Clarify parsing multiline messages (#10212)

make more clear the distinction between docker/cri splitting and merging, and multiline application log (stacktraces) merging

Signed-off-by: Martin Fleurke <[email protected]>

Author: Martin Fleurke

administration/configuring-fluent-bit/multiline-parsing.md

@@ -17,8 +17,8 @@ Fluent Bit exposes certain pre-configured parsers (built-in) to solve specific m
 
 | Parser | Description |
 | ------ | ----------- |
-| `docker` | Process a log entry generated by a Docker container engine. This parser supports the concatenation of log entries split by Docker.      |
-| `cri` | Process a log entry generated by CRI-O container engine. Like the `docker` parser, it supports concatenation of log entries.          |
+| `docker` | Process a log entry generated by a Docker container engine. This parser supports the concatenation of large log entries split by Docker. If you use this parser, and you also want to concatenate loglines like stacktraces, you can add the [multiline filter](../../pipeline/filters/multiline-stacktrace.md) to specify additional parsers. |
+| `cri`    | Process a log entry generated by CRI-O container engine. Like the `docker` parser, it supports concatenation of log entries.            |
 | `go`     | Process log entries generated by a Go based language application and perform concatenation if multiline messages are detected.          |
 | `python` | Process log entries generated by a Python based language application and perform concatenation if multiline messages are detected.      |
 | `java`   | Process log entries generated by a Google Cloud Java language application and perform concatenation if multiline messages are detected. |

pipeline/filters/multiline-stacktrace.md

@@ -16,7 +16,7 @@ Along with multiline filters, you can enable one of the following built-in Fluen
 When using this filter:
 
 - The usage of this filter depends on a previous configuration of a [multiline parser](../../administration/configuring-fluent-bit/multiline-parsing.md) definition.
-- To concatenate messages read from a log file, it's highly recommended to use the multiline support in the [Tail plugin](https://docs.fluentbit.io/manual/pipeline/inputs/tail#multiline-support) itself. This is because performing concatenation while reading the log file is more performant. Concatenating messages originally split by Docker or CRI container engines, is supported in the [Tail plugin](https://docs.fluentbit.io/manual/pipeline/inputs/tail#multiline-support).
+- To concatenate messages read from a log file, it's highly recommended to use the multiline support in the [Tail plugin](https://docs.fluentbit.io/manual/pipeline/inputs/tail#multiline-support) itself. This is because performing concatenation while reading the log file is more performant. Concatenating messages that were originally one line, but split by Docker or CRI container engines because of their size, is supported in the [Tail plugin](https://docs.fluentbit.io/manual/pipeline/inputs/tail#multiline-support) in combination with the `docker` or `cri` parser. To concatenate application logs like stacktraces on top of that, you can use this multiline filter.
 
 {% hint style="warning" %}
 
@@ -314,7 +314,8 @@ Lines that don't match a pattern aren't considered as part of the multiline mess
 
 ## Docker partial message use case
 
-When Fluent Bit is consuming logs from a container runtime, such as Docker, these logs will be split when larger than a certain limit, usually 16&nbspKB. If your application emits a 100K log line, it will be split into seven partial messages. If you are using the [Fluentd Docker Log Driver](https://docs.docker.com/config/containers/logging/fluentd/) to send the logs to Fluent Bit, they might look like this:
+When Fluent Bit is consuming logs from a container runtime, such as Docker, these logs will be split when larger than a certain limit, usually 16&nbspKB. 
+If your application emits a 100K log line, it will be split into seven partial messages. The docker parser will merge these back to one line. If instead you are using the [Fluentd Docker Log Driver](https://docs.docker.com/config/containers/logging/fluentd/) to send the logs to Fluent Bit, they might look like this:
 
 ```text
 {"source": "stdout", "log": "... omitted for brevity...", "partial_message": "true", "partial_id": "dc37eb08b4242c41757d4cd995d983d1cdda4589193755a22fcf47a638317da0", "partial_ordinal": "1", "partial_last": "false", "container_id": "a96998303938eab6087a7f8487ca40350f2c252559bc6047569a0b11b936f0f2", "container_name": "/hopeful_taussig"}]

pipeline/inputs/tail.md

@@ -151,7 +151,7 @@ Multiline core is exposed by the following configuration:
 
 ### Multiline and containers
 
-If you are running Fluent Bit to process logs coming from containers like Docker or CRI, you can use the built-in modes. This helps reassemble multiline messages originally split by Docker or CRI:
+If you are running Fluent Bit to process logs coming from containers like Docker or CRI, you can use the built-in modes. This helps reassemble large messages originally split by Docker or CRI:
 
 {% tabs %}
 {% tab title="fluent-bit.yaml" %}