Take the 2-minute tour ×
Database Administrators Stack Exchange is a question and answer site for database professionals who wish to improve their database skills and learn from others in the community. It's 100% free, no registration required.

Imagine a project which requires to be able to process 50,000+ (tiny ints) writes per second. The data, once inserted, won't change. The data is a measurement on a particular time. Sub second resolution is not required (two measurements of the same device can be a second apart).

We are willing to buy/rent more hardware but not more expensive hardware (no fancy RAID etc).

The data is ordered in sets (per device), a couple of hundreds. And should thus be shown/graphed. There is no relation between devices but we do want to group them (showing, for example, 10 graphs).

The only other requirement is that the datastore is open source and *nix based and has reasonable (community) support.

What kind of datastore would you use?

share|improve this question
    
Is graphing the data your only requirement, or do you want to do some kind of querying on the data? –  FreshPhilOfSO Mar 22 '12 at 9:59
    
Querying would be nice (eg select everything above a certain int). But initially having a list of numbers would be cool. –  harm Mar 22 '12 at 13:45
    
Would you be willing to do this in two stages? First stage is just to write the >50k records to a file, and then the second stage would be to process and load that to a database, but possibly at a rate slower than 50k/s. Basically, does it need to go to the database immediately? –  FrustratedWithFormsDesigner Mar 22 '12 at 15:08
    
I would be willing to do it in two steps. But wouldn't this lead to a lagging database? There are 50k writes coming in, if the db writes them slower then that it would introduce an ever bigger lag. –  harm Mar 22 '12 at 16:22
    
So your answer is that batch processing isn't a good choice because the measurements won't ever stop, correct? They will be processed constantly, so the read portion of the database needs to keep up? –  Myrddin Emrys Mar 22 '12 at 23:25
show 3 more comments

2 Answers

up vote 2 down vote accepted

A 'data logging' approach to the problem

For anything involving 50,000 inserts a second you will need to batch your inserts. If you have to run it on cheap hardware then you need to look at the efficiency of your I/O.

If you assume:

  • Up to the second latency is not necessary (i.e. batched writes are acceptable)

  • You have no requirement for analytics within the application except your graphs by device.

  • Any further analytic requirements can be met by batching data into an external system

  • Some administrative overhead is acceptable in adding devices to the system (i.e. they have to be registered).

  • A polled once per second read is acceptable - incoming items can be batched and recorded on a per-second basis with a null recorded if a datum is not produced by a given device within that period.

Then you can store one row per second with all the devices de-normalised into an array of readings. The application maintains a registry of offsets into this array by device. As you add more devices the BLOB simply expands.

This changes your problem to one of storing one 50K-ish BLOB per second indexed on the time, which can be done by pretty much any DBMS platform that supports BLOBs. You might even be able to use a key-value pair system such as Berkely DB.

50K per second is 180MB per hour, 4.3GB per day and approximately 1.5TB per year. This should be possible to manage even with fairly modest hardware. Depending on how much you need to archive you can periodically clear down historical data. You will need something that supports partitioned tables to do this efficiently, though.

Getting the data back out

One disadvantage of this approach is that you would have to read your entire data set to query the statistics for any given device, which would imply scanning 4GB of data for a single day. If you need to support a lot of users querying the devices on an ad-hoc basis you will need to find a way to supplement this store with a fast querying capability. Some possibilities for this are:

  • One hour's data is approximately 180MB in memory. Cache a rolling period of a few hours - either within your application, or in leading partitions on a fast store (maybe a SSD) that can be archived off on an hourly basis.

    If your applications that displayed the graphs queried this on a rolling basis (i.e. only got the most recent data) then this structure could be keyed on time and still support a significant query workload.

    You may need to benchmark the BLOB reader performance of your platform to judge whether you need to build additional caching within your middle-tier, although you will almost certainly need a middle tier server application of some description to avoid shipping the entire BLOB record out to the client.

  • If you only expect to monitor a small subset of sensors at any given time, then you may be able to run caching on a per-sensor basis, with the cache living for some period (e.g. 10 minutes) after the last read for that sensor. The main cache would push values out into the per-sensor cache until the per-sensor cache expires. Per-sensor caching could be implemented fairly efficiently as a ring buffer of 1-2 hours (or whatever period seems appropriate) worth of sensor readings with supplementary data for the beginning and end periods. A time period should then resolve into an offset within the ring buffer. In this case the memory overhead per sensor amounts to a few bytes of header information.

