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# Using Log4j in Cloud Enabled Applications

## The Twelve-Factor Application

The Logging Guidelines for [The Twelve-Factor App](https://12factor.net/logs) state that all logs should be routed 
unbuffered to stdout. Since this is the least common denominator it is guaranteed to work for all applications. However,
as with any set of general guidelines, choosing the least common denominator approach comes at a cost. Some of the costs
in Java applications include:

1. Java stack traces are multi-line log messages. The standard docker log driver cannot handle these properly. See 
[Docker Issue #22920](https://github.com/moby/moby/issues/22920) which was closed with the message "Don't Care".
Solutions for this are to:
    a. Use a docker log driver that does support multi-line log message,
    b. Use a logging format that does not produce multi-line messages,
    c. Log from Log4j directly to a logging forwarder or aggregator and bypass the docker logging driver.
1. When logging to stdout in Docker, log events pass through Java's standard output handling which is then directed 
to the operating system so that the output can be piped into a file. The overhead of all this is measurably slower
than just writing directly to a file as can be seen in these benchmark results where logging 
to stdout is 16-20 times slower over repeated runs than logging directly to the file. The results below were obtained by 
running the [Output Benchmark](https://github.com/apache/logging-log4j2/blob/release-2.x/log4j-perf/src/main/java/org/apache/logging/log4j/perf/jmh/OutputBenchmark.java)
on a 2018 MacBook Pro with a 2.9GHz Intel Core i9 processor and a 1TB SSD.  However, these results alone would not be 
enough to argue against writing to the standard output stream as they only amount to about 14-25 microseconds 
per logging call vs 1.5 microseconds when writing to the file. 
    ```
        Benchmark                  Mode  Cnt       Score       Error  Units
        OutputBenchmark.console   thrpt   20   39291.885 ±  3370.066  ops/s
        OutputBenchmark.file      thrpt   20  654584.309 ± 59399.092  ops/s
        OutputBenchmark.redirect  thrpt   20   70284.576 ±  7452.167  ops/s
    ```
1. When performing audit logging using a framework such as log4j-audit guaranteed delivery of the audit events
is required. Many of the options for writing the output, including writing to the standard output stream, do
not guarantee delivery. In these cases the event must be delivered to a "forwarder" that acknowledges receipt
only when it has placed the event in durable storage, such as what [Apache Flume](https://flume.apache.org/) 
or [Apache Kafka](https://kafka.apache.org/) will do.

## Logging Approaches

All the solutions discussed on this page are predicated with the idea that log files cannot permanently
reside on the file system and that all log events should be routed to one or more log analysis tools that will 
be used for reporting and alerting. There are many ways to forward and collect events to be sent to the 
log analysis tools. 

Note that any approach that bypasses Docker's logging drivers requires Log4j's 
[Docker Lookup](lookups.html#DockerLookup) to allow Docker attributes to be injected into the log events.  

### Logging to the Standard Output Stream

As discussed above, this is the recommended 12-Factor approach for applications running in a docker container.
The Log4j team does not recommend this approach for performance reasons.

![Stdout](../images/DockerStdout.png "Application Logging to the Standard Output Stream")

### Logging to the Standard Output Stream with the Docker Fluentd Logging Driver

Docker provides alternate [logging drivers](https://docs.docker.com/config/containers/logging/configure/), 
such as [gelf](https://docs.docker.com/config/containers/logging/gelf/) or 
[fluentd](https://docs.docker.com/config/containers/logging/fluentd/), that
can be used to redirect the standard output stream to a log forwarder or log aggregator. 

When routing to a log forwarder it is expected that the forwarder will have the same lifetime as the 
application. If the forwarder should fail the management tools would be expected to also terminate 
other containers dependent on the forwarder.

