Data Durability

A friend of mine half-jokingly says that the only reason to put data into a database is to get it back out again. In order to get data out, we need to ensure some kind of durability.

Relational databases offer single server durability through write-ahead logging and checkpoint mechanisms. These are tried and true methods of writing data to a replay log on disk as well as caching writes in memory. Whenever a checkpoint occurs, dirty data is flushed to disk. The benefit of a write ahead log is that we can always recover from a crash (so long as we have the log files, of course).

How does single server durability work with non-relational databases? Most of them don’t have write-ahead logging.

MongoDB currently has limited single server durability. While some people consider this a weakness, it has some strengths – writes complete very quickly since there is no write-ahead log that needs to immediately sync to disk. MongoDB also has the ability to create replica sets for increased durability. There is one obvious upside to replica sets – the data is in multiple places. Another advantage of replica sets is that it’s possible to usegetLastError({w:...}) to request acknowledgement from multiple replica servers before a write is reported as complete to a client. Just keep in mind that getLastError is not used by default – application code will have to call the method to force the sync.

Setting a w-value for writes is something that was mentioned in Getting Faster Writes with Riak. Although, in that article we were decreasing durability to increase write performance. In Amazon Dynamo inspired systems writes are not considered complete until multiple clients have responded. The advantage is that durable replication is enforced at the database and clients have to elect to use less security for the data. Refer to the Cassandra documentation onWrites and Consistency or the Riak Replication documentation for more information on how Dynamo inspired replication works. Datastores using HDFS for storage can take advantage of HDFS’s built-in data replication.

Even HBase, a column-oriented database, uses HDFS to handle data replication. The trick is that rows may be chopped up based on columns and split into regions. Those regions are then distributed around the cluster on what are called region servers. HBase is designed for real-time read/write random-access. If we’re trying to get real-time reads and writes, we can’t expect HBase to immediately sync files to disk – there’s a commit log(RDBMS people will know this as a write-ahead log). Essentially, when a write comes in from a client, the write is first written to the commit log (which is stored using HDFS), then it’s written in memory and when the in-memory structure fills up, that structure is flushed to the filesystem. Here’s something cunning: since the commit log is being written to HDFS, it’s available in multiple places in the cluster at the same time. If one of the region servers goes down it’s easy enough to recover from – that region server’s commit log is split apart and distributed to other region servers which then take up the load of the failed region server.

There are plenty of HBase details that have been grossly oversimplified or blatantly ignored here for the sake of brevity. Additional details can be found inHBase Architecture 101 – Storage as well as this Advanced HBasepresentation. As HBase is inspired by Google’s big table, additional information can be found in Chang et al. Bigtable: A distributed storage system for structured data and The Google File System.

Interestingly enough, there is a proposed feature for PostgreSQL 9.1 to add synchronous replication to PostgreSQL. Current replication in PostgreSQL is more like asynchronous database mirroring in SQL Server, or the default replica set write scenario with MongoDB. Synchronous replication makes it possible to ensure that data is being written to every node in the RDBMS cluster. Robert Haas discusses some of the pros and cons of replication in PostgreSQL in his post What Kind of Replication Do You Need?.

Microsoft’s Azure environment also has redundancy built in. Much like Hadoop, the redundancy and durability is baked into Azure at the filesystem. Building the redundancy at such a low level makes it easy for every component of the Azure environment to use it to achieve higher availability and durability. The Windows Azure Storage team have put together an excellent overview. Needless to say, Microsoft have implemented a very robust storage architecture for the Azure platform – binary data is split into chunks and spread across multiple servers. Each of those chunks is replicated so that there are three copies of the data at any given time. Future features will allow for data to be seamlessly geographically replicated.

Even SQL Azure, Microsoft’s cloud based relational database, takes advantage of this replication. In SQL Azure when a row is written in the database, the write occurs on three servers simultaneously. Clients don’t even see an operation as having committed until the filesystem has responded from all three locations. Automatic replication is designed into the framework. This prevents the loss of a single server, rack, or rack container from taking down a large number of customers. And, just like in other distributed systems, when a single node goes down, the load and data are moved to other nodes. For a local database, this kind of durability is typically only obtained using a combination of SAN technology, database replication, and database mirroring.

There is a lot of solid technology backing the Azure platform, but I suspect that part of Microsoft’s ultimate goal is to hide the complexity of configuring data durability from the user. It’s foreseeable that future upgrades will make it possible to dial up or down durability for storage.

While relational databases are finding more ways to spread load out and retain consistency, there are changes in store for MongoDB to improve single server durability. MongoDB has been highly criticized for its lack of single server durability. Until recently, the default response has been that you should take frequent backups and write to multiple replicas. This is still a good idea, but it’s promising to see that the MongoDB development team are addressing single server durability concerns.

Why is single server durability important for any database? Aside from guaranteeing that data is correct in the instance of a crash, it also makes it easier to increase adoption of a database at the department level. A durable single database server makes it easy to build an application on your desktop, deploy it to the server under your desk, and move it into the corporate data center as the application gains importance.

Logging and replication are critical technologies for databases. They guarantee data is durable and available. There are also just as many options as there are databases on the market. It’s important to understand the requirements of your application before choosing mechanisms to ensure durability and consistency across multiple servers.

References

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