Technically, ZooKeeper is only used to find the location of the
accumulo.root table, which tells the client where to find the
accumulo.metadata table. The accumulo.metadata table contains all of the
information about where "user" tables are being hosted. But yes, you
have the right idea.
Yes, in other words, a client does not need to consume all data in the
range or a Tablet in one RPC.
For example, if you are reading from a Tablet in a Table that contains
data in the range [a-z], the client might see batches such as:
[a - frank]
(frank - maria]
(maria - steve]
[steve - z]
Each Key-Value that exists in the Table (contained in the in-memory map
or RFiles in HDFS) is passed through the iterators you have configured.
In practice, Iterators are very analogous to filters and transformations
on a Java List. They take one entry (key-value) and pass it through a
series of Iterators (filters, transformations), and then queue the entry
to be sent to the client. The TabletServer, after any record is passed
through the complete Iterator "stack" (never only some of the
iterators), may choose to flush the queue entries back to the client.
Yamini Joshi wrote:
I see. So, foa a scan opertaion that span 2 tservers: the client knows
that the ts1 contains range a-h (via communication with zookeeper) and
once it gets the data it requests the next range from ts2?
Also, can the data be sent to the client in the middle of the pipeline
Tserver -> it1 -> it2 -> it3 -> client (if the max limit is reached) or
is it always at the the end of the pipeline?
Best regards,
Yamini Joshi
On Tue, Nov 22, 2016 at 12:36 PM, Josh Elser <josh.el...@gmail.com
<mailto:josh.el...@gmail.com>> wrote:
Scanners are sequentially communicating with TabletServers, as
opposed to BatchScanners which do this communication in parallel.
Scanners aren't so much "merging" data, but requesting it in sorted
order from the appropriate TabletServer.
All Iterators are applied to some batch of results from a
TabletServer before the results are sent to the client. So, the
action is more Tserver -> it1 -> it2 -> it3 -> client. Multiple
iterators does not increase the number of RPCs.
And yes, data is returned in batches from a TabletServer,
constrained by the max memory setting you listed. When the
boundaries of the Tablet that are currently be read from are reached
(this is rowId boundaries), the batch would also be returned
immediately.
Yamini Joshi wrote:
So, for a batch scan, the merge is not required but, for a scan,
since
it returns sorted data, data from tserver1 and tserver2 is
merged at the
client?
I know how to write iterators but I can't vsiualize the
workflow. Lets
say in the same example I have 3 custom iterators to be applied
on data:
it1, it2, it3 respectively. When are the iterators applied:
1. scan on tserver -> client -> it1 on tserver -> client -> it2 on
tserver -> client -> it3 on tserver -> client
I'm sure this is not the case, it adds a lot of overhead
2. scan on tserver -> it1 on tserver -> it2 on tserver -> it3 on
tserver -> client
The processing is done in batches?
Data is returned to the client when it reaches the max limit for
table.scan.max.memory even if it is in the middle of the
pipeline above?
Best regards,
Yamini Joshi
On Tue, Nov 22, 2016 at 11:56 AM, Christopher
<ctubb...@apache.org <mailto:ctubb...@apache.org>
<mailto:ctubb...@apache.org <mailto:ctubb...@apache.org>>> wrote:
That's basically how it works, yes.
1. The data from tserver1 and tserver2 necessarily comes
from at
least two different tablets. This is because tables are
divided into
discrete, non-overlapping tablets, and each tablet is
hosted only on
a single tserver. So, it is not normally necessary to merge
the data
from these two sources. Your application may do a join
between the
two tablets on the client side, but that is outside the
scope of
Accumulo.
2. Custom iterators can be applied to minc, majc, and scan
scopes. I
suggest starting here:
https://accumulo.apache.org/1.8/accumulo_user_manual.html#_iterators
<https://accumulo.apache.org/1.8/accumulo_user_manual.html#_iterators>
<https://accumulo.apache.org/1.8/accumulo_user_manual.html#_iterators
<https://accumulo.apache.org/1.8/accumulo_user_manual.html#_iterators>>
On Tue, Nov 22, 2016 at 12:05 PM Yamini Joshi
<yamini.1...@gmail.com <mailto:yamini.1...@gmail.com>
<mailto:yamini.1...@gmail.com <mailto:yamini.1...@gmail.com>>>
wrote:
Hello all
I am trying to understand Accumulo scan workflow. I've
checked
the official docs but I couldn't understand the workflow
properly. Could anyone please tell me if I'm on the
right track?
For example if I want to scan rows in the range e-g in
a table
mytable which is sharded across 3 nodes in the cluster:
Step1: Client connects to the Zookeeper and gets the
location of
the root tablet.
Step2: Client connects to tserver with the root tablet
and gets
the location of mytable.
the row distribution is as follows:
tserver1 tserver2 tserver3
a-g h-k
l-z
Step3: Client connects to tserver1 and tserver2.
Step4: tservers merge and sort data from in-memory
maps, minc
files and majc files, apply versioning iterator, seek the
requested range and send data back to the client.
Is this how a scan works? Also, I have some doubts:
1. Where is the data from tserver1 and tserver2 merged?
2. when and how are custom iterators applied?
Also, if there is any resource explaining this, please
point me
to it. I've found some slides but no detailed explanation.
Best regards,
Yamini Joshi