![Docker Fluentbit](../images/DockerFluentd.png "Logging via StdOut using the Docker Fluentd Logging Driver to Fluent-bit")

As an alternative the logging drivers could be configured to route events directly to a logging aggregator.
This is generally not a good idea as the logging drivers only allow a single host and port to be configured. 
The docker documentation isn't clear but infers that log events will be dropped when log events cannot be
delivered so this method should not be used if a highly available solution is required.

![Docker Fluentd](../images/DockerFluentdAggregator.png "Logging via StdOut using the Docker Fluentd Logging Driver to Fluentd")

### Logging to a File

While this is not the recommended 12-Factor approach, it performs very well. However, it requires that the 
application declares a volume where the log files will reside and then configures the log forwarder to tail 
those files. Care must also be taken to automatically manage the disk space used for the logs, which Log4j 
can perform via the "Delete" action on the [RollingFileAppender](appenders.html#RollingFileAppender).

![File](../images/DockerLogFile.png "Logging to a File")

### Sending Directly to a Log Forwarder via TCP

Sending logs directly to a Log Forwarder is simple as it generally just requires that the forwarder's
host and port be configured on a SocketAppender with an appropriate layout.

![TCP](../images/DockerTCP.png "Application Logging to a Forwarder via TCP")

### Sending Directly to a Log Aggregator via TCP

Similar to sending logs to a forwarder, logs can also be sent to a cluster of aggregators. However,
setting this up is not as simple since, to be highly available, a cluster of aggregators must be used.
However, the SocketAppender currently can only be configured with a single host and port. To allow 
for failover if the primary aggregator fails the SocketAppender must be enclosed in a 
[FailoverAppender](appenders.html#FailoverAppender),
which would also have the secondary aggregator configured. Another option is to have the SocketAppender 
point to a highly available proxy that can forward to the Log Aggregator.

If the log aggregator used is Apache Flume or Apache Kafka (or similar) the Appenders for these support 
being configured with a list of hosts and ports so high availability is not an issue. 

![Aggregator](../images/LoggerAggregator.png "Application Logging to an Aggregator via TCP")

## <a name="ELK"></a>Logging using Elasticsearch, Logstash, and Kibana

There are various approaches with different trade-offs for ingesting logs into
an ELK stack. Here we will briefly cover how one can forward Log4j generated
events first to Logstash and then to Elasticsearch.

### Log4j Configuration

### JsonTemplateLayout
Log4j provides a multitude of JSON generating layouts. In particular, [JSON
Template Layout](layouts.html#JSONTemplateLayout) allows full schema
customization and bundles ELK-specific layouts by default, which makes it a
great fit for the bill. Using the EcsLayout template as shown below will generate data in Kibana where
the message displayed exactly matches the message passed to Log4j and most of the event attributes, including
any exceptions, are present as individual attributes that can be displayed. Note, however that stack traces 
will be formatted without newlines.

    <Socket name="Logstash"
            host="${sys:logstash.host}"
            port="12345"
            protocol="tcp"
            bufferedIo="true">
      <JsonTemplateLayout eventTemplateUri="classpath:EcsLayout.json">
        <EventTemplateAdditionalField key="containerId" value="${docker:containerId:-}"/>
        <EventTemplateAdditionalField key="application" value="${lower:${spring:spring.application.name:-spring}}"/>
        <EventTemplateAdditionalField key="kubernetes.serviceAccountName" value="${k8s:accountName:-}"/>
        <EventTemplateAdditionalField key="kubernetes.containerId" value="${k8s:containerId:-}"/>
        <EventTemplateAdditionalField key="kubernetes.containerName" value="${k8s:containerName:-}"/>
        <EventTemplateAdditionalField key="kubernetes.host" value="${k8s:host:-}"/>
        <EventTemplateAdditionalField key="kubernetes.labels.app" value="${k8s:labels.app:-}"/>
        <EventTemplateAdditionalField key="kubernetes.labels.pod-template-hash" value="${k8s:labels.podTemplateHash:-}"/>
        <EventTemplateAdditionalField key="kubernetes.master_url" value="${k8s:masterUrl:-}"/>
        <EventTemplateAdditionalField key="kubernetes.namespaceId" value="${k8s:namespaceId:-}"/>
        <EventTemplateAdditionalField key="kubernetes.namespaceName" value="${k8s:namespaceName:-}"/>
        <EventTemplateAdditionalField key="kubernetes.podID" value="${k8s:podId:-}"/>
        <EventTemplateAdditionalField key="kubernetes.podIP" value="${k8s:podIp:-}"/>
        <EventTemplateAdditionalField key="kubernetes.podName" value="${k8s:podName:-}"/>
        <EventTemplateAdditionalField key="kubernetes.imageId" value="${k8s:imageId:-}"/>
        <EventTemplateAdditionalField key="kubernetes.imageName" value="${k8s:imageName:-}"/>
      </JsonTemplateLayout>
    </Socket>

