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According to the WSO2 patch application process, once the patches are created, you can apply them either as individual patches or through a service pack. However, when the number of patches increase, it is more convenient to apply patches through service packs.

What is a service pack?

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Applying patches to the Kernel

1. Copy the required patches and service packs to the relevant locations in your product directory as explained below.
   
   • To apply patches individually, copy the patches to <PRODUCT_HOME>/repository/components/patches.
     

   • To apply a service pack, copy the service pack file to <PRODUCT_HOME>/repository/components/servicepacks. For example, the image below shows how a new service pack, named "servicepack001", is added to this directory. 
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2. Start the Carbon server. If any patch changes exist, the following steps will be executed:
   

   1. Before applying any patches, the process first creates a backup folder named patch0000 inside the <PRODUCT_HOME>/repository/components/patches directory, which will contain the original content of the plugins folder that is in the <PRODUCT_HOME>/repository/components directory. This step enables you to revert back to the previous state if something goes wrong during operations.

   2. If any service packs are copied to the servicepacks directory, the latest service pack will be applied. That is, the patches in the service pack will be applied to the <PRODUCT_HOME>/repository/components/plugins directory.

   3. After the service pack is applied, any other patches that were not included in the service pack will be incrementally applied to the plugins directory. The metadata file available in the service pack will maintain a list of the applied patches by service pack. Therefore, the metadata file information will be compared against the <PRODUCT_HOME>/repository/components/patches directory, and only the patches that were not applied by the service pack will be incrementally applied to the plugins directory. The diagram below shows how the patch application is implemented when you start the server.
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   Info

   Prior to the Carbon 4.2.0 release, to apply patches in the Carbon server, the users were expected to start the server as follows:  wso2server.sh -DapplyPatches. Now, you do not have to issue a special command to trigger the patch process. It starts automatically if there are changes in either the  <PRODUCT_HOME>/repository/components/servicepacks directory or the <PRODUCT_HOME>/repository/components/patches directory. It verifies all the latest JARs in the servicepacks and patches directories against JARs in the plugins directory by comparing MD5s of JARs.

Verifying the patch application

After the patch application process is completed, the patch verification process ensures that the latest service pack and other existing patches are correctly applied to the <PRODUCT_HOME>/repository/components/plugins folder.

• All patch related logs are recorded in the <PRODUCT_HOME>repository/logs/patches.log file.
• The <PRODUCT_HOME>/repository/components/patches/.metadata/prePatchedJARs.txt meta file contains the list of patched JARs and their md5 values.
• A list of all the applied service packs and patches are in the <PRODUCT_HOME>/repository/components/default/configuration/prePatched.txt file.

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