Life of a Fluentd event
The following article describes a global overview of how events are processed by Fluentd using examples. It covers the complete cycle including Setup, Inputs, Filters, Matches and Labels.
The configuration file is the fundamental piece to connect all things together, as it allows to define which Inputs or listeners Fluentd will have and setup common matching rules to route the Event data to a specific Output.
We will use the in_http and the out_stdout plugins as examples to describe the events cycle. The following is a basic definition on the configuration file to specify an http input, for short: we will be listening for HTTP Requests:
<source>@type httpport 8888bind 0.0.0.0</source>
The definition specifies that a HTTP server will be listening on TCP port 8888. Now lets define a Matching rule and a desired output that will just print to the standard output the data that arrived on each incoming request:
<match test.cycle>@type stdout</match>
The Match sets a rule where each Incoming event that arrives with a Tag equals to test.cycle, will match and use the Output plugin type called stdout. At this point we have an Input type, a Match and an Output. Let's test the setup using Curl:
$ curl -i -X POST -d 'json={"action":"login","user":2}' http://localhost:8888/test.cycleHTTP/1.1 200 OKContent-type: text/plainConnection: Keep-AliveContent-length: 0
On the Fluentd server side the output should look like this:
$ fluentd -c in_http.conf2019-12-16 18:58:15 +0900 [info]: parsing config file is succeeded path="in_http.conf"2019-12-16 18:58:15 +0900 [info]: gem 'fluentd' version '1.8.0'2019-12-16 18:58:15 +0900 [info]: using configuration file: <ROOT><source>@type httpport 8888bind "0.0.0.0"</source><match test.cycle>@type stdout</match></ROOT>2019-12-16 18:58:15 +0900 [info]: starting fluentd-1.8.0 pid=44323 ruby="2.4.6"2019-12-16 18:58:15 +0900 [info]: spawn command to main: cmdline=["/path/to/ruby", "-Eascii-8bit:ascii-8bit", "/path/to/fluentd", "-c", "in_http.conf", "--under-supervisor"]2019-12-16 18:58:16 +0900 [info]: adding match pattern="test.cycle" type="stdout"2019-12-16 18:58:16 +0900 [info]: adding source type="http"2019-12-16 18:58:16 +0900 [info]: #0 starting fluentd worker pid=44336 ppid=44323 worker=02019-12-16 18:58:16 +0900 [info]: #0 fluentd worker is now running worker=02019-12-16 18:58:27.888557000 +0900 test.cycle: {"action":"login","user":2}
Fluentd event consists of tag, time and record.
tag: Where an event comes from. For message routing
time: When an event happens. Nanosecond resolution
record: Actual log content. JSON object
The input plugin has responsibility for generating Fluentd event from data sources. For example, in_tail generates events from text lines. If you have following line in apache logs:
192.168.0.1 - - [28/Feb/2013:12:00:00 +0900] "GET / HTTP/1.1" 200 777
you got following event:
tag: apache.access # set by configurationtime: 1362020400.000000000 # 28/Feb/2013:12:00:00 +0900record: {"user":"-","method":"GET","code":200,"size":777,"host":"192.168.0.1","path":"/"}
When a Setup is defined, the Router Engine already contains several rules to apply for different input data. Internally an Event will to pass through a chain of procedures that may alter its cycle.
Now we will expand our previous basic example and we will add more steps in our Setup to demonstrate how the Events cycle can be altered. We will do this through the new Filters implementation.
A Filter aims to behave like a rule to pass or reject an event. The following configuration adds a Filter definition:
<source>@type httpport 8888bind 0.0.0.0</source>​<filter test.cycle>@type grep<exclude>key actionpattern ^logout$</exclude></filter>​<match test.cycle>@type stdout</match>
As you can see, the new Filter definition added will be a mandatory step before to pass the control to the Match section. The Filter basically will accept or reject the Event based on its type and rule defined. For our example we want to discard any user logout action, we will care just about the logins. The way to accomplish this, is doing a grep inside the Filter that states that will exclude any message on which action key have the logout string.