#### Gelft Template

The JsonTemplateLayout can also be used to generate JSON that matches the GELF specification which can format 
the message attribute using a pattern in accordance with the PatternLayout. For example, the following
template, named EnhancedGelf.json, can be used to generate GELF-compliant data that can be passed to Logstash. 
With this template the message attribute will include the thread id, level, specific ThreadContext attributes, 
the class name, method name, and line number as well as the message. If an exception is included it will also 
be included with newlines. This format follows very closely what you would see in a typical log file on disk 
using the PatternLayout but has the additional advantage of including the attributes as separate fields that 
can be queried.

    {
        "version": "1.1",
        "host": "${hostName}",
        "short_message": {
            "$resolver": "message",
            "stringified": true
        },
        "full_message": {
            "$resolver": "message",
            "pattern": "[%t] %-5p %X{requestId, sessionId, loginId, userId, ipAddress, corpAcctNumber} %C{1.}.%M:%L - %m",
            "stringified": true
        },
        "timestamp": {
            "$resolver": "timestamp",
            "epoch": {
                "unit": "secs"
            }
        },
        "level": {
            "$resolver": "level",
            "field": "severity",
            "severity": {
                "field": "code"
            }
        },
        "_logger": {
            "$resolver": "logger",
            "field": "name"
        },
        "_thread": {
            "$resolver": "thread",
            "field": "name"
        },
        "_mdc": {
            "$resolver": "mdc",
            "flatten": {
                "prefix": "_"
            },
            "stringified": true
        }
    }
    
The logging configuration to use this template would be    

    <Socket name="Elastic"
            host="\${sys:logstash.search.host}"
            port="12222"
            protocol="tcp"
            bufferedIo="true">
      <JsonTemplateLayout eventTemplateUri="classpath:EnhancedGelf.json" nullEventDelimiterEnabled="true">
        <EventTemplateAdditionalField key="containerId" value="${docker:containerId:-}"/>
        <EventTemplateAdditionalField key="application" value="${lower:${spring:spring.application.name:-spring}}"/>
        <EventTemplateAdditionalField key="kubernetes.serviceAccountName" value="${k8s:accountName:-}"/>
        <EventTemplateAdditionalField key="kubernetes.containerId" value="${k8s:containerId:-}"/>
        <EventTemplateAdditionalField key="kubernetes.containerName" value="${k8s:containerName:-}"/>
        <EventTemplateAdditionalField key="kubernetes.host" value="${k8s:host:-}"/>
        <EventTemplateAdditionalField key="kubernetes.labels.app" value="${k8s:labels.app:-}"/>
        <EventTemplateAdditionalField key="kubernetes.labels.pod-template-hash" value="${k8s:labels.podTemplateHash:-}"/>
        <EventTemplateAdditionalField key="kubernetes.master_url" value="${k8s:masterUrl:-}"/>
        <EventTemplateAdditionalField key="kubernetes.namespaceId" value="${k8s:namespaceId:-}"/>
        <EventTemplateAdditionalField key="kubernetes.namespaceName" value="${k8s:namespaceName:-}"/>
        <EventTemplateAdditionalField key="kubernetes.podID" value="${k8s:podId:-}"/>
        <EventTemplateAdditionalField key="kubernetes.podIP" value="${k8s:podIp:-}"/>
        <EventTemplateAdditionalField key="kubernetes.podName" value="${k8s:podName:-}"/>
        <EventTemplateAdditionalField key="kubernetes.imageId" value="${k8s:imageId:-}"/>
        <EventTemplateAdditionalField key="kubernetes.imageName" value="${k8s:imageName:-}"/>
      </JsonTemplateLayout>
    </Socket>