  • Calculate aggregates of average readings for each minute, 10 minutes, hour or suitable period, and store those in a supplementary table. Graphing data over longer time periods can use the aggregates.

  • Take the aggregate values and un-pivot them into a dimensional structure in a supplementary data mart (i.e. with time and sensor ID dimensions). This can be done on (for example) a nightly basis. Ad-hoc queries on historical data can come from the data mart.

Some combination of these approaches will allow quick access to near-realtime data with a fair amount of read traffic. Even if you have to cache this with your application and write something hand built to do it.

Administrative overhead

A sensor will have to be registered with the system and given an ID (essentially the ID is - or maps to - an array index within the BLOB). As the BLOB is opaque the database doesn't need to care about columns. A KVP store that can deal with BLOBs efficiently may be sufficient.

A sensor can be registered with the system, which will then start recording data. You may need to explicitly record start/end dates of sensor connections, and maintain metadata that links sensor IDs to offsets within the array. You will need to recycle array slots as it's a static structure. Any middle-tier caching will have to be aware of this as well.

Message queues

You may wish to consider an asynchronous architecture using a transactional queue manager such as RabbitMQ to manage spikes in the read workload with the regular writes. The logging process and a query fronting process would collect requests to be processed in a batch queue. This would smooth out spikes in the system load. If you anticipate a significant ad-hoc query workload you may wish to look into this.

Pros and Cons - summary

This structure is optimised to minimise the storage space requirements, which should in turn minimise the I/O overhead on the system and allow it to run on relatively cheap hardware - in particular hardware with relatively cheap storage subsystems. In principle an ordinary PC should be able to handle the write workload from the sensors, although you may need faster disk hardware or SSDs to cope with any substantial query workload.

However, this architecture requires a fair bit of plumbing to get the data back out. The Sensor ID cannot be indexed in the database, as it is just an offset into an array of sensor readings.1 This means an entire periods worth of data must be read to get a single sensor reading out. The querying functionality may therefore require a fairly sophisticated caching function in the middle tier to get decent performance, so this approach comes at the price of some development overhead.

1 This is an egregious violation of first normal form, although a relational store may not be necessary or even desirable for this application.

share|improve this answer
add comment
  1. I have placed the answer here since I was restricted by the size of the comment field. This is based on an approach initially suggested by @FrustratedWithFormsDesigner. I would recommend MySQL 5.5 from a cost/performance basis, but PostGresql would work as well (free license).

  2. The two step approach to loading the data is as follows:

a) Write the >50k writes to a text file and then transfer it to same server as the database (for performance reasons)

b) This is an optional step depending on how the data is formatted in the text file - transform the data into a delimited format and move it to a loading directory

c) Use the LOAD DATA INFILE command (http://dev.mysql.com/doc/refman/5.0/en/load-data.html) to load the text files into your tables

  1. Schedule a) and b) as cron jobs, but remember to removed files which have been loaded so that you do not get duplicated data

  2. These are just guidelines and will be made more complicated depending on the formats of the data and of the database into which they are loaded

share|improve this answer
    
Can MySQL and/or PostgreSQL handle a 50k line LOAD DATA INFILE once per second? Is that command faster than direct writes to the database? –  Myrddin Emrys Mar 23 '12 at 15:56
    
When you use loading you can queue the files to be loaded as fast the database can handle them, but the files will be loaded. If you remove keys and index definitions from the tables you may be able to get that level of performance. –  ssmusoke Mar 23 '12 at 16:04
    
That still doesn't answer the question of whether this method is an improvement over a direct connection to the database and normal inserts, particularly since the user presumably must have indexing or they will be unable to get any useful information back out of the database... this is 129 billion entries per month at this rate. Do you want to query a 129 billion line table with no index? –  Myrddin Emrys Mar 23 '12 at 16:12
    
@MyrddinEmrys In my experience this method is an improvement, since it allows you to batch and throttle the inserts of the data. If the database slows down for some reason you do not lose data from the inserts. Also you can have indexes on the table then disable index updates as you load the data files and enable them after. –  ssmusoke Mar 24 '12 at 6:48
    
@Myrddin Emrys - If the OP's application can cope with batched writes than bulk inserts will be much faster. –  ConcernedOfTunbridgeWells Apr 22 '12 at 13:22
add comment

Your Answer

 
discard

By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.