One of the biggest projects I’ve been working on the last few months is an extension that came about trying to organize a per-table replication process that has been in use with several of our clients, but never really formalized. After nearly 300 hours of time logged working on it, and mentioning it several times in other blog posts, I figured it’s about time I talk more about it.
I got the name mimeo after searching a thesaurus for words similar to “copy” & “replicate” and came across a mimeograph. The terms “low cost” and copying “small quantities” seemed to fit with the theme of what I was trying to accomplish, so the name stuck. There are some other great add-ons for PostgreSQL that allow per-table replication (Bucardo being the other one I’m more familiar with), but their setup and use can be a bit daunting. And if you just need a few tables copied, a bit overkill. The goal with mimeo was to keep the installation, maintenance and monitoring as simple as possible. Honestly, the hardest part of the extension I’ve found, and had others report the same to me, is just managing the permissions. I’ve got some plans to make some of it easier, but the extension doesn’t assume or require any superuser privileges, which I think is another plus.
The existing code I was working off of had several different replication methods that were used, and each had their own merits. I’ll be covering them over several blog posts, along with some general tips on usage, since I think a single blog post discussing the entire thing would be a bit much. I’ve already done some pretty extensive documentation and even written a howto, so these blog posts will mostly be informational and not really focused too much on the entire setup and maintenance process.
Before I get into too much detail, I just have to thank the PostgreSQL team for getting the extension system into place with 9.1. The only reason I was able to even come close to organizing the existing processes into something formal like this was because of that. The versioning control of a group of objects within the database allows a much smoother upgrading (and downgrading) process. And also a big thanks to David Wheeler for the pgTAP suite. If you look in the tests folder you can see I made extensive use of it and it’s saved me a tremendous amount of development time (especially with the latest v0.10.0 release where I rewrote much of the refresh process). When dealing with data replication you want to be really sure you’re getting every bit of data across 100% of the time and not breaking anything as development progresses.
Now onto the details. The most basic way to copy a table from one database to another is to just grab the entire thing. That’s covered in mimeo with the snapshot replication method. A table setup with this method will have the entire contents refreshed every time it is run. To help make this processes more transparent to a user of these tables, a view with two underlying tables is used. The view only ever points to one table at a time. When the refresh runs, the table it’s not pointing to is truncated and refreshed. A brief lock is then taken to swap the view to that new table. If you’ve got to refresh a rather larger table, the lock that a truncate takes would make the table unusable during that time. And doing a delete instead of a truncate could lead to some very heavy bloat in addition to the locking. The view swapping minimizes both of these issues.
Some additions to the snapshot process I was able to make were to allow the automatic propagation of column changes. This includes adding & dropping columns as well as type changes. Indexes are also copied over as well at creation time (this will be optional soon). You shouldn’t need to worry about constraints on the destination end since that should be being controlled on the source table. That would make the replication needlessly take longer. Since the table is re-created from scratch on a column change, there is an extra configuration option to run some additional commands such as setting permissions on the view/table. This can also be handled with the default privileges settings that were added in 9.0, but the extension option allows you to control them for each individually replicated table.
The snapshot method is ideal for smaller tables. It’s also much more efficient than the DML replication method (replaying every insert/update/delete, which I’ll be covering in a later post) if almost the entire table is updated in the span of time between table refreshes. One of the things I am currently working on is a way to just skip the refresh process if nothing has changed on the source. This would then make the snapshot process ideal for static tables or ones that rarely ever change.
That’s all for this post. We’re using this extension in some of our production environments already, so I’m confident in the code even though it’s not “1.0”. Would appreciate any feedback to speed up my decision to make such a stable release.
My next post will cover the incremental replication methods.