ServiceComb provides Vert.x based access log. When developing with REST over Vert.x , access log printing can be enabled through a simple configuration.


The user may need the access log when debugging the application. When using REST over servlet, the web container provides the access log function; for REST over Vert.x, ServiceComb provides a set of access log functionalities.


Enable Access Log

Add the following configurations in the microservice.yaml file to enable access log:

    enabled: true  ## Enable access log

Access log Configuration Items

Configuration Item Values Default Value Description
servicecomb.accesslog.enabled true/false false true to enabled access log
servicecomb.accesslog.pattern the format of the log "%h - - %t %r %s %B" See log configuration items for more details


  • The 2 items are optional, if not configured, the default value will be applied.

Log format configuration

The currently available configuration items for log are describe in the following table Log configuration items(Apache & W3C) andLog configuration items(ServiceComb)

Log configuration items (Apache & W3C)

Item Apache log format W3C log format Description
HTTP method %m cs-method -
HTTP status %s sc-status -
Duration in second %T - -
Duration in millisecond %D - -
Remote hostname %h - -
Local hostname %v - -
Local port %p - -
Size of response %B - Print "0" if body size is 0
Size of response %b - Print "-" if body size is 0
First line of request %r - Include HTTP Method, Uri and HTTP version
URI path %U cs-uri-stem -
Query string %q cs-uri-query -
URI path and query string - cs-uri -
Request protocol %H - -
Datetime the request is received %t - Print time stamp by the default configuration, the format is "EEE, dd MMM yyyy HH:mm:ss zzz", in English and GMT time zone
Configurable datetime the request is received %{PATTERN}t - Print time stamp by specified format, in English and GMT time zone
Configurable datetime the request is received %{PATTERN|TIMEZONE|LOCALE}t - Print time stamp by the specified format, language and time zone. The items between vertical bar can be empty(while the \ should not be omitted)
Request header %{VARNAME}i - Print "-" if the specified request header is not found
Response header %{VARNAME}o - Print "-" if the specified response header is not found
Cookie %{VARNAME}C - Print "-" if the specified cookie is not found

Log configuration items(ServiceComb)

Element Placeholder Comment
TraceId %SCB-traceId Print the trace id generated by ServiceComb, if the id is not found, print "-"
Invocation Context %{VARNAME}SCB-ctx Print the invocation context value whose key is VARNAME, if the key is not found, print "-"

Output file configuration

The default log framework for Access log is Log4j which provides a default set of configurations for output files. Users can override these configurations in their own file. The configuration items for output files are as follows.

Log file configuration items

Item Default Value Description Remarks
paas.logs.accesslog.dir ${paas.logs.dir} The output path of the log file The common logs will be outputted to the same path
paas.logs.accesslog.file access.log Name of the log file -
log4j.appender.access.MaxBackupIndex 10 Max file numbers for log rotating -
log4j.appender.access.MaxFileSize 20MB Max size of log file When log file reaches the max size, log rotating is triggered
.appender.access.logPermission rw------- Log file permissions -

Note Since ServiceComb's log function relies only on the slf4j interface, users can select other log frameworks. For other frameworks, users need to configure the log file output options.

Switch to logback

For the project that uses logback, the log framework dependency should be changed from Log4j to logback with some extra configurations to make access log work.

1. Remove Log4j dependencies

Before switching to logback, check the dependencies of the project and remove Log4j related dependencies. Run the maven command dependency:tree in the project, find the ServiceComb components that depend on Log4j, and add the following configuration to its <dependency>:


2. Add a logback dependency

Add a dependency for the logback in the pom file:


3. Configure the logger for the access log component

Since the log component provided by ServiceComb obtains the logger named accesslog for log printing, the key to log framework switching is to provide a file called accesslog and configure the output file for it. The following is a sample configuration of the access log for logback. It only shows the configurations related to the access log. Other log configurations are omitted:

<?xml version="1.0" encoding="UTF-8"?>
  <!-- Users can customize the appender by their requirement -->
  <appender name="ACCESSLOG" class="ch.qos.logback.core.rolling.RollingFileAppender">
    <rollingPolicy class="ch.qos.logback.core.rolling.TimeBasedRollingPolicy">
    <!-- Note: the access log content is formatted in code, the pattern should only specify the message without extra format -->

  <!-- Provide a logger named "accesslog" for log printing -->
  <logger name="accesslog" level="INFO" additivity="false">
    <appender-ref ref="ACCESSLOG" />

Extending Access Log

Users can customize their AccessLogItem by ServiceComb's AccessLogItem extension mechanism.

  1. AccessLogItem
  public interface AccessLogItem<T> {
     * Get specified content from accessLogParam, generate the access log and return
    String getFormattedItem(AccessLogParam<T> accessLogParam);

The definition of AccessLogItem is as shown above. When request triggers Access Log printing, ServiceComb's Access Log mechanism will traverse a valid AccessLogItem, call the getFormattedItem method to get the item's Access Log fragment, concatenate all the the fragments into an Access Log, and output it to the log file.

