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The duration of the catch up is not deterministic, because it depends on the workload and the rate of incoming transactions to the group. This process is completely online and the server joining the group does not block any other server in the group while it is catching up. Therefore the number of transactions the server joining the group is behind when it moves to this stage can, for this reason, vary and thus increase or decrease according to the workload. 

When the server joining the group reaches zero queued transactions and its stored data is equal to the other members, its public state changes to online. 

<img src='content_image/716822.jpg'>

## 18.5.4 Network Partitioning 

The group needs to achieve consensus whenever a change that needs to be replicated happens. This is the case for regular transactions but is also required for group membership changes and some internal messaging that keeps the group consistent. Consensus requires a majority of group members to agree on a given decision. When a majority of group members is lost, the group is unable to progress and blocks because it cannot secure majority or quorum. 

Quorum may be lost when there are multiple involuntary failures, causing a majority of servers to be removed abruptly from the group. For example, in a group of 5 servers, if 3 of them become silent at once, the majority is compromised and thus no quorum can be achieved. In fact, the remaining two are not able 

