The Past
It was mid 2011, I had been developing
HDFS/
HBase for five years and I was in love with the
Hadoop ecosystem. Here is a system that can store and query hundreds of petabytes of data without blinking an eye. Hadoop has always focussed on scalability first and Hadoop's scalability was increasing by leaps and bounds. It was apparent that we will be able to store an exabyte of data in a single Hadoop cluster in a few years. I wanted to take on a new challenge...to see if we can extend HDFS's success story from Data Analytics to Query Serving workloads as well.
A Query Serving workload mostly consists of point lookups, random short range scans and point updates. The primary requirement of this workload is
low-latency queries. On the other hand, a Big Data Analytics query typically involves large sequential scans and joins of two or more data sets with a very low volume of updates if any. Thus, the following year I spent comparing HBase/HDFS and
MySQL for a Query Serving workload. The advantage of using HBase is that we can have multiple copies of data within a single data center. I wanted to determine what is needed to migrate a very large Query Serving workload from a cluster of MySQL servers to an HBase/HDFS cluster. This multi-petabyte dataset was stored on spinning disks. After multiple rounds of enhancements to HBase and were able to make it such that HBase latencies were only twice as slow as a MySQL server and HBase was using only three times more IOPs to serve the same workload on the same hardware. We were making steady progress towards our goal... but then something changed!
Flash storage became a reality. The data set migrated from spinning disks to flash. Now, the million dollar question that came up is whether HBase is capable of efficiently using flash hardware. I benchmarked HDFS and HBase with data on flash storage and posted the results in an
earlier post.The short story is that the HDFS/HBase of 2012 had a few software bottlenecks because of which it was not able to use flash storage efficiently. It became clear that if data is stored in flash storage, then we need a new storage engine to be able to serve a random workload on that data efficiently. I started to look out for techniques to build the next generation key-value store, especially designed to serve data from fast storage.
Why do we need an Embedded Database?
Flash is fundamentally different from spinning storage in performance. For the purpose of this discussion, let's assume that a read or write to spinning disk takes about 10 milliseconds while a read or write to flash storage takes about 100 microseconds. Network network-latency between two machines remains around 50 microseconds. These numbers are not cast in stone and your hardware could be very different from this one, but these numbers demonstrate the relative differences between two scenarios. What does this have anything to do with application-systems architecture? A client wants to store and access data from a database. There are two alternatives, it can store data on locally attached disks or it can store data over the network on a remote server that have disks attached to it. If we consider latency, then the locally attached disks can serve a read request in about 10 milliseconds. And in the client-server architecture, accessing the same data over a network results in a latency of 10.05 milliseconds, the overhead imposed by the network being only a miniscule 0.5%. Given this fact, it is easy to understand why a majority of currently-deployed systems use the client-server model of accessing data. (For the purpose of the discussion, I am ignoring network bandwidth limitations).
Now, lets consider the same scenario but with disks replaced by flash drives. A data access in the case of locally attached flash storage is 100 microseconds whereas accessing the same data via the network is 150 micros. Network data access is 50% higher overhead than local data access and 50% is a pretty big number. This means that databases that run embedded within an application could have much lower latency than applications that access data over a network. Thus, the necessity of an
Embedded Database.
The above hypothesis does not state that client-server models will become extinct. The client-server-data-access-model has inherent advantages in the areas of data management and will continue to remain prominent in application deployment scenarios.
Aren't there any existing embedded databases?
Of course there are many existing embedded databases:
BerkeleyDB,
KyotoDB,
SQLite3,
leveldb, etc.
Open-source benchmarks seem to state that leveldb is the fastest of the lot. But not all of them are suitable for storing data on Flash storage. Flash has limited write-endurance; updates to a block of data on Flash typically introduces write-amplification within the Flash driver. Given that we want to run our database on flash, we focussed on measuring write-amplification for evaluating databases technologies.
HBase and
Cassandra are Log Structured Merge (
LSM) style databases, but it will take a lot of engineering work to make HBase and Cassandra be an embeddable library. Both of them are servers with an entire ecosystem of built-in management, configuration and deployments. I was looking for a simple c/c++ library: leveldb was the apparent first choice for our benchmarking.
Why was leveldb insufficient for our purpose?
I started to benchmark leveldb and found that it was unsuitable for our server workloads. Leveldb was a cool piece of work but was not designed for server workloads. The open-source
benchmark results looks awesome at first sight, but I quickly realized that those results were for a database whose size was smaller than the size of RAM on the test machine, i.e. the entire database has to fit in the OS page cache. When I performed the same benchmarks on a database that was at least 5 times larger than main memory, the performance results was dismal.