The significant difference with this configuration from the first example is that it references the 
custom template and it specifies an event delimiter of a null character ('\0');   
    
**Note**: The level being passed with the above template does not strictly conform to the GELF spec as the
Level being passed is the Log4j Level NOT the Level defined in the GELF spec. However, testing has shown 
that Logstash, Elk, and Kibana are pretty tolerant of whatever data is passed to it.

#### Custom Template

Another option is to use a custom template, possibly based on one of the standard templates. The template 
below is loosely based on ECS but a) adds the spring boot application name, b) formats the message
using PatternLayout, formats Map Messages as event.data attributes while setting the event action based on
any Marker included in the event, includes all the ThreadContext attributes. 

**Note**: The Json Template Layout escapes control sequences so messages that contain '\n' will have those 
control sequences copied as "\n" into the text rather than converted to a newline character. This bypasses 
many problems that occur with Log Forwarders such as Filebeat and FluentBit/Fluentd. Kibana will correctly
interpret these squences as newlines and display them correctly. Also note that the message pattern does
not contain a timestamp. Kibana will display the timestamp field in its own column so placing it in the 
message would be redundant.

    {
      "@timestamp": {
        "$resolver": "timestamp",
        "pattern": {
          "format": "yyyy-MM-dd'T'HH:mm:ss.SSS'Z'",
          "timeZone": "UTC"
        }
      },
      "ecs.version": "1.11.0",
      "log.level": {
        "$resolver": "level",
        "field": "name"
      },
      "application": "\${lower:\${spring:spring.application.name}}",
      "short_message": {
        "$resolver": "message",
        "stringified": true
      },
      "message": {
        "$resolver": "pattern",
        "pattern": "[%t] %X{requestId, sessionId, loginId, userId, ipAddress, accountNumber} %C{1.}.%M:%L - %m%n"
      },
      "process.thread.name": {
        "$resolver": "thread",
        "field": "name"
      },
      "log.logger": {
        "$resolver": "logger",
        "field": "name"
      },
      "event.action": {
        "$resolver": "marker",
        "field": "name"
      },
      "event.data": {
        "$resolver": "map",
        "stringified": true
      },
      "labels": {
        "$resolver": "mdc",
        "flatten": true,
        "stringified": true
      },
      "tags": {
        "$resolver": "ndc"
      },
      "error.type": {
        "$resolver": "exception",
        "field": "className"
      },
      "error.message": {
        "$resolver": "exception",
        "field": "message"
      },
      "error.stack_trace": {
        "$resolver": "exception",
        "field": "stackTrace",
        "stackTrace": {
          "stringified": true
        }
      }
    }

    
Finally, the GelfLayout can be used to generate GELF compliant output. Unlike the JsonTemplateLayout it 
adheres closely to the GELF spec.    

