of data. For example, a query with a malformed filter should provide an indication that the filter was malformed and which filter it is, but it cannot include the values being filtered for as that risks leaking data.
* It is not always clear that something will leak information when the message is created. This is a shared responsibility and accidents can happen. If anything is discovered to leak or a message is suspected of be able to leak things, it should be opportunistically fixed. It is valid to include such fixes in PRs that are unrelated to the log-line specifically as it is more important that the fixes happen than that we have perfect separation of PRs.
2. Interpolated values should always be encased in a `[]` and come after a noun that describes what is being interpolated. This is to ensure that enough context on what is happening exists and to clearly demark that an interpolation has occurred. Additionally, this identifies the start and end of the interpolation, which is important because messages that attempt to mimic natural prose that also include interpolation can sometimes mask glaring problems (like the inclusion of a space).
* Note, the `[]` is an indication of interpolation. Its closest cousin in terms of grammatical structures is the [Apposition](https://en.wikipedia.org/wiki/Apposition). Given that the interpolation specifies a specific instance of the noun, it would be considered "restrictive" and normal English rules would dictate that it come immediately in the sentence without any sort of punctuation. If we were to follow these rules, we would lose track of what is in the interpolation and what is not, so we encase in `[]` to help identify the start and end of the Apposition. As with appositions, the interpolation is an ***optional*** addition to our messages that helps accelerate the identification of next steps.
* This means that when intepolating an array-like structure, we will see double `[]`. That is okay and a similar thing can happen no matter what we use to indicate interpolation.
3. Messages should read like a sentence and follow sentence-like structure.
* A nice way to validate this is that even if you were to remove interpolations, the message should still be readable and understandable. When a message requires interpolations to be meaningful, that is indicative of the message not carrying enough context.
* After interpolation, it is unclear what values were interpolated and what values were part of the message: `"null is not null cannot handle INTEGER"`
* With interpolations removed, it becomes completely incomprehensible: `" cannot handle "`
* Better, but still not wonderful: `log.info("column [%s] filter [%s] cannot handle type [%s]", "null", "is not null", "INTEGER")`
* After interpolation, clear separation: `"column [null] filter [is not null] cannot handle type [INTEGER]"`
* With interpolations removed, it becomes something that is difficult to penetrate: `"column filter cannot handle type"`
* Good: `log.info("Filter [%s] on column [%s] cannot be applied to type [%s]", "is not null", "null", "INTEGER")`
* After interpolation, clear separation: `"Filter [is not null] on column [null] cannot be applied to type [INTEGER]"`
* With interpolations removed, it is clear what happened, though still hard to figure out which specific thing to adjust: `"Filter on column cannot be applied to type"`
For the majority of style considerations, the Apache Druid documentation follows the [Google Developer Documentation Style Guide](https://developers.google.com/style). For more details, see [Contribute to Druid docs](../docs/development/docs-contribute.md#style-guide).