From a Terminal, run the following two Curl commands, please note that each one contains a different action value:
$ curl -i -X POST -d 'json={"action":"login","user":2}' http://localhost:8888/test.cycleHTTP/1.1 200 OKContent-type: text/plainConnection: Keep-AliveContent-length: 0​$ curl -i -X POST -d 'json={"action":"logout","user":2}' http://localhost:8888/test.cycleHTTP/1.1 200 OKContent-type: text/plainConnection: Keep-AliveContent-length: 0
Now looking at the Fluentd service output we can realize that just the one with the action equals to login just matched. The logout Event was discarded:
$ fluentd -c in_http.conf2019-12-16 19:07:39 +0900 [info]: parsing config file is succeeded path="in_http.conf"2019-12-16 19:07:39 +0900 [info]: gem 'fluentd' version '1.8.0'2019-12-16 19:07:39 +0900 [info]: using configuration file: <ROOT><source>@type httpport 8888bind "0.0.0.0"</source><filter test.cycle>@type grep<exclude>key "action"pattern ^logout$</exclude></filter><match test.cycle>@type stdout</match></ROOT>2019-12-16 19:07:39 +0900 [info]: starting fluentd-1.8.0 pid=44435 ruby="2.4.6"2019-12-16 19:07:39 +0900 [info]: spawn command to main: cmdline=["/path/to/ruby", "-Eascii-8bit:ascii-8bit", "/path/to/fluentd", "-c", "in_http.conf", "--under-supervisor"]2019-12-16 19:07:40 +0900 [info]: adding filter pattern="test.cycle" type="grep"2019-12-16 19:07:40 +0900 [info]: adding match pattern="test.cycle" type="stdout"2019-12-16 19:07:40 +0900 [info]: adding source type="http"2019-12-16 19:07:40 +0900 [info]: #0 starting fluentd worker pid=44448 ppid=44435 worker=02019-12-16 19:07:40 +0900 [info]: #0 fluentd worker is now running worker = 02019-12-16 19:08:06.934660000 +0900 test.cycle: {"action":"login","user":2}
As you can see, the Events follow a step by step cycle where they are processed in order from top to bottom. The new engine on Fluentd allows to integrate many Filters as necessary, also considering that the configuration file will grow and start getting a bit complex for the readers, a new feature called Labels have been added that aims to solve this possible problem.
This new implementation called Labels, aims to solve the configuration file complexity and allows to define new Routing sections that do not follow the top to bottom order, instead it acts like linked references. Talking the previous example, we will modify the setup as follows:
<source>@type httpbind 0.0.0.0port 8888@label @STAGING</source>​<filter test.cycle>@type grep<exclude>key actionpattern ^login$</exclude></filter>​<label @STAGING><filter test.cycle>@type grep<exclude>key actionpattern ^logout$</exclude></filter>​<match test.cycle>@type stdout</match></label>
The new configuration contains a @label key on the source indicating that any further steps takes place on the STAGING Label section. The expectation is that every Event reported on the Source, the Routing engine will continue processing on STAGING, for hence it will skip the old filter definition.
In this example, we use stdout non-buffered output. But in production, you use outputs in buffered mode, e.g. forward, mongodb, s3 and etc. Output plugin in buffered mode stores received events into buffers first and write out buffers to a destination by meeting flush conditions. So using buffered output, you don't see received events immediately unlike stdout non-buffered output.
Buffer is important for reliability and throughput. See Output and Buffer articles.
Once the events are reported by the Fluentd engine on the Source, can be processed step by step or inside a referenced Label, as well any Event may be filtered out at any moment. The new Routing engine behavior aims to provide more flexibility and makes easier the processing before reaching the Output plugin.
If this article is incorrect or outdated, or omits critical information, please let us know. Fluentd is a open source project under Cloud Native Computing Foundation (CNCF). All components are available under the Apache 2 License.
