pg_rewind, a utility introduced in PostgreSQL 9.5 allowing one to reconnect a master to one of its promoted standbys, making unnecessary the need of a new base backup and hence saving a huge amount of time, has gained something new in Postgres 9.6 with the following commit:
commit: e50cda78404d6400b1326a996a4fabb144871151 author: Teodor Sigaev <email@example.com> date: Tue, 1 Dec 2015 18:56:44 +0300 Use pg_rewind when target timeline was switched Allow pg_rewind to work when target timeline was switched. Now user can return promoted standby to old master. Target timeline history becomes a global variable. Index in target timeline history is used in function interfaces instead of specifying TLI directly. Thus, SimpleXLogPageRead() can easily start reading XLOGs from next timeline when current timeline ends. Author: Alexander Korotkov Review: Michael Paquier
The following document gives a short introduction about pg_rewind. You may want to look at it before moving on with this post and understand why here is described a damn cool feature. First, before entering in the details of this feature, let’s take the example of the following cluster:
Node 1 / \ / \ / \ Node 2 Node 3 / / / Node 4
Node 1 is a master node, with two standbys directly connected to it, presented here as nodes 2 and 3. Finally node 4 is a cascading standby connected to node
- Then say that all the nodes have been successively promoted. At promotion, a standby finishes recovery and jumps to a new timeline to identify a new series of WAL record generated by what is now a master node to avoid overlapping WAL records from another node. The promotion of each node in the cluster is done in the following order, and results in the following timelines being taken by each node:
- Node 2 promotes, at the end of recovery it begins to use timeline 2. Node 4 jumps as well to timeline 2, and continues to follow node 2.
- Node 3 promotes, it switches to timeline 3.
- Node 4 promotes, it switches to timeline 4.
At this point all the nodes have become master nodes, and are now generating their own history and making their own way in life. In terms of WAL history, the cluster becomes as follows:
----------------------------------> Node 1 (timeline 1) \ \ \ \ \ \------------------> Node 3 (timeline 3) \ \ \------------------------> Node 2 (timeline 2) \ \ \-----------------> Node 4 (timeline 4)
With 9.5’s pg_rewind, when trying to rewind a node, or to put it in other words to synchronize a target node with a source node so as the target node can become a standby of the target node, it is not possible to perform complicated operations like in the case of the cluster described above. For example, one can use node 1 as target and node 2 as source so as node 1 is recycled as a standby of node 2. Same for node 1 as source and node 3 as target, and similarly for node 4 as source and node 2 as target. Also, as a timeline history cannot be looked up backwards, it is not possible to rewind a promoted standby to its former master. In short, when trying more complicated operations, the following error will be quickly found out:
could not find common ancestor of the source and target cluster's timelines Failure, exiting
Finally, what does the feature aimed at being described in this blog post do? Well, to put it simply pg_rewind has been extended so as it can have a look at the timeline history graph like the one above. And it is able to find out the most recent shared point in the timeline history between a target and a source node, and then it performs a rewind operation from this point. In even shorter words, do you remember the four nodes of the same cluster promoted one after the other? In PostgreSQL 9.6, pg_rewind can allow one to rebuild a complete cluster using all the existing nodes, without limitation, even if they completely forked off. Any node can become the master, and have as standbys the other nodes. This abscence of limitation is what makes this feature a really cool thing, and something to count on.