Recently, the first rsyslog version 8 release was announced. Major changes in its core should give outputs better performance, and the one for Elasticsearch should benefit a lot. Since we’re using rsyslog and Elasticsearch in Sematext‘s own Logsene, we had to take the new version for a spin.

The Weapon and the Target

For testing, we used a good-old i3 laptop, with 8GB of RAM. We generated 20 million logs, sent them to rsyslog via TCP and from there to Elasticsearch in the Logstash format, so they can get explored with Kibana. The objective was to stuff as many events per second into Elasticsearch as possible.

Rsyslog Architecture Overview

In order to tweak rsyslog effectively, one needs to understand its architecture, which is not that obvious (although there’s an ongoing effort to improve the documentation). The gist of it its architecture represented in the figure below.

• you have input modules taking messages (from files, network, journal, etc.) and pushing them to a main queue
• one or more main queue threads take those events and parse them. By default, they parse syslog formats, but you can configure rsyslog to use message modifier modules to do additional parsing (e.g. CEE-formatted JSON messages). Either way, this parsing generates structured logs, made out of properties
• after parsing, the main queue threads push events to the action queue. Or queues, if there are multiple actions and you want to fan-out
• for each defined action, one or more action queue threads takes properties from events according to templates, and makes messages that would be sent to the destination. In Elasticsearch’s case, a template should make Elasticsearch JSON documents, and the destination would be the REST API endpoint

There are two more things to say about rsyslog’s architecture before we move on to the actual test:

• you can have multiple independent flows (like the one in the figure above) in the same rsyslog process by using rulesets. Think of rulesets as swim-lanes. They’re useful for example when you want to process local logs and remote logs in a completely separate manner
• queues can be in-memory, on disk, or a combination called disk-assisted. Here, we’ll use in-memory because they’re the fastest. For more information about how queues work, take a look here

Configuration

To generate messages, we used tcpflood, a small and light tool that’s part of rsyslog’s testbench. It generates messages and sends them over to the local syslog via TCP.

Rsyslog took received those messages with the imtcp input module, queued them and forwarded them to Elasticsearch 0.90.7, which was also installed locally. We also tried with Elasticsearch 1.0 Beta 1 and got the same results (see below).

The flow of messages in this test is represented in the following figure:

The actual rsyslog config is listed below. It can be tuned further (for example by using the multithreaded imptcp input module), but we didn’t get significant improvements.

module(load="imtcp")   # TCP input module
module(load="omelasticsearch") # Elasticsearch output module

input(type="imtcp" port="13514")  # where to listen for TCP messages

main_queue(
    queue.size="1000000" # capacity of the main queue
    queue.dequeuebatchsize="1000" # process messages in batches of 1000
    queue.workerthreads="2" # 2 threads for the main queue
)

# template to generate JSON documents for Elasticsearch
template(name="plain-syslog"
         type="list") {
           constant(value="{")
             constant(value=""@timestamp":"")      property(name="timereported" dateFormat="rfc3339")
             constant(value="","host":"")        property(name="hostname")
             constant(value="","severity":"")    property(name="syslogseverity-text")
             constant(value="","facility":"")    property(name="syslogfacility-text")
             constant(value="","syslogtag":"")   property(name="syslogtag" format="json")
             constant(value="","message":"")    property(name="msg" format="json")
             constant(value=""}")
         }

action(type="omelasticsearch"
       template="plain-syslog"  # use the template defined earlier
       searchIndex="test-index"
       bulkmode="on"
       queue.dequeuebatchsize="5000"   # ES bulk size
       queue.size="100000"
       queue.workerthreads="5"
       action.resumeretrycount="-1"  # retry indefinitely if ES is unreachable
)

You can see from the configuration that:

• both main and action queues have a defined size in number of messages
• both have number of threads that deliver messages to the next step. The action needs more because it has to wait for Elasticsearch to reply
• moving of messages from the queues happens in batches. For the Elasticsearch output, the batch of messages is sent through the Bulk API, which makes queue.dequeuebatchsize effectively the bulk size

Results

We started with default Elasticsearch settings. Then we tuned them to leave rsyslog with a more significant slice of the CPU. We measured the indexing rate with SPM. Here are the average results over 20 million indexed events:

• with default Elasticsearch settings, we got 8,000 events per second
• after setting Elasticsearch up more production-like (5 second refresh interval, increased index buffer size, translog thresholds, etc), and the throughput went up to average of 20,000 events per second
• in the end, we went berserk and used in-memory indices, updated the mapping to disable any storing or indexing for any field, to have Elasticsearch do as little work as possible and make room for rsyslog. Got an average of 30,000 events per second. In this scenario, rsyslog was using between 1 and 1.5 of the 4 virtual CPU cores, with tcpflood using 0.5 and Elasticsearch using from 2 to 2.5

Conclusion

20K EPS on a low-end machine with production-like configuration means Elasticsearch is quick at indexing. This is very good for logs, where you typically have lots of messages being generated, compared to how often you search.

If you need some tool to ship your logs to Elasticsearch with minimum overhead, rsyslog with its new version 8 may well be your best bet.

Additional resources:

• Using rsyslog to ship logs to Elasticsearch in Logstash format, to analyze them with Kibana
• Handling CEE-formatted logs (JSON over syslog) with rsyslog and Elasticsearch

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