    <Socket name="Elastic" host="${sys:elastic.search.host}" port="12222" protocol="tcp" bufferedIo="true">
      <GelfLayout includeStackTrace="true" host="${hostName}" includeThreadContext="true" includeNullDelimiter="true"
                  compressionType="OFF">
        <ThreadContextIncludes>requestId,sessionId,loginId,userId,ipAddress,callingHost</ThreadContextIncludes>
        <MessagePattern>%d [%t] %-5p %X{requestId, sessionId, loginId, userId, ipAddress} %C{1.}.%M:%L - %m%n</MessagePattern>
        <KeyValuePair key="containerId" value="${docker:containerId:-}"/>
        <KeyValuePair key="application" value="${lower:${spring:spring.application.name:-spring}}"/>
        <KeyValuePair key="kubernetes.serviceAccountName" value="${k8s:accountName:-}"/>
        <KeyValuePair key="kubernetes.containerId" value="${k8s:containerId:-}"/>
        <KeyValuePair key="kubernetes.containerName" value="${k8s:containerName:-}"/>
        <KeyValuePair key="kubernetes.host" value="${k8s:host:-}"/>
        <KeyValuePair key="kubernetes.labels.app" value="${k8s:labels.app:-}"/>
        <KeyValuePair key="kubernetes.labels.pod-template-hash" value="${k8s:labels.podTemplateHash:-}"/>
        <KeyValuePair key="kubernetes.master_url" value="${k8s:masterUrl:-}"/>
        <KeyValuePair key="kubernetes.namespaceId" value="${k8s:namespaceId:-}"/>
        <KeyValuePair key="kubernetes.namespaceName" value="${k8s:namespaceName:-}"/>
        <KeyValuePair key="kubernetes.podID" value="${k8s:podId:-}"/>
        <KeyValuePair key="kubernetes.podIP" value="${k8s:podIp:-}"/>
        <KeyValuePair key="kubernetes.podName" value="${k8s:podName:-}"/>
        <KeyValuePair key="kubernetes.imageId" value="${k8s:imageId:-}"/>
        <KeyValuePair key="kubernetes.imageName" value="${k8s:imageName:-}"/>
      </GelfLayout>
    </Socket>

#### Logstash Configuration with Gelf

We will configure Logstash to listen on TCP port 12345 for payloads of type JSON
and then forward these to (either console and/or) an Elasticsearch server.

    input {
      tcp {
        port => 12345
        codec => "json"
      }
    }

    output {

      # (Un)comment for debugging purposes.
      # stdout { codec => rubydebug }

      # Modify the hosts value to reflect where elasticsearch is installed.
      elasticsearch {
        hosts => ["http://localhost:9200/"]
        index => "app-%{application}-%{+YYYY.MM.dd}"
      }

    }

#### Logstash Configuration with JsonTemplateLayout    

When one of the GELF compliant formats is used Logstash should be configured as 

   gelf {
           host => "localhost"
           use_tcp => true
           use_udp => false
           port => 12222
           type => "gelf"
         }
       }
   
       filter {
         # These are GELF/Syslog logging levels as defined in RFC 3164. Map the integer level to its human readable format.
         translate {
           field => "[level]"
           destination => "[levelName]"
           dictionary => {
             "0" => "EMERG"
             "1" => "ALERT"
             "2" => "CRITICAL"
             "3" => "ERROR"
             "4" => "WARN"
             "5" => "NOTICE"
             "6" => "INFO"
             "7" => "DEBUG"
           }
         }
       }
   
       output {
         # (Un)comment for debugging purposes
         # stdout { codec => rubydebug }
         # Modify the hosts value to reflect where elasticsearch is installed.
         elasticsearch {
           hosts => ["http://localhost:9200/"]
           index => "app-%{application}-%{+YYYY.MM.dd}"
         }
       }
#### Filebeat configuration with JsonTemplateLayout

When using a JsonTemplateLayout that complies with ECS (or is similar to the custom template previously shown)
the configuration of filebeat is straightforward.

    filebeat.inputs:
    - type: log
      enabled: true
      json.keys_under_root: true
      paths:
        - /var/log/apps/*.log


### Kibana
Using the EnhancedGelf template, the GelfLayout or the custom template the above configurations the message 
field will contain a fully formatted log event just as it would  appear in a file Appender. The ThreadContext 
attributes, custome fields, thread name, etc. will all be available as attributes on each log event that can 
be used for filtering. The result will resemble
![](../images/kibana.png)

