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Effects of Replication being behind
Answer ID 11988   |   Last Review Date 10/11/2021

Oracle B2C Service
The replication server is a copy which is constantly being updated from the operational database. In most cases, your replication connection will be up to date. However, there may be times when the replication server falls behind and replication may be several days old. This is normal & expected behavior. See Answer ID 5609: Information on different database types used for reporting for additional information.
However, it's important to be aware of how replication being behind can affect different areas of the product.
If a report is using the replication server as a data source, the report may not reflect recent updates when replication is behind. For more information, see Answer ID 2817: Reporting data seems incorrect or behind.
Custom Objects
When deploying a Custom Object, it is not possible to predict the exact amount of time a deployment will take. A completely normal deployment can take 24 hours or more. If replication is behind, a deployment cannot complete. As it is possible for replication to be days behind, you should make any necessary changes to Custom Objects at least several days before they will be needed. Even back-to-back deployments can take very different amounts of time to complete, and this is rarely a cause for concern.  
Mailings and Surveys
Mailings and surveys with a high volume audience will use the replication server to reduce the impact of large processes on the production database. The mailer daemon uses a failsafe function before sending to ensure that the replication server is caught up. If it is not, the mailing or survey will be delayed until the replication server has been synchronized. For more information, see Answer ID 2665: Mailings and surveys using replication server.
When creating emails to send out for a Campaign, Oracle B2C Service uses queries against the replication server to select the Audience Segment (i.e. the email recipients). The campaign email content which may contain merge fields uses the live Operational database. Replication differences can cause unexpected campaign email results. For more information, see Answer ID 5289: Campaign emails may have blank merge field content.
ROQL Customizations
ROQL queries always run off of the Production database unless a query has been blocklisted.  If a query is blocklisted, it will either be blocked or it will be redirected to run off of the Replication database.  
Most blocklisted queries can be redesigned to be compliant.  ROQL queries are also run by product functionality behind the scenes, and those queries can be blocklisted as well.  If blocklisted queries are suspected, please submit an SR to Technical Support for further investigation. 
Creating a Clone Site
The length of time it takes to create a clone of a production site can be affected by replication being behind, as well as other factors such as how busy the site is, how many transactions need to be written, the shared server load, and database size. The process can take several hours or it can take more than a week. Due to the complexity of the process and the effect of multiple factors, we are unable to provide an estimate of when the process will be complete.