The parameter AccessLogParam<T> contains the request start time, the end time, and the request context of type T. In the REST over Vert.x communication mode, the type T is the RoutingContext of Vert.x.

  1. VertxRestAccessLogItemMeta
  // pattern placeholder prefix
  protected String prefix;
  // pattern placeholder suffix
  protected String suffix;
  // order number of priority
  protected int order;
  // AccessLogItem constructor
  protected AccessLogItemCreator<RoutingContext> accessLogItemCreator;

The VertxRestAccessLogItemMeta contains the properties listed above, it specifies how ServiceComb parse the pattern string to get specific AccessLogItem.

  • To define a AccessLogItem with placeholder %user-defined, declare a subclass of VertxRestAccessLogItemMeta,set prefix="%user-defined", suffix=null, when AccessLogPatternParser parses the "%user-defined", it will fetch the AccessLogItemCreator from the meta class and create the corresponding AccessLogItem. Note: since there is not variable in placeholder "%user-defined", the call to AccessLogItemCreator passes the configuration parameter null。

  • To get a AccessLogItem with placeholder %{VARNAME}user-defined, declare a subclass ofVertxRestAccessLogItemMeta, set prefix="%{", suffix="}user-defined". When AccessLogPatternParserparses "%{VARNAME}user-defined", it will extract the "VARNAME" as parameter to call AccessLogItemCreator, to create a AccessLogItem.

    VertxRestAccessLogItemMeta has a subclassCompositeVertxRestAccessLogItemMeta. When user needs to define multiple AccessLogItems, multiple VertxRestAccessLogItemMeta can be aggregated into CompositeVertxRestAccessLogItemMeta. When Parser loads AccessLogItemMeta of type CompositeVertxRestAccessLogItemMeta, it calls the meta class's getAccessLogItemMetas() method to get a set of AccessLogItemMeta. VertxRestAccessLogItemMeta is loaded by the SPI mechanism, and CompositeVertxRestAccessLogItemMeta allows user to load multiple meta infos with on one record in the SPI configuration file, which provides great flexibility.

  • AccessLogItemCreator

  public interface AccessLogItemCreator<T> {
    // Receive configuration values and return an AccessLogItem. The method receives a null if there is no variables in AccessLogItem placeholder
    AccessLogItem<T> createItem(String config);

The user instantiates his AccessLogItem by setting the AccessLogItemCreator in the custom VertxRestAccessLogItemMeta. Since this is a functional interface, when the AccessLogItem is initialized in a simple way, you can directly define the Creator using a Lambda expression to simplify development.

Matching rules of AccessLogItemMeta

Once AccessLogItemMeta is loaded into the Parser, it will be sorted once. Parser will match the meta list from front to back when parsing the pattern string. The general matching rules are as follows:

  1. Match metas with higher priority.
  2. Match the meta with suffix first. When metas with multiple suffixes are matched, take the one with the smallest suffix.
  3. Match the meta with a longer placeholder, for example, there are two metas, "%abc" and "%a". If "%abc" is matched, it will return directly.


  1. Extend AccessLogItem

    First, the user needs the AccessLogItem interface to implement their own item:

  public class UserDefinedAccessLogItem implements AccessLogItem<RoutingContext> {
    private String config;

    public UserDefinedAccessLogItem(String config) {
      this.config = config;

    public String getFormattedItem(AccessLogParam<RoutingContext> accessLogParam) {
      // Here is the user's custom logic, user needs to take relevant data from AccessLogParam or other places, generate and return access log fragments
      return "user-defined-[" + config + "]-[" + accessLogParam.getStartMillisecond() + "]";
  1. Define AccessLogItem meta class

Inherit the class VertxRestAccessLogItemMeta or CompositeVertxRestAccessLogItemMeta, define the prefix and suffix of the AccessLogItem:

  public class UserDefinedCompositeExtendedAccessLogItemMeta extends CompositeVertxRestAccessLogItemMeta {
    private static final List<VertxRestAccessLogItemMeta> META_LIST = new ArrayList<>();

    static {
      META_LIST.add(new VertxRestAccessLogItemMeta("%{", "}user-defined", UserDefinedAccessLogItem::new));

    public List<VertxRestAccessLogItemMeta> getAccessLogItemMetas() {
      return META_LIST;
  1. Configure the SPI load file

In the resources/META-INF/services/ directory, create a file named "" and fill in the full class path of the meta class defined in the previous step. Parser will use this file to load the meta class.

  1. Configure Access Log pattern

The configuration pattern in the microservice.yaml file is assumed to be "%{test-config}user-defined". The running service triggers the Access Log to print. If the request start time is 1, Access Log will print "user- Defined-[test-config]-[1]".

Sample code

Configurations in microservice.yaml

## other configurations omitted
    enabled: true  ## Enable access log
    pattern: "%h - - %t %r %s %B"  ## Custom log format

Configurations in

# access log configuration item
# access log File appender

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