Hi Anand! I see... sorry for being so curious, but since I started studying HBase I am curious about how people are modeling their tables, and in what kinds of systems HBase is in use.
Have you evaluated recording your reports in a distinct CF using timestamps as column qualifiers? It's my curiosity asking again! Thanks for sharing! Regards, Cristofer -----Mensagem original----- De: AnandaVelMurugan Chandra Mohan [mailto:[email protected]] Enviada em: quarta-feira, 18 de julho de 2012 13:04 Para: [email protected] Assunto: Re: Rowkey hashing to avoid hotspotting Hi Cristofer, Data i store is test cell reports about a component. I have many test cell reports for each model number + serial number combination. So to make rowkey unique, I added timstamp. On Wed, Jul 18, 2012 at 3:14 AM, Cristofer Weber < [email protected]> wrote: > So, Anand, there are some things that can help, but again, most of > them are related with the famous access patterns. > > Sometimes is not easy to get more information about them in advance, > but if you are replacing another system you can study its data > distribution, grouping for counts, mean, changes over time, etc. It is > possible to analyze with partial data too, but it is risky because you > will be subjected to the way this partial data was gathered; sample > data may not be representative. > > Salting your rowkey with a hash calculated over your model# will > probably result in an uniform distribution over a range (if using > modulus), and pre-spliting your table will balance your load over your Region > Servers. > Also, you will be able to recalculate your hash for your model# before > scanning for it, allowing for a scan over specific rowkey while > restricting this scan by startRow and stopRow. Remember that if your > rowkeys shares the same prefix they will probably be located in the > same region and your scan will be favored by this. > > I'm still curious about your need of adding a timestamp after your > model#,serial#... I have some background in manufacturing systems and > usually a serial number is unique. But, of course, it's just > curiosity. :-) > > Regards, > Cristofer > > -----Mensagem original----- > De: Alex Baranau [mailto:[email protected]] Enviada em: > terça-feira, 17 de julho de 2012 12:53 > Para: [email protected] > Assunto: Re: Rowkey hashing to avoid hotspotting > > The most common reason for RS hotspotting during writing data in HBase > is writing rows with monotonically increasing/decreasing row keys. > E.g. if you put timestamp in the first part of your key, then you are > likely to have monotonically increasing row keys. You can find more > info about this issue and how to solve it here: [1] and also you may > want to look at already implemented salting solution [2]. > > As for RS hotspotting during reading - it is hard to predict without > knowing what it the most common data access patterns. E.g. putting > model # in first part of a key may seem like a good distribution, but > if your web site used mostly by Mercedes owners, the majority of the > read load may be directed to just few regions. Again, salting can help a lot > here. > > +1 to what Cristofer said on other things, esp: use partial key scans > +were > possible instead of filters and pre-split your table. > > Alex Baranau > ------ > Sematext :: http://blog.sematext.com/ :: Hadoop - HBase - > ElasticSearch - Solr > > [1] http://bit.ly/HnKjbc > [2] https://github.com/sematext/HBaseWD > > On Tue, Jul 17, 2012 at 10:44 AM, AnandaVelMurugan Chandra Mohan < > [email protected]> wrote: > > > Hi Cristofer, > > > > Thanks for elaborate response!!! > > > > I have no much information about production data as I work with > > partial data. But based on discussion with my project partners, I > > have some answers for you. > > > > Number of model numbers and serial numbers will be finite. Not so many... > > As far as I know,there is no predefined rule for model number or > > serial number creation. > > > > I have two access pattern. I count the number of rows for a specific > > model number. I use rowkey filter for this. Also I filter the rows > > based on model, serial number and some other columns. I scan the > > table with column value filter for this case. > > > > I will evaluate salting as you have explained. > > > > Regards, > > Anand.C > > > > On Tue, Jul 17, 2012 at 12:30 AM, Cristofer Weber < > > [email protected]> wrote: > > > > > Hi Anand, > > > > > > As usual, the answer