## Managing Logging Configuration

Spring Boot provides another least common denominator approach to logging configuration. It will let you set the 
log level for various Loggers within an application which can be dynamically updated via REST endpoints provided 
by Spring. While this works in a lot of cases it does not support any of the more advanced filtering features of 
Log4j. For example, since it cannot add or modify any Filters other than the log level of a logger, changes cannot be made to allow 
all log events for a specific user or customer to temporarily be logged 
(see [DynamicThresholdFilter](filters.html#DynamicThresholdFilter) or 
[ThreadContextMapFilter](filters.html#ThreadContextMapFilter)) or any other kinds of changes to filters. 
Also, in a micro-services, clustered environment it is quite likely that these changes will need to be propagated
to multiple servers at the same time. Trying to achieve this via REST calls could be difficult.
  
Since its first release Log4j has supported reconfiguration through a file.
Beginning with Log4j 2.12.0 Log4j also supports accessing the configuration via HTTP(S) and monitoring the file 
for changes by using the HTTP "If-Modified-Since" header. A patch has also been integrated into Spring Cloud Config
starting with versions 2.0.3 and 2.1.1 for it to honor the If-Modified-Since header. In addition, the 
log4j-spring-cloud-config project will listen for update events published by Spring Cloud Bus and then verify
that the configuration file has been modified, so polling via HTTP is not required.

Log4j also supports composite configurations. A distributed application spread across microservices could 
share a common configuration file that could be used to control things like enabling debug logging for a 
specific user.

While the standard Spring Boot REST endpoints to update logging will still work any changes made by those 
REST endpoints will be lost if Log4j reconfigures itself do to changes in the logging configuration file.

Further information regarding integration of the log4j-spring-cloud-config-client can be found at 
[Log4j Spring Cloud Config Client](../log4j-spring-cloud-config/log4j-spring-cloud-config-client/index.html).

## Integration with Spring Boot

Log4j integrates with Spring Boot in 2 ways:

1. A Spring Lookup can be used to access the Spring application configuration from Log4j configuration files.
2. Log4j will access the Spring configuration when it is trying to resolve log4j system properties.

Both of these require that the log4j-spring-cloud-client jar is included in the application.

## Integration with Docker

Applications within a Docker container that log using a Docker logging driver can include special 
attributes in the formatted log event as described at 
[Customize Log Driver Output](https://docs.docker.com/config/containers/logging/log_tags/). Log4j 
provides similar functionality via the [Docker Lookup](lookups.html#DockerLookup). More information on
Log4j's Docker support may also be found at [Log4j-Docker](../log4j-docker/index.html). 

## Integration with Kubernetes

Applications managed by Kubernetes can bypass the Docker/Kubernetes logging infrastructure and log directly to 
either a sidecar forwarder or a logging aggragator cluster while still including all the kubernetes 
attributes by using the Log4j 2 [Kubernetes Lookup](lookups.html#KubernetesLookup). More information on
Log4j's Kubernetes support may also be found at [Log4j-Kubernetes](../log4j-kubernetes/index.html). 

## Appender Performance
The numbers in the table below represent how much time in seconds was required for the application to 
call `logger.debug(...)` 100,000 times. These numbers only include the time taken to deliver to the specifically 
noted endpoint and many not include the actual time required before they are available for viewing. All 
measurements were performed on a MacBook Pro with a 2.9GHz Intel Core I9 processor with 6 physical and 12 
logical cores, 32GB of 2400 MHz DDR4 RAM, and 1TB of Apple SSD storage. The VM used by Docker was managed 
by VMWare Fusion and had 4 CPUs and 2 GB of RAM. These number should be used for relative performance comparisons 
as the results on another system may vary considerably.

