Subject: CouchDB 1.0 work
Date: Monday 28th April 2008 16:27:40 UTC (over 10 years ago)
Here are my thoughts on what we need for before we can get to CouchDB 1.0. Feedback please. Must have: Incremental reduce: Maybe single biggest outstanding work item. Probably 2 weeks of development to get to a testable state Security/Document validation: We need a way to control who can update what documents and to validate the updates are correct. This is absolutely necessary for offline replication, where replicated updates to the database do not come through the application layer. View index compaction/management: View indexes currently just grow, need a compaction similar to storage compaction. Also, there is no way to purge old unused indexes, except via the OS. File sync problem: file:sync(), a call that flushes all uncommitted writes to disk before returning, doesn't work fully or at all on all some platforms (usually we just lack the flags to tell the OS to write to disk). Should be fixable by either patching the existing Erlang driver source, or using a replacement file driver. Optimizations. Right now HTTP overhead is huge, with HTTP latency/ overhead at about 80% of our document read time when loaded from local client (same machine). Once we can get this down to below 50%, we'll focus on optimizing the database and other component. Most core database operations, document reads, updates and view indexing are completely unoptimized so far, which the update speed being the biggest complaint. Testing: We need lots more tests. By the time we ship 1.0, we should have far more test suite code than production code. And we need to do load testing. Will the current browser based test suite can scale for this kind of heavy testing? Nice to have: Plugs in: Erlang module plug-in architecture, to make adding new server side code easy. Right now the code that maps special urls (_view, _compact, _search, etc) to the appropriate Erlang call is messy and convoluted, and getting worse as we go. We need a standard way to map the special urls to the appropriate Erlang call. Tail committed database headers: To optimize the updating of database by reducing the number and length of seeks required, the file header should be written to the end of the file, rather than the beginning. Depending on platform this can remove a full headseek and in the best case scenario a document insert/update can require zero head seeks (if the head is already positioned at the end of the file). But this can slow file opening speed as it may need to do a search in the file for the most recent valid header. In the result of a crash, the header scan/search cost at database open can be linear or logarithmic, depending on the exact implementation. Clustering: The ability to cluster CouchDB servers, to increase both reliability (failover-clustering) and client scalability (more servers to handle more concurrent user load). Clustering does not increase data scalability, which is (that's partitioning/sharding). Selective document purging/compaction: Deletion stubs are kept around for replication purposes. Need a way to purge the records of document that are old or deleted. Revision rev path pruning: Each document keeps a list of all previous revisions. We need a way to prune the oldest records of document revisions and remerge pruned lists during replication. Don't Need: Authentication. We can go to 1.0 without authentication, relying instead on local proxies to provide authentication. Partioning. Partitioning is a big project with lots of considerations. It's best to move this post 1.0.