is that 'it depends' :) > > > > > > I think that the main question here is: why are you afraid that > > > this > > setup > > > would lead to region server hotspotting? Is because you don't know > > > how > > your > > > production data will seems? > > > > > > Based on what you told about your rowkey, you will query mostly by > > > providing model no. + serial no., but: > > > 1 - How is your rowkey distribution? There are tons of different > > > modelNumbers AND serialNumbers? Few modelNumbers and a lot of > > > serialNumbers? Few of both? > > > 2 - Putting modelNumber in front of your rowkey means that your > > > data will be sorted by rowkey. So, what is the rule that > > > determinates a modelNumber creation? Is it a sequential number > > > that will be increased by time? If > > so, > > > are newer members accessed a lot more than older members? If not, > > > what > > will > > > drive this number? Is it an encoding rule? > > > 3 - Do you expect more write/read load over a few of these > > > modelNumbers and/or serialNumbers? Will it be similar to a Pareto > Distribution? > > > Distributed over what? > > > > > > Also, two other things got my attention here... > > > 1 - Why are you filtering with regex? If your queries are over > > > model > no. > > + > > > serial no., why don't you just scan starting by your > > > modelNumber+SerialNumber, and stoping on your next SerialNumber? > > > modelNumber+Or is there another access pattern that doesn't > > > apply to your composited rowkey? > > > 2 - Why do you have to add a timestamp to ensure uniqueness? > > > > > > Now, answering your question without more info about your data, > > > you can apply hash in two ways: > > > 1 - Generating a hash (MD5 is the most common as far as I read > > > about) and using only this hash as your rowkey. Based on what you > > > have told, this > > way > > > doesn't fit your needs, because you would not be able to do apply > > > your filter anymore. > > > 2 - Salting, by prefixing your current rowkey with a pinch of hash. > > Notice > > > that the hash portion must be your rowkey prefix to ensure a kind > > > of balanced distribution over something (where something is your > > > region servers). I'm working with a case that is a bit similar to > > > yours, and > > what > > > I'm doing right now is calculating the hashValue of my rowkey and > > > using a Java Formatter to create a hex string to prepend to my > > > rowkey. Something like a String.format("%03x", hashValue) > > > > > > In both cases, you still have to split your regions in advance, > > > and it will be better to work your splitting before starting to > > > feed your table with production data. > > > > > > Also, you have to study the consequences that changing your rowkey > > > will bring. It's not for free. > > > > > > There's a lot of words here and a lot of questions, so by now I > > > feel I started to shoot in the dark. Try to understand your > > > production data and > > if > > > you have more to share, for sure it will help! > > > > > > Regards, > > > Cristofer > > > > > > -----Mensagem original----- > > > De: AnandaVelMurugan Chandra Mohan [mailto:[email protected]] > > > Enviada em: segunda-feira, 16 de julho de 2012 02:30 > > > Para: [email protected] > > > Assunto: Rowkey hashing to avoid hotspotting > > > > > > Hi, > > > > > > I am using Hbase to store data about mechanical components. Each > > component > > > has model no. and serial no. and some other attributes. > > > > > > I would be querying my data mostly by model no. and serial no. So > > > I created a composite key with these two attributes and added > > > timestamp to make it unique. > > > > > > To filter the data, I use rowkey filter with regex string > > > comparator and it works well with sample seed data. Now I am > > > afraid whether this set up will lead to region server hotspotting > > > when we load production data in HBase. I read hashing may solve > > > this problem. Can some one help me in implementing hashing the row > > > key? Also I would want the row filter to > > work > > > as I have to display the number of components in a web page and I > > > use row key filter for implementing that functionality? Any > > > guidance would be of great help. > > > > > > -- > > > Regards, > > > Anand > > > > > > > > > > > -- > > Regards, > > Anand > > > > > > -- > Alex Baranau > ------ > Sematext :: http://blog.sematext.com/ :: Hadoop - HBase - > ElasticSearch - Solr > -- Regards, Anand
