Enterprise Manager Grid Control Architecture

Although Enterprise Manager Grid Control is viewed as a single entity, technically, it

is built with the following software components

   1)      Oracle Management Agent (Management Agent)

Management Agent is an integral software component that is deployed on each

monitored host. It is responsible for monitoring all the targets running on those

hosts, communicating that information to the middle-tier Oracle Management

Service, and managing and maintaining the hosts and its targets.

Configuration file for Management Agent is $OH/ <hostname>_<sid>/ sysman/ config/ emd.properties. Configuration file for Management Agent is $OH/ <hostname>_<sid>/ sysman/ config/ emd.properties. Agent uploads managed data (from server its monitoring) to Management Service via HTTP Server (URL of management Service HTTP Server is defined by REPOSITORY_URL parameter in emd.properties)
     Management Agent software also includes inbuilt HTTP Listener (different from standalone HTTP Server) to accept messages(data) from Management Service and this URL is defined by parameter EMD_URL in emd.properties.

Management Agent Log & Trace files are in $AGENT_HOME (or $OH/ <hostname>_<sid>) / sysman/ emagent.log, emagent.trc, emagentfetchlet.log, emagentfetchlet.trc , emagent.nohup

   2)      Oracle Management Service (OMS)

OMS is a J2EE Web application that orchestrates with Management Agents to

discover targets, monitor and manage them, and store the collected information in

a repository for future reference and analysis. OMS also renders the user interface

for the Enterprise Manager Grid Control console. OMS is deployed to the Oracle

Middleware home, which is the parent directory that has the Oracle WebLogic

Server home, the Web tier instance files, and, optionally, one or more Oracle

homes.

 Configuration file for Management Service is $OH/<hostname>_<sid>/ sysman/ config/ emoms.properties.
               Repository connection details are defined by parameter emdRepSID, emdRepServer, emdRepConnectDescriptor and emdRepUser in emoms.properties
  Management Service also monitors Management Agent (to check its up and running), submit enterprise manager jobs and other functions using EMD_URL (Management Agent runs inbuilt HTTP listener) defined in emoms.properties

Management Service Log & Tracefiles are in $AS_HOME (or $OH/ <hostname>_<sid>)  / sysman/ emoms.log, emoms.trc

    3)      Oracle Management Repository (Management Repository)

Management Repository is the storage location where all the information collected

by the Management Agent gets stored. It consists of objects such as database jobs,

packages, procedures, views, and tablespaces.

Management Service connects to Management Repository using JDBC

    4)      Enterprise Manager Grid Control Console

Enterprise Manager Grid Control Console is the user interface you see after you

install Enterprise Manager Grid Control. With the help of the console, you can

monitor and administer your entire computing environment from one location on

the network. All the services within your enterprise, including hosts, databases,

listeners, application servers, and so on, are easily managed from one central

5. HTTP Server: recieve requests from webcache, pulls data from Repository via Management Serviceand return response back to Webcache. HTTP Server also receive data from Management Agents, forward it to Management Service to store them in Management Repository.

6. Webcache : acts as web accelerator; forward request from Users (console) to HTTP Server and response back from HTTP server to Users (Console)

.
Communication between Management Agent & Service

Management Agent connect to Management Service via HTTP Server of Grid Control Middleware where as Management Service connect directly (using HTTP protocol) to Management Agent (Agent software include inbuilt http server)

Reference:  Oracle Enterprise Manager Grid Control Basic Installation Guide