Assuming a full-duplex, fault-tolerant implementation, data is sent from CallRouter A to Logger A and from CallRouter B to Logger B.

The Logger Central Database forwards (replicates) historical data to corresponding historical tables on the Historical Database Server in summary intervals. These data populate the historical interval and daily reports.

Two Administration & Data Servers are typically set up as HDS machines. A similar fault-tolerant strategy applies to the HDS—when the primary HDS fails, the Administration Client automatically switches over to use the backup HDS.

Each Historical Data Server (HDS) is connected to a single Logger.

Data loss manifests as data holes, which are one or more missing records in an historical database table.

There are two types of data loss: temporary and permanent:

• A temporary data hole can happen during the Logger recovery process. For example, Logger A goes down, then comes back up and contacts Logger B to synchronize and recover historical data that was written while it was down.

  While this recovery process is going on, the reporting database on Logger A may have temporary data holes, which will be filled when the recovery process completes.

• A permanent data hole can happen during an Emergency Purge. For example, there can be permanent data loss if an emergency purge deletes records on one Logger that have not been sent to the other Logger or to the HDS.

It is possible to monitor and tune Unified CCE to minimize the occurrence of data loss.

Fault Tolerance

To protect your system, see the information on duplexed Unified CCE fault tolerance in the Administration Guide for Cisco Unified Contact Center Enterprise.

Data Retention and Backups

Another way to safeguard against loss is to configure the amount of time that data is stored on the Logger Central Database and in the HDS in relation to the schedule for HDS backups.

The Central database stores data for less time than the HDS. For example, you might store two weeks of data on the Logger and a year of data on the HDS.

When the HDS recovers after going offline, it retrieves all of the data on the Logger for the interval for which data is missing from the backup. You must manually restore the rest of the data from the last HDS backup.

The amount of data retained on the Logger should cover, at a minimum, the time period between HDS backups. For example, if the Logger stores data for two weeks, then you need to back up at least every other week to ensure that you can recover all historical data.

CPU Utilization

It is possible that the process on one of the Loggers is slow because of space issues or an overload of the SQL Server. In this situation, the data on the Logger with the slower SQL Server will lag in persistence of the historical data with respect to the other Logger. This causes the HDS on the corresponding side to lag as well.

As a consequence, if both sides have an HDS set up and the same reports are run from both HDSs, the reports might differ. This is usually a temporary inconsistency, since the condition that causes the SQL server process to slow might be remedied. Autogrowing of the database and load conditions often remediate. The Loggers and the HDSs eventually catch up and are in sync. Running the reports later will result in consistent reports.

However, it the database server runs of disk space, the situation is quite serious and might cause data to be out of sync for a longer duration until the problem is remedied. A permanent loss of data can occur when data is purged from the peer Logger and never replicated on the slower side.

Scheduled Purge and Retention Settings on Loggers

The goal of the scheduled purge is to free up database space by purging the oldest data.

There are several reasons for data loss during a scheduled purge:

• Retention settings on Loggers

  Data inconsistencies and permanent data loss can occur if the number of days to retain the data differs on the Loggers.

  Assume that Logger A is set to retain 7 days' worth of data, while Logger B is set to retain 15 days worth of data.

  If Logger B is down for 6 days, a temporary data discrepancy exists when it is brought back up, until the Recovery process synchronized the data from Logger A. However, if Logger B is down for 10 days, when it comes back up, it can synchronize only the last 7 days worth of data, based on Logger A's retention setting. Three days are lost permanently from Logger B.

  Note

  The data might be lost from the system permanently, if the historical data was copied to the HDS database associated with Logger A. Although this situation appears as a discrepancy in the reports that are run from HDS servers that connect to side B, the system is functioning in a predictable manner. It can be considered as an issue of perception.

  To avoid this situation, make sure that the retention settings are the same on both Loggers are the same.

• Scheduled purge and Peripheral Gateway failure

  If multiple Peripheral Gateways (PGs) are configured, and if one of the PGs goes down for a brief period, it is possible to lose historical data permanently.

  Assume that there are three PGs in the system and that one goes down for a day and then comes back online. When that PG comes back online, it sends historical data for activity that occurred prior to it going offline.

  If the scheduled purge mechanism activates and determines that the oldest one hour of data needs to be purged, it is possible that the purge will delete data that was sent by the PG after it came online but before it was replicated to the HDS.

  Permanent data loss can occur is the HDS is down and the scheduled purge on the Logger deletes data that has not yet been replicated to the HDS.

Emergency Purge

The emergency purge mechanism is triggered when the Logger Central Database becomes full or reaches a configured threshold size. Its objective is to free up space by purging data from the historical tables so that the database has more free space than the allowed minimum.

The emergency purge goes through each historical table in a predefined order one at a time and purges one hour's worth of data from the table. As data is purged from each historical table, a check is made to verify if the free space is more than the minimum threshold value. Once adequate space has been recovered, the emergency purge procedure stops. Otherwise, it continues through to the next historical table and keeps looping as necessary.

Permanent loss of historical data can occur if the emergency purge removes historical data that has not yet made it to an HDS and has also not been replicated to the peer Logger that is "down" or in the recovery process.

Database used percentage is displayed as a status message in the replication process every few minutes. You can occasionally monitor this value to make sure that is does not grow too often or too fast. Emergency purge occurs when the percentage used is greater than the configured value (usually 90%).

Here are some reporting considerations when you experience data loss from PIM failure.

The Peripheral Interface Manager (PIM) is the process on the Peripheral Gateway responsible for the actual connection to the peripheral and for normalizing the CTI interface on behalf of Unified CCE .

If a PIM fails, if the link between the PIM and the ACD goes down, or if the ACD goes down, then all of the reporting data that has been gathered for the peripheral associated with the PIM is deleted.

When the PIM failures occur, the peripheral is marked offline to the central controller.

The state of all agents on that peripheral is set to logged out and is reported as such to the CallRouter.

The CallRouter has no way of determining what was going on at the ACD while the PIM was out of contact with the ACD. When the PIM reconnects to the ACD, the ACDS does not send the PIM sufficient information to allow the recording of accurate historical reporting data for the interval(s) in which the disconnect took place.

Note	When the PIM reconnects to the ACD, most ACDs do pass information to the PIM about each agent's state and duration in that state. While this is not enough to allow accurate historical reporting data to be recorded, it is enough to allow the CallRouter to make accurate call routing decisions.

When the PG is duplexed, either the Side A or Side B PIM is active for each peripheral. If one side loses connection, the other comes up and activates.