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MySQL 5.7 Reference Manual
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17.9.5.1 Group Replication Basics

The Group Replication distributed recovery procedure can be summarized as the process through which a new server gets missing data from a online server in the group, while listening for events happening in the group. During recovery a server listens to membership events and also to the transactions that are happening while recovery is happening. This is a high level summary. The following sections provide additional detail, by describing the two phases of the procedure.

Phase 1

In the first phase, the joining server selects one of the online servers from the group to be the donor of the state that it is missing. The donor is responsible for providing the joining server with all the data it is missing up to the moment it has joined the group. This is achieved by relying on a standard asynchronous replication channel, established between donor and joining server. Through that channel, binary logs are transferred up until the point that the view change happened when the joining server became part of the group. While the joining server receives binary logs from the donor, it is also applying them.

Furthermore, while the binary logs transfer is ongoing, the joining server is also caching every transaction that is exchanged within the group. In other words, storing the transactions that happen after it joined the group and while it is applying the missing state from the donor. When the first phase ends and the replication channel to the donor is closed, the joining server then starts phase two: the catch up.

Phase 2

In this phase, the joining server proceeds to the execution of the cached transactions and when the number of transactions queued for execution finally reaches zero, then the member is declared online.

Resilience

The recovery procedure withstands donor failures while the joining server is fetching binary logs from it. In such cases, whenever a donor fails during phase 1, the joining server fails over to a new donor and resumes from that one. When that happens the joining server closes the connection to the failed joining server explicitly and opens a connection to a new donor. This happens automatically.


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