The sample application used can be found under the log4j-spring-cloud-config/log4j-spring-cloud-config-samples
directory in the Log4j [source repository](https://github.com/apache/logging-log4j2).

| Test                    | 1 Thread | 2 Threads | 4 Threads | 8 Threads |
|------------------------ |---------:|----------:|----------:|----------:|
|Flume Avro |||||
|- Batch Size 1 - JSON    |49.11     |46.54      |46.70      |44.92      |
|- Batch Size 1 - RFC5424 |48.30     |45.79      |46.31      |45.50      |
|- Batch Size 100 - JSON  | 6.33     |3.87       |3.57       |3.84       | 
|- Batch Size 100 - RFC5424 | 6.08   |3.69       |3.22       |3.11       | 
|- Batch Size 1000 - JSON | 4.83     |3.20       |3.02       |2.11       |
|- Batch Size 1000 - RFC5424 | 4.70  |2.40       |2.37       |2.37       |
|Flume Embedded |||||
| - RFC5424               |3.58      |2.10       |2.10       |2.70       |
| - JSON                  |4.20      |2.49       |3.53       |2.90       |
|Kafka Local JSON |||||
| - sendSync true         |58.46     |38.55      |19.59      |19.01      |
| - sendSync false        |9.8       |10.8       |12.23      |11.36      |
|Console|||||
| - JSON / Kubernetes     |3.03      |3.11       |3.04       |2.51       |
| - JSON                  |2.80      |2.74       |2.54       |2.35       |
| - Docker fluentd driver |10.65     |9.92       |10.42      |10.27      |
|Rolling File|||||
| - RFC5424               |1.65      |0.94       |1.22       |1.55
| - JSON                  |1.90      |0.95       |1.57       |1.94       |
|TCP - Fluent Bit - JSON  |2.34      |2.167      |1.67       |2.50       |
|Async Logger|||||
|- TCP - Fluent Bit - JSON|0.90      |0.58       |0.36       |0.48       |
|- Console - JSON         |0.83      |0.57       |0.55       |0.61       |
|- Flume Avro - 1000 - JSON|0.76     |0.37       |0.45       |0.68       |

Notes:

1. Flume Avro - Buffering is controlled by the batch size. Each send is complete when the remote 
acknowledges the batch was written to its channel. These number seem to indicate Flume Avro could
benefit from using a pool of RPCClients, at least for a batchSize of 1.
1. Flume Embedded - This is essentially asynchronous as it writes to an in-memory buffer. It is
unclear why the performance isn't closer to the AsyncLogger results.
1. Kafka was run in standalone mode on the same laptop as the application. See  sendSync set to true
requires waiting for an ack from Kafka for each log event. 
1. Console - System.out is redirected to a file by Docker. Testing shows that it would be much
slower if it was writing to the terminal screen.
1. Rolling File - Test uses the default buffer size of 8K.
1. TCP to Fluent Bit - The Socket Appender uses a default buffer size of 8K.
1. Async Loggers - These all write to a circular buffer and return to the application. The actual
I/O will take place on a separate thread. If writing the events is performed more slowly than 
events are being created eventually the buffer will fill up and logging will be performed at 
the same pace that log events are written.

## Logging Recommendations

1. Use asynchronous logging unless guaranteed delivery is absolutely required. As 
the performance numbers show, so long as the volume of logging is not high enough to fill up the 
circular buffer the overhead of logging will almost be unnoticeable to the application.
1. If overall performance is a consideration or you require multiline events such as stack traces
be processed properly then log via TCP to a companion container that acts as a log forwarder or directly
to a log aggregator as shown above in [Logging with ELK](#ELK). Use the  
Log4j Docker Lookup to add the container information to each log event.
1. Whenever guaranteed delivery is required use Flume Avro with a batch size of 1 or another Appender such 
as the Kafka Appender with syncSend set to true that only return control after the downstream agent 
acknowledges receipt of the event. Beware that using an Appender that writes each event individually should 
be kept to a minimum since it is much slower than sending buffered events. 
1. Logging to files within the container is discouraged. Doing so requires that a volume be declared in 
the Docker configuration and that the file be tailed by a log forwarder. However, it performs 
better than logging to the standard output stream. If logging via TCP is not an option and
proper multiline handling is required then consider this option.
