Via Mulley. Magnet doing well, with UPC coming second; UPC have dropped a fair bit in the past month. Would love to see it broken down by region…
In practice there are two gotchas that are so painful I am looking for a replacement with a different featureset than couchdb provides. The location tracking project icecondor.com uses couchdb to store 20,000 new records per day. It has more write traffic than read traffic and runs on modest hardware. Those two gotchas are: 1. View Index updates. While I have a vague understanding of why view index updates are slow and bulky and important, in practice it is unworkable. Every write sets up a trap for the first reader to come along after the write. The more writes there are, the bigger the trap for the first reader which has to wait on the couchdb process that refreshes the view index on an as-needed basis. I believe this trade-off was made to keep writes fast. No need to update the view index until all writes are actually complete, right? Write traffic is heavier than read traffic and the time needed for that index refresh causes the webapp to crash because its not setup to handle timeouts from a database query. The workaround is as hackish as one can imagine – cron jobs to hit every map/reduce query to keep indexes fresh. 2. Append only database file Append only is in theory a great way to ensure on-disk reliability. A system crash during an append should only affect that append. Its a crash during an update to existing parts of the file that risks the integrity of more than whats being updated. With so many layers of caching and optimizations in the kernel and the filesystem and now in the workings of SSD drives, I’m not sure append-only gives extra protection anymore. What it does do is a create a huge operational headache. The on-disk file can never grow beyond half the available storage space. Record deletion uses new disk space and if the half-full mark approaches, vacuuming must be done. The entire database is rewritten to the filesystem, leaving out no longer needed records. If the data file should happen to grow beyond half the partition, the system has esentially crashed because there is no way to compact the file and soon the partition will be full. This is a likely scenario when there is a lot of record deletion activity. The system in question does a lot of writes of temporary data that is followed up by deletes a few days later. There is also a lot of permanent storage that hardly gets used. Rewriting every byte of the records that are long-lived due to compaction is an enormous amount of wasted I/O – doubly so given SSD drives have a short write-cycle lifespan.
Jonathan Ellis on some CouchDB negatives:
Here are some reasons you should think twice and do careful testing before using CouchDB in a non-toy project: Writes are serialized. Not serialized as in the isolation level, serialized as in there can only be one write active at a time. Want to spread writes across multiple disks? Sorry. CouchDB uses a MVCC model, which means that updates and deletes need to be compacted for the space to be made available to new writes. Just like PostgreSQL, only without the man-years of effort to make vacuum hurt less. CouchDB is simple. Gloriously simple. Why is that a negative? It’s competing with systems (in the popular imagination, if not in its author’s mind) that have been maturing for years. The reason PostgreSQL et al have those features is because people want them. And if you don’t, you should at least ask a DBA with a few years of non-MySQL experience what you’ll be missing. The majority of CouchDB fans don’t appear to really understand what a good relational database gives them, just as a lot of PHP programmers don’t get what the big deal is with namespaces. A special case of simplicity deserves mention: nontrivial queries must be created as a view with mapreduce. MapReduce is a great approach to trivially parallelizing certain classes of problem. The problem is, it’s tedious and error-prone to write raw MapReduce code. This is why Google and Yahoo have both created high-level languages on top of it (Sawzall and Pig, respectively). Poor SQL; even with DSLs being the new hotness, people forget that SQL is one of the original domain-specific languages. It’s a little verbose, and you might be bored with it, but it’s much better than writing low-level mapreduce code.
Good write up of CouchDB replication
There really isn’t a separate “protocol” per se for replication. Instead, replication uses CouchDB’s REST API and data model. It’s therefore a bit difficult to talk about replication independently of the rest of CouchDB. In this document I’ll focus on the algorithm used, and link to documentation of the APIs it invokes. The “protocol” is simply the set of those APIs operating over HTTP.
A good writeup of how to detect cases of copyright infringement for photography, art and other visual media.
Von Glitschka, Modern Dog and myriad others make clear that the support of the creative community is absolutely vital in raising awareness of copyright infringements. Sites like www.youthoughtwewouldntnotice.com name and shame clear breaches of copyright, while the Modern Dog case shows that there is no better IP tracing system than the eyes and ears of the design community itself. “It’s the industry at large that has kept me aware of infringements,” states Von. “Without that I would miss most of them because I don’t go looking – they find me via the eyes of others.”
an [LGPL] open-source data processing library following the spirit of NoSQL movement. It offers a set of searching functions supported by compressed bitmap indexes. It treats user data in the column-oriented manner similar to well-known database management systems such as Sybase IQ, MonetDB, and Vertica. It is designed to accelerate user’s data selection tasks without imposing undue requirements. In particular, the user data is NOT required to be under the control of FastBit software, which allows the user to continue to use their existing data analysis tools. The key technology underlying the FastBit software is a set of compressed bitmap indexes. In database systems, an index is a data structure to accelerate data accesses and reduce the query response time. Most of the commonly used indexes are variants of the B-tree, such as B+-tree and B*-tree. FastBit implements a set of alternative indexes called compressed bitmap indexes. Compared with B-tree variants, these indexes provide very efficient searching and retrieval operations, but are somewhat slower to update after a modification of an individual record. A key innovation in FastBit is the Word-Aligned Hybrid compression (WAH) for the bitmaps.[…] Another innovation in FastBit is the multi-level bitmap encoding methods.
The bit array data structure is implemented in Java as the BitSet class. Unfortunately, this fails to scale without compression. JavaEWAH is a word-aligned compressed variant of the Java bitset class. It uses a 64-bit run-length encoding (RLE) compression scheme. We trade-off some compression for better processing speed. We also have a 32-bit version which compresses better, but is not as fast. In general, the goal of word-aligned compression is not to achieve the best compression, but rather to improve query processing time. Hence, we try to save CPU cycles, maybe at the expense of storage. However, the EWAH scheme we implemented is always more efficient storage-wise than an uncompressed bitmap (as implemented in the BitSet class). Unlike some alternatives, javaewah does not rely on a patented scheme.
Links for 2013-04-09
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