Leveldb's single-threaded compaction process was insufficient to drive server workloads. Frequent write-stalls caused 99-percentile latency to be tremendously large. Mmap-ing a file into the OS cache introduced performance bottlenecks for reads. Leveldb was unable to consume all the IOs offered by the underlying flash storage.
On the other hand, I was seeing server storage hardware evolve fast in different dimensions, For example, an experimental system where a storage volume is striped across 10 flash cards can provide upwards of a million IOs per second. A NVRAM based storage can support a few million data accesses per second. I would like to use a database that can drive these types of fast storage hardware. A natural evolution of flash storage could lead us to storage that has a very limited erase cycles and I envisioned that a database that can allow elegant tradeoffs of read amplification, write amplification and space amplification would be a dire necessity for these type of storage. Leveldb was not designed to achieve these goals. The best path was to fork the leveldb code and change its architecture to suit these needs. So, RocksDB was born!
The vision for RocksDB
1. An embedded key-value store with point lookups and range scans
2. Optimized for fast storage, e.g. Flash and RAM
3. Server Side database with full production support
4. Scale linearly with number of CPU cores and with storage IOPs
RocksDB is not a distributed database. It does not have fault-tolerance or replication built into it. It does not know anything about data-sharding. It is upto the application that is using RocksDB to implement replication, fault-tolerance and sharding if needed.
Architecture of RocksDB
RocksDB is a C++ library that can be used to persistently store keys and values. Keys and values are arbitrary byte streams. Keys are stored in sorted runs. New writes occur to new places in the storage and a background compaction process eliminates duplicates and processes delete markers. There is support for atomically writing a set of keys into the database. Backward and forward iteration over the keys are supported.
RockDB is built using a "pluggable" architecture. This makes it easy to replace parts of it without impacting the overall architecture of the system. This architecture makes me confident that RocksDB will be easily tunable for different workloads and on different hardware.
For example, one can plug in various compression modules (snappy, zlib, bzip, etc) without changing any RocksDB code. Similarly, an application can plug in their own compaction filter to process keys during compaction; an example application can use it to implement a
expiry-time for keys in the database. RocksDB has pluggable
memtables so that an application can design a custom data structure to cache their writes, one example is prefix-hash-memtable where a portion of the key is hashed and the remainder of the key is arranged in the form of a btree. The implementation of a
sst file is pluggable too and an application can design their own format for sst files. RocksDB supports a MergeType record that allows applications to build higher level constructs (Lists, Counters, etc) by avoiding a read-modify-write.
RocksDB currently supports two styles of compaction: the level style compaction and the universal stye compaction. These two styles offers flexible performance tradeoffs. Compactions are inherently multi-threaded so that large databases can sustain high update rates. I will write a separate post on the pros and cons of these two styles of compaction.
RocksDB exposes interfaces for incremental online backup which is needed for any kind of production usage. It supports setting bloom filters on a sub-part of the key, which is one possible technique to reduce iops needed for range-scans.
RocksDB's apis are
stackable, which means that you can wrap lower level apis with higher level easy-to-use wrappers. This is the topic of a future post.
Potential Use-cases of RocksDB
RocksDB can be used by applications that need low latency database accesses. A user-facing application that stores the viewing history and state of users of a website can potentially store this content on RocksDB. A spam detection application that needs fast access to big data sets can use RocksDB. A graph-search query that needs to scan a data set in realtime can use RocksDB. RocksDB can be used to cache data from Hadoop, thereby allowing applications to query Hadoop data in realtime. A message-queue that supports a high number of inserts and deletes can use RocksDB.
The Road Ahead
There is work-in-progress to make RocksDB be able to serve data at memory speeds when the entire database fits in RAM. RocksDB would evolve to be compatible with highly multi-core machines. With the advent of super-low-endurance-flash storage, we expect RocksDB to store data with minimal write amplification. Maybe sometime in the future, someone will port RocksDB to the Android and iOS Platform. Another nice feature would be to support
Column Families to support better clustering of related data.
I am hoping that software programmers and database developers would use, enhance and customize RocksDB for their use-cases. The code base is in http://github.com/facebook/rocksdb. You can join the Facebook Group https://www.facebook.com/groups/rocksdb.dev to participate in the engineering design discussions about RocksDB.
